Tuesday, November 27, 2007

Vertebrates

Fish
Fish are
aquatic vertebrates that are cold-blooded, covered with scales, and equipped with two sets of paired fins and several unpaired fins. Fish are abundant in the sea and in fresh water, with species being known from mountain streams (e.g., char and gudgeon) as well as in the deepest depths of the ocean (e.g., gulpers and anglerfish). They are of tremendous importance as food for people around the world, either collected from the wild (see fishing) or farmed in much the same way as cattle or chickens (see aquaculture). Fish are also exploited for recreation, through angling and fishkeeping, and fish are commonly exhibited in public aquaria. Fish have an important role in many cultures through the ages, ranging as widely as deities and religious symbols to subjects of books and popular movies.
Definition
The term "fish" is most precisely used to describe any non-
tetrapod chordate, i.e., an animal with a backbone that has gills throughout life and has limbs, if any, in the shape of fins.[1] Unlike groupings such as birds or mammals, fish are not a single clade but a paraphyletic collection of taxa, including hagfishes, lampreys, sharks and rays, ray-finned fishes, coelacanths, and lungfishes
A typical fish is
cold-blooded; has a streamlined body that allows it to swim rapidly; extracts oxygen from the water using gills or an accessory breathing organ to enable it to breath atmospheric oxygen; has two sets of paired fins, usually one or two (rarely three) dorsal fins, an anal fin, and a tail fin; has jaws; has skin that is usually covered with scales; and lays eggs that are fertilized internally or externally.
Fish come in many shapes and sizes. This is a
sea dragon, a close relative of the seahorse. Their leaf-like appendages enable them to blend in with floating seaweed
To each of these there are exceptions.
Tuna, Swordfish, and some species of sharks show some warm-blooded adaptations, and are able to raise their body temperature significantly above that of the ambient water surrounding them.[4] Streamlining and swimming performance varies from highly streamlined and rapid swimmers which are able to reach 10-20 body-lengths per second (such as tuna, salmon, and jacks) through to slow but more maneuverable species such as eels and rays that reach no more than 0.5 body-lengths per second.[5] Many groups of freshwater fish extract oxygen from the air as well as from the water using a variety of different structures. Lungfish have paired lungs similar to those of tetrapods, gouramis have a structure called the labyrinth organ that performs a similar function, while many catfish, such as Corydoras extract oxygen via the intestine or stomach.[6] Body shape and the arrangement of the fins is highly variable, covering such seemingly un-fishlike forms as seahorses, pufferfish, anglerfish, and gulpers. Similarly, the surface of the skin may be naked (as in moray eels), or covered with scales of a variety of different types usually defined as placoid (typical of sharks and rays), cosmoid (fossil lungfishes and coelacanths), ganoid (various fossil fishes but also living gars and bichirs, cycloid, and ctenoid (these last two are found on most bony fish.[7] There are even fishes that spend most of their time out of water. Mudskippers feed and interact with one another on mudflats and are only underwater when hiding in their burrows.[8] The catfish Phreatobius cisternarum lives in underground, phreatic habitats, and a relative lives in waterlogged leaf litter.[9][10]
Fish range in size from the 16 m (51 ft)
whale shark to the 8 mm (just over ¼ of an inch) long stout infantfish.
Many types of
aquatic animals commonly referred to as "fish" are not fish in the sense given above; see Fish (disambiguation).
Digestive system
The advent of jaws allowed fish to eat a much wider variety of food, including plants and other organisms. In fish, food is ingested through the mouth and then broken down in the
esophagus. When it enters the stomach, the food is further broken down and, in many fish, further processed in fingerlike pouches called pyloric caeca. The pyloric caeca secrete digestive enzymes and absorb nutrients from the digested food. Organs such as the liver and pancreas add enzymes and various digestive chemicals as the food moves through the digestive tract. The intestine completes the process of digestion and nutrient absorption.
Respiratory system
Most fish exchange gases by using
gills that are located on either side of the pharynx. Gills are made up of threadlike structures called filaments. Each filament contains a network of capillaries that allow a large surface area for the exchange of oxygen and carbon dioxide. Fish exchange gases by pulling oxygen-rich water through their mouths and pumping it over their gill filaments. The blood in the capillaries flows in the opposite direction to the water, causing counter current exchange. They then push the oxygen-poor water out through openings in the sides of the pharynx. Some fishes, like sharks and lampreys, possess multiple gill openings. However, most fishes have a single gill opening on each side of the body. This opening is hidden beneath a protective bony cover called an operculum.
Juvenile
bichirs have external gills, a very primitive feature that they hold in common with larval amphibians.
Swim bladder of a Rudd (Scardinius erythrophthalmus)
Many fish can breathe air. The mechanisms for doing so are varied. The skin of anguillid eels may be used to absorb oxygen. The buccal cavity of the
electric eel may be used to breathe air. Catfishes of the families Loricariidae, Callichthyidae, and Scoloplacidae are able to absorb air through their digestive tracts.[13] Lungfish and bichirs have paired lungs similar to those of tetrapods and must rise to the surface of the water to gulp fresh air in through the mouth and pass spent air out through the gills. Gar and bowfin have a vascularised swim bladder that is used in the same way. Loaches, trahiras, and many catfish breathe by passing air through the gut. Mudskippers breathe by absorbing oxygen across the skin (similar to what frogs do). A number of fishes have evolved so-called accessory breathing organs that are used to extract oxygen from the air. Labyrinth fish (such as gouramis and bettas) have a labyrinth organ above the gills that performs this function. A few other fish have structures more or less resembling labyrinth organs in form and function, most notably snakeheads, pikeheads, and the Clariidae family of catfish.
Being able to breathe air is primarily of use to fish that inhabit shallow, seasonally variable waters where the oxygen concentration in the water may decline at certain times of the year. At such times, fishes dependent solely on the oxygen in the water, such as perch and cichlids, will quickly suffocate, but air-breathing fish can survive for much longer, in some cases in water that is little more than wet mud. At the most extreme, some of these air-breathing fish are able to survive in damp burrows for weeks after the water has otherwise completely dried up, entering a state of
aestivation until the water returns.
Tuna gills inside of the head. The fish head is oriented snout-downwards, with the view looking towards the mouth.
Fish can be divided into obligate air breathers and facultative air breathers. Obligate air breathers, such as the African lungfish, must breathe air periodically or they will suffocate. Facultative air breathers, such as the catfish Hypostomus plecostomus, will only breathe air if they need to and will otherwise rely solely on their gills for oxygen if conditions are favourable. Most fish are not obligate air breathers as there is an energetic cost in rising to the surface and a fitness cost of being exposed to predators.
[13]
Circulatory system
Fish have a
closed circulatory system with a heart that pumps the blood in a single loop throughout the body. The blood goes from the heart to gills, from the gills to the rest of the body, and then back to the heart. In most fish, the heart consists of four parts: the sinus venosus, the atrium, the ventricle, and the bulbus arteriosus. Despite consisting of four parts, the fish heart is still a two-chambered heart.[14] The sinus venosus is a thin-walled sac that collects blood from the fish's veins before allowing it to flow to the atrium, which is a large muscular chamber. The atrium serves as a one-way compartment for blood to flow into the ventricle. The ventricle is a thick-walled, muscular chamber and it does the actual pumping for the heart. It pumps blood to a large tube called the bulbus arteriosus. At the front end, the bulbus arteriosus connects to a large blood vessel called the aorta, through which blood flows to the fish's gills.
Excretory system
As with many aquatic animals, most fish release their nitrogenous wastes as
ammonia. Some of the wastes diffuse through the gills into the surrounding water. Others are removed by the kidneys, excretory organs that filter wastes from the blood. Kidneys help fishes control the amount of ammonia in their bodies. Saltwater fish tend to lose water because of osmosis. In saltwater fish, the kidneys concentrate wastes and return as much water as possible back to the body. The reverse happens in freshwater fish, they tend to gain water continuously. The kidneys of freshwater fish are specially adapted to pump out large amounts of dilute urine. Some fish have specially adapted kidneys that change their function, allowing them to move from freshwater to saltwater.
Sensory and nervous system

Central nervous system
Fish typically have quite small
brains relative to body size when compared with other vertebrates, typically one-fifteenth the mass of the brain from a similarly sized bird or mammal.[15] However, some fishes have relatively large brains, most notably mormyrids and sharks, which have brains of about as massive relative to body weight as birds and marsupials.[16]
The brain is divided into several regions. At the front are the
olfactory lobes, a pair of structure the receive and process signals from the nostrils via the two olfactory nerves.[15] The olfactory lobes are very large in fishes that hunt primarily by smell, such as hagfish, sharks, and catfish. Behind the olfactory lobes is the two-lobed telencephalon, the equivalent structure to the cerebrum in higher vertebrates. In fishes the telencephalon is concerned mostly with olfaction.[15] Together these structures form the forebrain.
Connecting the forebrain to the midbrain is the
diencephalon (in the adjacent diagram, this structure is below the optic lobes and consequently not visible). The diencephalon performs a number of functions associated with hormones and homeostasis.[15] The pineal body lies just above the diencephalon. This structure performs many different functions including detecting light, maintaining circadian rhythms, and controlling colour changes.[15]
The
midbrain or mesencephalon contains the two optic lobes. These are very large in species that hunt by sight, such as rainbow trout and cichlids.[15]
The hindbrain or
metencephalon is particularly involved in swimming and balance.[15] The cerebellum is a single-lobed structure that is usually very large, typically the biggest part of the brain.]Hagfish and lampreys have relatively small cerebellums, but at the other extreme the cerebellums of mormyrids are massively developed and apparently involved in their sense. The brain stem or myelencephalon is the most posterior part of the brain.[15] As well as controlling the functions of some of the muscles and body organs, in bony fish at least the brain stem is also concerned with respiration and osmoregulation.[15]
Sense organs
Most fish possess highly developed sense organs. Nearly all daylight fish have well-developed eyes that have color vision that is at least as good as a human's. Many fish also have specialized cells known as chemoreceptors that are responsible for extraordinary senses of taste and smell. Although they have ears in their heads, many fish may not hear sounds very well. However, most fishes have sensitive receptors that form the
lateral line system. The lateral line system allows for many fish to detect gentle currents and vibrations, as well as to sense the motion of other nearby fish and prey.[17] Some fishes such as catfishes and sharks, have organs that detect low levels electric current.[18] Other fish, like the electric eel, can produce their own electricity.
Pain reception in fish
In 2003, it was concluded by Scottish
scientists at the University of Edinburgh performing research on rainbow trout that fish exhibit behaviors often associated with pain.[19] Professor James D. Rose of the University of Wyoming found flaws in the study and published a critique of it.[20] Rose had published his own study a year earlier concluding that fish cannot feel pain as they lack the appropriate neocortex of the brain.[21] Other researchers have suggested that the question of pain in fish may be diffcult to answer since fish "pain" might be felt differently than human "pain."[22]
Muscular system
Most fish move by contracting paired sets of muscles on either side of the backbone alternately. These contractions form S-shaped curves that move down the body of the fish. As each curve reaches the back fin, backward force is created. This backward force, in conjunction with the fins, moves the fish forward. The fish's fins are used like an airplane's stabilizers. Fins also increase the surface area of the tail, allowing for an extra boost in speed. The streamlined body of the fish decreases the amount of friction as they move through water. Since body tissue is more dense than water, fish must compensate for the difference or they will sink. Many bony fishes have an internal organ called a
swim bladder that adjust their buoyancy through manipulation of gases.
Reproductive system
Organs
Fish reproductive organs include
testes and ovaries. In most fish species, gonads are paired organs of similar size, which can be partially or totally fused.[ There may also be a range of secondary reproductive organs that help in increasing a fish's fitness.
In terms of
spermatogonia distribution, the structure of teleosts testes has two types: in the most common, spermatogonia occur all along the seminiferous tubules, while in Atherinomorph fishes they are confined to the distal portion of these structures. Fishes can present cystic or semi-cystic spermatogenesis in relation to the phase of release of germ cells in cysts to the seminiferous tubules lumen. Fish ovaries may be of two types: gymnovarian or cystovarian. In the first type, the oocytes are released directly into the coelomic cavity and then eliminated. In the second type, the oocytes are conveyed to the exterior through the oviduct. Gymnovaries are the primitive condition found in lungfishes, sturgeons, and bowfins. Cystovaries are the condition that characterizes most of the teleosts, where the ovary lumen has continuity with the oviduct.[23]
Oogonia development in teleosts fish varies according to the group, and the determination of oogenesis dynamics allows the understanding of maturation and fertilization processes. Changes in the nucleus, ooplasm, and the surrounding layers characterize the oocyte maturation process.
[23]
Postovulatory follicles are structures formed after oocyte release; they do not have
endocrine function, present a wide irregular lumen, and are rapidly reabosrbed in a process involving the apoptosis of follicular cells. A degenerative process called follicular atresia reabsorbs vitellogenic oocytes not spawned. This process can also occur, but less frequently, in oocytes in other development stages.[23]
Some fish are
hermaphrodites, either having testes and ovaries at different phases in the life cycle. However, there are even some fish, such as hamlets, that are simultaneously male and female.
Reproductive method
Over 97% of all known fishes are
oviparous,[25] that is, the eggs develop outside the mother's body. Examples of oviparous fishes include salmon, goldfish, cichlids, tuna, and eels. In the majority of these species, fertilisation takes place outside the mother's body, with the male and female fish shedding their gametes into the surrounding water. However, a few oviparous fishes practise internal fertilisation, with the male using some sort of intromittent organ to deliver sperm into the genital opening of the female, most notably the oviparous sharks, such as the horn shark, and oviparous rays, such as skates. In these cases, the male is equipped with a pair of modified pelvic fins known as claspers.
The newly-hatched young of oviparous fish are called
larvae. They are usually poorly formed, carry a large yolk sac (from which they gain their nutrition) and are very different in appearance to juvenile and adult specimens of their species. The larval period in oviparous fish is relatively short however (usually only several weeks), and larvae rapidly grow and change appearance and structure (a process termed metamorphosis) to resemble juveniles of their species. During this transition larvae use up their yolk sac and must switch from yolk sac nutrition to feeding on zooplankton prey, a process which is dependent on zooplankton prey densities and causes many mortalities in larvae.
Ovoviviparous fish are ones in which the eggs develop inside the mother's body after internal fertilization but receive little or no nutrition from the mother, depending instead on the yolk. Each embryo develops in its own egg. Familiar examples of ovoviviparous fishes include guppies, angel sharks, and coelacanths.
Some species of fish are
viviparous. In such species the mother retains the eggs, as in ovoviviparous fishes, but the embryos receive nutrition from the mother in a variety of different ways. Typically, viviparous fishes have a structure analogous to the placenta seen in mammals connecting the mother's blood supply with the that of the embryo. Examples of viviparous fishes of this type include the surf-perches, splitfins, and lemon shark. The embryos of some viviparous fishes exhibit a behaviour known as oophagy where the developing embryos eat eggs produced by the mother. This has been observed primarily among sharks, such as the shortfin mako and porbeagle, but is known for a few bony fish as well, such as the halfbeak Nomorhamphus ebrardtii.[26] Intrauterine cannibalism is an even more unusual mode of vivipary, where the largest embryos in the uterus will eat their weaker and smaller siblings. This behaviour is also most commonly found among sharks, such as the grey nurse shark, but has also been reported for Nomorhamphus ebrardtii.[26]
Aquarists commonly refer to ovoviviparous and viviparous fishes as livebearers.
Immune system
Types of immune organs vary between different types of fish.
[27] In the jawless fish (lampreys and hagfishes), true lymphoid organs are absent. Instead, these fish rely on regions of lymphoid tissue within other organs to produce their immune cells. For example, erythrocytes, macrophages and plasma cells are produced in the anterior kidney (or pronephros) and some areas of the gut (where granulocytes mature) resemble primitive bone marrow in hagfish. Cartilaginous fish (sharks and rays) have a more advanced immune system than the jawless fish. They have three specialized organs that are unique to chondrichthyes; the epigonal organs (lymphoid tissue similar to bone marrow of mammals) that surround the gonads, the Leydig’s organ within the walls of their esophagus, and a spiral valve in their intestine. All these organs house typical immune cells (granulocytes, lymphocytes and plasma cells). They also possess an identifiable thymus and a well-developed spleen (their most important immune organ) where various lymphocytes, plasma cells and macrophages develop and are stored. Chondrostean fish (sturgeons, paddlefish and birchirs) possess a major site for the production of granulocytes within a mass that is associated with the meninges (membranes surrounding the central nervous system) and their heart is frequently covered with tissue that contains lymphocytes, reticular cells and a small number of macrophages. The chondrostean kidney is an important hemopoietic organ; where erythrocytes, granulocytes, lymphocytes and macrophages develop. Like chondrostean fish, the major immune tissues of bony fish (or teleostei) include the kidney (especially the anterior kidney), where many different immune cells are housed.[28] In addition, teleost fish possess a thymus, spleen and scattered immune areas within mucosal tissues (e.g. in the skin, gills, gut and gonads). Much like the mammalian immune system, teleost erythrocytes, neutrophils and granulocytes are believed to reside in the spleen whereas lymphocytes are the major cell type found in the thymus.[29][30] Recently, a lymphatic system similar to that described in mammals was described in one species of teleost fish, the zebrafish. Although not confirmed as yet, this system presumably will be where naive (unstimulated) T cells will accumulate while waiting to encounter an antigen.[31]
Evolution
The early fossil record on fish is not very clear. It appears it was not a successful enough animal early in its evolution to leave many fossils. However, this would eventually change over time as it became a dominant form of sea life and eventually branching to include land
vertebrates such as amphibians, reptiles, and mammals.
The formation of the hinged jaw appears to be what resulted in the later proliferation of fish because un-jawed fish left very few ancestors.
[32] Lampreys may be a rough representative of pre-jawed fish. The first jaws are found in Placodermi fossils. It is unclear if the advantage of a hinged jaw is greater biting force, respiratory-related, or a combination.
Some speculate that fish may have evolved from a creature similar to a coral-like
Sea squirt, whose larvae resemble primitive fish in some key ways. The first ancestors of fish may have kept the larval form into adulthood (as some sea squirts do today, see Neoteny), although the reversal of this case is also possible. Candidates for early fish include Agnatha such as Haikouichthys, Myllokunmingia, Pikaia, and Conodonts.
Amphibians
Amphibians (
class Amphibia; from Greek αμφις "both" and βιος "life") are a taxon of animals that include all living tetrapods (four-legged vertebrates) that do not have amniotic eggs, are ectothermic (term for the animals whose body heat is regulated by the external environment; previously known as cold-blooded), and generally spend part of their time on land. Most amphibians do not have the adaptations to an entirely terrestrial existence found in most other modern tetrapods (amniotes). There are around 6,200 described, living species of amphibians. The study of amphibians and reptiles is known as herpetology. Amphibians are able to breathe through their skin.
Systems

Reproductive
Caecilian from the San Antonio zoo
For the purpose of
reproduction most amphibians are bound to fresh water. A few tolerate brackish water, but there are no true seawater amphibians. Several hundred frog species in adaptive radiations (e.g., Eleutherodactylus, the Pacific Platymantines, the Australo-Papuan microhylids, and many other tropical frogs), however, do not need any water whatsoever. They reproduce via direct development, an ecological and evolutionary adaptation that has allowed them to be completely independent from free-standing water. Almost all of these frogs live in wet tropical rainforests and their eggs hatch directly into miniature versions of the adult, passing through the tadpole stage within the egg. Several species have also adapted to arid and semi-arid environments, but most of them still need water to lay their eggs. Symbiosis with single celled algae that lives in the jelly-like layer of the eggs has evolved several times. The larvae (tadpoles or polliwogs) breathe with exterior gills. After hatching, they start to transform gradually into the adult's appearance. This process is called metamorphosis. Typically, the animals then leave the water and become terrestrial adults, but there are many interesting exceptions to this general way of reproduction.
The most obvious part of the amphibian metamorphosis is the formation of four legs in order to support the body on land. But there are several other changes:
The gills are replaced by other
respiratory organs, i.e., lungs.
The skin changes and develops
glands to avoid dehydration.
The eyes develop eyelids and adapt to vision outside the water.
An
eardrum is developed to lock the middle ear.
In frogs and toads, the
tail disappears.
Conservation

The Golden Toad of Monteverde, Costa Rica was among the first casualties of amphibian declines. Formerly abundant, it was last seen in 1989.
Dramatic declines in amphibian populations, including population crashes and mass localized
extinction, have been noted in the past two decades from locations all over the world, and amphibian declines are thus perceived as one of the most critical threats to global biodiversity. A number of causes are believed to be involved, including habitat destruction and modification, over-exploitation, pollution, introduced species, climate change, destruction of the ozone layer (ultraviolet radiation has shown to be especially damaging to the skin, eyes, and eggs of amphibians), and diseases like chytridiomycosis. However, many of the causes of amphibian declines are still poorly understood, and are a topic of ongoing discussion.
Evolutionary history
The first major groups of amphibians developed in the Devonian Period from fishes similar to the modern coelocanth where the fins had evolved into legs. These amphibians were around five meters long in length, which is rare now except for some species of Japanese Salamander. The land was safe as the giant fishes and sharks in the ocean could not come onto land. However, there were two problems with living out their entire lives on land. Primarily, the food that these amphibians consumed was in the water, but also at this point the skin on most of these amphibians was not water-tight.
In the Carboniferous Period, the amphibians moved up in the food chain and began to occupy the ecological position where we now find crocodiles. These amphibians were notable for eating the mega-insects on land and many types of fishes in the water. Towards the end of the Permian Period and the Triassic Period, the amphibians started having competition with proto-crocodiles which led to their drop in size in the temperate zones or leaving for the poles. (Amphibians were able to hibernate during the winter whereas crocodiles could not, allowing the amphibians in higher latitudes protection from the reptiles.)
The modern
mudskipper provides a rough glimpse into the kind of lifestyle and adaptations that proto-amphibians may have taken.] (Mudskippers are not closely related to coelocanths.)
Reptiles are
tetrapods and amniotes, animals whose embryos are surrounded by an amniotic membrane, and members of the class Sauropsida

Modern reptiles inhabit every
continent except for Antarctica, although their main distribution comprises the tropics and subtropics. Though all cellular metabolism produces some heat, most modern species of reptiles do not generate enough to maintain a constant body temperature and are thus referred to as "cold-blooded" or ectothermic (the Leatherback Sea Turtle might be an exception, see also gigantothermy). Instead, they rely on gathering and losing heat from the environment to regulate their internal temperature, e.g, by moving between sun and shade, or by preferential circulation — moving warmed blood into the body core, while pushing cool blood to the periphery. In their natural habitats, most species are adept at this, and can usually maintain core body temperatures within a fairly narrow range. Reptiles are thick-skinned; unlike amphibians, they do not need to absorb water. While this lack of adequate internal heating imposes costs relative to temperature regulation through behavior, it also provides a large benefit by allowing reptiles to survive on much less food than comparably-sized mammals and birds, who burn much of their food for warmth. While warm-blooded animals move faster in general, an attacking lizard, snake or crocodile moves very quickly.
Except for a few members of the Testudines, all reptiles are covered by scales.Most reptile species are
oviparous (egg-laying).
Reptiles
Systems

Circulatory
Thermographic image of a monitor lizard.
All reptiles have closed circulation via a three-chamber
heart consisting of two atria and one, variably-partitioned ventricle. There is usually one pair of aortic arches. In spite of this, because of the fluid dynamics of blood flow through the heart, there is little mixing of oxygenated and deoxygenated blood in the three-chamber heart. Furthermore, the blood flow can be altered to shunt either deoxygenated blood to the body or oxygenated blood to the lungs, which gives the animal greater control over its blood flow, allowing more effective thermoregulation and longer diving times for aquatic species. There are some interesting exceptions among reptiles. For instance, crocodilians have an anatomically four-chambered heart that is capable of becoming a functionally three-chamber heart during dives (Mazzotti, 1989 pg 47). Also, it has been discovered that some snake and lizard species (e.g., monitor lizards and pythons) have three-chamber hearts that become functional four-chamber hearts during contraction. This is made possible by a muscular ridge that subdivides the ventricle during ventricular diastole and completely divides it during ventricular systole. Because of this ridge, some of these squamates are capable of producing ventricular pressure differentials that are equivalent to those seen in mammalian and avian hearts (Wang et al, 2003).
Respiratory
All reptiles breathe using lungs. Aquatic
turtles have developed more permeable skin, and some species have modified their cloaca to increase the area for gas exchange (Orenstein, 2001). Even with these adaptations, breathing is never fully accomplished without lungs. Lung ventilation is accomplished differently in each main reptile group. In squamates the lungs are ventilated almost exclusively by the axial musculature. This is also the same musculature that is used during locomotion. Because of this constraint, most squamates are forced to hold their breath during intense runs. Some, however, have found a way around it. Varanids, and a few other lizard species, employ buccal pumping as a complement to their normal "axial breathing." This allows the animals to completely fill their lungs during intense locomotion, and thus remain aerobically active for a long time. Tegu lizards are known to possess a proto-diaphragm, which separates the pulmonary cavity from the visceral cavity. While not actually capable of movement, it does allow for greater lung inflation, by taking the weight of the viscera off the lungs (Klein et al, 2003). Crocodilians actually have a muscular diaphragm that is analogous to the mammalian diaphragm. The difference is that the muscles for the crocodilian diaphragm pull the pubis (part of the pelvis, which is movable in crocodilians) back, which brings the liver down, thus freeing space for the lungs to expand. This type of diaphragmatic setup has been referred to as the "hepatic piston."
How
Turtles & Tortoises breathe has been the subject of much study. To date, only a few species have been studied thoroughly enough to get an idea of how turtles do it. The results indicate that turtles & tortoises have found a variety of solutions to this problem. The problem is that most turtle shells are rigid and do not allow for the type of expansion and contraction that other amniotes use to ventilate their lungs. Some turtles such as the Indian flapshell (Lissemys punctata) have a sheet of muscle that envelopes the lungs. When it contracts, the turtle can exhale. When at rest, the turtle can retract the limbs into the body cavity and force air out of the lungs. When the turtle protracts its limbs, the pressure inside the lungs is reduced, and the turtle can suck air in. Turtle lungs are attached to the inside of the top of the shell (carapace), with the bottom of the lungs attached (via connective tissue) to the rest of the viscera. By using a series of special muscles (roughly equivalent to a diaphragm), turtles are capable of pushing their viscera up and down, resulting in effective respiration, since many of these muscles have attachment points in conjunction with their forelimbs (indeed, many of the muscles expand into the limb pockets during contraction). Breathing during locomotion has been studied in three species, and they show different patterns. Adult female green sea turtles do not breathe as they crutch along their nesting beaches. They hold their breath during terrestrial locomotion and breathe in bouts as they rest. North American box turtles breathe continuously during locomotion, and the ventilation cycle is not coordinated with the limb movements (Landberg et al., 2003). They are probably using their abdominal muscles to breathe during locomotion. The last species to have been studied is red-eared sliders, which also breathe during locomotion, but they had smaller breaths during locomotion than during small pauses between locomotor bouts, indicating that there may be mechanical interference between the limb movements and the breathing apparatus. Box turtles have also been observed to breathe while completely sealed up inside their shells (ibid).
Most reptiles lack a
secondary palate, meaning that they must hold their breath while swallowing. Crocodilians have evolved a bony secondary palate that allows them to continue breathing while remaining submerged (and protect their brains from getting kicked in by struggling prey). Skinks (family Scincidae) also have evolved a bony secondary palate, to varying degrees. Snakes took a different approach and extended their trachea instead. Their tracheal extension sticks out like a fleshy straw, and allows these animals to swallow large prey without suffering from asphyxiation.
Excretory
Excretion is performed mainly by two small kidneys. In diapsids uric acid is the main nitrogenous waste product; turtles, like mammals, mainly excrete urea. Unlike the kidneys of mammals and birds, reptile kidneys are unable to produce liquid urine more concentrated than their body fluid. This is because they lack a specialized structure present in the nephrons of birds and mammals, called a Loop of Henle. Because of this, many reptiles use the colon to aid in the reabsorption of water. Some are also able to take up water stored in the bladder. Excess salts are also excreted by nasal and lingual salt-glands in some reptiles.
Nervous
The reptilian nervous system contains the same basic part of the
amphibian brain, but the reptile cerebrum and cerebellum are slightly larger. Most typical sense organs are well developed with certain exceptions most notably the snakes lack of external ears (middle and inner ears are present). All reptilians have advanced visual depth perception compared to other animals.
Reproductive
Most reptiles reproduce sexually. All male reptiles except turtles and tortoises have a twin tube-like sexual organ called the
hemipenes. Turtles and tortoises have a single penis. All testudines lay eggs, none are live bearing as some lizards and snakes are. All reproductive activity occurs with the cloaca, the single exit/entrance at the base of the tail where waste and reproduction happens.
Asexual reproduction has been identified in
squamates in six families of lizards and one snake. In some species of squamates, a population of females are able to produce a unisexual diploid clone of the mother. This asexual reproduction called parthenogenesis occurs in several species of gecko, and is particularly widespread in the teiids (especially Aspidocelis) and lacertids (Lacerta) In captivity Komodo dragons (varanidae) have reproduced by parthenogenesis.
Parthenogenetic species are also suspected to occur among
chameleons, agamids, xantusiids, and typhlopids.
Amniotic eggs are covered with leathery or calcareous shells. An
amnion, chorion and allantois are present during embryonic life. There are no larval stages of development.
Birds
Birds (
class Aves) are bipedal, warm-blooded, egg-laying vertebrate animals. Around 10,000 living and recently (after 1500) extinct species of birds compose the class Aves, making them the most diverse tetrapod vertebrates. They inhabit ecosystems across the globe, from Arctic terns to Antarctic penguins. Birds range in size from the tiny hummingbirds to the huge Ostrich. The fossil record indicates that birds evolved from theropod dinosaurs during the Jurassic period, c 200 to 150 Ma (million years ago), and the earliest known bird is the Late Jurassic Archaeopteryx, c 155–150 Ma.
Modern birds are
characterised by feathers, a beak with no teeth, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a lightweight but strong skeleton. All birds have forelimbs modified as wings and most can fly, though the ratites and several others, particularly endemic island species, have lost the ability to fly. Birds also have unique digestive and respiratory systems that are highly adapted for flight.
Many species of bird undertake long distance annual
migrations, and many more perform shorter irregular movements. Birds are social and communicate using visual signals and through calls and song, and participate in social behaviours including cooperative hunting, cooperative breeding, flocking and mobbing of predators. The vast majority of bird species are socially monogamous, usually one breeding season at a time, sometimes for years, and rarely for life. Other species have breeding systems that are polygynous ("many females") or, rarely, polyandrous ("many males"). Among some monogamous species, extra-pair copulations are common. Eggs are usually laid in a nest and incubated and most birds have an extended period of parental care after hatching.
Birds are economically important to humans: many are important sources of food, acquired either through hunting or farming, and they provide other products. Some species, particularly
songbirds and parrots, are popular as pets. Birds figure prominently in all aspects of human culture from religion to poetry and popular music. About 120–130 species have become extinct as a result of human activity since 1600, and hundreds more before this. Currently around 1,200 species of birds are threatened with extinction by human activities and efforts are underway to protect them.
Anatomy


External anatomy of a bird: 1 Beak, 2 Head, 3 Iris, 4 Pupil, 5 Mantle, 6 Lesser coverts, 7 Scapulars, 8 Median coverts, 9 Tertials, 10 Rump, 11 Primaries, 12 Vent, 13 Thigh, 14 Tibio-tarsal articulation, 15 Tarsus, 16 Feet, 17 Tibia, 18 Belly, 19 Flanks, 20 Breast, 21 Throat, 22 Wattle
Compared with other vertebrates, birds have a
body plan that shows many unusual adaptations, mostly to facilitate flight.
The skeleton consists of bones which are very light. They have large pneumatic (air-filled) cavities which connect with the
respiratory system. The skull bones are fused and do not show cranial sutures.[34] The orbits are large and separated by a bony septum. The spine has cervical, thoracic, lumbar and caudal regions with the number of cervical (neck) vertebrae highly variable and especially flexible, but movement is reduced in the anterior thoracic vertebrae and absent in the later vertebrae. The last few are fused with the pelvis to form the synsacrum. The ribs are flattened and the sternum is keeled for the attachment of flight muscles, except in the flightless bird orders. The forelimbs are modified into the wings.
Like the
reptiles, birds are primarily uricotelic, that is their kidneys extract nitrogenous wastes from their bloodstream and excrete it as uric acid instead of urea or ammonia. The uric acid is excreted along with feces as a semisolid waste and they do not have a separate urinary bladder or opening. Some birds such as hummingbirds however can be facultatively ammonotelic, excreting most of the nitrogenous wastes as ammonia. They also excrete creatine rather than creatinine as in mammals. This material, as well as the output of the intestines, emerges from the bird's cloaca. The cloaca is a multi-purpose opening: their wastes are expelled through it, they mate by joining cloaca, and females lay eggs out of it. In addition, many species of birds regurgitate pellets.[42] The digestive system of the bird is unique, with a crop for storage and a gizzard that contains swallowed stones for grinding food, given the lack of teeth. Most are highly adapted for rapid digestion, an adaptation to flight. Some migratory birds have the additional ability to reduce parts of the intestines prior to migration.
Birds have one of the most complex
respiratory systems of all animal groups. When a bird inhales, 75% of the fresh air bypasses the lungs and flows directly into a posterior air sac which extends from the lungs and connects with air spaces in the bones and fills them with air. The other 25% of the air goes directly into the lungs. When the bird exhales, the used air flows out of the lung and the stored fresh air from the posterior air sac is simultaneously forced into the lungs. Thus, a bird's lungs receive a constant supply of fresh air during both inhalation and exhalation. Sound production is achieved using the syrinx, a muscular chamber with several tympanic membranes, situated at the lower end of the trachea where it bifurcates.[ The bird's heart has four chambers and the right aortic arch gives rise to systemic aorta (unlike in the mammals where the left arch is involved).[ The postcava receives blood from the limbs via the renal portal system. Birds, unlike mammals, have nucleated erythrocytes, that is, red blood cells which retain a nucleus.[
The
nervous system is large relative to the bird's size.[34] The most developed part of the brain is the one that controls the flight related function while the cerebellum coordinates movement and the cerebrum controls behaviour patterns, navigation, mating and nest building. Most birds have a poor sense of smell with notable exceptions including kiwis,[49] New World vultures[50] and the tubenoses.[51] The visual system is usually highly developed. Water birds have special flexible lenses, allowing accommodation for vision in air and water.[34] Some species also have dual fovea. Birds are tetrachromatic, possessing ultraviolet (UV) sensitive cone cells in the eye as well as green, red and blue ones.[52] This allows them to perceive ultraviolet light; which is used in courtship. Many birds show plumage patterns in ultraviolet that are invisible to the human eye; so that some birds, whose sexes appear similar are distinguished by the presence of ultraviolet reflective patches of feathers. Male Blue Tits have an ultraviolet reflective crown patch which is displayed in courtship by posturing and raising of their nape feathers.[53] Ultraviolet light is also used in foraging—kestrels have been shown to search for prey by detecting the UV reflective urine trail marks left on the ground by rodents.[54] The eyelids of a bird are not used in blinking, instead the eye is lubricated by the nictitating membrane, the third eyelid that moves horizontally.[55] The nictitating membrane also covers the eye and acts as a contact lens in many aquatic birds.[34] The bird retina has a fan shaped blood supply system called the pecten.[34] Most birds cannot move their eyes, although there are exceptions, like the Great Cormorant.[56] Birds with eyes on the sides of their heads have a wide visual field while birds with eyes on the front of their heads like owls have binocular vision and can estimate field depth.[57] The avian ear lacks external pinnae but is covered by feathers, although in some birds (the Asio, Bubo and Otus owls, for example) these feathers form tufts which resemble ears. The inner ear has a cochlea but it is not spiral as in mammals.[58]
A few species are able to use chemical defenses against predators; some
Procellariiformes can eject an unpleasant oil against an aggressor,[59] and some species of pitohui from New Guinea secrete a powerful neurotoxin in their skin and feathers.[60]
Feathers and plumage

The plumage of the African Scops Owl allows it to blend in with its surroundings.
Uniquely found in birds,
feathers are epidermal growths attached to the skin that serve a variety of functions: they aid in thermoregulation by insulating from cold weather and water, they are essential to flight, and are also used in display, camouflage and signalling.[34] There are several different types of feather that serve different purposes. Feathers need maintenance, and birds preen or groom their feathers daily (they around 9.2% of their daily time budget on this),[61] using their bills to brush away foreign particles, and applying waxy secretions from the uropygial gland, which protects feather flexibility and also acts as an anti-microbial agent, inhibiting the growth of feather-degrading bacteria.[62] This may be supplemented with the secretions of formic acid from ants, which birds apply in a behaviour known as anting in order to remove feather parasites.[63]
Plumage is the term given to the arrangement and appearance of feathers on the body, and within species can vary with age, social status,[64] with higher ranked individuals displaying their status, or most commonly by sex.[65] Plumage is regularly moulted, the standard plumage of a bird that has moulted after breeding is known as the non-breeding plumage, or in the Humphrey-Parkes terminology, 'basic plumage'; breeding plumages or variations of the basic plumage are known under the Humphrey-Parkes system as 'alternate plumages'.[66] Moult is annual in most species but some species may have two moults a year, while large birds of prey may moult once in two or three years. Ducks and geese moult their primaries and secondaries simultaneously and become flightless for about a month.[67] Moulting patterns vary across species. Some drop and regrow wing flight feathers starting sequentially from the outermost feathers and progressing inwards while others replace feathers starting out from the innermost feathers. The rare few species lose all their flight feathers at once.[67] The first or centripetal moult as termed for the moult of tail feathers is seen for instance in the Phasianidae.[68] The second or centrifugal moult is seen for instance in the tail feathers of the woodpeckers and treecreepers, although it begins with the second innermost pair of tail-feathers and the central pair of feathers is molted last, so as to permits the continuous presence of a functional climbing tail.[69] The general pattern seen in the passerines is that the primaries are replaced outward, secondaries inward, and the tail from center outward.[70]
Feathers do not arise from all parts of the bird skin but grow in specific tracts or pterylae. The distribution pattern of these feather tracts or pterylosis is used in taxonomy and systematics. Prior to nesting, the females of most bird species gain a bare brood patch by loss of feathers close to the belly. The skin here is well supplied with blood vessels and helps in incubation.
[71]
Flight
Flight characterises most birds, distinguishing them from almost all other vertebrates with the exception of mammalian bats and the extinct pterosaurs. As the main means of locomotion for most bird species, flight is used for breeding, feeding, and predator avoidance and escape. Birds have a variety of adaptations to flight, including a lightweight skeleton, two large flight muscles, the pectoralis (which accounts for 15% of the total mass of the bird) and the supercoracoideus and a modified forelimb (the wing) serving as an aerofoil.[34] Wing shape and size primarily determines the type of flight each species is capable of. Many birds combine powered or flapping flight with less energy intensive soaring flight. About 60 species of extant birds are flightless, and many extinct birds were also flightless.[72] Flightlessness often arises in birds on isolated islands, probably due to the lack of land predators and limited resources, which rewards the loss of costly unnecessary adaptations.[73] Though flightless, penguins use similar musculature and movements to "fly" through the water, as do auks, shearwaters and dippers.[74]
Behaviour
Most birds are
diurnal, but some birds, such as many species of owls and nightjars, are nocturnal or crepuscular (active during twilight hours), and many coastal waders feed when the tides are appropriate, by day or night.[75]
Diet and feeding


Feeding adaptations in beaks. A:Nectarivore, B:Insectivore, C:Granivore, D:Specialist Seed-eater, E:Fishing, F:Netting, G:Filter feeding, H:Surface probing, I:Probing, J:Surface skimming, K:Raptorial
Birds feed on a variety of things, including
nectar, fruit, plants, seeds, carrion, and various small animals including other birds.[34] Because birds have no teeth, the digestive system of birds is specially adapted to process unmasticated food items that are usually swallowed whole.
Different species of birds use different feeding strategies. Many glean for insects, invertebrates, fruit or seeds. Some hunt insects by sallying from a branch.
Nectar feeders such as hummingbirds, sunbirds, lories and lorikeets amongst others are facilitated by specially adapted brushy tongues and in many cases bills designed to fit co-adapted flowers.[76] Kiwis and shorebirds with long bills probe for invertebrates; in the case of shorebirds, length of bill and feeding method are associated with niche separation.[34][77] Loons, diving ducks, penguins and auks pursue their prey underwater, using their wings or feet for propulsion,[26] while aerial predators such as sulids, kingfishers and terns plunge dive after their prey. Three species of prion, the flamingos and some ducks are filter feeders.[78][79] Geese and dabbling ducks are primarily grazers. Some species will engage in kleptoparasitism, stealing food items from other birds; frigatebirds, gulls,[80] and skuas[81] employ this type of feeding behaviour. Kleptoparasitism is not thought to play a significant part of the diet of any species, and is instead a supplement to food obtained by hunting; a study of Great Frigatebirds stealing from Masked Boobies estimated that the frigatebirds could at most obtain 40% of the food they needed, and on average obtained only 5%.[82] Finally, some birds are scavengers, either specialised carrion eaters like vultures or opportunists like gulls, corvids or other birds of prey.[83] Some birds may employ many strategies to obtain food, or feed on a variety of food items and are called generalists, while others are considered specialists, concentrating time and effort on specific food items or having a single strategy to obtain food.[34]
Migration
The routes of satellite tagged
Bar-tailed Godwits migrating north from New Zealand. This species has the longest known non-stop migration of any species, up to 10,200 km (6,300 mi).
Many bird species migrate to take advantage of global differences of seasonal temperatures to optimise availability of food sources and breeding habitat. These migrations vary among the different groups. Many landbirds,
shorebirds and waterbirds undertake annual long distance migrations, usually triggered by length of daylight as well as weather conditions. These are characterised by a breeding season spent in the temperate or arctic/antarctic regions, and a non-breeding season in the tropical regions or opposite hemisphere. Prior to migration, birds substantially increase body fats and reserves and reduce the size of some of their organs.[84][45] Migration is highly energetically demanding, particularly as birds need to cross deserts and oceans without refuelling; landbirds have a flight range of around 2,500 km (1,600 mi) and shorebirds can fly up to 4,000 km (2,500 mi),[34] although the Bar-tailed Godwit is capable of non-stop flights of up to 10,200 km (6,300 mi).[85] Seabirds also undertake long migrations, the longest annual migration being those of Sooty Shearwaters, which nest in New Zealand and Chile and spend the northern summer feeding in the North Pacific off Japan, Alaska and California, an annual round trip of 64,000 km (39,800 mi).[86] Other seabirds disperse after breeding, traveling widely but having no set migration route. Albatrosses nesting in the Southern Ocean often undertake circumpolar trips between breeding seasons.[87]
Birds also display other types of migration. Some species undertake shorter migrations, traveling only as far as is required to avoid bad weather or obtain food. Irruptive species such as the boreal finches are one such species, commonly found in one year while absent in others. This type of migration is normally associated with food availability.
[88] Species may also travel shorter distances over part of their range, with individuals from higher latitudes travelling into the existing range of conspecifics; others undertake partial migrations, where only a fraction of the population, usually females and subdominant males, migrates.[89] Partial migration can form a large percentage of the migration behaviour of birds in some regions; in Australia, surveys found that 44% of non-passerine birds and 32% of passerines were partially migratory.[90] Altitudinal migration is a form of short distance migration, in which birds spend the breeding season at higher altitudes elevations, and move to lower ones during suboptimal conditions. It is most often triggered by temperature changes and usually occurs when the normal territories become inhospitable also due to lack of food.[91] Some species may also be nomadic, holding no fixed territory and moving according to weather and food availability. Parrots as a family are overwhelmingly neither migratory nor sedentary but considered to either be dispersive, irruptive, nomadic or undertake small and irregular migration.[92]
The ability of birds to return to precise locations across vast distances has been known for some time; in an experiment conducted in the 1950s a
Manx Shearwater released in Boston returned to its colony in Skomer, Wales within 13 days, a distance of 5,150 km (3,200 mi).[93] Birds navigate during migration using a variety of methods. For diurnal migrants the sun is used to navigate by, at night a stellar compass is used instead. Birds that use the sun compensate for the changing position of the sun during the day, by the use of an internal clock.[34] Orientation with the stellar compass depends on the position of the constellations surrounding Polaris.[94] These are backed up in some species with the ability to sense the Earth's geomagnetism through specialised sensitive photoreceptors.[95]
Communication
Birds
communicate principally using visual and auditory signals. Signals can be interspecific (between species) and intraspecific (within species).


The startling display of the Sunbittern mimics a large predator
Visual communication in birds serves a number of functions and is manifested in both
plumage and behaviour.[44] Plumage can be used to assess and assert social dominance,[96] display breeding condition in sexually selected species, even make a threatening display, such as the threat display of the Sunbittern, which mimics a large possible predator. This display is used to ward off potential predators such as hawks, and to protect young chicks.[97] Variation in plumage also allows for the identification of birds, particularly between species.
Visual communication includes ritualised displays, such as those which signal aggression or submission, or those which are used in the formation of pair-bonds.
[34] These ritualised behaviours develops from non-signalling actions such as preening, the adjustments of feather position, pecking or other behaviour; used as forms of visual communication as indication of a particular trait or mood. The most elaborate displays are shown during courtship, such as the breeding dances of the albatrosses, where the successful formation of a life-long pair-bond requires both partners to practice a unique dance,[98] and the birds-of-paradise, where the breeding success of males depends on plumage and display quality.[99] Male birds can demonstrate their fitness through nest-site selection and construction; females of weaver species, such as the Baya Weaver, may choose mates with good site selection and nest-building skills,[100] while bowerbirds attract mates through constructing bowers and decorating them with bright objects.[101]
In addition to visual communication, birds are renowned for their auditory skills. Calls, and in some species
song, are the major means by which birds communicate with sound; though some birds use mechanical sounds, for example driving air through their feathers, as do the Coenocorypha snipes of New Zealand,[102] the territorial drumming of woodpeckers,[44] or the use of tools to drum in Palm Cockatoos.[103] Bird calls and songs can be very complex; sounds are created in the syrinx, both sides of which, in some species, can be operated separately, resulting in two different songs being produced at the same time.[47]


Call of the House Wren, a common songbird from North America
Calls are used for a variety of purposes, several of which may be tied into an individual song.
[104] They are used to advertise when seeking a mate, either to attract a mate, aid identification of potential mates or aid in bond formation (often with combined with visual communication). They can convey information about the quality of a male and aid in female choice.[105] They are used to claim and maintain territories. Calls can also be used to identify individuals, aiding parents in finding chicks in crowded colonies or adults reuniting with mates at the start of the breeding season.[106] Calls may be used to warn other birds of potential predators; calls of this nature may be detailed and convey specific information about the nature of the threat.[107]
Flocking


Red-billed Queleas, the most numerous species of bird,[108] form enormous flocks—sometimes tens of thousands strong.
While some birds are essentially territorial or live in small family groups, other birds often form large
flocks. The benefits of aggregating in flocks are varied and flocks will form explicitly for specific purposes. Flocking also has costs, particularly to socially subordinate birds, which are bullied by more dominant birds; birds may also sacrifice feeding efficiency in a flock in order to gain other benefits.[109] The principal benefits are safety in numbers and increased foraging efficiency.[34] Defence against predators is particularly important in closed habitats such as forests where predation is often by ambush and early warning provided by multiple eyes is important, this has led to the development of many mixed-species feeding flocks.[110] These multi-species flocks are usually composed of small numbers of many species, increasing the benefits of numbers but reducing potential competition for resources. Birds also form associations with non-avian species; plunge diving seabirds associate with dolphins and tuna which push shoaling fish up towards the surface,[111] and a mutualistic relationship has evolved between Dwarf Mongooses and hornbills, where hornbills seek out mongooses in order to forage together, and warn each other of birds of prey and other predators.[112]
Resting and roosting
The high metabolic rates of birds during the active part of the day is supplemented by rest at other times.
Sleeping birds often utilise a type of sleep known as vigilant sleep, where periods of rest are interspersed with quick eye-opening 'peeks' allowing birds to be sensitive to disturbance and enable rapid escape from threats.[113]
It has been widely believed that
swifts may sleep while flying, however this is not confirmed by experimental evidence. It is however suggested that there may be certain kinds of sleep which are possible even when in flight.[114] Some birds have also demonstrated the capacity to fall into slow-wave sleep one hemisphere of the brain at a time, tending to exercise this ability depending upon one's position relative to the outside of the flock. This may allow the eye opposite the sleeping hemisphere and viewing the outer margins of the flock to remain vigilant for predators. This adaption is also known in marine mammals.[115]
Birds do not have sweat glands and they may cool themselves by moving to shade, standing in water, panting, increasing their surface are, fluttering their throat or by using special behaviours like
urohidrosis to cool themselves.
Many sleeping birds bends their heads over their backs and tuck their
bills in their back feathers, others cover their beaks among their breast feathers. Many birds rest on one leg, some may pull up their legs into their feathers, especially in cold weather. Communal roosting is common, it lowers the loss of body heat and decreases the risks associated with predators.[116] Roosting sites are often chosen with regard to thermoregulation and safety.[117]
Perching birds have a tendon locking mechanism that helps hold on to the perch when they are asleep. Many ground birds such as quails and pheasants roost in trees. A few parrots of the genus Loriculus roost hanging upside down.
[118] Some hummingbirds go into a nightly state of torpor with a reduction in their metabolic rates.[119] This physiological adaptation is shown in around a hundred of other species, including owlet-nightjars, nightjars, and woodswallows. One species, the Common Poorwill, even enters a state of hibernation.[120]
Breeding
Social systems


Red-necked Phalaropes have an unusual polyandrous mating system where males care for the eggs and chicks and brightly coloured females compete for males.[121]
The vast majority (95%) of bird species are
socially monogamous; although polygyny (2%) and polyandry (<>polygamy, polygynandry (where a female pairs with several males and the male pairs with several females) and promiscuity systems also occur.[34] Some species may use more than one system depending on the circumstances. Monogamous species of males and females pair for the breeding season; in some cases, the pair bonds may persist for a number of years or even the lifetime of the pair.[122]
The advantage of monogamy for birds is bi-parental care. In most groups of animals, male parental care is rare, but in birds it is quite common; in fact, it is more extensive in birds than in any other vertebrate class.
[34] In birds, male care can be seen as important or essential to female fitness; in some species the females are unable to successfully raise a brood without the help of the male.[123] Polygamous breeding systems arise when females are able to raise broods without the help of males.[34] There is sometimes a division of labour in monogamous species, with the roles of incubation, nest site defence, chick feeding and territory defence being either shared or undertaken by one sex.[124]
While social monogamy is common in birds,
infidelity, in the form of extra-pair copulations, is common in many socially monogamous species.[125] These can take the form of forced copulation (or rape) in ducks and other anatids,[126] or more usually between dominant males and females partnered with subordinate males. It is thought that the benefit to females comes from getting better genes for her offspring, as well as an insurance against the possibility of infertility in the mate.[127] Males in species that engage in extra-pair copulations will engage in mate-guarding in order to ensure parentage of the offspring they raise.[128]
Breeding usually involves some form of courtship display, most often performed by the male.
[129] Most are rather simple, and usually involve some type of song. Some displays can be quite elaborate, using such varied methods as tail and wing drumming, dancing, aerial flights, and communal leks depending on the species. Females are most often involved with partner selection,[130] although in the polyandrous phalaropes the males choose brightly coloured females.[131] Courtship feeding, billing and preening are commonly performed between partners, most often after birds have been paired and mated.[44]
Territories, nesting and incubation
Many birds actively defend a
territory from others of the same species during the breeding season. Large territories are protected in order to protect the food source for their chicks. Species that are unable to defend feeding territories, such as seabirds and swifts, often breed in colonies instead; this is thought to offer protection from predators. Colonial breeders will defend small nesting sites, and competition between and within species for nesting sites can be intense.[132]

The nesting colonies of the Sociable Weaver are amongst the largest bird-created structures.
All birds lay
amniotic eggs with hard shells made mostly of calcium carbonate.[34] The colour of eggs is controlled by a number of factors, those of hole and burrow nesting species tend to be white or pale, while those of open nesters such as Charadriiformes are camouflaged. There are many exceptions to this pattern, however; the ground nesting nightjars have pale eggs, camouflage being provided instead by the bird's plumage. Species that are victims of brood parasites like the Dideric Cuckoo will vary their egg colours in order to improve the chances of spotting a cuckoo's egg, and female cuckoos need to match their eggs to their hosts.[133]
The eggs are usually laid in a
nest, which can be highly elaborate, like those created by weavers and oropendolas, or extremely primitive, like some albatrosses, which are no more than a scrape on the ground. Some species have no nest, the cliff nesting Common Guillemot lays its egg on bare rock and the egg of the Emperor Penguin is kept between the body and feet; this is especially prevalent in ground nesting species where the newly hatched young are precocial. Most species build more elaborate nests, which can be cups, domes, plates, beds scrapes, mounds or burrows.[134] Most nests are built in shelter and hidden to reduce the risk of predation, more open nests are usually colonial or built by larger birds capable of defending the nest. Nests are mostly built out of plant matter, some species specifically select plants such as yarrow which have chemicals that reduce nest parasites such as mites, leading to increased chick survival.[135] Nests are often lined with feathers in order to improve the retention of heat.[134]
Incubation, which regulates temperature to keep it optimum for chick development, usually begins after the last egg has been laid.
[34] Incubation duties are often shared in monogamous species; in polygamous species a singe parent undertakes all duties. Warmth from parents passes to the eggs through brood patches, areas of bare skin on the abdomen or breast of the incubating birds. Incubation can be an energetically demanding process, for example adult albatrosses lose as much as 83 g of body weight a day.[136] The warmth for the incubation of the eggs of megapodes comes from the sun, decaying vegetation or from volcanic sources.[137] Incubation periods last between 10 days (in species of woodpeckers, cuckoos and passerine birds) to over 80 days (in albatrosses and kiwis).[34]
Parental care and fledging

A female Seychelles Sunbird with arachnid prey attending its nest.
Chicks can be helpless or independent at hatching, or be at any stage in between. The helpless chicks are known as
altricial, and tend to be born, small, naked and blind; chicks that are mobile and feathered at hatching are precocial, chicks can also be semi-precocial and semi-altricial. Altricial chicks require help in thermoregulation and need to be brooded for longer than precocial chicks.
The length and nature of parental care varies widely amongst different orders and species. At one extreme, parental care in
megapodes ends at hatching; the newly-hatched chick digs itself out of the nest mound without parental assistance and can fend for itself immediately.[138] At the other extreme many seabirds have extended periods of parental care, the longest being Great Frigatebird, the chicks of which take up to six months to fledge and are fed by the parents for up to another 14 months.[139]
In some species the care of young is shared between both parents; in others it is the responsibility of just one sex. In some species other members of the same species will help the breeding pair in raising the young. These
helpers are usually close relatives such as the chicks of the breeding pair from previous breeding seasons.[140] Alloparenting is particularly common among the Corvida, including such birds as the true crows, Australian Magpie and Fairy-wrens,[141] but has been observed in as different species as the Rifleman and Red Kite.


This Reed Warbler is raising the young of a Common Cuckoo, a brood parasite
The point at which chicks
fledge varies dramatically. The chicks of the Synthliboramphus murrelets, like the Ancient Murrelet, leave the nest the night after they hatch, following their parents calls out to sea, where they are raised away from terrestrial predators.[142] Some other species, especially ducks, move their chicks away from the nest at an early age. In most species chicks leave the nest soon after, or just before, they are able to fly. Parental care after fledging varies; in albatrosses chicks leave the nest alone and receive no further help, other species continue some supplementary feeding after fledging.[143] Chicks may also follow their parents during their first migration.[144]
Brood parasites
Although some
insects and fish engage in brood parasitism, most brood parasites are birds.[145] Brood parasites are birds which lay their eggs in the nests of other birds. These eggs are often accepted and raised by the host species, often at the cost of their own brood. There are two kinds of brood parasite, obligate brood parasites, which are incapable of raising their own young and must lay their eggs in the nests of other species; and non-obligate brood parasites, which are capable of raising their own young but lay eggs in the nests of conspecifics in order to increase their reproductive output.[146] The most famous obligate brood parasites are the cuckoos, although in total 100 species of cuckoos, honeyguides, icterids, estrildid finches and ducks are obligate parasites.[145] Some brood parasites are adapted to hatching before their hosts and pushing their hosts' eggs out of the nest, destroying the egg or killing their chicks, ensuring that all the food brought to the nest is fed to them.[147]
Ecology
The South Polar Skua (left) is a generalist predator, taking the eggs of other birds, fish, carrion and other animals. This skua is attempting to push an Adelie Penguin (right) off its nest
The diverse food habits and life-histories of birds are associated with a range of ecological positions.
[108] While some birds are generalists, others are highly specialized in their habitat or food requirements. Even within a habitat such as a forest, the niches occupied by different groups of birds are varied with some species using the forest canopy, others using the space under the canopy, while still others may use the branches and so on. In addition forest birds may be classified into different feeding guilds such as insectivores, frugivores and nectarivores. Aquatic birds show other food habits such as fishing, plant eating and piracy or kleptoparasitism. The birds of prey specialize in hunting mammals or other birds while the vultures have specialized as scavengers.
Some nectar-feeding birds are also important pollinators of plants and many frugivores play a key role in seed dispersal.
[148] Numerous plants have adapted to using birds as their primary pollinators, and both flower and plant have coevolved together,[149] in some cases to the point where the flower's primary pollinator is the only species capable of reaching the nectar.[150]
Birds have important impacts on the ecology of islands. In many cases they reach islands that mammals do not, and in which they may fulfill ecological roles played by larger animals; for example in New Zealand the
moa were important browsers, as are the Kereru and Kokako today.[148] Today the plants of New Zealand retain the defensive adaptations evolved to protect them from the extinct moa.[151] Large concentrations of nesting seabirds also have an impact on the ecology of islands and the surrounding seas, principally through the concentration of large quantities of guano, which can have appreciable impacts on the richness of the local soil,[152] and of the surrounding seas.[153]
Relationship with humans
Industrial farming of chickens.
Birds are highly visible and common animals, and humans have had a long relationship with them. In some cases the relationship has been
mutualistic, such as the cooperative relationship between honeyguides and tribesmen in obtaining honey,[154] or commensal, as found in the numerous species that benefit indirectly from human activities.[155] For example, the domestic pigeon thrives in urban areas around the world. Human activities can also be detrimental, threatening some bird species with extinction.
Birds can act as vectors for spreading diseases such as
psittacosis, salmonellosis, campylobacteriosis, mycobacteriosis (avian tuberculosis), avian influenza (bird flu), giardiasis, and cryptosporidiosis over long distances. Some of these are zoonotic diseases that can also be transmitted to humans.[156] Recent research suggests that the saliva of birds is a better indicator of avian influenza than are their faecal samples. [157]
Birds are also commercially important
pests on agricultural crops,[158] as well as posing a hazard to aviation through bird strikes They are also important food and income sources.
Some birds are apex predators of their respective food chains making them very sensitive indicators of
pollution. The decline in bird populations in the US, as a result of pesticide use is a famous example.[161] Birds and their diversity have therefore been considered as good indicators of ecosystem health and, in the UK, bird diversity is used as one of 15 quality of life indicators.[162]
Economic importance
Birds are an important food source for humans. The most commonly eaten species is the domestic
chicken and its eggs, as well as geese, pheasants, turkeys, ducks and quail. Hunting remains an important method of obtaining birds, as it has been throughout human history,[163] and has led to the extinction or endangerment of dozens of species. However, muttonbirding in Australia and New Zealand is an example of an ongoing sustainable harvest of two seabird species.[165]
Besides meat and eggs, birds provide
feathers for clothing, bedding and decoration, guano-derived phosphorus and nitrogen that is used in fertiliser and gunpowder.Colourful birds (such as parrots, and mynas) are bred in captivity or kept as pets, and this practice has led to the illegal trafficking of some endangered species. Other birds have long been used by humans to perform tasks; falcons for hunting, and cormorants to catch fish. Pigeons were used as a messenger as early as 1 AD, according to Pliny and played an important role as recently as World War II. Today, such activities are more common as a hobbies, or for entertainment and tourism or for sport including pigeon racing.


Cormorants used by fishermen in Southeast Asia. The practice is in steep decline but survives in some areas as a tourism attraction.
The scientific study of birds is called
ornithology. Birds are among the most extensively studied of all animal groups; chickens and pigeons are popular as experimental subjects, and are often used in biology and comparative psychology research. Hundreds of academic journals and thousands of scientists are devoted to bird research, while amateur enthusiasts (called birdwatchers, twitchers or, more commonly, birders) number in the millions Many homeowners erect bird feeders near their homes to attract various species. Bird feeding has grown into a multimillion dollar industry; for example an estimated 75% of households in Britain provide food for birds at some point during the winter.
Mammals
Mammals (
class Mammalia) are warm-blooded, vertebrate animals characterized by the presence of sweat glands, including milk producing sweat glands, and by the presence of: hair, three middle ear bones used in hearing, and a neocortex region in the brain. Most mammals also possess specialized teeth and utilize a placenta in the ontogeny. The mammalian brain regulates endothermic and circulatory systems, including a four-chambered heart. Mammals encompass approximately 5,400 species (including humans), distributed in about 1,200 genera, 153 families, and 29 orders,[1] though this varies by classification scheme.
Most mammals belong to the
placental group. The four largest orders within the placental mammals are Rodentia (mice, rats, and other small, gnawing mammals), Chiroptera (bats), Carnivora (dogs, cats, bears, and other mammals that primarily eat meat), and Cetartiodactyla (including numerous herbivore species, such as deer, sheep, goats, and buffalos, plus whales).
Phylogenetically, Mammalia is defined as all descendants of the most recent common ancestor of monotremes (e.g., echidnas and platypuses) and therian mammals (marsupials and placentals). This means that some extinct groups of "mammals" are not members of the crowngroup Mammalia, even though most of them have all the characteristics that traditionally would have classified them as mammals. These "mammals" are now usually placed in the unranked clade Mammaliaformes.
The first true mammals appeared in the
Jurassic period. Modern mammalian orders appeared in the Palaeocene and Eocene epochs of the Palaeogene period.
Anatomy and morphology
Skeletal system
The majority of mammals have seven
cervical vertebrae (bones in the neck); this includes bats, giraffes, whales, and humans. The few exceptions include the manatee and the two-toed sloth, which have only six cervical vertebrae, and the three-toed sloth with nine cervical vertebrae.
Respiratory system
The lungs of mammals have a spongy texture and are honeycombed with
epithelium having a much larger surface area in total than the outer surface area of the lung itself. The lungs of humans are typical of this type of lung.
Breathing is largely driven by the muscular
diaphragm at the bottom of the thorax. Contraction of the diaphragm pulls the bottom of the cavity in which the lung is enclosed downward. Air enters through the oral and nasal cavities; it flows through the larynx and into the trachea, which branches out into bronchi. Relaxation of the diaphragm has the opposite effect, passively recoiling during normal breathing. During exercise, the diaphragm contracts, forcing the air out more quickly and forcefully. The rib cage itself also is able to expand and contract to some degree, through the action of other respiratory and accessory respiratory muscles. As a result, air is sucked into or expelled out of the lungs, always moving down its pressure gradient. This type of lung is known as a bellows lung as it resembles a blacksmith's bellows.
Circulatory system
The mammalian
heart has four chambers: the right atrium, right ventricle, left atrium, and left ventricle. Atria are for receiving blood; ventricles are for pumping blood to the lungs and body. The ventricles are larger than the atria and their walls are thick, because muscular walls are needed to forcefully pump the blood from the heart to the body and lungs. Deoxygenated blood from the body enters the right atrium, which pumps it to the right ventricle. The right ventricle pumps blood to the lungs, where carbon dioxide diffuses out, and oxygen diffuses in. From the lungs, oxygenated blood enters the left atrium, where it is pumped to the left ventricle (the largest and strongest of the 4 chambers), which pumps it out to the rest of the body, including the heart's own blood supply.
Nervous system
All mammalian brains possess a
neocortex, a brain region that is unique to mammals.
Integumentary system
Mammals have
integumentary systems made up of three layers: the outermost epidermis, the dermis, and the hypodermis. This characteristic is not unique to mammals, since it is found in all vertebrates.
The
epidermis is typically ten to thirty cells thick; its main function being to provide a waterproof layer. Its outermost cells are constantly lost; its bottommost cells are constantly dividing and pushing upward. The middle layer, the dermis, is fifteen to forty times thicker than the epidermis. The dermis is made up of many components such as bony structures and blood vessels. The hypodermis is made up of adipose tissue. Its job is to store lipids, and to provide cushioning and insulation. The thickness of this layer varies widely from species to species.
Although mammals and other animals have
cilia that superficially may resemble it, no other animals except mammals have hair. It is a definitive characteristic of the order. Some mammals have very little, albeit in obscure parts of their bodies, but nonetheless, careful examination reveals the characteristic. None are known to have hair that naturally is blue or green in color although some cetaceans, along with the mandrills appear to have shades of blue skin. Many mammals are indicated as having blue hair or fur, but in all known cases, it has been found to be a shade of gray. The two-toed sloth and the polar bear may seem to have green fur, but this color is caused by algae growths.
Reproductive system


Goat kids will stay with their mother until they are weaned, this is usually about one month
Most mammals give birth to live young (
vivipary), but a few, such as the monotremes lay eggs. Live birth also occurs in some non-mammalian species, such as guppies, snakes, and hammerhead sharks; thus it is not a distinguishing characteristic of mammals.
Mammals have sweat glands, a defining feature present only in mammals. Some of these glands produce
milk (in what are called mammary glands), a liquid used by newborns as their primary source of nutrition. The monotremes branched from other mammals early on, and do not have the nipples seen in most mammals, but they do have mammary glands.
Physiology
Endothermy ("warm-blooded")
Nearly all mammals are endothermic. Most mammals also have hair to help keep them warm. Like birds, mammals can forage or hunt in cold weather and climes where reptiles and large insects cannot.
Endothermy requires plenty of food energy, so pound for pound mammals eat more food than reptiles. Small insectivorous mammals eat prodigious amounts for their size.
A rare exception, the
naked mole rat is exothermic ("cold-blooded"). Birds are also endothermic, so endothermy is not a defining mammalian feature.
Intelligence
In intelligent mammals, such as
primates, the cerebrum is larger relative to the rest of the brain. Intelligence itself is not easy to define, but indications of intelligence include the ability to learn, matched with behavioral flexibility. Rats, for example, are considered to be highly intelligent as they can learn and perform new tasks, an ability that may be important when they first colonize a fresh habitat. In some mammals, food gathering appears to be related to intelligence: a deer feeding on plants has a brain relatively smaller than a cat that must "think" to outwit its prey.
Social Structure
The dependence of the young mammal on its
mother for nourishment has made possible a period of training. Such training permits the nongenetic transfer of information between generations. The ability of young mammals to learn from the experience of their elders has allowed a behavioral plasticity unknown in any other group of organisms and has been a primary reason for the evolutionary success of mammals. The possibility of training is one of the factors that has made increased brain complexity a selective advantage. Increased associational potential and memory extend the possibility of learning from experience, and the individual can make adaptive behavioral responses to environmental change. Individual response to short-term change is far more efficient than genetic response.
Some types of mammals are solitary except for brief periods when the female is in estrus. Others, however, form social groups. Such groups may be reproductive or defensive, or they may serve both functions. In those cases that have been studied in detail, a more or less strict hierarchy of dominance prevails. Within the social group, the hierarchy may be maintained through physical combat between individuals, but in many cases stereotyped patterns of behaviour evolve to displace actual combat, thereby conserving energy while maintaining the social structure.
A pronounced difference between sexes (sexual dimorphism) is frequently extreme in social mammals. In large part this is because dominant males tend to be those that are largest or best-armed. Dominant males also tend to have priority in mating or may even have exclusive responsibility for mating within a “harem.” Rapid evolution of secondary sexual characteristics, including size, can take place in a species with such a social structure.
A complex behavior termed “play” frequently occurs between siblings, between members of an age class, or between parent and offspring. Play extends the period of maternal training and is especially important in social species, providing an opportunity to learn behaviour appropriate to the maintenance of dominance.
[3]
Locomotion
Mammals evolved from four-legged ancestors. They use their limbs to walk, climb, swim, and fly. Some land mammals have toes that produce claws and hooves for climbing and running.
Terrestrial
Specialization in habitat preference has been accompanied by locomotor adaptations. Terrestrial mammals have a number of modes of progression. The primitive mammalian stock walked plant grade—that is, with the digits, bones of the midfoot, and parts of the ankle and wrist in contact with the ground. The limbs of ambulatory mammals are typically mobile, capable of considerable rotation.
Mammals modified for running are termed cursorial. The stance of cursorial species may be digitigrade (the complete digits contacting the ground, as in
dogs) or unguligrade (only tips of digits contacting the ground, as in horses). In advanced groups limb movement is forward and backward in a single plane.
Saltatory (leaping) locomotion, sometimes called “ricochetal,” has arisen in several unrelated groups (some marsupials, lagomorphs, and several independent lineages of rodents). This mode of locomotion is typically found in mammals living in open habitats. Jumping mammals typically have elongate, plantigrade hind feet, reduced forelimbs, and long tails. Convergent evolution within a given adaptive mode has contributed to the ecological similarity of regional mammalian faunas.
Mammals of several orders have attained great size (
elephants, hippopotamuses, and rhinoceroses) and have converged on specializations for a ponderous mode of locomotion referred to as “graviportal.” These animals have no digit reduction and deploy the digits in a circle around the axis of the limb for maximum support, like the pedestal of a column. [3]
Arboreal
Well-adapted arboreal mammals frequently are plantigrade, five-toed, and equipped with highly mobile limbs. Some species, including many
New World monkeys, have a prehensile tail, which is used like a fifth hand. Brachiation, or “arm walking,” in which the animal hangs from branches and moves by a series of long swings, is an adaptation seen in gibbons. The primitive opposable anthropoid thumb is reduced as a specialization for this method of locomotion. Tarsiers are highly arboreal primates that have expanded pads on the digits to improve grasping, whereas many other arboreal mammals have claws or well-developed nails. [3]
Sloths travel slowly along branches rather than swinging energetically.
Aquatic
Three mammalian groups are fully aquatic.
Sirenians (dugongs and manatees) eat plants. Cetaceans (whales and dolphins) and pinnipeds (seals and walruses) eat krill or fish. Some semiaquatic mammals are very similar to their close land-borne relatives (otters, muskrats, and water shrews, for example). Other mammals have undergone profound adaptation for swimming and life at sea. Walruses and seals) give birth to and nurse their young on land, but cetaceans are completely helpless out of water. They depend on water for mechanical support and thermal insulation. [3] Bouyed by their aquatic environment, whales have evolved into the largest mammals and indeed the largest animals ever.
Aerial
Bats are the only truly flying mammals. Only with active flight have the resources of the aerial habitat been successfully exploited. Mammals belonging to other groups (colugos, marsupials, rodents) are adapted for gliding. A gliding habit is frequently accompanied by scansorial (climbing) locomotion. Many nongliders, such as tree squirrels, are also scansorial. [3]
Feeding
To maintain a high constant body temperature is energy expensive- mammals therefore need a nutritious and plentiful diet. While the earliest mammals were probably predators, different
species have since adapted to meet their dietary requirements in a variety of ways. Some eat animal prey- this is a carnivorous diet (and includes insectivorous diets). Other mammals, called herbivores, eat plants. An herbivorous diet includes sub-types such as fruit-eating and grass-eating. An omnivore eats both prey and plants. Carnivorous mammals have a simple digestive tract, because the proteins, lipids, and minerals found in meat require little in the way of specialized digestion. Plants, on the other hand, contain complex carbohydrates, such as cellulose. The digestive tract of a herbivore is therefore host to bacteria that ferment these substances, and make them available for digestion. The bacteria are either housed in the multichambered stomach or in a large cecum. The size of an animal is also a factor in determining diet type. Since small mammals have a high ratio of heat losing surface area to heat generating volume, they tend to have high-energy requirements and a high metabolic rate. Mammals that weigh less than about 18oz (500g) are mostly insectivorous because they cannot tolerate the slow, complex digestive process of a herbivore. Larger animals on the other hand generate more heat and less of this heat is lost. They can therefore tolerate either a slower collection process (those that prey on larger vertebrates) or a slower digestive process (herbivores). Furthermore, mammals that weigh more than 18oz (500g) usually cannot collect enough insects during their waking hours to sustain themselves. The only large insectivorous mammals are those that feed on huge colonies of insects (ants or termites).[2]

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