Animal Science curriculum

Biologists define an animal as a heterotrophic eukaryote that is multicellular by obligation, can take in food particles by phagocytosis, and can change body shape through coordinated cell contraction. With no rigid cell wall, animal cells can push out flexible projections such as microvilli and pseudopods, something walled plant cells rarely do.

A sponge has a skin of flattened cells and chambers lined with flagellated choanocytes, but these cells are not organized into true tissues or organs. Individual sponge cells take on jobs that tissues, organs and a nervous system handle in other animals; carnivorous sponges even digest prey by sending single cells to engulf its fragments.

More than 99 percent of animal species are bilaterally symmetric. Radial symmetry shows up in sessile, drifting or slow-moving animals such as cnidarians and echinoderms, whose body parts can meet forces from every direction equally well. One hypothesis holds that bilateral bodies persist because they can push off hardest when changing direction, so they maneuver best.

Heads did not appear in one step. Across bilaterians, nervous tissue, sense organs and feeding structures around the mouth gathered at the front end progressively. In arthropods and vertebrates the head grew by specializing the unsegmented front tip and then recruiting neighboring segments into it, steps linked to the anterior Hox genes.

A three-layered embryo places the mesoderm between an outer layer that becomes skin and an inner layer that lines the gut, and the mesoderm supplies the tissues in between, such as muscle and blood. Cnidarians such as corals and sea anemones have no separate mesoderm: the lining of their gut does double duty, with traits of both bilaterian gut cells and muscle-like cells.

A fluid-filled cavity works as a hydrostatic skeleton: muscles press against fluid that cannot be compressed, so contracting circular muscles makes a worm longer and thinner, while longitudinal muscles make it shorter and fatter. Flatworms show the alternative, a solid acoelomate body with no cavity, where molecules move by diffusion.

Repeating units let an animal control its body locally. In earthworms, muscular walls called septa keep each segment's fluid separate, so damage stays contained: if a fish bites off a worm's tail, the front end's hydrostatic skeleton still works and it can escape. The cost is flexibility, since fixed segment volumes limit some movements.

A tree is a hypothesis about relationships, not a record. Its shape depends on the characters and models used to build it, and new data can rearrange it, as happened with the placement of turtles. Trees also mark which traits are ancestral and which are derived, which is what lets you trace how a feature changed along a lineage.

Homologous traits are similar because they were inherited from a shared ancestor; analogous traits are similar for another reason, such as convergent evolution. Grouping animals by an analogous trait gives a misleading group: “flying tetrapods,” birds plus bats, would have two separate origins of flight, so their wings cannot mark a single branch.

The names refer to whether the mouth forms first, but the traits used to split the groups are leakier than the names suggest. Chaetognaths have radial cleavage, coeloms formed by enterocoely and a mouth that forms second, yet sequence data place them among protostomes. Molecular data strongly support protostomes; deuterostomes have far weaker support, and some recent analyses do not recover them as a group.

Ecdysozoans, including nematodes and arthropods, grow through periodic molts in which the exoskeleton is shed and made anew. Lophotrochozoa, a subclade of Spiralia, is less settled: in one large genomic analysis its internal branching shifted sharply with the taxa and genes used, though the lophophore-bearing groups consistently formed one clade.

Anatomy misleads most when a group is highly modified: the shell-centered body plan of turtles long blocked their placement, and genome-scale data now put them as the sister group of birds and crocodiles. Conflicts can also dissolve rather than be won: once characters that were not independent were handled properly, anatomy alone recovered hagfish and lampreys as one clade, matching molecular trees.

A paraphyletic group shares one common ancestor but leaves out some of its descendants, so no single branch of the tree belongs to it alone. “Lizards” is one: within squamates, lizards are paraphyletic while snakes are monophyletic. Older classifications kept such groups as grades, united by a shared set of adaptations rather than by an exclusive branch.

Flagellated choanocytes packed into chambers pump water through inflowing and outflowing canals, and the canal layout adds resistance that shapes how the pump works. Some deep-sea carnivorous sponges abandon the plan entirely, losing both the canal system and the choanocytes and trapping small crustaceans on a velcro-like cover of hooked spicules.

Sponge skeletons are built from siliceous or calcareous spicules, collagen-based spongin fibers, or both, and rarely from solid limestone, which is why species range from soft and compressible to rock hard. A dissociated sponge's cells can reassemble into a working individual, yet its regeneration is not unlimited; reproduction and environment both affect it.

Most medusozoans alternate a sessile polyp with a free-swimming medusa, and the medusa usually carries the more complex nervous system, with nerve rings, eyes and balance organs. The cycle is flexible: Hydra lives as a polyp that reproduces mainly by budding, and in Aurelia even sexually mature medusae have been seen turning back into polyps.

Each stinging cell holds one capsule under an osmotic pressure of about 150 bars. In Hydra's stenoteles, one of the most elaborate capsule types, the barbed tubule accelerates in under a microsecond, fast enough to pierce a crustacean's cuticle. Firing is not a simple tripwire: mechanoreceptors and chemoreceptors adjust it, and a well-fed animal suppresses discharge.

A coral polyp's stomach opens at one end only, so food enters and waste leaves by the same mouth, ringed by tentacles. Most coral polyps feed at night, catching prey with stinging cells, and many also trap fine organic particles in mucus strands that they draw into the mouth.

Cnidarians, the sister group of bilaterians, have a simple diffuse nervous system whose neurons arise from both the outer and inner cell layers. The net is not uniform, though: medusae add nerve rings and sense organs grouped in rhopalia, where information is processed, which may be an independent origin of nervous system centralization.

Reef-building corals house zooxanthellae, which pass up to 90 percent of the organic material they photosynthesize to the coral. That dependence on light is why reef builders need clear, low-nutrient water and mostly live above about 30 meters. Stressed polyps expel the algae and turn white, and a coral left bleached too long can starve.

Planarians are three-layered animals with a solid body and no body cavity, and they lack both circulatory and respiratory systems, so their metabolism depends on molecules diffusing through the body. Their fairly uniform makeup and thin shape have even made them a test subject for measuring diffusion inside a living animal.

Rebuilding tissue and deciding which end becomes the head are separate jobs. Stem cells in the wound blastema differentiate into the missing cell types, but polarity is set by Wnt and BMP signaling and by bioelectric signals, so a tail fragment with its follistatin gene knocked down fails to rebuild its head-to-tail axis.

In Taenia tapeworms, cattle or pigs swallow eggs, the larvae settle in muscle as cysticerci, and people are infected by eating raw or undercooked meat; the adult then anchors in the small intestine by its scolex. Schistosoma flukes instead pass through snails, and the released cercariae swim and penetrate human skin.

Protonephridia are branched organs spread through the body: each ciliated flame cell connects to a long tubule that opens at the body surface. The flame cell is the filter, built with proteins related to those of the vertebrate kidney's glomerulus; when planarians lose those proteins, fluid builds up in their tissues.

Beneath the cuticle lie four quadrants of body-wall muscle, with the gut and gonad sitting in the fluid of the pseudocoelom. Motor neurons drive dorsal and ventral muscles against each other to bend the body into waves, and internal pressure sets how stiff the worm is: hyperosmotic shock lowers the pressure and softens it.

A nematode's cuticle is a collagen-based layer secreted by the epidermis. Unlike an arthropod's rigid shell, it is elastic enough for the larva to grow between molts, though structures such as the mouth cavity enlarge only at a molt. Molting is staged: the larva goes quiet, loosens the old cuticle and escapes it with a forward thrust.

Bdelloid rotifers are among the smallest animals, most with about 1,000 cells and under half a millimeter long, yet they have distinct nervous, muscular, digestive, excretory and reproductive systems, and their cell number is fixed at maturity. Surviving desiccation appears ancestral in bdelloids, but two Rotaria species tested were sensitive to it.

The mantle builds the shell continuously: an embryo's first shell forms from a dorsal shell field, while the adult shell grows from secretory cells along the mantle margin. The Ediacaran fossil Kimberella has been read as an early mollusc relative with a foot surrounded by a mantle and mantle cavity, though that reading is disputed.

The radula is a chitinous membrane studded with small teeth and moved by muscles, a feature unique to molluscs. Feeding wears the teeth, so the ribbon keeps growing forward and old tooth rows break away and are replaced. Filming 20 species of snails and chitons as they fed revealed six distinct radular motions, including rotating and bending the ribbon.

Beyond snails, clams and squids, molluscs include worm-like aplacophorans covered in spicules, chitons with shell plates, monoplacophorans with a single cap-like shell, and tusk shells. Sequence data split the phylum into Aculifera, the spiny and plated forms, and Conchifera, the mainly single-shelled groups.

An octopus's nervous system holds roughly 500 million neurons, the most of any invertebrate and in the range of a ferret's brain. It is not all in the head: besides a central brain between the eyes there is a nerve cord in each arm and ganglia in the mantle, and the whole system expanded independently of mammalian brains.

Septa wall off each segment's coelomic fluid, so pressure can differ along the body and only the segments doing the work need to be pressurized. Forward crawling needs waves of contraction traveling backward along the body, called retrograde peristalsis. Several annelid lineages have lost septa in part or all of the body, gaining flexibility.

An earthworm keeps hemoglobin dissolved freely in its blood rather than packed into cells, in a closed circulation. Each segment has a pair of nephridia that form urine, a tissue called chloragog stores carbohydrate much as a liver does, and the setae and gizzard are stiffened with chitin.

The old division into polychaetes, oligochaetes and leeches does not describe three equal branches. Oligochaetes and leeches together form Clitellata, with leeches a derived subgroup of oligochaetes, and clitellates themselves appear to be derived polychaetes. Molecular work also suggests some groups once ranked as separate phyla belong inside Annelida.

The exoskeleton is the defining arthropod feature: it supports the segmented body and appendages and protects against predators and environmental stress. Because the chitin cuticle is rigid, it must be shed and replaced as the animal grows, so arthropod life is paced by discrete molts, each running from preparation through ecdysis to post-molt maturation.

After a molt, the new cuticle stays soft for up to a few days until tanning or mineral deposits harden it. In that window the animal is vulnerable, moves less and, on land, must hold itself up with internal pressure. The new cuticle also has to be swelled first: aquatic species drink, while land arthropods may swallow air or, in some isopods, absorb water vapor.

Tagmata differ by lineage. Insects have a six-segment head, a three-segment thorax and an abdomen; chelicerates such as spiders have a prosoma and an opisthosoma; myriapods keep just a head and a long, mostly uniform trunk. In many crustaceans a carapace covers segments behind the head, and that covered region is what gets called a cephalothorax.

A spider's prosoma carries four pairs of walking legs plus two pairs of feeding or sensory appendages in front, the pedipalps and chelicerae. Antennae belong to the mandibulate head of insects and their relatives. In mites and harvestmen there is no clear border between prosoma and opisthosoma, so count legs rather than body regions.

Crustaceans and insects are not separate branches: insects sit inside Pancrustacea along with them, and all share a six-segment mandibulate head. Behind the head, crustaceans show a bewildering variety of body regions; in crabs, most of the abdomen is reduced and folded under the body.

Myriapods have a head plus a long, mostly uniform trunk in which most segments carry walking legs. Specializations exist, such as the gonopods millipedes use to transfer sperm, but none changes the two-part plan. Their family tree is still argued: morphology links millipedes with pauropods, while several molecular studies disagree.

Insect mouthparts come in far more forms than four tidy types. In the Triassic, most newly appearing types were tied to plant feeding, such as the piercing-sucking mouthparts of Thysanoptera, probably used on pollen. Some researchers avoid sorting them into a few classes at all, because similar types arose separately in distant insects.

Tracheae carry oxygen directly to metabolically active tissues, an efficient and adjustable supply. Giant insects of the late Paleozoic led to the idea that oxygen limits insect size, but whether oxygen sets limits for tracheated insects is still contentious; in one study, the heat tolerance of a stonefly's aquatic stages did shift with oxygen supply.

In incomplete metamorphosis the nymph usually resembles the adult but lacks wings and genitalia, and its wing pads become working wings only at the final molt. One developmental view lines the two patterns up stage for stage, matching larva, pupa and adult to the hemimetabolous pronymph, nymph and adult. Juvenile hormone keeps an insect in its immature form.

Echinoderm larvae start out bilateral, pass through an asymmetric phase, and only then become five-part radial adults. The fossil record agrees: plated Cambrian animals near the base of the echinoderm line were bilateral, showing that the radial body plan came after a bilaterian start.

The water vascular system is unique to echinoderms and serves locomotion, circulation and feeding. Its five-part layout, a canal ringing the mouth with radial canals running outward, also defines the main body axis, because echinoderm axes are traced along the ambulacra that carry it.

A sea star's body wall is built from magnesium calcite ossicles joined by collagen and muscle. Its stiffness can change because that collagen is mutable: under nervous control it can alter its tension within seconds, letting the same skeleton go rigid or slack.

Autotomy, deliberate self-detachment, works by rapidly destabilizing mutable collagen at a set breaking plane. A brittle star then regrows the arm in stages, with the radial water canal and nerve back in about six days and half the new arm differentiated after two to three weeks. One study proposed larger sea stars autotomize less because their arms resist damage better.

The notochord is the trait that best sets chordates apart. Some members of Xenambulacraria, the group joining hemichordates and echinoderms, also have pharyngeal slits, a tail past the anus and a form of endostyle, so those features may be older than chordates. The notochord itself is a flexible rod that resists compression, giving muscles something to bend the body against.

Amphioxus keeps its notochord into adulthood, as do appendicularian tunicates, whereas ascidian tunicates have one only as tadpole-like larvae and settle as sessile adults. Even so, tunicates, not amphioxus, are probably the closest living relatives of vertebrates.

Hagfish and lampreys keep a notochord as adults, as sturgeons do, instead of largely replacing it with a spinal column like most vertebrates. A fossil hagfish from the Cretaceous helped settle their relationship: reanalyzed with it, anatomy alone groups hagfish and lampreys as cyclostomes, the same answer molecular trees give.

The idea that jaws evolved from a gill arch was challenged, then supported again by the pseudobranch, a small gill-like structure behind the jaw. In skates it develops from the jaw-forming mandibular arch and shares gene expression and cell types with gills, pointing to a gill-arch-like ancestry for the jaw.

Shark skeletons are cartilage, much lighter than bone, yet most sharks deposit calcium salts in it as they age, which is why a dried shark jaw feels solid and shark skeletons can fossilize. The skin is covered in placoid scales, or dermal denticles, tooth-like structures that point toward the tail and reduce friction as the shark swims.

Teleosts number about 29,000 species, nearly half of all vertebrates. One main explanation is a whole-genome duplication in their ancestor, and one shift in diversification rate does sit at the base of the teleost radiation, but two other increases are much younger than the duplication, so it cannot be the whole story.

Filling a swim bladder at depth means pushing gas in against high pressure. Physoclist fish, whose bladder has no open duct to the gut, concentrate gas with the rete mirabile, a countercurrent capillary network, which is how some fish keep gas-filled bladders hundreds or even more than 1,000 meters down.

Countercurrent flow is a highly efficient way to aerate a gill, while concurrent flow, with water and blood moving the same way, exchanges oxygen less efficiently. The design is old and widespread: modeling suggests the Late Ordovician trilobite Triarthrus eatoni used countercurrent exchange, and preserved gills of the Cambrian Haikouella fit the same model.

Lateral line sensors, called neuromasts, sit either on the skin surface or inside canals. In a model, canal neuromasts saturated less than surface ones during movement in one direction, and in cichlids, fish with larger canal pores and fewer surface neuromasts swam closer to their neighbors in groups.

Genome data confirm lungfish as the closest living relatives of four-limbed vertebrates. Lungfish also carry preadaptations to life on land, including limb-like expression of developmental genes such as hoxc13 and sall1 in their lobed fins.

Amphibian skin is far more permeable than mammal skin: in one study, frog skin let several chemicals through 26 to 302 times faster than pig skin. The tie to water is not universal, though. Lungless salamanders make up more than two thirds of living salamander species, and most of them are fully terrestrial.

Some amphibians breathe with no lungs at all: lungless salamanders exchange gases only across the skin and the lining of the mouth and throat, and lunglessness has evolved several times in salamanders, frogs and caecilians. How they meet their oxygen needs while reaching large sizes and varied climates is still an open question.

Thyroid hormone works together with corticosteroids, and the organs answer on their own schedules rather than all at once. That timing matters: giving thyroid hormone to a young tadpole can kill it, likely because its organs then change out of step with one another.

Living amphibians split into Batrachia, the frogs and salamanders, and the limbless, worm-like caecilians. Teeth sort them too: caecilians and salamanders always have them, while frogs have lost teeth completely more than 20 times, more often than any other vertebrate group, a loss tied to diets of small invertebrates.

The amnion gives the embryo its own small pool of water; the yolk sac supplies nutrients; the allantois stores waste and, with the chorion, handles gas exchange through the shell. Together these membranes supply food and gas while preventing dehydration. In eutherian mammals the allantois is largely vestigial.

In lizards and snakes, the hard outer beta-layer of the skin mainly protects against wear; the barrier against water loss lies in the softer, lipid-rich layers beneath it, and the deepest layer lets scales stretch during movement. That is why studies of desert lizards such as the Gila monster analyze the lipids in shed skin.

Low heat production lets ectotherms run on low metabolic rates, which helps them survive cold, low oxygen and long fasts. They control body temperature mostly by choosing microclimates: small reptiles bask, and crocodiles leave the water to warm in the sun or cool by evaporation in the shade.

Among non-avian reptiles, squamates, meaning lizards, snakes and amphisbaenians, hold about 96 percent of species, while the tuatara is a single species. Birds belong inside the reptile tree: defining reptiles to exclude birds makes the group paraphyletic, which is why a phylogenetic classification counts birds as reptiles.

Feathers serve heat retention, mate attraction, communication, camouflage and skin protection as well as flight. Display or heat retention may have been their original job, with flight a later use of a structure that already existed.

The sternum anchors the major flight muscles, and across living birds a large sternum with a deep or forward-projecting keel is needed for powered flight. Sternum shape also tracks how a bird moves, though only loosely: deeper keels go with slower but stronger flight, and narrower sterna with running.

Aspect ratio, span divided by mean chord, is a key determinant: long, narrow wings in fast or soaring birds cut induced drag and raise the best lift-to-drag ratio. In wind-tunnel tests of real wings, flight-style groups differed mainly in drag rather than maximum lift, and birds fly at lower Reynolds numbers than most aircraft.

One-way airflow is not just a flight adaptation. Alligators and Nile crocodiles also move air in one direction through open-ended tubes, even though crocodiles have neither air sacs nor air-filled bones, so the pattern is probably older than birds within the archosaurs.

Birds hold a high body temperature without brown adipose tissue, the heat-producing fat many mammals use, so research on avian heat production centers on skeletal muscle, where cycling of calcium and proton ions generates heat. Feathers help keep that costly heat in, since heat retention is one of their main roles.

Darwin's finches made the beak the classic case of diet shaping form, and within that island radiation skull shape fits food closely. Across all living birds, though, diet and foraging behavior proved significant but very weak predictors of skull form, and earlier work pointed to ancestry and body size as stronger influences.

Birds use three compasses, based on the sun, the stars and the earth's magnetic field. The magnetic compass reads the inclination of field lines, not their polarity, so it distinguishes poleward from equatorward. One proposed strategy, the magnetoclinic route, has a bird keep a fixed heading relative to the inclination angle it senses.

The middle-ear bones record a reuse: as mammals evolved a new jaw joint, the original joint was taken into the middle ear, joining the stapes that reptiles and birds also have. Eardrum-based middle ears evolved independently in different amniote groups, so the three-bone chain is a mammalian solution, not an inherited reptile ear.

The labels can mislead: marsupials also have a fully functional placenta, but gestation is short and the highly altricial newborn crawls to the pouch and attaches to a teat. Its milk changes through a long lactation and supports lung maturation that eutherians complete before birth. Monotremes are the only living outgroup to all other mammals.

Heterodonty arose gradually during synapsid evolution, and incisors, canines, premolars and molars were all present by the Mesozoic. Many mammals have since dropped whole classes; a mouse, for example, has no canines or premolars at all. Dental formulas, such as the opossum's 5134 in the upper jaw, record which classes remain.

Rumination relies on density sorting in the forestomach, a mechanism that lets ruminants eat more without losing digestive efficiency. The rumen, the first and most important compartment, holds cellulose-digesting bacteria such as Fibrobacter and Ruminococcus. The four-chamber layout also varies: camels lack an omasum.

Horses and donkeys ferment fiber in the hindgut, beyond the small intestine, rather than in a chambered stomach. Rabbits rely heavily on cecal fermentation and add cecotrophy, eating soft feces to recover more nutrients. The trade-off is that non-ruminants digest fibrous feed less efficiently and need more digestible diets.

Insulation is tuned, not maximized. Ruminants from arid regions carry thinner fat under the skin, seals huddle as temperatures drop, and many eutherian mammals make extra heat in brown adipose tissue. Uninsulated appendages such as seal flippers serve as thermal windows, where blood flow can be adjusted to shed heat quickly.

Marsupials show how investment can be timed. A tammar wallaby invests little during its brief pregnancy and early lactation, which lets the mother respond to changing conditions, then supports the young through a long lactation whose milk composition shifts as it develops. Tammars also have the longest embryonic diapause of any mammal.

Placental orders fall into four large groups. Euarchontoglires holds primates, rodents, rabbits and their relatives; Laurasiatheria includes cows, dogs, bats and shrews; Xenarthra and Afrotheria complete the set. One genome-scale study dated the first splits to 95 to 100 million years ago, in quick succession, which is why some branches, such as bats, are hard to place.

Each type is defined by how its cells work together. Epithelium covers surfaces and lines cavities with tightly packed cells; connective tissue is mostly matrix with relatively few cells; muscle tissue contracts; and nervous tissue is built around neurons, the cells that generate and conduct impulses.

Epithelial cells are tightly packed, with one free surface, and rest on a basement membrane. Simple columnar epithelium lines the stomach and intestines, simple cuboidal lines kidney tubules and glands, and pseudostratified columnar lines parts of the respiratory tract. Transitional epithelium can be stretched, and glandular epithelium secretes.

Connective tissue is defined by abundant matrix with relatively few cells, which is why blood and bone share a category. It binds and supports organs, stores fat, transports substances and helps repair damage. Its common cells include fibroblasts, macrophages and mast cells, and most types have a good blood supply, though some do not.

Skeletal fibers are cylindrical, multinucleated, striated and under voluntary control. Cardiac muscle is also striated but branched, with one nucleus per cell and intercalated disks, and it is involuntary. Smooth muscle cells are spindle-shaped, with one central nucleus and no striations, so striation alone does not tell you a muscle is voluntary.

When shape stays the same, surface area grows with volume to the 2/3 power. Across 54 shark species spanning about a 19,000-fold range in body mass, whole-body surface area followed that prediction almost exactly, so a bigger shark has less surface for each unit of its volume.

Metabolic rate rises more slowly than body mass, and the physiological cause is still debated. The usual reading is that cells run slower in big animals, but in planarians, whose adult mass can change over three orders of magnitude, the scaling came instead from larger animals packing more mass into each cell.

A sagittal plane divides the body into right and left, a coronal or frontal plane into front and back, and a transverse plane into upper and lower parts. Proximal and distal are measured from the trunk or a part's point of origin. Human anatomy treats anterior as ventral and superior as cranial, so check which body plan a term assumes.

Hormone control shows the pattern: a gland senses the level of the substance it regulates and reverses any rise or fall. The control has to be tight because hormones are so potent that tiny amounts have large effects. Walter Cannon coined the word homeostasis for the relatively constant states that loops like this maintain.

In positive feedback, a response raises the very signal that triggered it instead of reversing it. Nursing is one case: receptors in the nipple, activated by a suckling newborn, drive pulses of hormone release that lead to still more release, and milk let-down follows 30 to 60 seconds after suckling begins.

An ectotherm facing unfavorable temperatures can acclimatize, reversibly adjusting its thermal sensitivity, or thermoregulate by behavior. Set points move too: skinks in wind regulated between cooler set-point temperatures than skinks without wind, despite the same available temperatures.

At the same body temperature, a mammal's basal metabolic rate is seven to eight times that of a similar-sized ectotherm, which is the price of internal heat. The line is not absolute, since some ectotherms warm parts of their bodies with localized endothermy. One review argues whole-body endothermy is shared by descent in birds and mammals rather than evolved twice.

Animals shift which route carries their heat. Crocodiles leave the water to warm in sunlight or cool by evaporation in the shade, and rabbits shed heat through long, well-vascularized ears. Seals in cold water lose heat mainly from the trunk, while their flippers take a growing share of convective heat loss as the water warms.

A countercurrent exchanger can shield a body region from heat as well as hold heat in. A blood-feeding kissing bug has one in its head, where closely aligned circulatory and feeding channels cool the warm blood meal before it reaches the abdomen, keeping the abdomen near air temperature. It was the first such exchanger found in an insect head.

Hibernating mammals must survive low blood flow, low body temperature and immobility without forming clots, so during torpor they reversibly lower platelets and clotting factors and speed clot breakdown, then restore normal clotting on arousal. Estivation is a comparable dormancy timed to heat and drought: milk snails seal their shell with mucus and cut metabolism below 30 percent of resting.

Filter feeding turns up in sponges, ram-feeding fishes and balaenid whales, which is why feeding categories cut across phyla. A filter's main problem is clogging. Balaenid whales and ram-feeding fishes use cross-flow filtration, in which water sweeps along the filter surface, which delays clogging and concentrates prey near the throat.

In one radiation of fishes, herbivores did not converge on a shared body shape, yet all needed significantly longer intestines, so gut length tracked diet when outward form did not. Anatomy can also lag behind diet: the giant panda eats bamboo through a short, simple, carnivore-like gut.

Digesting inside cells means making new lysosomes, with acidified contents and fresh enzymes, for each meal, which costs energy. Enzymes released into a cavity can break down much larger particles, and the cavity keeps enzymes and nutrients from diffusing away. Bilaterian gut cells then take up the products through membrane transporters for sugars and amino acids.

The usual story is that a blind gut like a cnidarian's digestive sac turned into the through-gut of bilaterians, but how that happened is debated. Ctenophores expel undigested particles mainly through anal pores opposite the mouth, a working through-gut that lacks the genes marking a bilaterian anus.

Mechanical digestion starts with chewing and continues as the stomach churns and mixes, breaking food into smaller particles that enzymes can act on. Chemical digestion is the enzymes' part: they speed up the splitting of carbohydrates, proteins and fats, a reaction that would otherwise be very slow. Peristalsis, rhythmic waves of muscle contraction, moves the mixture along.

Enzymes from several organs work in sequence along the tract. The small intestine's own lining adds peptidase for proteins, sucrase, maltase and lactase for sugars, and lipase for fats, finishing work that enzymes from upstream organs began. The large intestine, by contrast, makes no digestive enzymes at all, so breakdown there is left to its bacteria.

Three levels of folding enlarge the small intestine's absorptive surface: circular folds called plicae circulares, then villi, then microvilli. Absorbed products do not all leave by the same route, since fatty acids enter the lymph rather than the blood. The large intestine has no villi at all.

Herbivores cannot break down cellulose on their own, so their guts house microbes that ferment it into short-chain fatty acids the animal uses for energy. The rumen and the hindgut chambers of horses and rabbits are separate solutions to housing those microbes, one placed ahead of the small intestine and the other behind it.

Sometime between 500 and 550 million years ago, the animal lineage gave up the pathways for 9 of the 20 amino acids used to build proteins. One perspective argues the trade paid because food was plentiful enough that the energy saved outweighed the risk of shortage. That loss is why those amino acids are essential: an animal must get them from its food.

Gas crosses a respiratory surface by diffusion, which is why the exchange tissue in your lungs is a single layer of flat cells. Birds go further on both counts: next to a mammal of similar size, a bird has a blood-gas barrier about 2.5 times thinner and roughly 15% more area. The bird lung's rigidity, since it does not stretch with each breath, has been proposed as what made that extreme thinning possible.

Water holds much less oxygen than air and is about 50 times more viscous, so every breath of water costs a fish more effort than a breath of air costs you. Gills compensate with countercurrent flow, blood running opposite to the water, which lets them extract up to about 90% of the dissolved oxygen; parallel flow is less efficient.

Your breathing is tidal: air moves through the same passages into and out of the lungs, and ventilation and gas exchange are coupled. A bird uses its air sacs as bellows to drive air one way through rigid tubes on both inhalation and exhalation, and pairs that with cross-current blood flow. Do not file one-way flow as uniquely avian: crocodilians and other reptiles have it too.

You inhale by suction: the diaphragm contracts, the chest cavity enlarges, and the lower pressure in the lungs pulls air in. A frog works the other way, as a positive-pressure pump. It lowers the floor of its mouth to draw air in through the nostrils, then raises it to press that air into the lungs; small throat movements that never reach the lungs are not breaths.

Hemoglobin binds oxygen on iron and is packed inside red blood cells in vertebrates. Hemocyanin binds it between two copper atoms and floats freely in the hemolymph of some arthropods and mollusks. Myoglobin is the store rather than the carrier: it holds oxygen inside muscle, and diving mammals and birds carry far more of it than land mammals do.

Hemoglobin's binding curve is S-shaped, and the molecule works by shifting between a high-affinity, mostly oxygenated state and a low-affinity, mostly deoxygenated one; myoglobin gives a plain hyperbola. The Bohr effect lowers affinity in acid, which promotes unloading in working tissue. Penguins evolved higher affinity plus a much stronger Bohr effect, a pairing that should aid oxygen pickup in the lung without hurting delivery.

Inside red blood cells, carbonic anhydrase converts carbon dioxide into bicarbonate and a hydrogen ion, and the bicarbonate leaves the cell in exchange for chloride. Bicarbonate dissolves better in plasma than the gas does, the exchange spares the tissues from acidifying, and the acid left inside the cell nudges hemoglobin to release oxygen. In the lungs the steps run backward and the carbon dioxide is exhaled.

In an insect the heart is a dorsal tube that empties hemolymph into the main body cavity, where it moves slowly between the organs along ill-defined paths. Your blood stays inside closed tubes that begin and end at the heart and flows down a pressure gradient from arteries to veins. Open does not mean unregulated, though: in one beetle, some body movements made abdominal pressure run well above thoracic pressure.

A fish heart has one atrium and one ventricle, and an adult amphibian's has two atria and one ventricle. Lizards and snakes also have two atria over a single, partly divided ventricle, where oxygen-rich and oxygen-poor blood can mix. A fully divided four-chambered heart arose more than once by convergence, and researchers link it to the high output that warm-bloodedness demands.

A teleost heart pumps blood to the gills, and from there it flows on to the body with no second pump, which is enough for a fish's needs. With two separate ventricles running two circuits, pressure in the body circuit can sit well above lung pressure without rupturing the thin lung surface. Many reptiles sit between: they can shunt blood past the lungs when, for example, they dive.

Arteries leave the heart under high pressure and have a thick smooth-muscle middle layer that changes their diameter. Arterioles are the control valves, deciding how much blood enters each capillary bed, and capillaries do the exchange. Veins return blood at lower pressure, so their walls are thinner and less rigid, which lets them hold more blood; in medium and large veins, valves keep it moving toward the heart.

Gases cross the capillary wall by diffusion, but fluid follows the balance of hydrostatic and osmotic pressure, so it leaves the blood at one end of a capillary and returns at the other. Flow is slowest in capillaries because their combined cross-section is largest, which buys time for exchange. When protein leaks out, it raises the osmotic pull of the tissue fluid and slows that return.

Spin a blood sample and it separates: plasma, about 55% of the volume, on top; packed red cells, the hematocrit, at the bottom; and a thin buffy coat of white cells and platelets between. Do not assume every animal matches yours. Bird blood carries nucleated red cells and thrombocytes, features that challenge automated counters designed for mammals.

Each heartbeat is a cycle of systole, when the chambers contract, and diastole, when they relax and refill. Output can rise through a faster rate, a bigger stroke, or both. Birds, whose hearts are nearly twice as heavy as those of mammals their size, are thought to raise output during exercise mostly by beating faster, possibly because their stroke is already near its maximum at rest.

About 90% of the fluid that leaves a capillary is reabsorbed; lymph capillaries collect the rest, plus leaked protein, and return it to the veins. When leaked protein holds fluid in the tissues faster than it drains, the fluid pools as edema. On its way back, lymph passes through nodes where lymphocytes filter out microorganisms, so the drainage system doubles as a surveillance route.

Most marine invertebrates, and sharks, skates and hagfish, keep their body fluids at the same osmotic pressure as seawater, using organic solutes such as urea and trimethylamine oxide. Most vertebrates and freshwater invertebrates instead hold an internal level that differs from the water around them. The categories are not fixed per species: one mud crab conforms at some salinities and regulates at others.

A freshwater animal is saltier than its water, so water floods in and ions leak out across the gills; ion transporters hold its internal salts up and it gets rid of water as dilute urine. A marine bony fish loses water to the sea, so it drinks seawater, absorbs the water in its gut, excretes the excess sodium and chloride across its gills, and makes only a little urine.

Nitrogen metabolism yields ammonia, a potent poison that takes a lot of water to flush away. Aquatic animals can let it diffuse into the surrounding water. Life on land needed a water-saving route, so many land animals convert it to urea or uric acid, both far less toxic. The labels ammonotelic, ureotelic and uricotelic are tendencies: many snails, turtles and fish excrete more than one.

In animals without a true coelom, beating cilia in a terminal cell drive fluid through a filter into a tubule. Annelids filter blood into the coelom and drain it through open-ended tubes called metanephridia. Insects work by secretion instead: Malpighian tubules pump ions into their lumen, water follows, and together with the hindgut they do the job of a kidney.

Each nephron starts with a glomerulus, a knot of capillaries that filters blood into the tubule without regard to what is useful. The tubule then does the selective work, returning needed substances to the blood and moving wastes into the urine. So what the filter lets through is not what leaves: the tubule decides which of it the body keeps.

The thick ascending limb of the loop of Henle pumps salt into the surrounding tissue but cannot let water follow, building a salty gradient in the medulla. Water then leaves the collecting ducts into that gradient under the control of vasopressin. A thicker medulla, an index of loop length, predicts how concentrated the urine can get, and that anatomy varies with how arid a species' habitat is.

Many birds have nasal salt glands that excrete a hypertonic salt solution, which clears excess salt while losing little water. A sea turtle's gland wraps the rear of the eye socket and empties concentrated salt through a duct at the eye. The glands' size tracks habitat salinity and diet; a coastal snake without them instead tolerates very salty blood.

Dendrites carry signals toward the cell body and, being short and branched, offer a large receiving surface; the single axon carries them away to terminals that release transmitter. Sensory neurons tend to have long dendrites and short axons, motor neurons the reverse. Glial cells conduct no impulses; they support neurons and, as Schwann cells or oligodendrocytes, make the myelin.

A membrane potential exists because ions are unevenly distributed and the membrane lets some cross more easily than others. The resting value is where the net flow of sodium, potassium and chloride balances out. Every action potential lets some ions run down those gradients, and ATP-driven pumps have to restore them, which is why signaling is a real energy cost for nervous tissue.

When the membrane reaches threshold, voltage-gated sodium channels open, and the inflowing sodium depolarizes the membrane further. Sodium channels then inactivate and potassium flows out, bringing the voltage back down. During the refractory period that follows, the patch cannot fire again, which keeps each spike separate and makes the signal travel one way along the axon.

Myelin wraps the axon between small gaps called nodes, and the impulse jumps from node to node, which is far faster than creeping along bare membrane. Invertebrate neurons typically lack myelin, and many hasten conduction with giant-diameter axons instead, a trait the lamprey, an early-diverging vertebrate, still shares. In jawed vertebrates, myelin delivers speed without that bulk.

An arriving action potential opens calcium channels in the axon terminal, and the calcium triggers release of transmitter into the synaptic cleft, where it binds receptors on the next cell. The response there is graded, not all-or-none, and strong activation can strengthen a synapse while weaker patterns can weaken it, which makes the synapse a key point where signals are adjusted.

The central nervous system is the brain and spinal cord; the peripheral system is every nerve that branches out from them. The peripheral side splits by target: somatic nerves serve skin and skeletal muscle and handle conscious activity, while autonomic nerves serve organs such as the heart and gut and run below awareness. A single mixed nerve can carry both sensory and motor fibers.

A reflex needs no conscious thought, so it is faster than a considered reaction. Many reflexes are handled in the spinal cord without going to higher brain centers: in the withdrawal reflex, your hand is already moving before you feel the pain, because the cord acts before the signal reaches awareness.

Hydra's neurons form two separate networks, one in each tissue layer, that never touch. Bilaterians usually have far more nerve cells, organized into a centralized system with large ganglia. Treat that as a pattern, not a ladder: a study of one comb jelly found its nerve net to be a single continuous membrane, with no synapses detected between its neurons, a design unlike either.

Sensory cells are classified by what they respond to: photoreceptors to light, mechanoreceptors to deformation, chemoreceptors to molecules. In the mouse trigeminal ganglion, sensory neurons split further into touch receptors, thermoreceptors for heat and cold, receptors for irritants, and nociceptors for noxious heat, cold or chemicals. A hair cell in your ear is a mechanoreceptor: bending is what it detects.

Transduction turns a stimulus into an electrical change in the receptor, as when bent hair-cell stereocilia open channels and ions rush in. Because each spike is all-or-none, a touch receptor reports a stronger stimulus by firing more often, not by firing bigger spikes. Its range is limited at both ends: below a lower threshold it stays silent, and above an upper one its rate stops rising.

Sensory adaptation is a drop in a receptor's response when the same stimulus is repeated or held. In the mammalian main olfactory system it already shapes coding at the earliest processing stages, and in the accessory olfactory system of mice, sensory neurons respond less when urine is presented again within about 30 seconds. Adaptation varies in time course, from seconds to longer-lasting effects.

Eye designs differ so much between animal groups that, as far as current evidence goes, most evolved independently. An early step is direction: screening pigment shading a light-sensitive cell lets an animal steer toward or away from light. True images need many receptors behind focusing optics; the nautilus manages a pinhole eye with no lens or cornea, while arthropods, cephalopods and vertebrates reach high resolution.

Most daytime land vertebrates other than mammals have four cone types; most mammals have two and a rod-heavy retina, likely a legacy of nocturnal ancestors. Primate trichromacy rests on an X-linked gene. Some Australian marsupials appear trichromatic by another route, though no third cone pigment has been identified. Among birds, ultraviolet and violet sensitivity have switched several times.

Sound sets the cochlear fluid rippling, and a traveling wave moves along the basilar membrane carrying hair cells with it. When their stereocilia bend against an overlying structure, channels at their tips open and an electrical signal starts. Position sorts pitch: hair cells near the wide end of the cochlea respond to high frequencies, those toward the center to low ones.

A dolphin sends high-pitched clicks from its melon; they bounce off anything denser or less dense than water and return to build a picture of what is ahead. Bats and toothed whales evolved echolocation separately, a classic case of convergence. Not every bat does it: many Old World fruit bats rely largely on vision, and one genus echolocates with tongue clicks instead of the larynx.

Smell and taste both sample chemicals: olfactory neurons in a small patch high in the nose connect directly to the brain, while taste cells carry receptors tuned to a handful of basic qualities. Pheromones are secretions that change the behavior or hormones of another member of the same species. In pigs, steroids in boar saliva bind carrier proteins in the sow's nasal mucus and help bring on estrus behavior.

Nerves act through impulses and neurotransmitters, with effects that last seconds and stay local. Hormones travel in the blood and act over minutes, hours or weeks, with broader reach, which suits growth, development and metabolism. The two are not rivals: some glands are triggered by a nerve, as when sympathetic nerves make the adrenal medulla release epinephrine.

Protein hormones bind receptors on the cell surface, and their effects follow relatively quickly. Steroid hormones typically bind receptors inside the cell and act by changing which proteins the cell makes, so their effects are slower to appear. In your body the steroids are the sex hormones and those of the adrenal cortex; every other hormone is a protein or protein derivative.

A hormone reaches nearly every cell the blood reaches, yet only cells with a matching receptor respond, like a key that opens only its own lock. The set of responsive cells is the hormone's target tissue, which can be one organ or cells scattered through the body. So the same hormone can act on a single gland in one case and on many tissues in another.

The pituitary hangs from the hypothalamus on a slender stalk and has two parts with different controls. Releasing hormones from the hypothalamus run the anterior lobe, whose tropic hormones in turn drive other glands, such as the thyroid and gonads. The posterior lobe is run by nerve stimulation instead, and releases antidiuretic hormone and oxytocin.

In negative feedback a gland senses the level of what it controls and reverses any drift. Circulating thyroid hormone acts back on the hypothalamus and pituitary to set its own output, so a shortfall raises the upstream drive and an excess cuts it. The loop can backfire: if the thyroid lacks a raw material it needs, the drive stays high and the gland enlarges without making more hormone.

Pancreatic islet cells work as a pair: one cell type releases insulin when blood glucose is high, another releases glucagon when it is low, so each hormone corrects the opposite drift. Thyroid hormones set how the body uses energy more broadly, so a shortfall slows many of the body's functions at once rather than one process.

The adrenal gland is two glands in one. Its medulla, built from neural tissue, releases catecholamines on a direct signal from sympathetic nerves, especially in stressful situations. Its cortex answers a hormone from the pituitary instead, under negative feedback through the hypothalamus. The cortex is essential to life; the medulla can be removed without life-threatening effects.

Insect molting and metamorphosis are governed by two hormones working together. Juvenile hormone decides what each molt produces: while it is present, the larva molts into a bigger larva. When ecdysteroids act without juvenile hormone, the insect commits to metamorphosis, so a silkworm mutant that lacks juvenile hormone metamorphoses early, after fewer larval molts.

An earthworm's skeleton is pressurized fluid: circular and longitudinal muscles squeeze against it to drive peristaltic crawling. Arthropods use a rigid exoskeleton with muscles attached inside; it protects against injury and drying, but it has to be shed for the animal to grow. A bony endoskeleton grows with the animal and keeps being remodeled after full size is reached.

Compact bone is packed osteons, rings of matrix around canals that carry blood vessels; spongy bone is a lighter lattice of plates and bars around marrow-filled spaces. Osteoblasts build bone and osteoclasts remove it, and the balance between them maintains it. The spongy bone's struts follow lines of stress and can realign if the direction of load changes.

Joints first sort by how much they allow: immovable joints held by thin fibrous tissue, slightly movable ones bridged by cartilage, and freely movable synovial joints with a lubricated cavity. Synovial joints are then named by shape and motion: a hinge only bends and straightens, a pivot lets one bone rotate around another, a ball-and-socket moves in several planes and rotates, and a plane joint lets flat surfaces glide.

A sarcomere contains two overlapping sets of filaments, actin and myosin, and when the muscle shortens they slide past each other while each set stays nearly the same length. The key evidence was that as sarcomere length changed, the dark bands kept their length and the light bands took up the change. Myosin heads on the filament surface work like rowers, making the filaments slide.

The nerve-triggered action potential spreads inward along the muscle fiber's tubules, where voltage sensors signal calcium-release channels in the fiber's internal calcium store to open. The released calcium binds troponin C, switching on the contractile machinery. Relaxation is not passive: an ATP-driven pump, SERCA, has to take the calcium back into storage before the muscle can let go.

Muscles can only pull, so a movement made by one muscle has to be undone by another pulling the other way. Typically a muscle spans a joint, anchored by tendons at both ends, and moves one bone while the other stays fixed. A soft-bodied animal follows the same rule without bones: an earthworm's circular and longitudinal muscles work against each other through its fluid skeleton.

Slow-twitch oxidative fibers are packed with mitochondria and myoglobin and well supplied with capillaries. Fast-twitch glycolytic fibers have less oxidative capacity, fewer capillaries and usually a larger cross-section, and fast oxidative-glycolytic fibers sit between. The mix can shift: in rats held at a simulated 4,000 m altitude, a slow muscle gained intermediate fibers, though such hypoxia effects remain debated.

Water is about 50 times more viscous than air, so cutting drag matters more to a swimmer than to a walker or a flier. Drag rises with the square of speed, which makes fast swimming expensive, and behavior can cut it: in one study, fish swimming in a school lowered their total cost of transport by up to 53% compared with fish swimming alone.

Skin is usually the first line of defense, and its cells secrete antimicrobial proteins as well as forming a wall. Behind the barriers, innate cells carry inherited receptors that recognize broad danger patterns shared by viruses, bacteria and fungi. That makes innate defense immediate, but it cannot tell one bacterial strain from another, which is the job adaptive cells take over later.

Inflammation is the innate system's coordinated local response, and a normal part of defense against injury or infection; in a cut it shows as swelling, redness and pain that usually fade within hours. Neutrophils gather at an injury within minutes and signal one another into swarms, and mast cells release histamine and other inflammatory chemicals. The same response turns harmful when it hits healthy tissue or lasts too long.

Neutrophils patrol the blood and engulf bacteria, digesting them inside vesicles; macrophages, whose name means big eater, ingest and degrade bacteria too and also clear away dead cells. Natural killer cells take a different approach, punching holes in virus-infected or tumor cells and triggering their programmed death, which removes them without releasing danger signals.

The genes for B- and T-cell receptors are randomly rearranged as each cell matures, so every cell carries a unique receptor and the population can recognize almost anything, including pathogens never met before. The cost of that precision is narrowness: an adaptive cell may recognize only one strain of a pathogen, and it needs time to expand, so the adaptive response arrives after the innate one.

B cells make antibodies, the same molecule as their surface receptor released in soluble form, which coat a pathogen to block it from entering cells, flag it for engulfment and trigger complement. A neutralized microbe cannot bind and infect host cells. For cells already infected by a virus, cytotoxic CD8 T cells remove the cell itself by releasing granules that trigger its programmed death.

After B and T cells expand to fight an infection, they stop dividing and are kept as memory cells, already primed so that the next encounter is cleared before the pathogen can establish itself. Vaccination builds that memory without the infection. Because each receptor is unique, some memory cells are better than others at eliminating the target, and an effective vaccine engages both innate and adaptive responses.

Insects lack T and B lymphocytes and the genes for antibodies. Their hemocytes engulf small invaders, wall off objects too big to engulf by encapsulation, and clump around groups of microbes into nodules, a step tied to melanization and antimicrobial peptide production. Their defenses are not one-shot either: in immune priming, activation stays raised after a first challenge and boosts resistance to a second.

Corals show two clonal routes: a polyp that reaches a certain size divides and buds off new polyps, and a broken piece of colony can settle and grow into a new one, which reef restoration exploits by farming fragments. Asexual reproduction also happens in species that usually mate: female sharks, birds and snakes have produced young with no male, called facultative parthenogenesis.

Most gamete-producing stony corals release massive numbers of eggs and sperm into the water, and because colonies cannot move toward each other, timing is everything: along many reefs, species spawn in one synchronized event cued by temperature, day length and the moon. Among fishes, both modes persist: cartilaginous fishes such as stingrays fertilize internally, while chondrostean fishes such as the sterlet spawn externally.

Live birth has evolved from egg-laying more than 150 times among vertebrates, mostly in fishes and squamate reptiles. The real variable is how the embryo is fed: some live-bearers rely entirely on egg yolk, others on a continuing maternal supply, and many fall in between. The common lizard has both egg-laying and live-bearing populations, so one species can sit on both sides.

Animal gametes come in two sizes, large eggs and small sperm, a condition called anisogamy. The classic argument says competition for mates is stronger in the sex with the smaller gametes, the basis for traditional sex roles. Treat that as a tendency, not a law: in jacanas and buttonquails the roles reverse, and individuals within one sex vary in their choices, which some researchers read as flexible decisions.

Releasing hormones from the hypothalamus drive the anterior pituitary, whose gonadotropins act on receptors in the ovaries and testes to control their development, growth and function. Signals from outside the body feed into this system too: steroids in a boar's saliva advance puberty in young gilts and promote estrus behavior in gilts and sows, which helps farms detect estrus and time insemination.

Cleavage divides the fertilized egg into ever-smaller cells, the blastomeres, and in the moon jellyfish a cavity, the blastocoel, opens inside the ball as division continues. Gastrulation then moves cells to new places: in invagination, a sheet of cells folds inward to form the future endoderm, and the opening it leaves, the blastopore, later closes. That folding varies at the cellular level even between a jellyfish and a sea anemone.

In their classic 1924 transplant experiment, Spemann and Mangold showed that dorsal lip tissue could induce a complete central nervous system in the host's responding tissue, so one tissue can instruct its neighbors what to become. Induction needs two things: a signal from the inducing tissue and responding cells competent to read it, a competence that one frog study suggests is set by even earlier signals.

In direct development the embryo grows straight into a form resembling a young adult; in indirect development it passes through a larva that drifts in open water before settling into the adult's habitat. Polyclad flatworms show the range, with some species adding an intermediate route in which the larval stage runs inside the egg capsule. Some frogs skip the aquatic tadpole stage entirely and hatch as small froglets.

At one end, broadcast-spawning corals release massive numbers of gametes and give no care; their larvae drift through a period of exceptionally high mortality. At the other, poison frogs lay small clutches, and some species guard eggs, carry tadpoles and feed them unfertilized eggs. Few offspring and intensive care appear to have evolved together, especially where nursery sites are small and resource-poor.

Ernst Mayr's 1951 split separates how a trait works from why it evolved, and Tinbergen in 1963 turned it into four questions: mechanism and development on the proximate side, evolutionary history and adaptive value on the evolutionary side. A full answer needs all four. Mayr later preferred "evolutionary" to "ultimate", because the word invited confusion.

Tinbergen and Lorenz broke instinctive behavior into three parts: a sign stimulus, the key feature needed to set a response off; an innate releasing mechanism that turns that input into action; and the fixed action pattern itself, a hard-wired sequence triggered by the cue. Innate does not mean unchangeable, though: predatory hunting in rodents is regarded as innate, yet experience makes it markedly more efficient.

In filial imprinting a newly hatched chick or duckling follows a conspicuous object, learns its features through exposure alone, and then restricts its social preferences to it. This happens in a sensitive period of typically several days, though its length varies widely. Imprinting is hard but not impossible to reverse: a second stimulus can partly replace the first, with some memory of the original left behind.

In classical conditioning an animal learns that one stimulus predicts another: honeybees given a scent paired with a feeding stimulus come to extend their tongue, the proboscis, at the scent alone. In operant conditioning the animal's own responses are rewarded or not, and researchers use shaping, rewarding closer and closer approximations, to build a response the animal would not otherwise produce.

Foraging trades food gained against the risk of being eaten, and optimal foraging theory predicts where, when and how long an animal should feed. Ecologists measure the trade with giving-up density, the food left in a patch when an animal stops feeding. Individuals of one species differ consistently in how much risk they accept, so an apparently suboptimal forager may be choosing well for its own state.

Any signal can be overheard. Most territorial mammals communicate by scent, and in a field test predators that hunt by smell found patches of house-mouse scent sooner and in greater numbers than unscented spots. The study suggests the cost can fall unevenly: a mouse that comes to read a scent mark must visit the risky patch, while the signaler need not.

Ornaments, colors and weapons are secondary sexual traits: they are not directly involved in reproduction itself. In polyandrous birds such as spotted sandpipers and jacanas, females usually face the stronger sexual selection. The ruff shows that one sex can hold several strategies, with male forms differing in size and plumage that match distinct mating tactics.

Hamilton's rule says a gene for helping can spread when b > c/r: the benefit to the recipient, weighted by relatedness r, outweighs the cost to the helper. In most social insect species, haplodiploidy makes sisters share more genes with each other than with their own offspring, which fits the evolution of sterile workers, though how much haplodiploidy explains is still disputed.

With no density feedback, the per-capita growth rate r stays fixed and numbers grow exponentially. Logistic models let r fall as density rises, reaching about zero at the carrying capacity, K, which can be exceeded only briefly. In one simulation study of long-lived vertebrates, logistic fits still tracked trends when K fluctuated but were thrown off by steady declines from outside mortality.

Species sit along a fast-slow continuum: slow species are large, long-lived and late to mature, and are expected to be more sensitive to fishing than fast ones. Offspring number does not always line up with that axis, though: in the Barents Sea, Arctic fishes were faster than boreal species yet had larger offspring and lower fecundity.

In classic predator-prey cycles the predator's numbers track the prey's with a lag of about a quarter of a cycle; when prey can evolve defenses quickly, the cycles can fall out of step or even reverse. On Isle Royale, wolves and moose swung in opposition until a virus cut the wolves from 50 to 12 in two years, after which climate, forage and other factors gained the upper hand over moose numbers.

A fundamental niche is the range of conditions a species can live in without competitors; the realized niche is that range trimmed by competition and other limits, and a test across 105 reptile and amphibian species mostly found the realized niche smaller. Coexisting species must differ somewhere: diet analysis suggested three bat species used different feeding grounds and hunting techniques.

Symbiosis is two species living closely together: mutualism benefits both, commensalism benefits one and leaves the other unaffected, and parasitism benefits one at the other's expense. Corals and their zooxanthellae are mutualists; the coral shelters the algae, and up to 90% of what the algae make by photosynthesis passes to the coral. Reef fish that pick parasites off sea turtles are another mutualism.

Only about 10% of the energy at one feeding level reaches the next; the rest is spent as heat and work or passes through undigested. That loss is why each level supports fewer organisms than the one below, building a pyramid: in one example, 10,000 pounds of phytoplankton support 1,000 of zooplankton and only 10 of large fish. Food chains branch into webs, since animals eat more than one kind of food.

A keystone species has an effect far beyond its numbers. After wasting disease killed the keystone sea star Pisaster, star counts on the Pacific coast neared their former levels, but biomass, its capacity to eat mussels, stayed low, so numbers are not function. Indicator species work the other way: mayfly, stonefly and caddisfly larvae are sensitive to freshwater degradation.

Habitat loss, overexploitation for trade and invasive species are three of the biggest drivers of biodiversity loss. Well-managed protected areas reduce losses, and eradicating invasive species has clearly helped island wildlife. The gap is coverage: a global review found 91% of threatened species have too little of their habitat inside protected areas, and many at-risk species lack the actions their threats call for.

Each species has its own production clock. Beef calves are usually born in spring and weaned at 3 to 7 months. Pigs are nursed for about 3 weeks and then fed for about 4 months to reach 270 to 285 pounds. Turkeys often reach 4 months before slaughter, and egg-laying hens start at about 4 to 4.5 months on a cycle that can last over a year. Sheep are raised for both meat and wool.

Breeds are selected for measurable output, and the targets differ: among dairy cattle, one breed may give more milk per cow while another gives milk with more fat and other solids, and the richer-milk breeds have gained popularity in US herds in recent years. Commercial poultry are bred in tiers: primary breeders chosen for genetic traits produce the breeders whose offspring become meat birds or layers.

Prolactin from the anterior pituitary drives milk production, while a hormone from the posterior pituitary triggers milk ejection, the letdown. The udder responds to how often it is emptied, locally and most strongly in early lactation: cows milked four times a day for just the first 3 weeks have given more milk across that whole lactation than cows milked twice a day throughout.

A ruminant hosts microbes that ferment the structural carbohydrates in forage into fatty acids the animal burns for energy, which is why a beef cow can raise a calf on pasture with little or no grain. Pigs and poultry are at an inherent disadvantage with fiber, so their rations are built on corn and soybean meal.

Feed is generally the largest cost in livestock production, so feed conversion, the feed eaten per unit of product, matters. Feedlot cattle gain 2.5 to 4 pounds a day on about 6 pounds of dry-matter feed per pound of gain, with rations of 70 to 90 percent grain and protein concentrates. Poultry convert feed more efficiently than larger livestock, and modern genetics have sharply improved that conversion.

Gestation length matters downstream: in a study of over 250,000 dairy lactations, first-calf cows with unusually short gestations gave less milk than those with longer ones. Hormone programs that synchronize ovulation let farms inseminate at a fixed time, which works around the weak estrus signs of many dairy cows that make heat detection unreliable.

The freedoms began as a narrow list of five minimal behaviors confined farm animals should be able to make and were later broadened into freedoms from hunger and thirst, discomfort, pain, injury and disease, and fear and distress, plus freedom to express normal behavior. Each is paired with a provision, the husbandry that supports it. The newer Five Domains makes the animal's mental state, including positive experiences, the measure of welfare.

The oldest space test is behavioral: an animal in confinement should be able to stand up, lie down, turn around, stretch its limbs and groom itself. Housing trades against it. Confinement lets pigs be raised year-round, sheltered from weather, disease and predators, yet state laws such as California's Proposition 12 now limit crate time for sows and require more floor space per animal.

Biosecurity splits in two. External measures keep pathogens from entering the farm, through physical barriers and rules on animals, people and vehicles; internal measures stop spread once a pathogen is inside. Bringing in new animals is the single highest-risk route, which is why a well-run quarantine, built to contain anything it holds, is the most effective external measure.

A zoonosis is an infection that passes naturally between vertebrate animals and people, in either direction: rabies moves from animals to humans, while tuberculosis has passed from humans to cats and monkeys. More than 60% of human pathogens are zoonotic in origin. One Health is the response, bringing veterinarians, physicians, ecologists and others together on animal, human and environmental health.