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Adaptive Radiation

Fields of Study : the visible expression of the genetic makeup of an individual ; Genetics; Environmental Sciences; Ecology; : a taxonomic subdivision of a , con- Botany taining populations of similar organisms that interbreed and that usually do not interbreed with Abstract other species Many different forms of evolutionary may occur among that started with a common ancestor. In this way, evolutionary divergences can take place, and the In 1898, Henry F. Osborn developed the concept of occupation of a variety of ecological niches is made possible. adaptive radiation. According to Osborn, many dif- The ability to adapt is not shared by all species. Therefore, ferent forms of evolutionary adaptations may occur in many instances, either evolutionary divergence has been among animals that started with a common ancestor. modest or the species involved has become extinct. In this way, evolutionary divergences can take place, and the occupation of a variety of ecological niches is Key Concepts made possible according to the adaptive nature of the invading species. As may be seen with certain forms allele: an alternative form of a gene that is located at of , however, the ability to adapt is not the same position on a chromosome shared by all species. Therefore, in many instances, : the process of change through time by either evolutionary divergence has been modest or which the characteristics of a species of plant or the species involved has become extinct. animal are altered by adaptation to the environ- ment and produce a new species distinct from the THE PRINCIPLES OF parent species In order to understand how adaptive radiation oper- fossil: any recognizable remains of an organism pre- ates, it is necessary to become familiar with the prin- served in the earth’s crust; it may be a footprint, ciples of natural selection. The concept of natural bones, or even feces selection, frequently expressed as “survival of the gene: the biological unit of heredity, which is com- fittest,” is at the core of ’s theory of posed of DNA and is located on a chromosome evolution. Darwin did not mean to suggest that there genotype: the total genetic composition of an was a physical struggle among organisms in order to organism survive. Instead, he meant that organisms compete habitat: the place where an organism normally for food, space, shelter, water, and other things neces- or where individuals of a population live sary for existence. Rather, he meant only that those natural selection: the process of evolution whereby organisms best adapted for a particular habitat will organisms that are the best adapted are the most have the greatest ability to survive in a particular successful in reproducing and therefore in passing environment. According to the concept of natural along their genotypes to successive generations selection, all organisms of a given species will show niche: the role of an organism in an ecological variation in color, size, physiology, and many other ­community—­its unique way of life and its relation- characteristics; in nature, all organisms produce ship to other biotic and abiotic factors more offspring than can survive, so the offspring must

1 Adaptive Radiation Principles of Zoology therefore compete for the limited environmental resources. Organisms that are the best adapted (most fit) to compete are most likely to live to reproduce and pass their successful traits on to their offspring. The others, which are less fit, will be most likely to die without reproducing. When different parts of an animal population are faced with slightly different environments, they will diverge from one another and in time will become different enough to form new species, typically unable to interbreed. Natural selection also has the effect of producing different patterns of evolution. It may bring about widely dif- ferent (variable characteristics) in closely related animals, for example, or similar phenotypes in distantly related organisms. The organisms them- selves may also become forces of selection through Darwin's or Galapagos finches. Darwin, 1845. Journal of researches into the natural history and geology of the countries visited during the voyage of H.M.S. their interrelationships with other species. Beagle round the world, under the Command of Capt. Fitz Roy, R.N. 2d edition. The process of adaptive radiation illustrates how natural selection operates. The most frequently is the woodpecker . A true woodpecker has cited example is the evolution of Darwin’s finches an extremely long tongue that it uses to probe for on the Galápagos Islands, off the west coast of . Since the does not have a South America. The islands were formed from vol- long tongue, it has learned to use a cactus spine for canic lava about one million years ago. At first they probing, and it can therefore occupy a niche were devoid of life, but bit by bit, several species of normally filled by true woodpeckers. plants and animals arrived at them from the South A more recent example of adaptive radiation in American mainland. Since the nearest island is its early stages has taken place in an original popula- about 950 kilometers from the coast of Ecuador, it tion of brown bears. The brown bear can be found is anybody’s guess how the different species arrived. throughout the Northern Hemisphere, ranging from It has been suggested that the may have been the deciduous forests up into the tundra. During one carried to the islands by strong winds, since finches of the glacier periods, a small population of the brown are not known for their lengthy flights. Other organ- bear was separated from the main group; according isms may have been carried by floating debris. In any to fossil evidence, this small population, under selec- event, the islands became populated. The mainland tion pressure from the Arctic environment, evolved ancestor of the finches is not known, but it was no into the polar bear. Although brown bears are classi- doubt a nonspecialized finch (a finch is about the fied as carnivores, their diets are mostly vegetarian, size of a sparrow). Since there were no other birds with occasional fish and small animals eaten as with which to compete on the islands, the original supplements. On the other hand, the polar bear is population of finches began to adapt to the various unoccupied niches. The early offshoots of the orig- mostly carnivorous. Besides its white coat, the polar inal population were modified again and again as bear is different from the brown bear in many ways, adaptations continued. This process resulted in the including its streamlined head and shoulders and evolution of fourteen species of finches. The main the stiff bristles that cover the soles of its feet, which feature that makes each species different is the size provide traction and insulation, enabling it to walk of their , which have adapted for the various on ice. types of available foods. Today the finches live on fif- teen different islands. Some of the species are found EVOLUTION in the same area (sympatric), while others occur in All the genes of any population of living organisms at different areas (allopatric). The most noteworthy any given time make up its gene pool, and the ratio of example of an adaptation to a particular niche alternative characteristics (alleles) in the gene pool

2 Principles of Zoology Adaptive Radiation can change because of selection pressures during the designed to test the concept of adaptive radiation, passage of time. As the ratio of alleles changes, evo- the scientist could not be around thousands or mil- lution occurs. Evolution may be a random change, lions of years from now to gather the data. Therefore, or it may occur because of the directive influences scientific observation of animal remains is the of natural selection. In the former case, occasional best method. and unpredictable permanent random changes Based upon scientific observations, it has been called take place in the DNA molecules well established that the phenomenon known as that compose the genes. These mutations also may be adaptive radiation is a general feature of the evolu- selected for by the environment or selected against by tion of most organisms. Studies of the morphological the environment. It is simply an accident if the newly features of fossilized remains help determine rela- mutated genes help the organism to become better tionships among prehistoric animals and enable the adapted to its particular habitat niche. Genes may not scientist to trace adaptive radiations from a more change or become mutated through several genera- primitive ancestral stock. In order to establish time tions (the Hardy-­Weinberg law), but may change in intervals, techniques such as radioactive carbon terms of survival value if the environment changes or dating, potassium-argon­ dating, and fluorine dating the species population is subjected to new mutations have been used. or natural selection. Zoologists have also made use of the uneven The relative numbers of one form of allele distribution of blood groups (A, B, AB, and O) decrease in a divergent population, while the relative among different groups of animals. As more blood numbers of a different gene increase. This progres- subgroups were discovered, they became useful in sive change is all-important­ in the evolutionary pro- helping chart migrations and indicating relationships cess that takes place between the origin of a new gene between species. by random and the replacement of the original form of the gene by descendants having the THE EVIDENCE AND ITS IMPLICATIONS newer, better-­adapted form of the gene. The result Adaptive radiation as an important aspect of evolu- in the long term is that enough of the DNA changes, tion means that modern organisms have attained either slowly or rapidly, through divergent popula- their diversity in form and behavior through heredi- tions or organisms, that the new generations have tary modifications after having been separated from become so different from the original population ancestral populations. Adaptive radiation, therefore, that they are considered new species. Many times in is attributable to the genetic changes in isolated earth’s history, a single parental population has given groups of organisms or, more specifically, to a change rise not to one or two new species but to an entire in the relative frequency of their genes from one family of species. The rapid multiplication of related generation to the next that eventually results in the species, each with its unique specializations that fit formation of new species. it for a particular , is called adaptive Evidence in several areas supports the concept of radiation, or . adaptive radiation as an important aspect of evolu- tion: the fossil record (the most direct evidence), bio- STUDYING ADAPTIVE RADIATION geographic distribution of organisms, comparative Not all scientific information is gained by experimen- anatomy and embryology, homologous and analo- tation: A considerable portion of science is descrip- gous structures, vestigial organs, and comparative tive and is based upon observation. In determining biochemistry. Regarding comparative biochemistry, that adaptive radiation has occurred and is indeed scientists agree that blood group similarities confirm taking place among living species, much supporting evolutionary relationships among the nonhuman evidence has come from the study of fossils and from primates. It has been shown that the blood of higher observations of the structural, physiological, and primates, such as orangutans and chimpanzees, behavioral adaptations of current animals. Clearly, is closer to human blood than to that of the more wide-­scale experimentation would be out of the primitive monkeys. question. No matter how well an experiment may be —­Jon P. Shoemaker

3 Aging Principles of Zoology

Further Reading Hou, Lianhai, Larry D. Martin, Zhonghe Zhou, Curtis, Helena, and N. Sue Barnes. Invitation to and Alan Feduccia. “Early Adaptive Radiation of Biology. 5th ed. Worth, 1994. Birds: Evidence from Fossils from Northeastern Darwin, Charles R. The Illustrated Origin of Species. China.” Science 274, no. 5290 (November 1996): Abridged and introduced by Richard E. Leakey. 1164–­1168. Hill & Wang, 1979. Kimball, John W. Biology. 6th ed. Wm. C. Brown, 1994. Dobzhansky, Theodosius. Genetics of the Evolutionary Rainey, Paul B., and Michael Travisano. “Adaptive Process. Columbia University Press, 1970. Radiation in a Heterogenous Environment.” Farrell, Brian D., and Charles Mitter. “Adaptive Radi- Nature 394 (July 2, 1998): 69–­72. ation in Insects and Plants: Time and Opportu- Wahrheit, Kenneth I., Jonathan D. Forman, Jonathan nity.” American Zoologist 34, no. 1 (February 1994): B. Losos, and Donald B. Miles. “Morphological 57–­70. Diversification and Adaptive Radiation: A Com- Givnish, Thomas J., and Kenneth J. Sytsma, eds. parison of Two Diverse Lizard .” Evolution and Adaptive Radiation. Cam- 53 (August 1999): 1226–­1234. bridge University Press, 1997. Wallace, Robert A., Robert J. Ferl, and Gerald Hickman, Cleveland P., Larry S. Roberts, and Frances P. Sanders. Biology: The Science of Life. 4th ed. Harper- M. Hickman. Integrated Principles of Zoology. 11th Collins, 1996. ed. McGraw-­Hill, 2001.

Aging

Fields of Study CHANGE AND AGING Progressive and irreversible change has been called Medicine; Biology; Geriatrics; Thanatology the single common property of all aging systems. When change is reversible or self-maintaining,­ such Abstract as one would see in a forest, for example, the effects When change is reversible or self-maintaining,­ the effects of aging are often not observable. Growth of the of aging are often not observable. However, in animals forest is evident, but with the right conditions, trees some change is not reversible. The changes in the cells of within the forest may grow for hundreds of years in the body accumulate over time and result in a steady down- the absence of disease. Certain conditions of the ward trend. The end point of this trend is the death of the forest system help to regenerate, renew, and reverse organism. Aging occurs within body systems as a result changes that happen within that system. of unseen changes at the molecular and cellular levels. However, in animals some change is not revers- Although the mechanisms through which aging occurs may ible. The changes in the cells of the body accumulate be understood, the causes are less clear. over time and result in a steady downward trend. The end point of this trend is the death of the organism. Key Concepts Aging is a normal part of the life cycle. This is known to be true because aging changes within populations aging: a process common to all living organisms, are rather predictable. The changes associated with eventually resulting in death or conclusion of the aging that are seen in all animal species may occur life cycle for similar reasons. These may include chemical cognition: ability to perceive or understand aging, extracellular aging, intracellular aging, and death: the cessation of all body and brain functions aging of cells. function: ability, capacity, performance Aging occurs within body systems as a result of life span: length of life from birth to death unseen changes at the molecular and cellular levels. longevity: length of life Although the mechanisms through which aging

4 Principles of Zoology Aging

because of the notion that aging patterns may closely reflect those of humans. Aging monkeys show changes in their circulatory systems similar to those found in humans: There is notable atherosclerosis and arteriosclerosis, or hard- ening of the arteries. The heart pumps less effectively, and vessels show buildup of plaque. These changes often result in cardiac problems, including heart attacks. The respiratory system also shows a decrease in elasticity. Senile emphysema has been noted. The kidneys show signs of atrophy and sclerosis in aged monkeys. The kidneys of humans may lose up to half Before hatching Newly hatched larvæ hanging on to water-weed With external gills External gills are covered over and are absorbed Limbless about a month old of the functioning nephrons with advanced age and with internal gills with hind-legs, about two months old With the fore-limbs thus become less effective in filtering waste products emergin. from the body. Physical function or capacity tends to decline with occurs may be understood, the causes are less clear. age. This is largely due to the atrophy of muscles, The fact remains that due to changes in chemical bal- which is more common as the body gets older. The ances such as those of hormones, and to the dying of joints tend to become stiffer and less mobile. Range cells within the body, each of the bodily systems shows of motion may be restricted. Changes in bone den- deterioration over time. sity may lead to loss of teeth, osteoporosis, and sub- Changes that occur in domestic animals over the sequent fractures. Tooth loss and osteoporosis have life span can be similar to those that occur in humans. been documented in monkeys over the age of twenty Dogs experience the graying of their hair, a decrease years. Pictures of such older monkeys reveal a stooped in vision, and a slowing of movement with age. They posture, with shoulders hunched forward, similar to also experience cataract formation, arthritis, skin the kyphosis observed in many older human women. problems, cancer, and diabetes. Certain breeds of Physical function among animals has been less animals may demonstrate a tendency toward specific studied than that in humans, but certain physiolog- illnesses or diseases. For example, German shep- ical characteristics are similar. For example, survival herds often develop hip problems, and collies com- times after severe physical injury with blood loss and monly develop progressive arthritis that may seriously trauma decreases in both humans and animals as inhibit mobility by around ten years of age. age increases. Male monkeys do not lose reproduc- tive capabilities until toward the end of the life span, COMMON EFFECTS OF AGING while females have a more restricted period of time to bear offspring. Fertility among all females tends to There are many variations in the effects of aging decline with age after its peak. among the species of animals. The life span of animals The immune system functions less effectively as may range from a few days (among insects) to thirty age increases. This leaves the body more susceptible years or more, with great variation depending upon to a range of illnesses and diseases. Neoplasms, or many factors. Animals that live in captivity, as pets or tumors, are most common among as they in zoos where they are sheltered from the effects of age. An impaired immune system allows various types , disease, and adverse climate, also tend to of tumors or cancers to spread more rapidly in the live significantly longer than animals in the wild. older body. Response to stress and ability to adapt to Very little research has been done on the aging stressors also decline with age. For example, older of most animal species. The reasons for this include mice become less able to adapt to cold temperatures. the difficulty of observing animals over a long period Social roles and behaviors among animals may of time in their natural habitat. Aging in monkeys also change with age. Longitudinal studies on ani- has been studied more than that in other animals mals in the wild are scarce, so only generalities may

5 Principles of Zoology be speculated upon. Even studies done within con- Predator-­prey relationships among animals are trolled laboratory settings yield only broad sugges- particularly significant as causes of death. Thus, tions, since numbers of animals available for study the effect of the environment on animal aging and are limited. Males generally tend to dominate the death requires more investigation. Do animals age females in both physical strength and social ranks. more quickly if they are objects of prey? Do animals Some nonhuman primates show different character- relate to stress in ways similar to those of people, thus istics with advanced age. That is, some monkeys and showing signs of wear and tear that are seen with baboons allow older males to remain part of the social premature aging under stress? Are there risk factors group, while other species support the male leader in among animals that affect their life span? These are the group only as long as the female harem supports some of the questions that remain to be answered on him, whether younger or older. Individual monkeys the topic of aging among animals. in stable groups have been observed to resort less —­Kristen L. Mauk frequently to aggressive behavior to maintain their status within the group. Further Reading CAUSES OF DEATH Bowden, Douglas M. Aging in Nonhuman Primates. Van Among nonhuman primates, the leading cause of Nostrand Reinhold, 1979. spontaneous death is digestive problems. Older ani- Kohn, Robert R. Principles of Mammalian Aging. mals that die do not always show advanced signs of Prentice-­Hall, 1971. tissue aging. Since much less research has been done Schmidt-Nielsen,­ Knut. Animal Physiology: Adaptation on aging among animals than among humans, data and Environment. Cambridge University Press, 1997. about causes of death are rare. However, it appears Slater, P. J. B. Essentials of Animal Behavior. Cambridge that there is an increased probability of dying from University Press, 1999. trivial illnesses, perhaps due to decreased resistance Slobodkin, Lawrence B. Growth and Regulation of Animal factors, as animals age. Populations. Holt, Rinehart and Winston, 1961.

Amphibians

Fields of Study convergence: a phenomenon in which two forms that are not closely related evolve structures that Biology; ; Ecology; Environmental Studies appear similar disjunct distribution: a geographic distribution pat- Abstract tern in which two closely related groups are sepa- The term is derived from the Greek word rated by large areas that are devoid of either group amphibios, which means “to live two lives.” The majority metamorphosis: the complex developmental process of amphibians spend the first part of their lives as aquatic, of morphological change in which larval amphib- gill-­breathing larvae and then transform into terrestrial ians are transformed into adults adults. The larval stage can be as short as a few weeks or neoteny: the retention of larval features by adults, a as long as several years. Completion of the larval stage is process that has played a major role in the evolu- triggered by hormonal events that initiate some dramatic tion of amphibians developmental processes, collectively termed metamorphosis. phylogeny: the determination of the evolutionary history of a particular group of organisms Key Concepts adaptive radiation: rapid that occurs as the AMPHIBIAN LIFE result of a particular group being able to exploit a The term amphibian is derived from the Greek word new resource amphibios, which means “to live two lives.” The majority

6 Principles of Zoology Amphibians of amphibians spend the first part of their lives as One scenario envisions the early amphibian ­ancestors aquatic, gill-breathing­ larvae and then transform into in an environment that was gradually becoming more terrestrial adults. The larval stage can be as short as a and more arid. To survive, it would have been advan- few weeks or as long as several years. Completion of tageous to be able to crawl on land for short distances the larval stage is triggered by hormonal events that to escape drying pools in favor of more permanent initiate some dramatic developmental processes, col- bodies of water. Those that could migrate would have lectively termed metamorphosis. survived in higher numbers than those that lacked As adults, most amphibians seek out aquatic envi- this adaptation. A second scenario suggests that heavy ronments in which to deposit their . These can predation pressures from the jawed, carnivorous fish range from fast-flowing­ mountain streams to ephem- that are known to have been abundant in the shallow eral roadside ditches. Most male have species-­ freshwater lakes of the time may have selected for specific mating calls that serve both to attract females individuals that could leave the water, even if only and to prevent interbreeding. Frogs reproduce by briefly. A third scenario depicts for food external fertilization: the male typically grasps the as strong in the aquatic environments and much female and encourages her to deposit her eggs, weaker on land, where several groups of invertebrates which he promptly fertilizes. Normally, both parents were known to be abundant. abandon the eggs, but some variations of this pattern exist. In contrast, most practice internal TAXONOMY OF FOSSIL AMPHIBIANS fertilization, accomplished after the male has per- The taxonomy of fossil amphibians is confusing formed a type of species-specific­ courtship dance that because of a series of problems. First, on the whole, culminates with the deposition of a packet of sperm the range of morphological variations within the cells, called a spermatophore. The female squats on group has been conservative. Striking differences of the spermatophore, transferring the spermatozoa to taxonomic value (which would have to fossilize) are a specialized holding structure called a spermatheca. not numerous. Second, many of the skeletal elements The spermatozoa can be used to fertilize her eggs up that are important in determining the relationships to several months after mating. of living amphibians are composed of cartilage rather than bone, and this material does not fossilize well. EVOLUTION OF AMPHIBIANS Because of this limitation, bony elements such as the Amphibians were the first vertebrates to possess vertebrae and skull have played a major role in deter- adaptations that allowed them to spend considerable mining amphibian phylogeny. The molecular tech- periods of time out of the water. The earliest fossil niques that have greatly assisted modern taxonomists, amphibians, members of the order Ichthyostegalia, such as electrophoresis, immunology, karyotyping, appear in the geologic record during the and deoxyribonucleic acid (DNA) sequencing, period, about 320 million years ago. Among experts, cannot be used on fossilized materials. there is a general consensus that the ancestor The class Amphibia is further divided into orders. of the amphibians is to be found in the primi- The number of orders recognized by various authori- tive, lobe-finned­ fish (class Osteichthyes, subclass ties ranges from eight to thirteen. All but three of ). This conclusion is based on a detailed these are extinct. One extinct order is Ichthyostegalia, analysis of the comparative anatomy of hard body which includes the earliest recorded fossil amphib- parts that fossilized, such as the vertebrae, shoulder ians. Most were small, with elongate bodies and girdles, teeth, and skulls. Characteristics that the first weakly developed limbs. Many were almost assuredly amphibians shared with their fish ancestors include aquatic, but at least some were capable of spending internal nasal openings (nares); a strange, hinged extended periods of time out of the water. By the time skull; and a distinctive tooth structure, in which the of the Devonian period, when they first appear in the enamel is folded into intricate patterns. fossil record, they were already diverse, with several The environmental conditions that led to the aban- different genera and species present. donment of aquatic habitats in favor of a more ter- Another extinct group is the order , restrial existence remain a major topic of discussion. which was abundant during the late period

7 Amphibians Principles of Zoology and persisted until the end of the period. Most independently from two or more separate stocks of in this order were of moderate size (0.5 to 1.0 meter fish. Based on the presence of unique features, such in body length), with low, stout profiles and flattened as pedicellate teeth, a distinctive part of the inner ear, skulls. Some were highly aquatic and had short, weak and specialized eye receptors, most experts have con- appendages. One group, the Trematosauria, cluded that all living amphibians are monophyletic was marine, and they are apparently the only amphib- and can be grouped in the superorder . ians to have been successful at invading the oceans. As recognized, living amphibians are placed in the The order appears in the fossil orders Caudata, Anura, and Gymnophiona. record during the period and was extinct by the end of the Permian. Common names of SALAMANDERS members of this group include the seymouriamorphs Salamanders are grouped in the order Caudata. They and the embolomeres. This order contains a mixture are distributed over temperate parts of Europe, Asia, of terrestrial and aquatic amphibians. From an evo- and North and South America. There are approxi- lutionary standpoint, Anthracosauria is important mately 550 recognized species, grouped into eight because it gave rise to the ancestors of the . different families. Eastern North America has the Members of the order Aistopoda were eel-­like, greatest overall diversity, with seven of the eight aquatic amphibians with elongate, limbless bodies. described families represented there. One family, They are characterized by a large number of Plethodontidae, invaded South America and under- vertebrae—­more than one hundred—­that are clearly went a period of such tremendous speciation that divisible into cervical, trunk, and caudal regions. approximately 60 percent of the living species of sala- The order Nectridea consists of fully aquatic, manders are members of this family. -­like amphibians that persisted during Almost everyone is quick to recognize a based the Carboniferous period. Appendages were weak on its appearance, but the same cannot be said for sala- or absent, and most fossils indicate body forms that manders. The families Proteidae, Cryptobranchidae, were flattened dorsiventrally. They probably per- Sirenidae, and Amphiumidae are entirely aquatic. sisted by slowly crawling about the bottoms of ponds The family Cryptobranchidae is disjunctly distributed and lakes, where they preyed on unwitting animals and occurs today only in eastern Asia and eastern that crossed their paths. North America. It contains the genus Andrias, which Amphibians of the order Microsauria were a includes the largest living salamanders; one spe- diverse group of elongate, weak-limbed­ amphibians. cies, Andrias davidianus, has been known to reach Fossils of this group are fairly abundant in habitats lengths as great as 1.8 meters. Members of the family that were swamplike during the Carboniferous Proteidae are today isolated in Europe and eastern period. The remarkable physical similarities between North America. They are commonly called “water some microsaurs and some of the earliest reptiles dogs” and are frequently dissected in comparative are considered by most authorities to be the result of anatomy classes. Members of the families Sirenidae convergence of body form rather than an indication and Amphiumidae are restricted to the southeastern of a true evolutionary relationship. United States, where they are called sirens and amphi- The order Proanura consists of a single froglike umas, respectively. Sirens have external gills and two fossil that dates from the Triassic period on the island front legs, while amphiumas have minute front and of Madagascar. The skull is distinctly froglike, but back legs and lack external gills. a tail is present, and the hind limbs have not been The family Ambystomatidae is entirely North modified for jumping. American in its distribution. Most species are highly The taxonomy of the living members of the class secretive and are only encountered under objects Amphibia is still under considerable debate. A central or intercepted as they migrate to breeding ponds question revolves around whether all currently living during spring rains. amphibians are of monophyletic origin, meaning they The family Hynobildae is exclusively Asian in its derive from a single common ancestor, or whether present distribution. Reproduction in this family is they are of polyphyletic origin, meaning they arose considered to be primitive, in that the female lays

8 Principles of Zoology Amphibians eggs that are enclosed in a loose sac and are subse- streams, and arboreal vegetation. Several families quently fertilized externally by the male. have undergone tremendous adaptive radiation in The family Salamandridae is widely distributed in the tropics, so that today almost 80 percent of living Europe, North Africa, Asia, and North America, with amphibians are anurans. the greatest diversity in the Eastern Hemisphere. Families that are widely distributed include Many species have developed highly toxic skin secre- Bufonidae (toads), (tree frogs), tions to protect themselves from predators; human (mostly small frogs), and Ranidae (true frogs). These fatalities have been recorded from eating only one families are almost worldwide in their geographic salamander. Many species advertise their toxicity distributions, although lacks Bufonidae, by being very brightly and distinctly colored, a phe- Ranidae, and Microhylidae and Africa lacks Hylidae. nomenon known as aposematic coloration, while Toads often have dry, warty skins containing members of other, less toxic families also display numerous glands that produce noxious, protective these color patterns for protection from predators, a secretions. Tree frogs have expanded disks on the process called mimicry. tips of their toes that have allowed them to occupy Members of the family Plethodontidae all share arboreal habitats unavailable to many other families. the unique feature of being lungless; respiration is The families , , accomplished by diffusion across their moist skins. Rhinodermatidae, Pseudidae, Centrolenidae, and Many species have abandoned laying eggs in water Dendrobatidae reach their greatest abundance in in favor of damp, terrestrial nests. Females guard Central and South America. The leptodactylids are a the eggs until they hatch as miniatures of the adult, diverse assemblage of nearly seven hundred species. having completed their abbreviated metamorphosis Many of these lay eggs in specially constructed foam while still in the . nests; other species have taken this a step further and deposit their eggs in damp, terrestrial locations, thereby avoiding aquatic predators almost com- pletely. The dendrobatids are often small, brightly The order Gymnophiona consists of a highly special- colored frogs that have been given the common name ized group of wormlike, limbless amphibians called of poison arrow frogs because of their extremely toxic caecilians. They inhabit tropical regions of North and skin secretions, which have been used by some indig- South America, Asia, and Africa. Most are terrestrial enous tribes to poison the tips of hunting arrows. burrowers and are rarely observed. Some primi- Members of the family Pseudidae are unique in pro- tive forms have dermal scales embedded in their ducing very large that metamorphose into skin. All caecilians possess a unique sensory organ rather small frogs. The rhinodermatids consist of called a tentacle. Fertilization is internal, and male only two species, but one is unique in that the larvae caecilians possess a copulatory organ that is derived do not feed and are carried in the mouth of the adult from the cloaca. In the majority of species, until they complete metamorphosis. the females retain their eggs in the oviduct and give The Discoglossidae are found mostly in Europe, birth to fully developed young. Fossil caecilians are and Pelodytidae are native to southwestern Europe extremely rare. and western Asia. The midwife toad (Alytes obstetri- cians), a discoglossid, has an unusual reproductive FROGS mode: after fertilizing the eggs, the male cements The order Anura is composed of tailless amphibians them on his back and carries them to and from the called frogs. Their hind legs are typically modified for water until they are ready to hatch. jumping, their presacral vertebrae are usually eight The are a moderate-­sized family of in number, and their postsacral vertebrae are fused to about 180 species, distributed over southern Africa form a coccyx. Frogs occur on all continents except and southeast Asia. Most members have expanded Antarctica but reach their greatest diversity in the terminal digits, and some even have extensive web- tropics of South America, Africa, and Asia. They have bing between their toes, which allows them to glide successfully invaded deserts, rivers, cold mountain between arboreal perches. Diverse reproductive

9 Amphibians Principles of Zoology tactics occur in this family. Several species use The taxonomic relationships of frogs are also ­water-­filled tree holes in which to deposit their eggs. based largely on differences in bony anatomy. Skull The family is a diverse group of morphology, the shape of the vertebral centrum about ninety-­nine species that occur in Australia and and its manner of development, and the arrange- New Zealand. One species, Rheobatrachus silus, has a ment of the bones that make up the pectoral girdle unique reproductive mode that includes brooding are important diagnostic characters. In living forms, eggs in the stomach of the female. molecular techniques are also shedding new light on The family Leiopelmatidae is a small group of relationships. The number of chromosomes is more frogs that are disjunctly distributed in western North variable and has more value as a diagnostic tool in America and New Zealand. The (Ascaphus frogs than in salamanders. The morphology of the truei) is the only frog to possess an intromittent organ larvae is another important taxonomic tool. that is used to transfer sperm to the female for . This organ apparently evolved in response THE IMPORTANCE OF AMPHIBIANS to the swift, cold streams in which the frog lives. There are about seven thousand recognized species of living amphibians. This number represents only a small BASES OF CHARACTERIZATION fraction of the number of species that have been present As a group, amphibians are easier to characterize by on the earth over the past 350 million years. In many hab- the morphological features that they lack than by itats, amphibians still represent a major portion of the the unique characteristics that they possess. Missing biomass, and because ecologists often relate a group’s are the scales that cover fish and reptiles (although “worth” to its biomass, amphibians can be considered these are not closely related structures), as well as major members of many terrestrial communities, often the hair and feathers associated with mammals and serving as keystone species for their ecological niches. birds. Amphibians’ skin is relatively thin and con- Amphibians often exhibit traits such as low mobility, tains numerous glands. Large amounts of water can fidelity to breeding sites, and species-­specific behaviors be lost or gained via the epidermis. In many forms, that are sought by ecologists and animal behaviorists for the skin serves as a major organ for respiration. their studies. As a whole, salamanders and frogs repre- Amphibians are ectothermic, which means that they sent some of the most thoroughly studied vertebrates. do not have internal physiological mechanisms for Areas to which amphibians have contributed a maintaining a constant body temperature. The circu- significant portion of current knowledge include the latory system is closed, and the heart is composed of evolution of mating systems, , repro- three chambers—­two atria and one ventricle. ductive isolation mechanisms, niche partitioning, The taxonomic relationships of salamanders are and community structure. Embryologists have long based primarily on the arrangement of bones of the used amphibians to gain a basic understanding of skull, which of these bones possess teeth, and the complex development processes. shape of the centrum of the vertebrae. Living forms One area of extreme interest is the apparent are further compared by the manner of reproduc- decline of many populations of frogs and salaman- tion. In general, salamanders have been relatively ders over large geographic areas, notably in western conservative, and characteristics such as the number North America. Loss of breeding sites through habitat of chromosomes have not proved especially useful in modification, acid rain, and competition from exotic determining phylogenetic relationships. However, species have all contributed to their demise. Another modern molecular techniques, such as electro- likely cause is chytridiomycosis, an infectious and phoresis, immunology, and the use of restriction deadly skin disease caused by the chytrid fungus enzymes, are adding to the understanding of selected Batrachochytrium dendrobatidis. The disease was first dis- groups. With these techniques, it has been possible to covered in frogs in Queensland, Australia, in 1993 and show that several forms that were indistinguishable was first described by Lee Berger et al. in 1998. The based on morphological data are in fact genetically specific fungus was identified by Joyce E. Longcore, isolated from one another and are actually distinct Allan P. Pessier, and Donald K. Nichols in 1999. sibling species. —­Robert E. Herrington

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