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Frogs are amphibians belonging to the order anura. The anuran group has nearly 2,700 species throughout the world and includes both frogs and toads. The word anura means "without a tail," and the term applies to most adult frogs. The anura are distinguished from tailed amphibians (urodeles) such as salamanders because the latter retain a tail as an adult.

One of the most studied and best understood frogs is the northern leopard frog, Rana pipiens. This species is well-known to most children, to people who love the outdoors, and to scientists. Leopard frogs live throughout much of the United States as well as Canada and northern Mexico. Inhabiting a diverse array of environmental conditions, the order anura exhibits an impressive display of anatomical and behavioral variations among its members. Despite such diversity, the leopard frog is often used as a model that represents all members of the group.

Leopard frogs mate in the early spring. The frogs deposit their eggs in jelly-like masses. These soft formless clumps may be seen in temporary ponds where frogs are common. Early embryonic development occurs within the jelly mass after which the eggs hatch releasing small swimming tadpoles. The tadpoles feed on algal periphyton, fine organic detritus , and yolk reserves through much of the spring and early summer. Next, metamorphosis begins a few weeks or up to two years after the eggs is hatched depending of the species. Metamorphosis is the process whereby amphibious tadpoles lose their gills and tails and develop arms and legs. The process of metamorphosis is complex and involves not only the loss of the tail and the development of limbs, but also a fundamental reorganization of the gut. For example, in the leopard frog, the relatively long intestine of the vegetarian tadpole is reorganized to form the short intestine of the carnivorous adult frog because nutrients are more difficult to extract from plant sources. Additionally, metamorphosis profoundly changes the method of respiration in the leopard frog. As the animal loses it gills, airbreathing lungs are formed and become functional. A significant portion of respiration and gas exchange will occur through the skin of the adult frog. When metamorphosis of a tadpole is complete, the result is a terrestrial, insecteating, air-breathing frog.

Frogs are important tolls for biological learning and research. Many students first encounter vertebrate anatomy with the dissection of an adult leopard frog. Consequently physiologists have used frogs for the study of muscle contraction and the circulation of blood which is easily seen in the webbing between the toes of a frog. Embryologists have used frogs for study because they lay an abundance of eggs. A mature R. pipiens female may release 3,000 or more eggs during a single spawning. Frog eggs are relatively large and abundant which simplifies manipulation and experimentation. Another anuran, the South African clawed frog, Xenopus laevis, is useful in research because it can be cultivated to metamorphosis easily and can be bred any time of year. For these reasons, frogs emerge as extremely useful laboratory test animals that provide valuable information about human biology.

Frogs and biomedical research

Many biological discoveries have been made or enhanced by using frogs. For example, the role of sperm in development was studied in the late 1700s in frogs. Amphibians in general, including frogs, have significantly more deoxyribonucleic acid (DNA) per cell than do other chordates. Thus, their chromosomes are large and easy to see with a microscope. In addition, induced ovulation in frogs by pituitary injection was developed during the early 1930s. Early endocrinologists studied the role of the hormone thyroxine (also found in human beings) in vertebrate development in 1912. The role of viruses in animal and human cancer is receiving renewed interestthe first herpes virus known to cause a cancer was the frog cancer herpes virus. Furthermore, mammalian and human cloning , a controversial topic, has its foundations in the cloning of frogs. The first vertebrate ever cloned was Rana pipiens, in an experiment published by Thomas King and Robert Briggs in 1952. As such, experimentation with frogs has contributed greatly to biomedical research.

Unfortunately, the future of amphibians, like the northern leopard frog, appears to be jeopardized. Most amphibians in the world are frogs and toads. Since the 1980s, scientists have noted a distinct decline in amphibian populations. Worldwide, over 200 species of amphibians have experienced recent population declines. At least 32 documented species extinctions have occurred. Of the 242 native North American amphibian species, the U.S. Nature Conservancy has identified three species that are presumed to be extinct, another three classified as possibly extinct, with an additional 38% categorized as vulnerable to extinction . According to the United States Fish and Wildlife Service , there are four frog species listed as endangered and three listed as threatened.

The actual cause of the reductions in frog and amphibian populations remains unclear, but many well-supported hypotheses exist. One speculation is the run-off of chemicals that poison ponds producing defective frogs that cannot survive well. A second possibility is an increase in amphibian disease caused by virulent pathogens. Viral and fungal infection of frogs has led to recent declines in many populations. A third explanation involves parasitic infections of frogs with flatworms, causing decreased survival. Another theory blames atmospheric ozone depletion for frog decline. Yet another implicates a toxic combination of intensified agriculture, drainage of habitat , and predator changes as the cause for the drastic declines in frog populations. While each argument has merits of its own, it is unlikely that any single cause can adequately explain all amphibian decline.

The poisoned pond hypothesis brought the issue of frog population decline to the forefront. Several years ago, students on a field trip at a pond near the Minnesota community of Henderson discovered a number of frogs with extremely abnormal limbs. Some frogs had three or more hind legs, some had fused appendages, and some had no legs at all. Concerned with what they had found, their teacher, Cindy Reinitz, and her students contacted the Minnesota Pollution Control Agency. Soon, state agency biologists and officials from the University of Minnesota confirmed the presence of many abnormal frogs at the site. The word spread, and by late 1996, many sites in Minnesota were known to have abnormal frogs.

Frog developmental abnormalities are not new to science. What made the Minnesota case of frog malformations different, however, was the extraordinary concentration of abnormal frogs in an unusually large number of locations. Concern became more urgent when abnormal animals also were reported in several other states, then Canada, and finally in Japan. Many of the abnormal frogs were found in agricultural areas where large amounts of fertilizers and pesticides were used.

One reason for concern is that frogs and humans metabolize toxic xenobiotic chemicals, such as fertilizers or pesticides, in similar ways. Also, human beings and frogs have very similar early developmental stages, which is why frogs are used as model animals in embryology. Because of this, worry exists regarding the potential for human developmental abnormalities from such chemicals. Some of the chemicals implicated in the frog malformations were retinoids. Retinoids are derivatives of vitamin A that are potent and crucial hormonal regulators of vertebrate embryological development. Numerous laboratory studies using retinoic acid (a form of retinoid) have reproduced the abnormalities seen in the Minnesota frogs, and the role of retinoids in the regulation of genes involved in limb development is well-characterized. Frog anatomical defects are regarded as a warning sign for potential problems in humans exposed to the same chemicals, since retinoic acid regulates human limb development as well.

Disease is a growing problem for frogs. Recently, two important frog pathogens have been identified and are suspected to play a role in global frog decline. Iridiovirus, a virus that commonly infects fish and insects, has now been found to infect frogs. An aquatic fungus, chytrid, is also implicated in frog decline. Infection with the fungus is called chytridiomycosis. The chytrid fungus, Batrachochytrium dendrobatidis, which normally resides in decaying plant material, causes the degeneration of tadpole mouthparts. This results in the death of post-metamorphic frogs, and is reportedly involved in amphibian decline in Australia , Europe, and recently in the Sierra Nevada region of California. In Australia the depletion of the frog population may have been caused by a disease introduced through infected fish. The Australian cane toad, Bufo marinus, may be responsible for spreading a virus to other frogs outside its native range.

Another potential cause for the striking decline of frog populations involves parasitism, the act of being a parasite. Parasitism is a type of symbiosis in which a parasitic organism gains benefit from living within or upon a host organism. The host, in turn, gains no benefit, and in fact may be harmed by the symbiosis. Scientists have discovered that a parasitic trematode or flatworm is threatening many frog populations. Trematode larvae erupt from snails inhabiting ponds, which then infect frog tadpoles. Once inside the tadpoles, the flatworm larvae multiply and physically scramble developing limb bud cells in the hind quarters of the tadpole. If the limb bud cells are not in their proper places, malformations are the consequence. The result of such parasitism by flatworms is adult frogs with multiple legs or fused legs. It is believed that the flatworms derive benefit from the relationship because frogs with defective legs are easier for birds to prey upon. Birds that eat infected frogs in-turn become infected. The cycle is complete when bird droppings littered with flatworm larvae are dropped into pond water.

A fourth possible explanation for the decline of frogs involves ozone depletion. Some scientists believe that atmospheric ozone loss has led to an increase in ultraviolet light penetration to the surface of the earth. Frogs are exquisitely sensitive to ultraviolet light. It is believed that due to increased UV-B penetration, mutations in frogs has been increased, resulting in limb malformations. Evidence for this hypothesis exists in laboratory experiments that have been able to reliably replicate the limb malformations observed in the Minnesota ponds using UV-B radiation on experimental frogs.

Many scientists believe, however, that multiple factors are to blame for frog decline. They believe that synergy, or the combination of many factors is the most plausible explanation for the decrease in the number and diversity of frogs. Climate change, urbanization, prolonged drought , global warming, secondary succession , drainage of habitat for housing developments, habitat fragmentation , introduced predators, loss of territory, and the aforementioned infectious and poisoning reasons may all simultaneously contribute to the decline of frog populations worldwide. Humans perpetuate the decline by hunting frogs for food. Because the decrease in numbers and diversity of frogs is so striking, conservationists are concerned that it is an early indicator of the consequences of human progress and overpopulation.

[Robert G McKinnell ]



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