Amphibians are tetrapod or, in the case of some limbless salamanders and caecilians, derived from tetrapod ancestors. They have a moist, glandular skin without epidermal scales, feathers or hair. Frogs lack a true tail as adults and have disproportionately long hind legs and exaggerated mouths. Most salamanders have a more typical vertebrate body, with elongated bodies and tail and relatively equal-sized front and hind legs. Although easily identified by their lack of scales, salamanders are sometimes mistaken for lizards, which they resemble in body form. All frogs and most salamanders have 4 toes on the forefeet, 5 on the hind feet; some salamanders have fewer toes (4 on the hind feet in four-toed salamanders and the mudpuppy in Canada) or lack limbs entirely (some species beyond Canada). Caecilians are limbless and have little or no tail. They usually have folds or grooves, which give the body a segmented, wormlike appearance.
Modern amphibians are small: the largest salamanders attain 160 cm; caecilians, 120 cm; and frogs, 30 cm. Typically, living amphibians have 2 lungs, although the left lung is reduced in caecilians and members of one salamander family (Plethodontidae) are lungless. The heart has 2 atria and one (sometimes partly divided) ventricle. Poison glands are often abundant in the skin; the poison is distasteful but rarely fatal to predators.
Reproduction and Development
Most species breeding in aquatic situations form large aggregations in spring or after heavy rains. Male frogs typically have distinctive breeding calls; those defending calling sites may also have territorial calls. In contrast, salamanders make little sound but have evolved elaborate courtship behaviour.
Eggs are fertilized externally in most frogs; most salamanders deposit small packets of sperm, which are taken up by the female through her vent and held for internal fertilization. Most amphibians deposit eggs in water or in moist, terrestrial sites; eggs are rarely retained in the female until hatching. Most terrestrial eggs hatch only after the larval stage has been passed within them, but the typical amphibian life history features a gill-breathing, aquatic larva metamorphosing into the lung-breathing adult. Hence, the name of the group [Gk amphi, "both"; bios "life"].
Striking differences in larval development between groups show their long, divergent evolution. Larval salamanders have forelegs appearing first, hind ones later, and are similar to adults in form and in being carnivorous. Some salamanders remain aquatic, retaining gills throughout their lives.
Frog metamorphosis, from aquatic larva to terrestrial adult, is more dramatic. Larval frogs hatch with external gills and without legs. The tadpole intestine is long and coiled. The gills soon become internal; the body becomes globular, with no discernible neck; a small mouth with rasping teeth and beak appears for grazing on vegetation. Later hind legs appear as buds and complete their development externally by the time the forelegs, developed internally, push through the body wall; the intestine shortens for a carnivorous diet; the tadpole teeth and beak are lost; the mouth divides; gills and tail are absorbed. The mouth enlarges for gulping whole animals and the lungs take over respiration.
Distribution and Habitat
Amphibians have lost much of their ancestral diversity and size and have survived by accepting a small predator's role, largely in moist or aquatic habitats. Only frog tadpoles are plant eaters but some of these may be carnivorous; all salamander larvae and adults in both groups are carnivorous. There are over 5000 species of "frogs" (including TOADS, etc), and they occur on all major landmasses except Greenland and Antarctica. There are about 500 species of salamanders, largely restricted to the north temperate zone, are most diverse in Eurasia and North America, but one family (Plethodontidae) has successfully invaded the tropics of Central and South America and radiated into many species there. Caecilians number about 170 species, all entirely tropical. The total of amphibian species is constantly being revised upward, partly because herpetologists are collecting more from remote regions and more difficult to sample habitats, and partly through DNA analysis.
Canada has 46 species of native amphibians: 25 frogs and 21 salamanders. None are unique to Canada and most have more extensive ranges in the US. None occur in the northern TUNDRA, but several are abundant in the BOREAL FOREST. The deciduous forests of southwestern Ontario, the coastal rain forest and interior valley grasslands of British Columbia and the central plains of Saskatchewan and Alberta contain many species that barely range into Canada. No amphibians survived GLACIATION in Canada; therefore, all this fauna has spread from southern refugia and have been in Canada less than 18 000 years.
Amphibians are of minimal direct economic importance. Some frogs are caught for food, particularly the bullfrog native to southern parts of eastern Canada and introduced in British Columbia. Small frogs are used for fishing bait. Bullfrogs, leopard frogs and mudpuppies are used in university and high-school dissection and experimental physiology. Amphibians are a vital part of the ECOSYSTEM, forming a significant part of the terrestrial, aquatic and semiaquatic biomass and acting as a biological control on invertebrate members. They are widely used to monitor the possible environment for affects of ACID RAIN, GLOBAL WARMING, ultraviolet light and widespread parasite and fungal infections. Drastic population reductions and some extinctions have been widely studied throughout the world.
Author F.R. COOK
F.R. Cook, An Introduction to Canadian Amphibians and Reptiles (1984); W.E. Duellman and L. Trueb, Biology of Amphibians (1982); D.M. Green, ed, Amphibians in Decline: Canadian Studies of a Global Problem (1997); T.R. Halliday and K. Adler, The Encyclopedia of Reptiles and Amphibians (1986); W.R. Heyer et al, Measuring and Monitoring Biological Diversity: Standard Methods for Amphibians (1994); G. Linder, S.K. Krest and D.W. Sparling, Amphibian Decline: An Integrated Analysis of Multiple Stressor Effects (2003); G.R. Zug, Herpetology: An Introductory Biology of Amphibians and Reptiles, 2nd ed (2001).
Links to Other Sites
Canada's Biodiversity: Focus on Amphibians
A very extensive resource about Canadian species of frogs, salamanders, and newts. Provides illustrated notes about their life-histories, habitats and more. From Canada's Digital Collections.
Canadian Biodiversity Website
A great information source for all budding biologists. Learn about biodiversity theory, natural history, and conservation issues. From McGill’s Redpath Museum.
This multimedia website provides tips on monitoring frog populations and identifying frog species found in each region of Canada.
Why survey Herptiles?
This Parks Canada website is devoted to the study of amphibian and reptilian species native to Canada.
Amphibian Specialist Group
The Amphibian Specialist Group focuses on conservation of amphibians and their habitats around the world. Hop over to the link for “Froglog,” a bi-monthly newsletter which offers current information about the decline of amphibian species. Part of the International Union for Conservation of Nature and Natural Resources.
This website offers access to the “Red List,” a detailed, searchable database of flora and fauna facing extinction. Categories range from critically endangered to vulnerable. Note: select “Canada” for an extensive list of Canadian species at risk of extinction. From the International Union for the Conservation of Nature and Natural Resources, the world’s largest conservation network.
The AmphibiaWeb website provides access to information on amphibian biology and conservation. From the University of California.
Amphibian Species of the World
A detailed, highly technical information source about amphibian species of the world. From the American Museum of Natural History.
Ontario Herpetofaunal Atlas
The website for the Ontario Herpetofaunal Atlas. Offers a searchable database of detailed distribution maps of Onatario's amphibians and reptiles.
This site offers descriptions of mammals, birds, amphibians and reptiles, fish, and plant life found in boreal forest regions. Also includes a glossary of terms related to forestry.
Besides hockey and the maple leaf, there is little as symbolically Canadian as the CBC – the Canadian Broadcasting Corporation. It grew out of a developing nation's need to express its identity and find its voice.