What are Amphibians?
Amphibians form a class of vertebrates. Other classes include reptiles, birds and mammals. There are over 4700 species of amphibians worldwide, making this group more diverse than mammals. Amphibians have no unique structure, like the feathers of birds, which sets them apart from all other animals, but there are a few characteristics which all amphibians share:
- They are ectotherms (often referred to as cold-blooded). This means that internal temperature of amphibians depends upon that of the surrounding environment, unlike mammals that hold their body temperature constant
- Amphibians have soft, generally moist skin without scales
- Their eggs do not have shells and so they must be laid in water or a damp environment to keep from drying out
- Amphibians go through a two-stage life cycle: 1) when an amphibian hatches, it is in a gilled larval form. In frogs and toads this larva is called a tadpole. 2) After a few weeks or months, the larvae transform into the adult form; however, the larvae may still take a few years to become mature
Biologists divide amphibians into three orders:
- Frogs, including toads, found in Canada
- Salamanders, found in Canada.
- Caecilians or limbless amphibians, found only in the tropics
Frogs are by far the most diverse group of amphibians, with over 4000 species worldwide.
For more information about Amphibians in Canada visit the Toronto Zoo’s Adopt-a-Pond and Canadian Amphibian and Reptile Conservation Network websites.
Frogs, toads, salamanders and newts are all amphibians. The word amphibian means “double-life” –a reference to living both in water and on land. Most amphibians lay their eggs, protected only by jelly, in the water. The young go through a larval stage, breathing through gills and swimming with fins, before transforming into adults. Although most species have lungs, they also breathe through their moist skin and the lining of their mouths. Amphibians do not drink water; instead they absorb it through their mucous-moistened skin. Amphibians are therefore very sensitive to pollution and other environmental changes.
For example, the amphibians of Ontario hibernate in winter. With the warmth of spring, they emerge and migrate to traditional breeding ponds. The males of most species call to attract mates in a lively chorus, while voiceless salamanders and toads search for their mates by smell. Generally, mating and egg-laying take place in the water, where eggs and young are left to fend for themselves. However, some salamanders lay their eggs in rotten logs or in moss on the edge of ponds and several salamander species stay with their eggs until they hatch.
The eggs develop rapidly and hatch into larvae. Frog and toad larvae, called tadpoles, are good swimmers and eat mostly plants. Some tadpoles in temporary pools grow quickly and transform 30 – 45 days, such as American toads. Others, such as the Bullfrog tadpole, live in ponds with fish and do not transform for three years! In fact, some bullfrogs live longer as tadpoles than they do as frogs!
Salamander larvae, which eat insects, develop legs quickly so that they can walk on the bottom of the pond. The tadpoles of some species forage in schools and use chemicals to communicate and warn of danger when wounded. Over several weeks the larvae transform into air-breathing, land-dwelling animals. Adult frogs, toads and newts are active in the daylight hours in Ontario and feed on insects and other small creatures. Salamanders are nocturnal, hiding under logs and leaf litter. With the onset of freezing temperatures in late autumn, amphibians retreat to their hibernating sites.
In winter, northern amphibians survive by hibernating underground, in ponds or under leaf litter beneath a thick blanket of insulating snow. While most amphibians freeze to death if cooled below -1∫ C or -2∫ C for more than a brief period, a few northern species actually have a sugary antifreeze in their cells to allow them to survive to a temperature of -5∫ C to -7∫ C. Wood Frogs and Boreal Chorus Frogs, for example, use glucose in their cells as antifreeze that prevents the cells from bursting in these freezing temperatures. In effect, these frogs can be ‘frozen solid’ during hibernation and still emerge safe and sound in spring.
Northern amphibians take full advantage of the warmth of the spring and summer sun. Eggs are large and dark to help absorb heat. They are submerged in warmer shallow water, safely below the surface, which might freeze. Tadpoles love to bask in the sun in the shallow while adult frogs and toads bask on land or in shallow water in the heat of the day. Some amphibians, which are nocturnal in warmer climates, prefer the warmest and brightest part of the day in the North.
For more information about Amphibians in Canada, visit the Canadian Amphibian and Reptile Conservation Network website. For more information about amphibians in Ontario, visit the Toronto Zoo’s Adopt-A-Pond website