Frogs and toads are both members of the order anuran. There are 25 species of anurans found in Canada, and more than 7,000 (and counting) worldwide. “While it can be hard to tell frog and toad tadpoles apart, their eggs are often easy to tell apart. Toads lay their eggs in long strings, sort of like black pearls on a necklace. It can be metres long,” says Donnell Gasbarrini, Adopt-A-Pond Coordinator with the Toronto Zoo. “Frogs, on the other hand, lay their eggs in big clumps.”
The larval stage of both frogs and toads are called tadpoles, though in some areas they’re known as polliwogs. Both terms derive from old Middle English words for “toad head” and “wiggle head.” Salamanders and newts have a similar larval stage, but they’re simply call larvae.
The length of time a frog or toad spends as a tadpole varies by species. Some species, such as the chorus frog, lay their eggs in vernal pools – temporary wetlands that form from snowmelt and spring rains that completely dry up by the end of the summer — so their tadpole stage is just a couple of months long. Others, including the American bullfrog, the mink frog, and the green frog will spend a year or more as tadpoles before completing their metamorphosis. But the climate they live in will impact precisely how long. “American bullfrogs generally overwinter one to two years, and up to three years in Canada. But in southern states, such as Arizona, transformation can occur after just three months,” says Stéphanie Tessier, Collections Manager in Vertebrate Zoology for the Canadian Museum of Nature.
After its legs emerge, a tadpole will absorb its tail using a process called apoptosis. “They’re able to basically recycle the tissue,” says Gasbarrini.
“A bullfrog tadpole can fill most of the palm of your hand,” says Gasbarrini. The largest recorded in Canada measured 162 mm. The largest ever documented, found in Arizona, was more than 200 mm long. Alas, according to Tessier, at those large sizes the tadpoles likely used up too much energy to complete their transformation to terrestrial frogs. The paradoxical frog from South America has the largest tadpole in the world at up to 27 cm long. That’s actually about two-thirds longer than it is as an adult. The tiny Paedophryne amauensis frog from Papua New Guinea, on the other hand, doesn’t even have a tadpole stage. “Their eggs hatch as miniature adults,” says Tessier.
Most tadpoles that live in lakes or ponds embark on a daily migration, moving from deep waters where they shelter for the night to the shallows where they feed on algae. You can watch for yourself in this beautifully shot short film following the daily migration of western toads in British Columbia. The thumbnail-sized strawberry poison dart frog, found in Central America, doesn’t spend its time as a tadpole in a lake or pond. Instead, the mother carries the tadpole on her back and climbs a tree to deposit her tadpole in a pool of water captured inside a leaf.
Some frog eggs can detect danger in the form of vibration and actually hatch early to escape. The eggs of the red-eye tree frog, which is found between Mexico and Columbia, will emerge early to escape predators.
One sure sign of spring is spotting dozens of tiny dark tadpoles wiggling their way around the lakes and ponds of cottage country. While you’re waiting for them to transform into the frogs and toads that keep you up at night during the summer, here are seven facts you might not know about tadpoles.
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