The Cold Blooded News

The Newsletter of the Colorado Herpetological Society

Volume 28, Number 11; November, 2001

Unusual Life Cycle of an Unusual Puerto Rican Amphibian

by Byron Spice

Reprinted from The Michigan Herpetologist, the newsletter of the Michigan Society of Herpetologists, August 2001.
Originally published in the Pittsburgh Post-Gazette, June 11, 2001.

Legend has it that the coqui, a tree frog that abounds in Puerto Rico, will die of a broken heart if taken off its beloved island. But the little frog has no such affection for its own tadpole -- it doesn’t have one.

At some point in evolution, the coqui eliminated its own larval stage. So while other frogs emerge from their floating eggs as tadpoles and only later metamorphose into four-legged creatures, this one emerges from the egg as a tiny, but fully formed, frog.

Even more curious, the coqui's eggs are laid on dry land. It almost seems dishonest to call this tree frog an amphibian.

The coqui is hardly alone in lacking a tadpole. It is but one of about 500 species of a family of frogs called Eleutherodactylus found in the tropics of Central and South America and the southern reaches of Florida, all of which are so-called "direct developers." And it is not unique among amphibians in not venturing into the water at any point in its life cycle.

A typical frog egg measures about 1.5 millimeters in diameter, but the coqui eggs measure 3.5 millimeters. That doesn't sound like much, but in terms of volume, these eggs are 20 times larger.

That's significant in the coqui's ability to eliminate its tadpole.

The smaller eggs of other frogs contain very little yolk. The tiny tadpoles they produce are feeding machines. They swim around gobbling up plant matter until they are ready for metamorphosis, when they take on an adult shape, switch to a diet of insects, and continue to grow.

The coqui eggs, by contrast, contain much more yolk. When the frogs emerge after three weeks, they are fully formed, though small enough that five can fit atop a dime. Like a tadpole, the newly hatched tree frogs have tails. But the coqui tail is not muscular, like a tadpole's. Instead, it is almost leaf-like and functions as a respiratory organ until it is jettisoned.

The tree frogs, which have disks on the ends of their web-less toes to help them adhere to surfaces, grow to an inch or two in length.

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