It’s finally Spring, and in our part of the world, we’re ecstatic. We’ve had a rough winter—we had snow and ice on our city streets every day for about 40 days in a row! The crocuses are blooming in my backyard, and outside the city, frogs are springing to life.
In a way, Spring is a good time to think about our “inner amphibian.” After all, mammals are descended from animals that moved from aquatic environments onto the land over three million years ago. As embryos, our heads in the early stages of development look remarkably similar to shark embryos—with gill arches and all. The metamorphosis of frogs is a process that’s not all that different from what all vertebrates go through, but the difference is that most of the development of birds, reptiles, and mammals—such as the growth of the lungs and limbs—takes place inside an egg or inside the mother instead.*
FROM EGG TO TADPOLE
A female frog first lays eggs underwater, sometimes hundreds of eggs, which form into a jelly-like clump called frogspawn, which floats on the water. Most of these eggs become food for other pond life, but some survive.
Tadpoles developing in eggs. © Dan L. Perlman/EcoLibrary.org
The tiny animal inside the egg grows for about a month, then hatches out of the egg. It looks like a small black fish and breathes underwater with feathery gills on each side of its head.
The tadpole’s tail begins to grow; it wiggles its tail to swim. Tadpoles are also called polliwogs. (The word “polliwog” is from Middle English polwygle. Pol means “head” and wiglen means to “to wiggle”). The tadpole eats algae and other plants that grow underwater.
Tadpole. Photo from Wikipedia.
FROM TADPOLE TO FROGLET
After several weeks, tadpoles begin their metamorphosis. Two tiny bumps appear near the tadpole’s tail—these will grow into back legs.
Two more bumps appear near the frog’s head—these will grow into front legs. Lungs begin to grow inside the tadpole’s body and the feathery gills disappear so that the tadpole will be able to breathe air.
The tadpole now has legs for hopping and walking, lungs for breathing air, but its long tail is awkward on land. Until the tail shrinks and is absorbed into its body, the froglet stays in or near the water.
Froglet with tail. Photo courtesy of www.scienceprojectlab.com
FROGLET TO FROG
When the tail is gone, the frog has completed its metamorphosis. The young frog will now feed on small insects, caught with their long, sticky tongues. It will eventually move away from the pond and find a safe place to grow.
This Spring, take some time to visit a nearby pond or swamp and see this process yourself. And check out the poster below that Susan designed for Earth Day, inspired by a photo by FROGS ARE GREEN photographer friend Joe Furman. All proceeds go to help our amphibian friends.
*I found this idea in Thomas Marent’s lovely and informative book, FROG.