Bullfrog Mid-Metamorphosis

Growing up is hard to do…imagine what it must be like for a bullfrog! To become an adult frog, a tadpole must undergo an astounding transformation called metamorphosis.

This bullfrog, spotted at Nature Boardwalk at Lincoln Park Zoo, is nearing the end of this process and is sporting brand new land-worthy limbs as a result. That is why this frog doesn’t look like the typical adult bullfrog that you might be more familiar with (see the picture below for a reminder). If you compare the two, you can see the young frog still bears some of the characteristics of a tadpole, but nonetheless is well on its way to adulthood.


Photo by Joel Pond

To become an adult, the tadpole must lose some body parts and grow some new ones. The tadpole’s long tail is absorbed. Meanwhile, hind limbs and forelimbs form.

Tadpoles are herbivores, but adult frogs are omnivores. To prepare for this change, the tadpole’s teeth, which are designed to scrape algae and vegetation, are lost. They will be replaced with a different sort of teeth more appropriate for the frog’s adult diet. Likewise, the frog’s tongue develops and the jaw becomes functional.

Many subtle changes are occurring during this process, but some are quite obvious too, such as changes in skin texture and pigment, which you can see in the comparison between the two frogs. Eyes and ears develop and become more prominent in the adult form as well. Finally, the frog’s heart changes, going from two chambers to three!

In addition to the litany of physical changes the tadpole undergoes to become an adult frog, the frog’s lifestyle changes dramatically as well. While the tadpole is restricted to the underwater realm, the adult frog is amphibious, free to spend time both in the water and on land.

It can take several years for a bullfrog tadpole to become an adult frog, so it is a special treat to get to see a frog with brand new legs. You never know what you’ll see next at Nature Boardwalk!

Vicky Hunt

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