Creature Feature: The Frog that Ate the Midwest

Bullfrog - Photo Tom Sheley

Bullfrog - Photo Tom SheleyBullfrogs. They’re our ideal mental image of a frog: squat, slick, and green, with big eyes and a loud croak. They make homes in all our man-made habitats: farm ponds, reservoirs, and even roadside ditches. Plus they’re good to eat…or at least their legs are, so that they’re actually managed as a game animal. But there are still a lot of things you don’t know about these ur-frogs. So let’s meet the American Bullfrog.

Firstly, Bullfrogs (Rana catesbeiana) are one big frog. Unlike most of our frogs, Bullfrogs grow large, often exceeding 1/4 pound for a big adult. They’re part of the frog family Ranidae that favor slow or static warm water for breeding, and they spend most of their lives near these watery retreats. This group includes the widespread Green Frog (R. clamitans) and southern Pig Frogs (R.grylio), both big bruisers but still not as big as Bullfrogs. Bullfrogs get this size from two things: the fast-growing tadpoles that can graze algae for multiple years, and adults that aren’t fussy eaters, basically swallowing anything smaller than themselves. Since they metamorphose from a big tadpole, Bullfrogs can start eating big items right away, often crayfish, big insects, and other frogs, and the menu items only get bigger after that. A big population of bullfrogs in a pond can decimate other amphibians and invertebrates that aren’t used to their predation.

Despite this catholic diet, bullfrogs are not as ubiquitous as you might think. Like most frogs, they have tadpole young; unlike many small frogs, however, their tadpoles take 2-3 years to grow before metamorphosing. Because of this, they need permanent water. The result is that you won’t hear Bullfrogs calling from the shallow marshes or vernal pools favored by spring-calling frogs like peepers or chorus frogs. Nor can they reproduce in the flood pools or rain puddles avored by Toads and Treefrogs. In fact, you often won’t hear their booming ‘Jug-o-rum’ mating calls until warmer weather – late May or June – after the water has warmed enough for their mating and egg-laying. If you note where you hear them, it’s mostly in permanent ponds and swamps, places where their tadpoles have a chance to lead a full life of several years.

Bullfrog - Photo Craig StanfillLiving in ponds for that long means that young bullfrog tadpoles have to dodge two big problems: predatory fish and winter cold. Bullfrog eggs are eaten by lots of predators, but the tadpoles produce a skin chemical that makes them unpalatable for most fish. They can still be eaten by water snakes, herons, and predaceous water bugs, though. If they avoid predation their first year, they burrow into the mud bottoms of ponds and marshes to wait out the winter in a cold torpor. Most will metamorphose late in their second summer, but their troubles aren’t over yet. Young bullfrogs are heavily preyed upon by garter snakes and older bullfrogs, so most have an innate dispersal behavior that will drive them to move up to 10 miles away from their birth pond, even in dry conditions. So a crop of young bullfrogs are almost like wind-blown seeds, spreading out from their birth-pond in search of new habitat.

What does all this add up to? Bullfrogs are now the Starlings of amphibians. They evolved in the warm swamps of the southeastern U.S., but have taken advantage of human-designed habitats to spread throughout the eastern U.S. and into the Great Plains. All of those farm ponds, drainage rockpits, and reservoirs make for perfect Bullfrog habitat. Humans have also deliberately introduced them all over the western U.S., as well as Europe, Asia, and Australia in a misguided attempt to provide sport, food, or insect-control. Their biology makes them the ‘perfect invader’, and their colonization of many of these habitats has come at the expense of many native amphibians. Bullfrogs have now been implicated as major factors in the decline of many other amphibians, especially other Ranid frogs adapted to slow-water habitats. If you’re hearing calling bullfrogs from your pond, it’s a good bet that some native frogs have gone missing. Perhaps it’s time to harvest some of your Bullfrogs.