Wednesday, May 6, 2015

American Bullfrog

A large male American Bullfrog floats, calling in a pond. Note that the dorsolateral skin fold goes around the eardrum (tympanum) rather than going down the back.

     The American Bull Frog (Lithobates (formerly Ranacatesbeianus) is the largest native frog in North America. It can grow to 8" (203mm) and has traits that make it quite different from our other frogs besides its size. It is quite variable in color, particularly when young. It is often confused with its smaller and much more common cousin, the Green Frog (Lithobates clamitans). The skin fold that begins behind its eye (the dorsolateral fold) however goes around the eye rather than down the back as in the Green Frog. More on the Green Frog from a previous post:

A male Green Frog floats on a pond. Note that the dorsolateral fold of skin goes part way down the back. 

     Unlike other North American frogs where the female, in order to be able to carry more eggs, is larger than the male, just the opposite is true of Bull Frogs. The males are bigger due to having territorial disputes where the larger males force or bully the smaller ones away. The larger males have thus been selected for and have greater mating success.
     Males are quite easy to tell from females once they are adults. Not only are they bigger and have a yellowish throat during the breeding season, but their tympanum (ear drum) is larger than their eye. Female have eyes that are about the same size as their tympanums. These are all sexual differences their smaller Green Frog cousins also have.

The larger male Bullfrog with its eardrum (tympanum) larger than its eye courts the smaller female who's tympanum is about the same size as the eye. 

     Males are now giving their now giving their "jug-o-rum" calls in our permanent ponds. You can hear and see them do this in this video clip from the Capital Naturalist YouTube Channel:
     Also unusual in frogs, the females will sometimes also call, though this is normally the scream both sexes may give if they are caught by a predator. Some suspect that this release call attracts an even larger predator that might break up the attack and allow the frog to escape in the scuffle. Indeed, I've several times heard the screaming and gone to investigate, interrupting a feeding snake in the process. In most species of frog or toads, only the males vocalize.

A Bull Frog tadpole already developing its back legs.

     Once a female has chosen her suitor, she will lay a large floating mass of eggs numbering between 3,000-50,000 eggs. In less than 3 weeks, the tadpoles emerge. These tadpoles take 2 and sometimes 3 (up North) years to metamorphose into an adult. This again differs from most of our frogs which change over in a single growing season. The olive colored tadpoles can also grow to be our biggest. Some polliwogs can be up to 6" long. Although they typically eat algae or detritus just like other polliwogs, they are also unusual once again in occasionally scavenging meat and eating other tadpoles. They are distasteful to some fish as well which allows them to live in permanent ponds with them.

Bull Frog tadpoles are the largest in North America and take the longest to develop (2-3 years) of all our native tadpoles.

     Bull frogs are notorious for eating anything smaller than themselves, including each other. I've seen them eat small mice, frogs, and even small snakes. I once was leading a group of kids and watching a Red-winged Blackbird land next to the water, when a large Bull Frog grabbed it. They struggled for a while before the ruffled bird got away. This voracious trait has resulted in some local extinctions of animals. These are the most commercially available frog tadpoles because they take so long to change over and can be kept with small fish in aquarium pet stores. For this reason, they are the ones most bought for release into home ponds.
     Bull Frogs will eat any smaller species there however. They have been outlawed for sale in some countries like England where they have become an invasive predator of small ponds. They have even been implicated in the demise of California's Red-legged Frog. Bull Frogs from Maryland were taken to California for their edible legs and for frog jumping contests like that at the famous Calaveras Fair. Now they pose a threat to the smaller Red-legged Frogs who were already in trouble due to habitat destruction. It is important to realize that being native does not mean they belong everywhere, as that can upset the natural balance.
A female Bull Frog.

     Bull Frogs, if they survive being eaten by each other, snakes, humans, or the numerous other creatures who consume them, can live 15 or so years. It takes a male 1-2 years to reach sexual maturity (2-3 for a female). Their Latin name was given to them in honor of the great American naturalist Mark Catesby. 
     I truly appreciate these large frogs and their calls. Many times I've put aside my fishing rod or stopped what I was doing to try and sneak up on them and see what they were doing. I've been rewarded by many observations of them eating or being eaten. Luckily, these are tough amphibians who can survive in less than ideal conditions including farm ponds and park lakes. But they do not belong in every body of water because of their great size and appetites. Now that you know a bit more about them, you hopefully can appreciate them more as well.  

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