Many of the animals I write about in this blog are rare, endangered species. Somehow it often seems that the most interesting animals are the ones that are in danger of dying out. Of course, it could just be that these animals are studied to a much greater extent than more common species. It’s difficult to get funding to study a species that is super common and not in danger of extinction. Still, I try and keep this blog balanced, so today I’m going to talk about a very common amphibian, the American bullfrog.

American bullfrogs originally lived in the eastern part of the U.S., but have since spread into a much wider area, and is considered an invasive species in some areas. The map below shows the original range of the bullfrog (in red) and its current range (in dark green). Bullfrogs need to live in water, and so live in permanent bodies of water, such as lakes, ponds, or bogs. They have a preference for warm, still and shallow water with dense vegetation for cover. Fortunately for the bullfrog, water polluted by humans is often warmer with more vegetation, which means bullfrogs are doing very well these days.

Map of bullfrog range. Red is the bullfrog’s natural range, dark green where it has been introduced. Image CC BY-SA 3.0,

American bullfrogs are big frogs, and are the largest species of frog in North America. They can weigh up to half a kilogram and be over 20 centimetres in length. They are mostly olive green coloured, with a bright green upper lip and pale underside. In the breeding season the chin of male bullfrogs is bright yellow, which makes them easy to tell apart from females. They have enormous back legs, which gives them incredible jumping power. Bullfrogs are often hunted for frog legs in the United States, and in some areas is even used for racing. I want to go to a frog race someday.

A male bullfrog. You can tell it’s a male because the eardrum of the frog (that strange circle behind his eye) is bigger than the frog’s eye. In females the eardrum is the same size as the eye.
Image By Carl D. Howe – Carl D. Howe, Stow, MA USA, CC BY-SA 2.5

Breeding season for bullfrogs is between May and July in northern populations and between February and October in the south. To breed, males congregate in hopping places, like nice little ponds or lakes. They space themselves out nicely, and aggressively defend their territory from any incoming males. Basically the males sit in their territories and call loudly, hoping a lonely female will be impressed by their voices. It is these loud calls, which can be heard over a kilometre, that gives the bullfrog its name, as it sounds a bit like the roar of a bull. Tadpoles emerge from from eggs after three to four days, and can stay as tadpoles for up to three years. The tadpoles apparently don’t taste very good, so fish tend to leave them alone. After they’ve changed into adult frogs, a further two years is needed to reach sexual maturity. Luckily bullfrogs are fairly long lived, with one animal reaching 16 years in captivity.

Although it’s nice that bullfrogs can be so successful in human-altered environments, their tenacity has caused problems for other amphibians in bullfrog habitats. This is especially true where bullfrogs have been introduced, as bullfrogs will not hesitate to eat other frogs (they even eat other bullfrogs). Still, we should celebrate the ability of this species to adapt to a rapidly changing environment.

Cover image CC BY-SA 2.5,