American Bullfrog

Description

The American bullfrog is a heavy-bodied frog. It can reach up to 17 cm in length, making it the largest frog in North America. They are pale green, green-brown or brownish in colour, with variable dark speckling. The stomachs of bullfrogs are generally creamy white with dark mottling. During the mating season, males have bright yellow throats and chins. American bullfrogs always have a distinct fold of skin that starts at the eye, curves around the ear drum and ends at the shoulder. This skin fold, along with the absence of dorsolateral folds down the length of their back, is a helpful identifying feature, especially in young frogs.

The larval form of bullfrogs, known as tadpoles, can grow to be more than 10 cm long. They have an oval body and a long tail fin that ends in a point. When they are smaller than 2.5 cm in length, they are black with gold blotches. Once the tadpoles grow larger, they become olive green in colour, with black specks on their back and a cream-yellow stomach.

Distribution and Habitat

The American bullfrog is native to eastern and central North America. Within Canada, it can be found in southern Ontario, Quebec, New Brunswick and Nova Scotia. It has also been introduced (now considered invasive) to British Columbia, where it can be found in disjunct pockets on Vancouver Island, in the southern mainland and on some of the gulf islands. In the United States, bullfrogs are native to the central and eastern portions of the country but can be found in the Midwest as well, where they have been introduced. They are considered invasive in parts of the western US. They have also been introduced to Mexico, as well as other locations around the world.

Bullfrogs require habitats with large, permanent bodies of water. These may be in the form of wetlands, ponds, lakes, creeks and rivers. Bullfrogs prefer well-vegetated shorelines.

In winter, bullfrogs typically hibernate underwater as both tadpoles and adults. They settle at the bottom of permanent bodies of water, such as deep ponds, lakes and rivers. Here, the temperature never drops below freezing and oxygen levels remain high enough to sustain the bullfrogs until spring.

Reproduction and Development

Bullfrogs breed from mid-June to late July. Males will gather to call to females, usually on warm, humid or rainy nights. The male’s call resembles the roar of a bull from a distance, hence the English name “bullfrog.” The French-Canadian name for bullfrog, Ouaouaron (a word of Iroquoian origin), is likely meant to imitate the bullfrog’s call.

A female will lay between 5,000 and 20,000 eggs a year. The eggs are black and surrounded by a single jelly envelope. As they are laid, they spread out over the surface of the water in a mass that can be up to 1 m in diameter. As with most species of frog, the males fertilize the eggs externally as they are being laid by the female. The eggs often sink to the bottom of the pond as they develop and hatch in a few days to a week.

In Canada, the larval stage of the American bullfrog can last for more than two years, a period of time that is unusually long for frogs. In the southern parts of its range, the larval stage can be as short as a few months. After metamorphosis, males reach maturity after one to four years, and females after one to five years or longer. In the wild, American Bullfrogs can live for between 5 and 10 years after metamorphosis. In captivity, they can live up to 16 years.

Diet and Predation

The appetite of the American Bullfrog is often described as “voracious.” They hunt their prey using a sit-and-wait strategy and will attack any animal smaller than itself that passes by. A large part of their diet consists of other frogs, and often other American bullfrog tadpoles and juveniles, which can comprise up to 80 per cent of their diet in some parts of their range. They will also eat invertebrates, mammal, and, occasionally, birds. As tadpoles, bullfrogs eat suspended organic matter, such as algae and small aquatic invertebrates.

Did you know?
American bullfrogs aren’t the only animals to eat members of their own species. In nature, it is quite common for animals of the same species to eat one another. There are more than 1,300 species worldwide known to engage in cannibalism.

Predators of bullfrogs and their tadpoles include insects, fish, bullfrogs, snapping turtles, snakes, birds, and mammals such as raccoon, skunk and coyotes.

When threatened, bullfrogs will cry out as a form of defense. Juveniles, when alarmed, will let out a quick and startling squeak as they flee into the water. When grabbed, adult bullfrogs may let out a very loud, high-pitched sound, which can last as long as seven seconds.

Status and Threats

Internationally, American bullfrogs are listed as “least concern” by the International Union for Conservation of Nature. They have not been assessed by the Committee on the Status of Endangered Wildlife in Canada.

In some parts of their native range, bullfrogs experienced a decline in the 20th century. This decline was attributed to the unregulated overharvesting of the frogs for food and educational materials as well as habitat loss and pollution. Today, bullfrogs have returned to many of the areas where they were depleted, but it is unknown if overall numbers are increasing or decreasing.