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Friday, Apr. 29, 2016

Bullfrogs: Big, bad boys of the frog world

Thursday, June 26, 2008

The bullfrog (Rana catesbeiana) might not be the king of the forest, but it is the top dog in the frog pecking order. Their superiority to other species in various ways gives this robust species a competitive edge.

Size is the bullfrog's most obvious advantage. Females, which are larger than males, can grow to nearly 8 inches long, not counting their lanky hind legs. They can weigh a pound or more.

With that superior size goes a voracious appetite. Bullfrogs have been known to eat insects, worms, snails, salamanders, snakes, turtles, birds, bats, mice and small minks. They also are fond of other frogs. What better way to ensure your own survival than to eat the competition? Unfortunately for the bullfrog, they are not discriminating enough to distinguish between competitor species and their own kind. They readily prey on smaller individuals of their own species.

Bullfrogs are native to eastern North America, from Nova Scotia to Florida, but they have been widely introduced as far west as California and British Columbia, not to mention South America, Europe and Asia. Frog species native to Colorado have suffered significant declines due to competition from imported bullfrogs.

Besides eating virtually anything they can cram down their gullets with the help of their muscular front legs, bullfrogs have developed a fairly impressive array of strategies to avoid being eaten. Adult bullfrogs have glands in their skin that secrete a toxin that discourages predators.

Tadpoles apparently share this unpalatability.

A species that multiplies faster than its competitors also has an advantage, and the bullfrog has this covered, too. Females lay up to 25,000 eggs at a time, and can produce two broods a year.

Like all wild animals, bullfrogs need to produce lots of young to compensate for dramatic losses in the first years of life.

Tadpoles and young frogs are prey to an impressive array of creatures, including herons, turtles, kingfishers, snakes and raccoons.

Nevertheless, bullfrog tadpoles have a higher-than-average survival rate, and once they reach adulthood, their chances of living to a ripe old age go way up.

Various experts estimate the average lifespan of a bullfrog at 4 to 9 years, and one captive specimen lived 16 years.

It doesn't hurt the bullfrog that it thrives in conditions created by human modification of their environment. They prefer warmer water than many other frog species can tolerate, so they get the upper hand in streams that have been polluted with excess nutrients from agricultural runoff and where shading trees have been removed from banks.

They thrive in waters choked with vegetation, which also is favored by excess water fertility. Furthermore, the creation of thousands of small, sun-washed ponds has provided countless acres of new, prime bullfrog habitat.

Bullfrog tadpoles are believed to have hitched rides to some new areas in trucks transporting fish from hatcheries to stocking sites. Not that they need mechanical help in colonizing new areas.

Adult bullfrogs can cover as much as 6 feet in one leap, and they are prone to leave their homes and migrate overland to new waters during rainstorms.

All this explains why bullfrogs have prospered in the past, but new challenges face this and other amphibian species.

Of the 6,100 known species of amphibians worldwide, approximately 42 percent are declining in numbers.

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