Monday, February 26, 2024

Bullfrogs have a ridge of skin that runs from each eye, curving around the external eardrum. (Photo: Brian Rusnica)

The American bullfrog is the largest frog in North America, and for this reason it is probably the frog you dissected in ninth-grade biology class. It is native to Massachusetts and all of eastern North America. It’s not called a bullfrog because of the way it looks, but rather because of the way it sounds: It makes a call like a bellowing cow (or perhaps a foghorn?).

Bullfrogs live in swamps, ponds and lakes. They are found all over the state except on offshore islands near the Cape (excluding Nantucket, where they have been introduced). They are olive green on top, sometimes with brown mottling, and blotchy white or yellow on the underside.

A bullfrog at Black’s Nook in Cambridge. Its front toes are not webbed, but the back toes are, except for the unwebbed fourth toe. (Photo: Ann Schlesinger)

Groups of male bullfrogs are called choruses. Males arrive first at the breeding site in late May or early June. There they call loudly for females. Females initiate physical contact, maybe sexually receptive for only one night. For this reason, the males compete intensely with each other for females.

When the density of male frogs is low, females choose a mate based on the quality of the male’s territory. But when the density of male frogs is high, the females choose based on differences in male display behaviors and the male’s hierarchy within the chorus. Dominant males display their yellow throats while nondominant males stay in the water with only their heads showing. When two dominant males encounter each other, they wrestle to determine their position within the chorus.

Bullfrogs are green and brown, which helps them blend in with their surroundings. (Photo: Laura Pagano)

After a female chooses a male, he climbs on top of her, grasping her with his front legs until she lays up to 20,000 eggs. Males do not have penises, so they release sperm over the eggs to fertilize them in the water. The entire mating process takes about 20 minutes.

The eggs form a floating jellylike mass on the water. They develop and hatch in three to five days. When water temperatures are above 90 degrees Fahrenheit, developmental abnormalities occur. If the water temperature falls below 59 degrees, development stops. Therefore, the water needs to be between 60 and 90 degrees for normal development.

Bullfrogs can jump 10 times their body length to ambush prey. (Photo: Richard George)

Young tadpoles prefer living in shallow water, but as they grow, they move into deeper water. The tadpoles reach about an inch in length by the end of their first summer. They have three pairs of gills. When they pump water through them, it traps bacteria, algae, grains of pollen and other small particles. As they grow, they ingest larger particles. It takes tadpoles from three months to three years to metamorphose into adult bullfrogs, depending on water temperature (it’s most commonly two years in our region). Bullfrog tadpoles grow extraordinarily large before they metamorphose: up to 6 inches in length!

A female bullfrog has an external eardrum that is about the same size as the eye. A male’s eardrum is much larger than the eye. (Photo: Ann Schlesinger)

Bullfrogs will eat just about any animal smaller than they are. Scientists have found in bullfrog stomachs rodents, lizards, snakes, small birds, spiders and even bats. Their usual food sources are snails, worms and insects, though.

We’ve all probably seen cartoons of bullfrog tongues flicking out to catch prey. Bullfrogs take into account the refraction at the water-air interface to capture submerged prey. They are better at this than other types of frogs.

Bullfrogs are often observed in this classic bullfrog pose – eyes protruding above the water, alert for movement. (Photo: Richard George)

Motion causes a bullfrog to strike. When it sees motion, the bullfrog leaps toward the prey, mouth agape and eyes closed. You may have seen this person get his just desserts when teasing a bullfrog. The sticky tongue torpedoes out, engulfing the prey, while the mouth closes just as the tongue retracts. Bullfrogs stuff large prey into the mouth with their foreleg toes. They capture most mammals underwater, where the mammal struggles for air and then suffocates.

Bullfrogs cannot move their heads from side to side, but they don’t need to; the bulging eyes on top of the skull give them a wide field of vision. (Photo: Joe MacIndewar)

When a bullfrog opens its mouth, its tongue is propelled out like a slingshot because of the force stored up in the tongue and jaw. The speed is much faster than if muscles alone were the only force behind it. A bullfrog completes its strike and retrieval in only 0.07 seconds – five times faster than you can blink.

Many birds, including herons and kingfishers, prey on bullfrogs, but because bullfrog eggs and tadpoles taste terrible, many salamanders and fish leave them alone. When a predator attacks a bullfrog, the bullfrog emits a piercing scream that alerts other bullfrogs to retreat into deeper water.

Predators must look carefully to find this bullfrog camouflaged in its green and brown habitat. (Photo: Benny Albro)

These giants can grow up to 8 inches long and weigh 1.5 pounds. Because they are so large, people sometimes eat bullfrogs. Bullfrog legs look and taste like small chicken drumsticks (I am told). They can be found on local menus at some French and seafood restaurants. It is difficult to raise bullfrogs commercially because they will not eat pelleted feed, and they are cannibalistic. Most bullfrogs are, therefore, captured in the wild, making them an expensive delicacy. You need a fishing license to hunt bullfrogs in Massachusetts.

If you see our largest frog, be amazed that this amphibian, without sharp teeth or claws, is able to consume just about any prey it can stuff into its mouth – no matter how poisonous, toothsome or fast that prey may be.

Male bullfrogs have yellow throats, while females have lighter colored, whiter throats. (Photo: Brian Rusnica)


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Jeanine Farley is an educational writer who has lived in the Boston area for more than 30 years. She enjoys taking photos of our urban wild things.