The bullfrog is Missouri’s largest frog, known to reach 8 inches from snout to vent. The general color ranges from green to olive to brown. The back may have small brown spots or indistinct, irregular, dark blotches. The hind legs are marked with distinct dark brown bars. The belly is white, and the throat may have some gray mottling. The external eardrum is large and round. On adult males this tympanum is much larger than the eye; on females, it’s about the same size as the eye, or smaller. Bullfrogs lack a dorsolateral fold (which is a glandular fold or ridge of skin along each side of the back; in other members of the genus, the dorsolateral fold is common).
The call of a male bullfrog is a deep sonorous “ger-a-a-rum” or “jug-o-rum,” which can be heard a half mile away (or more). In the summer, males call during the day and often right after sunset, but the most intense vocalization takes place between midnight and three or four in the morning during the breeding season.
Similar species: Missouri has eight members of the true frog family. These are typically medium- to large-sized, have long legs, smooth skin, and well-developed webbing between the hind toes. Another common characteristic is a glandular fold or ridge of skin along each side of the back (these are called dorsolateral folds). The American bullfrog, the largest member of the genus in the United States, does not have these ridges of skin.
Adult length (snout to vent): 3½ to 6 inches; occasionally to 8¾ inches.
Habitat and Conservation
The bullfrog is Missouri's most aquatic species of frog. Individuals spend most of their time in or very near lakes, ponds, rivers, large creeks, drainage ditches, sloughs, and permanent swamps or marshes. Because bullfrogs require permanent water, their populations can decline during drought years.
When disturbed, bullfrogs escape by powerful jumps into the water. Young bullfrogs will often give out a short, high-pitched yelp as they leap to safety.
Bullfrogs sometimes move overland during heavy summer rains and can be seen crossing roads.
Bullfrogs burrow into the mud at the bottom of rivers or ponds to avoid winter temperatures. Adults usually enter winter retreats in late October; young bullfrogs follow a week or two later. In spring, young bullfrogs generally emerge from their retreats during the second half of March, and larger individuals appear about three weeks later.
Bullfrogs will try to eat just about any moving creature they can get their mouths around. The size and age of the frog, the season, and the type of habitat influence their diet. Frogs in farm ponds have different foods available than ones in impoundments or in rivers. In general, foods include insects, spiders, crayfish, fish, amphibians, birds, and even small mammals. Bullfrogs commonly eat other frogs, and they don’t hesitate to eat their own kind.
Bullfrogs take advantage of the most abundant prey in a given habitat and do not have a preference for one particular prey species. Bullfrogs are opportunistic feeders, and generally speaking, their voracious appetite and size allow them to eat nearly any animal small enough to be captured and swallowed.
The American bullfrog is classified as a game animal in Missouri and is protected by a season and bag limit. Consult the Wildlife Code of Missouri for regulations.
Taxonomy: The true frog family (Ranidae) is the largest and most widespread family of frogs. It contains 365 species in 14 genera and probably originated in Africa. Representatives of this cosmopolitan family occur on every major land mass except New Zealand, Antarctica, most oceanic islands, the West Indies, and southern South America. The largest genus in the family in the New World (North and South America) is Lithobates (formerly Rana), with about 50 species. Missouri’s species, formerly in genus Rana, are all in genus Lithobates. As of taxonomic understandings in 2016, the Rana genus is considered restricted to the eastern hemisphere and western North America. In Missouri, the genus Lithobates is represented by eight species.
In Missouri, bullfrogs are active from the middle of March to October. Males begin calling in early April, with large choruses being heard by mid-May into July. Breeding is in late-May to the end of August, sometimes into September; it generally peaks in late June. Calling is most intense between midnight and early morning (before sunrise). Males are highly territorial and physically aggressive to each other as they defend calling stations.
Eggs are laid in shallow water in a wide, floating mass. Females can lay over 20,000 eggs per clutch. Clutches are usually between 10,000 and 12,000 eggs. These hatch in 4–5 days. Tadpoles grow quickly and turn into froglets in about 11–14 months, usually from June through August in Missouri. Adult size isn’t reached for another 2–3 years. Lifespan may be slightly over 7 years.
The bullfrog is the official Missouri State Amphibian. It is also the state amphibian of Ohio and Oklahoma.
The legs are edible and considered by many to be a delicacy. Bullfrogs are classified as game animals in Missouri and are protected by a season and bag limit under the state’s Wildlife Code.
Bullfrogs are named because their calls were likened to the sound of a bellowing bull, plus no doubt for their large size.
Bullfrogs are a widespread, vigorous species, and their biology, behavior, and genetics are well studied.
Bullfrogs are often used in science classes as a model vertebrate animal for dissection.
Bullfrogs as weather omens? In the early 20th century, folklorist Vance Randolph collected a multitude of "weather signs" from old-time Ozark residents. These people had grown up in rural areas when meteorological science was undeveloped by today's standards. They looked to the animals and plants around them for signs that it was about to rain, snow, or be windy or sunny. Among the many folklore omens Randolph recorded was an observation by bullfrog hunters that bullfrogs' skin would darken about twelve hours before a rain.
Although these large, powerful amphibians will gulp down practically any creature that will fit in their mouths, they also become food for many other organisms. Even if female bullfrogs lay about 20,000 eggs, most of the eggs, tadpoles, and froglets are chomped by other animals.
Among the animals known to eat adult bullfrogs, their tadpoles, or both are other bullfrogs, wading birds such as herons, river otters, turtles, watersnakes, gartersnakes, and giant water bugs and fishing spiders (Dolomedes spp.). Apparently, bullfrog tadpoles are toxic and get too large for most fish to eat.
Bullfrog tadpoles, newly transformed froglets, and adult bullfrogs apparently are resistant to the venom of copperheads and, to a lesser extent, of cottonmouths.
Although bullfrogs are native to Missouri, this species has been introduced into many western states and countries around the world. In addition, bullfrogs appear to be resistant to lethal infectious diseases (such as amphibian chytrid fungus) but are often carriers of diseases infecting other amphibians.
Due to their voracious appetites and ability to spread infectious diseases, they are a threat to many other amphibian species and are considered an invasive species in many parts of their current range. One method for controlling them in places where they are invasive is to introduce genetically sterile males into the population.