Amphibians and Reptiles of Minnesota

A Complete On-line Field Guide to Minnesota's Amphibians and Reptiles

Bullfrog - Lithobates catesbeiana PDF Print E-mail
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Bullfrog, Rana catesbeiana, from Houston County, MN


 

 

 

 

 

 

 

Synonyms: American Bullfrog.

Status

Bullfrogs were listed as a species of Special Concern due to their limited natural range in Minnesota, however, natural populations have shown little fluctuation and there are now introduced populations. Bullfrogs now have no status in Minnesota.

Description

The bullfrog is Minnesota's largest frog species reaching 8 inches in body length and over a foot long with legs outstretched. The bullfrog's skin color can be anywhere from light green to dark olive to nearly brown. The shade of color may vary in any individual at different temperatures. Typically, darkness or cool weather will cause a frog to lighten up. Some bullfrogs may not have a dorsal pattern, but others may be heavily reticulated with dark brown or black. There may be dark bars on the front and back legs. Ventrally, they may be white, gray, or yellowish. Some may have plain bellies, but most of them have gray or brown reticulation networking from the sides to the center. There are markings on the undersides of the hind lles and young, but some big males develop a very bright yellow throat.

Further, and more reliable, sexual dimorphism is displayed by the size of the tympanum (the tympanum is the circular disk just behind the eyes; this is the eardrum). Males have tympanums larger than their eyes. The tympanums of females are the same size or slightly smaller then their eyes. Bullfrogs do not have dorsolateral ridges. The hind foot is extensively webbed except for the last digit on the fourth toe. The size of the adult bullfrog and the absence of dorsolateral ridges distinguish it from the very similar green frog.

Subspecies

There are no subspecies of Lithobates catesbeiana recognized. This species was formerly placed in the genus Rana.


Range

The natural range of the bullfrog in Minnesota is only two counties in the southeast, along the Mississippi River in Winona and Houston counties. There are introduced populations in nearly a dozen other counties and will probably spread to more in the future. These are not centralized introductions; they are scattered about the central and south central part of the state. Introductions are the result of humans releasing adults or tadpoles because they have used them for bait or stocked them in backyard ponds, etc. Introduced populations are very destructive - they compete with other frog species for natural resources, are "carriers" of chytrid fungus (they are resistant to chytrid but can spread it), and even outright eat other frogs!

Habitat

Bullfrogs require permanent bodies of water in which to breed and live. Lakes, ponds, oxbows, Mississippi River backwaters, and sometimes slow parts of large rivers are favorite haunts.

Habits

Bullfrogs are active in early May, but they do not breed until June through July. The breeding calls of males are often described as a deep "jug-o-rum" A deep "rum" seems more appropriate. The call is a "rum" given once every second and usually repeated consecutively five or six times. Sometimes, the last few calls stutter. Even large bullfrog choruses do not have many frogs, and the calls rarely overlap as extensively as in nearly all other Minnesota frogs. This is partly because a male bullfrog will establish and defend a large nine foot radius territory.

Female bullfrogs lay 10,000 to 20,000 eggs in large masses. The tadpoles take two or three years to transform. They are the largest tadpoles found in Minnesota. They are green with dark spotting on the body and tail. The underside is white or yellow. Bullfrogs are active day and night, although movement along water courses usually occurs at night and overland movements often take place on rainy nights. Otherwise, they usually do not move far, staying in or close to their home pond. When approached, they may slip into the water and swim away silently, but more often they make a fantastic leap. They usually emit a loud high-pitched "meap" and make several jumps over the surface of the water before diving and remaining submerged for several minutes. If picked up, rare individuals will "scream" with an open mouth. Adults and tadpoles alike overwinter at the bottom of their home lake or pond. The adults are inactive at this time, but the tadpoles may swim and even feed under the ice.

Food

Bullfrogs are voracious and eat anything they can fit into their mouths. Aside from smaller "normal" prey, they have been known to eat other frogs and toads, tadpoles, fish, small snakes, and turtles, even small birds and rodents. Anglers and "froggers" use this appetite to their advantage. A hook baited with almost anything (red plastic worms or red cloth works well) is dangled in front of them and they are caught like fish. A valid fishing license is required to take frogs in Minnesota. Introduced populations are destructive - see "Range" section above.

 
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