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Upper Midwest Environmental Sciences Center

Farm ponds as critical habitats for native amphibians
A Field Guide to Amphibian Larvae and Eggs of Minnesota, Wisconsin, and Iowa
Field guide contents

American Bullfrog Rana catesbeiana

Status: Wisconsin – Common
Minnesota – Range expanding through introductions
Iowa – Increasingly common
American Bullfrog
~16.2 cm total length

field mapAmerican Bullfrogs are the largest frogs in our area, produce the greatest number of eggs, and have very large larvae that usually overwinter at the bottom of ponds and mature after a couple of years in the tadpole stage. They are now found statewide in Iowa because of introductions with stocked fish. Introductions of American Bullfrogs in the western United States have caused the decline of native species (Hayes and Jennings 1986) because adult bullfrogs will eat other frogs, as well as other large prey. American Bullfrogs are at home along permanent bodies of water and are able to sustain populations in the presence of fish predators. American Bullfrogs and Green Frogs have either skin toxins or behavioral defenses against larval predation (Kats et al. 1988; Werner and McPeek 1994).

American Bullfrogs lay their eggs in large (may exceed 1 m diameter), flat (about one egg deep) floating rafts in June or July. The eggs are small (0.12–0.17 mm in diameter) with up to 20,000 eggs in a mass. The only other frog with a large floating mass is the Green Frog, but this species has smaller eggs (0.10–0.15 mm diameter) and fewer of them (< 5,000 eggs) in the mass. American Bullfrogs have only one weak jelly layer; Green Frog eggs have two jelly layers per egg. American Bullfrog tadpoles get quite large (16 cm or larger) and are greenish brown with dorsal eyes. Intestines are visible in small tadpoles, but the belly becomes opaque in larger larvae. American Bullfrogs have distinct black spots on the dorsum of the body and on the tail, these spots are concentrated on the dorsal half of the tail. The spots are round and distinct, whereas the Green Frog has fuzzier, less distinct spotting.

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Page Last Modified: December 29, 2010