American bullfrog (Rana catesbeiana)

American Bullfrog (Rana catesbeiana), female
Photo © by Dave Huth, some rights reserved. Click image for licensing information.

American Bullfrog (Rana catesbeiana), female - tympanic membrane and skin fold
Photo © by Dave Huth, some rights reserved. Click image for licensing information.

American Bullfrog (Rana catesbeiana), female
Photo © by Dave Huth, some rights reserved. Click image for licensing information.

American Bullfrog (Rana catesbeiana), female - profile
Photo © by Dave Huth, some rights reserved. Click image for licensing information.

American Bullfrog (Rana catesbeiana) - female in pond shallows
Photo © by Dave Huth, some rights reserved. Click image for licensing information.

Bullfrog
Photo © by Dave Huth, some rights reserved. Click image for licensing information.

Bullfrog (Rana catesbeiana)
Photo © by Dave Huth, some rights reserved. Click image for licensing information.

American Bullfrog, tadpole stage (Rana catesbeiana)

Photo © by Dave Huth, some rights reserved. Click image for licensing information.

Description

This frog is the largest in North America and is distinguished by lacking dorsolateral folds and having very large tympanums, larger than the eye in males. The tips of the fingers and toes are blunt. The webbing is well developed. The skin on the back of this species is rough with random tiny tubercles. There is no dorsolateral fold, but there is a prominent supratympanic fold. The mean snout to vent length for males is 152 mm (range 111-178) and for females it is 162 mm (range 120-183). The males have pigmented nuptial pads. The vocal openings are at the corner of the mouth.

The dorsum is green, with or without a netlike pattern of gray or brown on top. The venter is slightly white, sometimes mottled with gray or yellow. Coloration varies widely depending on the locality of the bullfrog (Conant and Collins 1975).

Distribution and Habitat

R. catesbeiana is widely distributed in eastern North America, ranging from Nova Scotia to central Florida and west to eastern Wyoming, Colorado and New Mexico. It occurs throughout most of Texas and into northwestern Mexico. It has been widely introduced for a variety of purposes, and is now common in many parts of western North America and many other countries, including those in Europe, Asia and South America (e.g., see Lever 2003). Rana catesbeiana is strongly aquatic, and can be found primarily at the edges of lakes, marshes, or cypress bays (Conant and Collins 1975).

Life History, Abundance, Activity, and Special Behaviors

The breeding season begins in spring and lasts throughout early summer, but can vary according to latitude. Bullfrogs breed on the surface of shallow, permanent water covered with vegetation. Males make distinctive resonant low-pitched calls with a single note that lasts 0.8 seconds at a frequency of 1.0 kHz. Males also display aggressive territorial behavior in defending good oviposition sites. One clutch consists of up to 20,000 eggs and one quarter of the female’s body weight. Duration of the larval stage varies greatly depending on the temperature. Metamorphosis is not synchronized.

Bullfrogs are often the predominant species in interspecific relationships, contributing to the decline of other amphibians and excluding them from the habitat. Bullfrog juveniles are adept at colonizing new ponds, and they are believed to disperse throughout an environment this way.
Bullfrogs are opportunistic predators, and prey on any animal smaller than themselves. While smaller bullfrogs eat mostly insects, larger bullfrogs consume aquatic species such as fish and crayfish, mice, and other frogs (for a video, see the account on Bufo californicus). Cannibalism is prevalent in a bullfrog’s diet, sometimes comprising up to 80% of its food.

Life History, Abundance, Activity, and Special Behaviors

Introduced populations present great threats to native frogs, due to the bullfrog’s voracious feeding habits and the size and competitive ability of the larvae. Although aquatic species and frogs constitute a major portion of its diet, other native species are also likely affected because bullfrogs have been reported to eat snake, birds, and small mammals as well. Furthermore, as bullfrogs are being introduced worldwide, they serve as carriers of the pathogenic fungusBatrachochytrium dendrobatidis (chytrid), which causes the lethal disease chytridiomycosis, believed to be a major factor in recent global amphibian declines (e.g., Garner et al. 2006 found that bullfrogs were consistently chytrid-infected in multiple countries). Infected bullfrogs appear to be rather resistant to chytridiomycosis, whereas the disease is lethal to many other amphibians, making the bullfrog an efficient carrier of the chytrid fungus (Daszak et al. 2004).

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