The American Bullfrog (Lithobates catesbeianus) is the largest true frog in North America, known for its imposing size and impressive voice. Its characteristic sound is a deep, resonant bellow, often described as a “jug-o-rum.” This powerful vocalization is a defining feature of wetlands and ponds during warmer months. The bullfrog’s croak serves as a primary method of communication, dictating social structures and reproductive success.
The Anatomy of Vocalization
Producing such a loud, low-pitched sound relies on a specialized biological structure. Frogs generate calls by pushing air from the lungs over the vocal cords, located in the larynx, or voice box. Unlike humans, who rely on a diaphragm, bullfrogs use a positive pressure system, coordinating movements of the throat and glottis to cycle air.
The air is forced back and forth between the lungs and a large, inflatable sac located under the throat. This vocal sac acts as a highly efficient resonator, amplifying the sound vibrations created by the vocal cords. The bullfrog’s large size, with males exhibiting a yellow throat sac, allows for the low frequency and high volume of their call. This anatomical setup enables the “jug-o-rum” to carry for long distances, sometimes over half a mile, especially over water.
Primary Function: Advertisement Calls for Mating
The most common and recognizable vocalization is the advertisement call, performed almost exclusively by males during the breeding season, which typically runs from late spring through summer. This loud, low-pitched bellow is primarily a sexual signal, serving the dual purpose of attracting females and establishing the male’s presence. The calls are often made in a “chorus,” where multiple males call simultaneously near permanent bodies of water.
The quality and intensity of a male’s croak provides information to listening females. Females assess potential mates by listening for a deeper or more powerful call, which indicates a larger body size. Since size correlates with age, experience, and the ability to defend a territory, the female uses this acoustic information to select a male with high fitness. The advertisement call acts as a filter for mate selection, ensuring reproductive effort is directed toward the most robust males. By calling from a desirable location, the male also advertises the quality of his territory as a suitable place for the female to lay her eggs.
Other Calls: Establishing Dominance and Defense
While the advertisement croak is for long-distance communication, bullfrogs employ other vocalizations for closer-range social interactions. Males use shorter, harsher sounds, known as territorial or encounter calls, to warn off rival males who approach their calling site. These aggressive calls establish a dominance hierarchy, often preventing physical conflict over prime breeding spots. If the warning is ignored, the calls may be followed by a physical chase or wrestling.
Another distinct call is the high-pitched release call, used to correct mating errors. If one male mistakenly grabs or mounts another male, the grabbed frog emits a sharp, brief squeak or chuckle. This signal identifies the frog as male and not receptive to amplexus, prompting the other frog to release it. Juveniles and stressed frogs may also produce a fast, high-pitched squeak or alarm call before leaping into the water to escape a perceived threat.