Without words, bullfrogs communicate through stutters in their croaks

Male bullfrogs communicate with other bullfrogs through calls made up of a series of croaks, some of which contain stutters, according to a new Brown University study which describes a pattern not previously identified in scientific literature.

Researchers recorded 2,536 calls from 32 male bullfrogs in natural chorus and analyzed the number of croaks in each call and the number of stutters in each croak. It is known that the male bullfrog’s call attracts females for mating, maintains territorial boundaries with other males, and indicates that the frog is healthy and aggressive.

“Some animals have evolved large, complex vocabularies to communicate, while others say a lot with very limited numbers of calls,” said Andrea Simmons, professor of psychology, who presented the findings at 75th meeting of the Acoustical Society of America Monday, May 24, 2004. “A fundamental question in the study of communication by sound is ‘how much information can a sender convey in a single sound’?”

Within a single vocalization, the frogs exhibited a pattern of croaks with and without stutters that appeared to have a communication function and did not simply represent that a male was getting tired, Simmons said.

An acoustic analysis showed the stutters followed certain rules: 100 percent of the recorded calls began with a croak containing no stutters; when the frogs started stuttering they generally did so within a croak that contained one stutter only; when they increased or decreased stutters from croak-to-croak, they did so by only a single stutter.

Stuttering did not occur because a frog was “running out of breath,” said the researchers. If that were the case, a less structured pattern of stutters would occur. More likely, the frog inserted stutters in the call to extend the length of his individual calls while reducing the amount of air exchange needed, similar to what occurs when opera singers insert vibrato in extended notes.

To determine how the frog’s calls were perceived, researchers played pre-recorded stuttering and non-stuttering calls through a loudspeaker to individual males. The frogs appeared to use non-stuttered calls for aggressive or territorial purposes. Males produced the stuttered calls more frequently at certain points during the breeding season, indicating the stuttering may be involved in attracting a mate.

Simmons conducted the study with Dianne Suggs, a graduating senior in psychology, who used it as the basis of her undergraduate honors thesis. Suggs was supported by a grant through the Undergraduate Teaching and Research Assistantship (UTRA) program. Simmons’ laboratory is supported by a research grant from the National Institutes of Health.