Too Many Suitors Leave Female Treefrogs Struggling to Choose

During the breeding season, male Cope’s gray treefrogs gather around ponds and advertise with high-pitched musical calls. For females searching for a mate, the resulting chorus may create a problem that has less to do with noise than with the sheer number of available suitors. Researchers tested that possibility by bringing 60 female frogs collected from ponds in eastern Tennessee into a laboratory sound chamber. The team played combinations of mating calls, including a longer “target” call that females are known to prefer and shorter calls intended as less appealing alternatives. When the frogs heard two choices, they moved toward the preferred call 77 percent of the time. Their performance declined as more calls were added. With four calls playing, roughly half selected the target. When eight calls were presented, only about a quarter did so. The number of options also affected how quickly the frogs made decisions. Three percent failed to choose when given two calls, compared with 18 percent when eight calls were playing. The researchers repeated the trials while adding overhead background noise designed to imitate a chorus of competing frogs. According to the study, the noise did not alter the females’ choices. Instead, their performance declined when more calls were presented, whether or not background sound was present. The findings, published in iScience in August and reported by Smithsonian Magazine, point to “choice overload” as a factor in the frogs’ mating decisions. Julianne Pekny of the Amphibian and Reptile Conservancy, who was not involved in the study, said the work offers insight into the cognitive abilities of Cope’s gray treefrogs. Another outside expert, Brown University behavioral ecologist Luke Larter, described the experiment as well designed but noted that laboratory tests cannot reproduce every detail of mate choice in the wild. The results could also have implications for evolution. Females’ preference for longer calls might be expected to reduce the number of short-calling males over time. But if females are less able to identify preferred calls when many options are available, less desirable males may still find opportunities to mate. The researchers suggest that similar decision-making limits could occur in other animals that choose among many potential mates or resources.