Female Treefrogs Experience 'Choice Overload' Amid Too Many Mating Calls From Males

To win over a female, a male Cope’s gray treefrog will puff out its throat and release a musical trill of high-pitched vibrations. The competing males crowd together around ponds during the mating season, producing a noisy chorus of calls. Under such conditions, the female frogs can have a hard time choosing their mates.

But researchers weren’t sure whether the background noise or the sheer number of suitors was giving them trouble. To find out, Hemingway and her team brought 60 female frogs, collected from ponds in eastern Tennessee, into the lab and played them various mating calls in a sound chamber. Their results, published in the journal iScience in August, revealed the frogs struggle with the number of options—a conundrum called choice overload—regardless of the presence of background sound.

The work “offers new insight into the cognitive capacity of Cope’s gray treefrogs,” Julianne Pekny, director of conservation and science at the Amphibian and Reptile Conservancy who was not involved with the study, tells Smithsonian magazine. From previous research, the team understood that female Cope’s gray treefrogs prefer males with longer calls. So, they played a long, “target” call alongside some shorter calls meant to distract the females.

When presented with just two choices—one target and one less appealing alternative—the female frogs hopped toward the target call 77 percent of the time. But adding more options gave the females more trouble: With four calls, only about half of the frogs selected the target, and just a quarter of them selected the target when the team played eight calls. The females also took longer to choose a mate when faced with more calls.

Some neglected to pick at all—with two calls playing, 3 percent of the females did not select any mate, and with eight calls, 18 percent did not make a decision. The researchers repeated these trials with background noise played out of an overhead speaker to mimic a chorus of other competing frogs. But they didn’t see an impact on the females’ choices.

The team conducted a “well thought-out experiment,” Luke Larter, a behavioral ecologist at Brown University who was not involved with this paper, tells Smithsonian magazine. The roles of background noise and choice overload “are often not explicitly differentiated,” in discussions about frog mate choice, Larter adds, and “few studies empirically tease these alternatives apart.” He notes, however, that no laboratory study can perfectly capture all the details of mate choice in the wild. Found in parts of the United States and Canada, the Cope’s gray treefrog grows to be roughly two inches long and has bumpy gray or green skin that helps it blend in with tree bark.