Much like an indecisive single person on a dating app, female treefrogs struggle to pick the most desirable mates when they have too many options to choose from. According to a new study publishing August 12 in the Cell Press journal iScience, when female Cope's gray treefrogs gather around ponds during mating season to listen to large groups of males chirping in noisy "choruses," they struggle to pick the most desirable male. The researchers call this phenomenon "choice overload," and it may impact the way species evolve over time.
"There are many animals that are confronted with a lot of different options simultaneously and have to compare them," says first author Claire Hemingway of the University of Tennessee, Knoxville.
Cope's gray treefrogs (Hyla chrysoscelis) live across wide swaths of the United States, ranging from Virginia, west to Texas, and north into Canada. They can reach about 2 inches long and have bumpy gray or green skin that resembles tree bark. During mating season, males serenade females by puffing up their throats and emitting a series of high-pitched vibrations.
"You may encounter them coming up to your window at night to eat the insects that fly toward the light," says lead author Jessie Tanner of the University of Tennessee, Knoxville.
To study how female treefrogs choose the best mates, the researchers built a dating arena fit for amphibians: a 2-meter-wide enclosure in a soundproof, temperature-controlled room. The team placed female frogs one by one in the middle of that enclosure. Then, the serenade began: a series of speakers spread around the enclosure played recordings of real male frogs calling. Females could choose which call they preferred by hopping toward a speaker. Because previous research has shown that female treefrogs tend to prefer males with longer calls, the team mostly played these longer "target" calls, as well as one or more "distractor" calls that were much shorter.
The researchers observed that more male calls led to more confusion for the females. When females had just two options to choose from—one target and one distractor, for example—they hopped toward the longer target call 77% of the time. When females had to listen to eight different males' calls, they selected the target call just 25% of the time. In some cases, the females got so overwhelmed that they didn't make any choice at all.
Were the females simply having trouble hearing individual males, akin to what happens when you try to have a conversation in a crowded restaurant? The researchers don't think so. To test the idea, they ran an identical experiment but added in background noise—the sound of numerous male calls overlapping—which played from an overhead speaker. The noisy environment didn't have much effect on how females chose mates, suggesting that they were instead struggling with the number of options available.
Choice overload could shape the genetics of animal species in profound ways, say the researchers. Evolutionary theory, for example, suggests that if females have strong preferences for males with long calls, males with short chirps should disappear from populations over time. But that hasn't happened yet.
The new study offers a possible explanation. Choice overload could be keeping the males with short calls in the gene pool by causing enough confusion that the females choose a mate who is, in theory, less optimal.
"The decisions that females make are really biologically important," says Tanner. "By choosing a male with a good-sounding call, the females can get genetic benefits for their offspring. But maybe they're not able to exercise that choice effectively all the time."
