Female Tree Frogs Experience "Choice Overload" When Too Many Males Are Calling

They could also alter the background noise to see if the females found it harder to make a choice when there was more going on. Previous work found that females prefer males with longer calls, so these calls became a crucial part of the experiment. 

“This experiment was conducted with wild treefrogs that we brought back to the lab,” Dr Claire Hemingway, Assistant Professor at the University of Tennessee and study author, told IFlScience.

“We collected the females (and males) in amplexus from several field sites, so all individuals had prior experience in this context. That said, prior experience does not seem to be particularly informative for guiding mate choice decisions in this system.”

In typical tree frog mating, the female must listen, process, and compare multiple male tree frog calls before making her decision. However, this gets considerably harder when multiple males are all calling at once, as their calls overlap. 

For each experiment, the team gave the female frogs a choice based on call duration. Normally, the females prefer males with longer calls, but in the experiment the females had the choice between one long “target” call and one, three, or seven shorter “distraction” calls. 

Choice paralysis

The results show that the females were less likely to choose the target caller as the number of shorter options increased, even if the longer call was still chosen more often than expected by chance. 

“One of the most striking findings was the relative importance of the number of choices compared with background noise,” explained Hemmingway. “We expected both factors to influence female decision-making, and we anticipated that they might interact in complex ways.” 

“Instead, the overwhelming pattern was that the number of options had the strongest effect on performance. As the number of potential mates increased, females became less accurate, took longer to make decisions, and were more likely to defer choosing.”

The results showed that the females took longer to decide when more males were available, increasing from 114.4 seconds in the two-choice trials to 128 in four-choice, and 162 in the eight-choice trials. Interestingly, in the noisy conditions females chose faster in the two- and four-choice trials but took longer in the eight.

Too much of a good thing

As Hemmingway points out, this could have a knock-on impact on the success of female frogs. 

“From the female’s perspective, choice overload could matter because mate choice can have important fitness consequences,” said Hemmingway. 

“Females gain genetic benefits for their offspring by choosing males with more attractive calls, so anything that interferes with their ability to discriminate among potential mates could potentially affect those benefits.”

Overall, the results suggested that more choice might be a bad thing, since the females took longer to decide, were less likely to choose the target signal, and became more likely not to decide at all. 

The experiments with the background trials showed that the females decided faster and were slightly less likely to pick the target with fewer options. 

“Animal decision-making is shaped not just by the quality of the options available, but also by the number of options and the sensory environment in which those decisions must be made,” said Hemmingway. 

“Animals, like humans, have finite cognitive and perceptual resources, and those constraints can become particularly important in complex environments. We’re excited to continue investigating how these constraints operate in ecologically relevant ways that reflect the complex, realistic environments animals encounter.”

The paper is published in iScience.