An international study led by the University of Tennessee and in collaboration with Tel Aviv University finds that noise and selection pressure make it difficult for female toads to choose mates – a phenomenon that may slow the pace of evolution.
Female toads generally prefer a mate with a faster, more consistent call, but when there are multiple calls in the chorus, they have difficulty finding the right call, according to a new international study led by Dr. Jessie Tanner, an assistant professor at the University of Tennessee, Knoxville.
When given a choice between just two males, female gray toads almost always choose the male with the faster, more consistent calls. But when they are forced to choose between four or eight different call types, their choices become inconsistent, according to a recent paper published in the Proceedings of the Royal Society B: Biological Sciences.
Noisy choirs in nature
In the wild, toads choose mates in crowded, noisy environments—“choruses”—where many males call at the same time. “Our study suggests that female toads may not get what they want when they choose the natural, noisy environment of a choir,” explained Tanner, a faculty member in the departments of Ecology and Evolutionary Biology and Psychology and Neuroscience. She collaborated on the study with researchers from Colorado State University, the University of Minnesota, and Tel Aviv University.
She said, "Since female mate choice can drive evolutionary processes, our findings suggest that evolution may occur at a slower rate than previously thought. Males who are not particularly attractive can still succeed in reproducing – especially if they take advantage of crowded environments, where it is more difficult for females to distinguish between attractive and unattractive males."
Choice Overload
The study was designed to investigate the phenomenon of “choice overload” – a condition in which multiple options impair the ability to make a decision. In humans, multiple types of toothpaste in a supermarket or profiles on a dating app can lead to poorer choices, delaying or even postponing a decision.
"We wanted to understand how decision-making in the courtship arena, in a noisy, crowded environment, might influence the evolution of male calls," Tanner said. "Since males chosen by females pass on their genes to the next generation, female choice could change toad populations over time."
Expanding research into animal behavior
Tanner is also researching the phenomenon with Dr. Claire Hemingway as part of a collaboration at CoLAB (Collaborative for Animal Behavior) at the University of Tennessee, which led to another publication in Trends in Cognitive Sciences.
"Our lab is investigating how selection load and noise may make it harder for female toads to make courtship decisions," she said. "We are also trying to understand whether selection load is a general phenomenon that occurs across many species, or whether it is unique to certain species."
Tanner is collaborating with Hemingway on a study of selection pressure in bumblebees, and both are working with Professor Todd Friberg to understand whether wood cockroaches experience similar phenomena.
Tanner began her career as a behavioral researcher in mate selection as an undergraduate, while also becoming interested in acoustic phenomena—out of a love of language and music. In postdoctoral research, she led a study on how noise affects mate selection in crickets, which also use vocal signals for courtship.
“During that study, it was clear that our work was consistent with two possibilities: one, that multiple calls at once create noise that makes it difficult for females to hear, and the other, that females experience choice overload,” she said. “The current study seeks to separate these two possibilities.”
Additional sources:
- Jessie C. Tanner et al, Complex choice environments shelter unattractive signallers from sexual selection, Proceedings of the Royal Society B: Biological Sciences (2025). DOI: 10.1098/rspb.2025.0585
- Jessie C. Tanner et al, Choice overload and its consequences for animal decision-making, Trends in Cognitive Sciences (2025). DOI: 10.1016/j.tics.2025.01.003
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