Female cognitive ability can limit how melodious or handsome males become over evolutionary time, biologists from the University of Texas at Austin, Louisiana State University Health Sciences Center and the Smithsonian Tropical Research Institute have observed.
Studying neotropical tungara frogs, the scientists found that females lose their ability to detect differences in male mating calls as the calls become more and more elaborate.
"We have shown the female tungara frog brains have evolved to process some kinds of information and not others," said Mike Ryan, professor or integrative biology at the University of Texas at Austin, "and that this limits the evolution of those signals."
The group of researchers provided an example: Imagine looking at a group of five oranges next to a group of six. At a glance, you would quickly notice that one group has one more orange than the other. Now, imagine looking at a pole of 100 oranges next to a pile of 101. It would be almost impossible to distinguish the difference in size (one orange) between those two piles at a glance. This is known as Weber's Law, which states that stimuli are based on proportional differences rather than absolute differences.
In a new study, published in the latest issue of the journal Science, tungara male frogs gather en masse to attract female frogs with a call that is made up of a longer "whine' followed by one or more short "chucks."
Through a series of experiments conducted in Panama, Ryan and his collaborators found that females prefer male calls with the most chucks, but their preference was based on the ratio of the number of the chucks. As makes elaborate their call by adding more chucks, their relative increase in attractiveness decreases due to a perceptual constraint on the part of females.
But the male tungara frog calls attract more than just their female counterparts. They also attract a predator: the frog-eating, fringe-lipped bat. So, the researchers studied how the bats responded to the additional "chucks" in the male call, discovering that hunting bats choose their prey based on chuck number ratio, just as the female frogs do.
By elaborating their call by adding chucks, the relative increase in predation risk decreases with each additional chuck.
"What this tells us is that predation risk is unlikely to limit male call evolution," said Karin Akre, a lecturer at the University of Texas at Austin. "Instead, it is the females' cognition that limits the evolution of increasing chuck number."