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.
Males across the animal world have evolved elaborate traits to
attract females, from huge peacock tails to complex bird songs and
frog calls. But what keeps them from getting more colorful
feathers, longer tails, or more melodious songs? Predators, for
one. Increased elaboration can draw predators in, placing an
enormous cost to males with these sexy traits.
In a new paper appearing this week in Science, a group of
biologists have shown that females themselves can also limit the
evolution of increased elaboration.
Studying neotropical túngara frogs, they found that females lose
their ability to detect differences in male mating calls as the
calls become more elaborate.
"We have shown that the female túngara frog brains have evolved to
process some kinds of information and not others," says Mike Ryan,
professor of integrative biology at The University of Texas at
Austin, "and that this limits the evolution of those signals."
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 pile of 100
oranges next to a pile of 101. It would be nearly impossible for
you to notice 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 compared based on proportional differences rather than
absolute differences (one orange in the case above).
In túngara frogs, males 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 chucks. As males elaborate their call by adding more chucks,
their relative increase in attractiveness decreases due to a
perceptual constraint on the part of females.
Male túngara frog calls also attract a predator: the frog eating
fringe-lipped bat. To confirm that male song elaboration wasn't
limited by these predators, the researchers also studied how the
bats respond to additional "chucks" in the male call.
They discovered that hunting bats choose their prey based on chuck
number ratio, just as the female frogs do. So, as males elaborate
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," says Karin Akre, lecturer at The University
of Texas at Austin. "Instead, it is the females' cognition that
limits the evolution of increasing chuck number."
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Thanks to University of Texas at Austin for this article.