Washington, DC--We all know a "good-looker" when we see one. But why do some traits, "say, large eyes and broad shoulders" seem attractive, as opposed to nose hair and a paunch? According to University of Texas zoologist Michael J. Ryan, the answer may be that mate choice is colored by perceptual biases hard-wired into an animal's sensory system for reasons other than sexual thrill. Beauty, that is, may be literally in the eye of the beholder.

Ryan's article is part of a special section on the evolution of sex that appears in the 25 September issue of Science. He discusses a growing body of evidence in support of the "preexisting preferences" or "sensory exploitation" theory of sexual selection. This model emphasizes that animals choose mates using sensory systems that primarily have been shaped to meet the daily demands of survival, foraging for food, or example, or avoiding predators. According to this argument, animals looking for potential mates do so with a built-in "receiver bias" toward traits that may not have anything to do with their mate's genetic fitness.

For example, water mites feed on tiny organisms called copepods, which the mites locate by sensing their prey's water-borne vibrations. Males of at least one species of water mite mimic copepod vibrations as part of their mating display. As it turns out, the mites' smooth moves do indeed boost their chances of attracting a mate. Another example can be found among moths that have evolved the ability to detect the ultrasonic echolocation calls of bats, a key predator. A subset of this group, more recently evolved, can also produce calls in the bats' own frequency, essentially jamming their predators' radar. Even further along the family tree are moths that have adapted this sensory channel for communication between the sexes, males and females who woo each other in ultrasonic displays of affection.

To Ryan and others, the theory's main strength is its simple logic. "Any sensory system is goi
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Contact: Diane Dondershine
ddonders@aaas.org
(202) 326-6421
American Association for the Advancement of Science
24-Sep-1998