Like a personal ad proclaiming: "Tall, good looking, disease-free," brightly colored male animals are advertising something of importance to their prospective mates. Should the female assume the gaudiest male has parasite-resistance genes that will benefit her offspring? Or that she simply won't pick up bugs from the guy? A Cornell University biologist's analysis of mating-success studies, as reported in the March 5, 1996, Proceedings of the National Academy of Sciences (Vol. 93, pp. 2229-2233), suggests that the direct benefit -- avoiding parasites for herself and her young ones -- may have a role in the evolution of male flamboyance.

"Accounting for conspicuous male displays -- the elk with the huge rack of antlers or the peacock with the bright train of feathers -- was a problem for Darwin's theory of evolution by natural selection," said David J. Able, a Cornell doctoral student in animal behavior and sole author of the PNAS article, "The contagion indicator hypothesis for parasite- mediated sexual selection." Darwin recognized that characteristics that make animals a target for predators also reduce survivability and reproduction, Able noted.

Darwin's rationale, that conspicuous male displays persist and grow ever more conspicuous because mating females prefer them, never fully explained the benefit of choosing the brightest or loudest. However, in the last 15 years, evolutionary biologists began to focus on debilitating parasite infections and inherited resistance to infection. For example, red jungle fowl (the ancestors of barnyard chickens) have glossy feathers and bright eyes when they are healthy but dull plumage and glassy eyes when certain parasites infest them. Experiments show that the brightest jungle fowl males have the greatest mating success.

In his analysis, Able examined previous studies of parasite transmittability in 15 species with conspicuous male displays. Fourteen studies were conducted by others and ranged from ring-necked pheasants and zebra finches to fence lizards and guppies; one study was his own, with red-spotted newts. His species-by-species analysis questioned whether the parasites are transmitted by association (during mating or by frequenting the male's territory), the level of parasite infection and the conspicuousness of the male trait, and whether the potential for parasite transmission is related to male mating success.

When the analysis was complete, six of eight species with associatively transmittable parasites showed a negative correlation between parasite intensity and male mating success. In other words, males with the fewest transmittable parasites have the most conspicuous traits and the best luck. And in species where parasites are not transmitted by association (including Able's newts, which get blood parasites from leech bites and not directly from each other), the number of parasites tended not to affect female choice or the male traits.

"This is far from conclusive, and it doesn't explain everything about conspicuous male displays," Able acknowledged. "But it is beginning to look like, when a female selects a flamboyant male, avoiding contagion may be more important than getting good genes."

A graduate of Indiana University with a degree in biology, Able has been conducting his research and teaching at Cornell for seven years with teaching assistantships in introductory biology, animal behavior and animal social behavior. Research reported in the PNAS paper was supported in part by a National Institutes of Health Neurobiology and Behavior Training Grant and U.S. Department of Agriculture (Hatch) Funds.