Best males have big wings, small bodies.

Impatient for procreation, some male Heliconius butterflies attempt to claim a mate before she has even emerged from her cocoon. The male settles on a female's cocoon, often jamming its rear end into the casing to stabilize its perch. Rivals will then circle the cocoon, sometimes landing as well. One male can displace another if the suitor tires and falls off before the female makes her appearance.

For 4 years, Erika I. Deinert and her colleagues at the University of Texas at Austin followed these courtship tactics in 235 marked male butterflies. Each courtship involved several males. In all, the researchers witnessed 27 mating events.

It turns out that older, more experienced males fared no better than younger ones. The larger males more often cornered a cocoon, but because of their more flexible abdomens, smaller males were more successful in holding on to their potential mates, Deinert, John T. Longino, and Lawrence E. Gilbert report in the July 7 NATURE.

Thus, natural selection favors neither large nor small males. Instead, it has led to a change in the relative size of wings and bodies, say the researchers. They reached this conclusion by comparing the wing and body lengths of three species of butterflies that engage in pupal courtship with four related species that do not. The two groups had similar wing and body lengths, but pupal-mating species had much longer wings relative to body size than did the other species, they note.