Did males get bigger or females smaller?

Don’t just say he’s bigger. When it comes to bird species with jumbo-size males, we may have to rethink evolutionary history and say that in some cases the female’s smaller, according to Chicago researchers.

Biologists have long assumed that these male-female size differences arose from pressures on males to bulk up for macho competitions or greater appeal to females, explains Jordan

Karubian of the University of Chicago. “We really challenge that assumption,” he says.

Karubian and his Chicago colleague John P. Swaddle have analyzed the family tree of 126 cardueline finch species. Scattered around the globe, this group includes such North American favorites as goldfinches, crossbills, and redpolls. The accepted family tree, rooted in extensive work by other researchers, suggests that male-female size differences evolved independently at least 15 times.

Karubian and Swaddle reconstructed these events by turning to a statistical analysis of species that extrapolates backward from current male-female comparisons to find the most-likely forms in ancestors. The method, called squared-change parsimony, assumes the least change necessary to create the birds’ current appearance.

Other researchers have applied this method to birds’ plumage colors and spiders’ sizes. As far as Karubian and Swaddle know, they’re the first to do this with size differences in birds, using the lengths of wings and leg bones.

“The dominant paradigm is that males got bigger,” Karubian says, but the new work shows that in roughly half of the cases, the current size difference came from the females’ shrinking. The histories of species with two sizes should be sorted out on a case-by-case basis, the researchers urge in the April 7 Proceedings of the Royal Society of London B.

Finch evolutionist Alexander Badyaev of Auburn University in Alabama says, “It’s a really strong message.” He points out that much research with current species shows that larger males indeed reap plenty of benefits in fighting and flirting. “But you can’t assume that’s how [the size difference] got started,” he says.

These lessons may also apply to the so-called reverse dimorphisms in birds, such as raptors, where females grow bigger than males. “The prevalent assumption has been that selection acted on the bigger sex,” he says. After looking at the new finch study, though, ornithologists may have to start looking for shrinking males.