For ornithologist Richard Prum, manakins are among the most beautiful creatures in the world. He first started studying these small South American birds in 1982, and he’s been privy to many of their flamboyant performances. One species has a golden head and moonwalks. Another puffs up a white ‘beard’ and hops about like a “buff gymnast.” Yet another makes alarmingly loud noises with its club-shaped wing bones. Each of the 54 species has its own combination of costumes, calls, and choreography, which males use in their mating displays. To Prum, this is a great example of “aesthetic radiation,” where a group of animals has evolved “54 distinctive ideals of beauty.”
That’s not a common view among evolutionary biologists. Most of Prum’s colleagues see outrageous sexual traits as reliable advertisements. The logic goes that only the fittest manakins could coordinate their movements just so. Only the healthiest peacocks could afford to carry such a cumbersome tail. Their displays and dances hint at their good genes, allowing females to make adaptive decisions.
But Prum says that view is poorly supported by years of research, and plainly makes no sense when you actually look at what birds do. How could there be adaptive value in every single minute detail of a manakin’s plumage and performance? And why have some species replaced certain ancestral maneuvers (like pointing one’s tail to the sky) with new moves (like pointing one’s bill to the sky) that surely provide no better information? “It’s clearly arbitrary,” says Prum. “I wrote that in a 1997 paper, but the reviewers hated it. They said you can’t claim that unless you falsify every adaptive hypothesis we can imagine. And if you can’t find an adaptive explanation, you haven’t worked hard enough to discover it.”