In the classic 1969 buddy movie Butch Cassidy and the Sundance Kid, the Kid gets the girl, but Butch still does some courting, especially when he takes Etta on a bicycle ride while the soundtrack plays "Raindrops Keep Falling on My Head." It's that way too with lance-tailed manakins, a bird found in South America. Two males team up to woo the female, but the alpha male is the only one who scores. A new study suggests that the beta male benefits from this supporting role, although the payoff can take a long time.
The evolutionary advantage of cooperative behavior is often a mystery, especially when there is no immediate perk to being someone's buddy. To see why beta males might lend a helping wing to their alpha pals, behavioral ecologist Emily DuVal, now at the Max Planck Institute for Ornithology in Seewiesen, Germany, spent several years studying lance-tailed manakins on a small island off the coast of Panama. The males of this species team up in pairs and perform elaborate duets and dances to attract females, including leapfrogging over each other and showing off with short, swooping flights. Between 1999-2004, DuVal captured 457 birds, banded them for identification, and took blood samples from their wing veins for DNA analysis.
DuVal wanted to test three hypotheses for why the beta male would help the alpha even though he never gets to mate with the female: The beta's own immediate reproductive chances could increase; the beta male could be a close relative of the alpha, thus helping to pass on related genes; or the beta male might be increasing his own chances of reproducing sometime down the line. As DuVal reports in the April issue of The American Naturalist, the third hypothesis seems to fit the data best. All of the matings she observed in the field were performed by alpha males, meaning that betas rarely if ever got the girl--a finding that was confirmed by the genotyping of offspring. Nor were the betas helping their kin, because they were not closer genetically to the alphas than were randomly chosen pairs of males. But betas did have a better shot at the top spot later: 15% of the betas became alphas the following year, compared to only 4% of males who were neither betas nor alphas.
It's not that betas were simply next in line to become alphas, DuVal says. When she removed alpha males from eight randomly selected mating areas, seven of the eight beta buddies began acting like alphas for a while--but they did not maintain this status into the next breeding season and thus did not have an automatic advantage over other male birds within the same mating territory. DuVal suggests that the betas must benefit indirectly, possibly by learning the intricate courtship dances by "apprenticing" with their alpha partners. These skills could then be put to good use when the betas move to other mating areas.
"This study is incredibly important," says Janis Dickinson, a behavioral ecologist at Cornell University. "It solves a riddle that has piqued the interest of ornithologists since Charles Darwin described manakin courtship behavior in the late 1800s."