Male birds that impress females with complex acrobatic movements tend to be smaller than females of the same species, probably because their reduced body size makes them more agile.
In birds, it is common for males to be larger than females, particularly when there is strong competition between males for mates. Unusually, females are larger than males in some species of manakin, a family of birds that live in tropical forests in Central and South America.
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at the University of Rochester in New York suspected that this size difference might be related to their mating displays. Manakin males court females with dance movements that can range from just a few rhythmic feather displays in some species to elaborate aerial routines in others – often too fast for the human eye to see.
“If you think about a gymnast, or a figure skater, a lot of times being small and compact might allow you to do flips and turns more efficiently or effectively,” she says. “And the movements that these birds do are things like high-speed dives, backflips and pirouettes, while the females go around checking out different males. It’s fascinating to watch!”
Shogren and her colleagues gave manakin species an agility score depending on the number of aerial movements in the males’ courtship dances. The team trapped 3051 wild birds from 22 species and held each bird just long enough to measure its body mass, wingspan and leg length.
They found that higher agility scores were associated with lighter body weight in males compared with females of the same species. For example, lance-tailed manakin (Chiroxiphia lanceolata) males were on average 2 grams lighter than the females they courted with rapid aerial flips and dives, often in synchronised teams of males. But red-headed manakin (Ceratopipra rubrocapilla) males, which court mainly by lifting their wings and moving their feet on a perch, were on average 1.5 grams heavier than their females.
“That doesn’t seem like a lot, but when you’re talking about a bird that’s between 10 and 15 grams, that’s a pretty big percentage of body weight,” says Shogren.
Environmental factors, such as precipitation, temperature and elevation, didn’t correlate with the difference in mass between males and females. However, males had slightly longer wings than females in rainy environments. Longer wings might help males soar to drier places to forage after using up energy to court females, she says, but more research is needed.
The findings run counter to the popular belief that females always prefer bigger mates, leading to sexual selection for larger males. “You have to know how sexual selection is acting, and what is being selected, to be able to make those kinds of inferences,” says Shogren.
Proceedings of the Royal Society B
