The peacock’s tail is a classic example of an overly exaggerated, sexually selected trait that comes at a cost -- all that elaborate plumage must slow them down, right? That's not the case according to a new study published in Journal of Experimental Biology this week.
Intuitively, the weight and aerodynamic drag of a male peafowl’s fanned train should reduce locomotor performance. More elaborate ornaments may indicate superior genetic quality, but these decorations weigh about 300 grams and can exceed 1.5 meters. “It’s expected that the male birds would be making a significant sacrifice in their flight performance for being attractive -- possibly giving up their lives if the train restricts escape from predators such as tigers and leopards in their natural environment,” Graham Askew from the University of Leeds says in a news release. But no one's ever quantified this cost, so he looked into the effect an extravagant train has on take-off flight.
Using two high-speed 3D video cameras, Askew filmed five Indian peacocks (Pavo cristatus) with intact trains taking off after being startled with a stick. Then he clipped off their “feather burden” to mimic natural molting at the end of breeding season, then filmed them again. This allowed him to compare the potential and kinetic energies of the birds' bodies with trains and without.
First, he analyzed the birds' trajectories over the first three wing beats, and then he calculated the position of their center of mass, wing motions, and movement of the train. Turns out, the loss of the train had little impact on their escape performance.
The amount of power used by the birds to accelerate and gain height over the first two wing beats were essentially the same, regardless of the train’s presence or absence, Inside JEB explains. Compare a startled long-tailed peacock with a short-tailed one in this video via Science.
The train does incur some drag during take-off, so to investigate further, Askew mounted a detached train in a wind tunnel. This produced a twofold increase in “parasite drag,” doubling the power its bearer would have to produce. However, overcoming that drag takes only 0.1 percent of total aerodynamic power -- which means the impact of the train on overall take-off performance is negligible. Birds with and without trains invest the same amount of power in their ascent.
“Not finding a detectable effect was a bit surprising,” Askew says. “These birds do not seem to be making quite the sacrifices to look attractive we thought they were.” But that doesn’t necessarily mean there’s no cost associated with having spectacular plumage. The train makes up 6.9 percent of body weight, and peacocks invest 3 percent of their basic daily metabolic budget on its growth. Maybe it affects how stable they fly or how fast they run -- a small price for sexiness?
Image: Jason Mrachina via Flickr CC BY-NC-ND 2.0
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