Take-off with train is no drag for peacocks

Kathryn Knight

doi:10.1242/jeb.112342

Inside JEB

Head of a peacock (Pavo cristatus). Photo credit: G. Askew.

Peacocks are in a league of their own when it comes to performing for the ladies, strutting about with their ostentatious fanned trains; what female could resist? Graham Askew from the University of Leeds, UK, explains that males from many branches of the animal kingdom advertise themselves with elaborate ornaments, but these extravagant displays come at a price. ‘It is thought that such sexual traits have a negative effect on an individual's performance…but that more elaborate ornaments indicate superior genetic quality’, says Askew. In the case of the peacock, he suspected that the train could restrict their flight – possibly costing them their lives in the event of an impaired escape bid – yet no one had ever measured the impact of the peacock train on the bird's ability to take-off. ‘Trying to measure the effect that it [the train] had on performance seemed like a worthwhile effort’, says Askew, who decided to investigate how much an escaping peacock is incapacitated by his feather burden (p. 3237).

Selecting five Indian peacocks with intact feather trains, Askew startled the birds into take-offs as he filmed them in 3D with two high-speed video cameras. He then relieved the birds of their elaborate plumage, mimicking the natural loss of their trains at the end of the breeding season, and filmed their now unencumbered take-offs. Analysing the birds' trajectories over the first three wing beats, Askew calculated the position of each bird's centre of mass, their wing motions and the movement of the train. Then, he calculated how the loss of the train had altered the birds' take-off and was amazed to see that it 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 was essentially the same (~200 W kg⁻¹), regardless of the presence or absence of the train.

‘Intuitively, you expect that the train would affect flight performance and so not finding a detectable effect was a bit surprising’, admits Askew. Puzzled, he investigated how much the train affected drag on the birds during take-off. Mounting a detached train in a wind tunnel and measuring the drag on the feathers, he found that the drag increased by 200%, doubling the amount of power that the birds have to produce. However, the power that the birds have to produce to overcome drag is only 0.1% of their total aerodynamic power. So, the impact of the train on their overall take-off performance is negligible, allowing birds with and without trains to invest the same amount of power in the ascent, rather than having to divert some of it to overcome the effects of drag on the extravagant feathers.

Having shown that the train does not affect the peacock's take-off, Askew says, ‘These results do not necessarily mean there are no costs associated with possessing an ornate train’. He points out that there is a range of other aspects of peacock performance that the train could affect, such as flight stability, running and the shear cost of producing such an impressive ornament – the peacocks invest 3% of their basic daily metabolic budget in train growth – and he suspects that all of these factors could contribute to some extent to the price that proud peacocks pay to lure in the ladies.