Science: Are birds with long tails sexier?
点击量： 时间：2019-03-01 09:17:01
By GEORGIA MASON Why do birds have long tails? Males often have far longer tails than females so the usual idea is that long tails have evolved because females prefer them. According to the theory, a male is likely to make a good mate and have ‘good genes’ if he is able to be active and healthy, despite being handicapped by extra-long feathers. But the reason why birds have long tails may not be as simple as this, say British zoologists. It depends on the shape of the tail. Although in some species, tails are long because females prefer them that way, in other species tails are long for aerodynamic reasons. Andrew Balmford, Adrian Thomas and Ian Jones of the University of Cambridge set out to discover if the long tails really are a burden which can be shouldered only by the fittest males. Using computer models, they calculated the lift and drag that birds’ tails of different shapes and sizes produced (Nature, vol 361, p 168). Tails come in three main shapes. Tails which are triangular in flight, such as the house martin’s, have shallow forks when closed. Other birds such as the pheasant have tails with a long tapering extension beyond the widest point. Still other tails have narrow streamer-like extensions – either paired, as in the swallow, or single, as in the pintail duck. The zoologists’ computer simulation showed that long tails with a shallow fork are no burden at all. The amount of lift provided by a tail depends on its widest span. When a bird’s tail feathers are lengthened, making a wider triangle, the bird gets far more lift than drag in flight, so such designs help birds to fly. Because such a tail helps both males and females equally, the difference between the sexes should be far less marked than in birds with other types of long tails. This is just what the Cambridge zoologists found when they examined birds in museum collections. However, long streamers and tapered extensions are a real drag. They do not make a tail wider, so they provide no extra lift; they serve only to make flying harder work. Such tails could be explained by the ‘good genes’ idea, because a male would have to be strong and healthy to carry this type and still live a successful life. If the argument is true, these tails should be a characteristic of males. The zoologists did not have access to enough museum species to test this idea for tapered tails. However, they measured museum specimens with long paired streamers and found that the longer a species’ tail extensions, the greater their development in the male relative to the female. But could the idea explain the evolution of both these tail-types from their very beginnings? Balmford’s group thinks not. Short streamers do not create much drag because they are so narrow. They would not hinder a bird’s flight, and so even the puniest male should be able to sport them with ease. This is a problem for the ‘good genes’ idea. Balmford and his colleagues believe that in their earlier stages these useless ornaments cannot have been preferred by females as a sign of quality, so they must have begun to evolve for another reason. The earliest streamers may have attracted females for a different reason. Females may have tended to opt for the most eye-catching of potential mates, for example. So, for whatever reason, some long tails are sexy. But this does not mean males should really handicap themselves in the name of sex appeal. When long tails do not help in flight, the maximum tail length is limited because at some point they become a serious burden, even for the best males. This limitation should be particularly evident in species with tapering rather than streamer-like extensions, because such tails produce the most drag. Among the museum specimens,