Hairy legs are sexier, especially for spiders

Male animals go to extreme lengths to attract the opposite sex. Sanjida O'Connell reports
Females are fickle. Once they decide something's attractive, any male who wants a look-in has to have it - or grow it. Witness the beautiful tail of the peacock. In the mating game, males fight or display to females who then choose the biggest or the brightest male. Widow birds, for example, have exceptionally long tails and females go for the male with the longest, even if the tail is beyond the realms of biological possibility and has been enhanced with glue, spare feathers and the aid of a biologist. Female zebra finches are so capricious, they will pick males with red plastic rings around their legs.

Now animal behaviourists Dr Sonja Scheffer, from the State University of New York at Stony Brook, George Uetz, from Albion College, and Gail Stratton, from the University of Cincinnati, have discovered that male brush-legged wolf spiders have evolved black bristly tufts of hair on their front legs to attract females.

The spiders, which live in leaf litter in east American forests, have an elaborate courtship that involves waving and arching their legs at a female. The aim of the game is to mate with her, but in the case of these spiders, as in many of their brethren, avoiding ending up as brunch is of equal consideration. Females that are not ready to mate will lunge towards males with fangs bared and, if given the chance, will kill and eat them. Nearly half of all males are cannibalised after copulation - a male spider's tactic is to freeze in the hope she'll think he's a leaf and not lunch.

Dr Scheffer and her colleagues wondered whether the male's bristly bits were crucial to a female spider's idea of sexiness. They offered females a choice of two males: one complete with tufts, and one that the researchers had shaved. The females seemed to show no preference for males with or without tufts. They mated with the bravest - or most foolhardy - male who was the first to court them. The researchers then tried another approach. They prevented the females from listening to the males.

As well as the elaborate leg waving procedure, males signal to the females by sending vibrations along the ground using their stridulatory organs. The researchers deprived the spiders of sound by placing them on insulating foam. Without the benefit of this extra sense, the females chose males who still had tufts and spurned the shorn spiders. Dr Scheffer concluded that tufts are essential in the mating arena. Female spiders have got good eye-sight, but vibrations do not travel well along the forest floor. A male will thus come into a female's visual range long before she can hear who he is. Since females are prone to eat males once they have performed their function, and are partial to other species of spider, it makes sense for a male to advertise as well as he can who he is and what he is about. A spider's tufts may also act as signal to other males. When spiders live in high densities, they establish the arachnid equivalent of a pecking order. Presumably those with the biggest bristles get to be top of the leaf litter.