DIFFERENTIAL COSTS OF A SECONDARY SEXUAL CHARACTER - AN EXPERIMENTAL TEST OF THE HANDICAP PRINCIPLE

The evolution of reliable signaling can be explained by the handicap p rinciple, which assumes that(1) the cost of a signal guarantees its re liability, and (2) cheating is prevented because the cost of a unit of display is greater for low-quality than for high-quality individuals. A test of these two assumptions was performed using manipulations of the length of the outermost tail feathers of male barn swallows Hirund o rustica, a trait currently subject to a directional female mate pref erence. We found that survival decreased with tail elongation and incr eased with tail shortening of males, supporting the assumption that th e secondary sexual character is costly. Naturally long-tailed males we re better able to survive with an elongated tail, whereas naturally sh ort-tailed males improved their survival following tail shortening. Th is observation supports the second assumption of a differential cost o f a signal. One mechanism imposing differential costs on sexually sign aling barn swallows is foraging. Males with elongated tails captured s maller, less profitable Diptera, whereas males with shortened tails ca ptured large, profitable prey items. The conditions for reliable sexua l signaling by the tail ornament of male barn swallows are thus fulfil led.