Handicap hypothesis of evolution of female preferences
The handicap hypothesis, which was formulated in 1975 by A. Zahavi (Zahavi 1975), assumes that, under certain conditions, it may be advantageous for the female to choose, as the father of her offspring, a handicapped male, for example, a male with long tail feathers. The long feathers represent a substantial handicap for their bearer in the fight for survival. Thus, if a male with abnormally long tail feathers, i.e. with an abnormally large handicap, has survived to reproductive age, it is almost certain that this must be an abnormally fit individual.
Simultaneously, this handicap need not be only a secondary sexual trait. It could, for example, be a physical defect incurred through an injury or even the old age of the individual (Kokko & Lindstrom 1996; Sundberg & Dixon 1996). It has been observed, for example, for sparrows and finches, that, in extra-pair parentage (EPP), females prefer old males (Wetton et al. 1995; Sundberg & Dixon 1996). Simultaneously, the frequency of extra-pair copulation (EPC) with old and young individuals is the same. In perching birds (Passeriformes), the female determines whether copulation will leads to transfer of sperm or not. The fact that, for the same frequency of EPC, a greater number of offspring are fathered by older males suggests that the purpose of this “gerontophilia” lies in an attempt of the female to obtain the best genes for her offspring.
The handicap hypothesis on the origin of secondary sexual traits has been subject to fundamental criticism in the past. Mathematical analysis of the effect of a handicap of the father on the fitness of offspring has shown that the advantage represented by the greater fitness of the father is exactly compensated in the offspring by the existence of the handicap that the progeny also inherit. However, at the present time, it seems that the model could work in a number of situations (Pomiankowski 1987; Siller 1998; Hastings 1994). At the same time, it is important that the coefficient of heritability of the handicap, a factor reflecting the probability that the offspring will inherit the particular trait, for example long feathers, is less than the average coefficient of heritability of the other traits determining the fitness of the individual. If, in addition to genetic factors, the effect of the environment also has a substantial effect on feather length, the coefficient of heritability of this trait can be very low. Under these conditions, it is really advantageous for the female to prefer reproduction with a handicapped male.s