SPERM COMPETITION AND SEXUAL SELECTION - A METAANALYSIS OF PATERNITY STUDIES OF BIRDS

Sperm competition (the competition among the sperm of different males for fertilization of the eggs of a female) has been suggested to be an important component of sexual selection, but no general assessment ha s been made of this proposition. We used a meta-analytic approach to a ssess the extensive literature on paternity (the proportion of offspri ng in a focal nest sired by an attending male) in birds based on alloz yme and molecular techniques. The relative variance in male mating suc cess was on average increased by a factor of 4.6 over the variance in apparent male success. Males with more extravagant secondary sexual ch aracters had higher paternity in their own nests than less adorned mal es. There was a weak effect of male age being positively associated wi th paternity in own nests. Male body size measured as the length of wi ng and tarsus was weakly positively associated with paternity in own n ests. Male survival prospect was positively associated with paternity in own nests. Polygynous males generally had decreased paternity of th eir broods compared to monogamous males. Paternity of the resident mal e decreased with increasing population density and breeding asynchrony . The intensity of paternity guards such as within-pair copulation rat e and mate guarding were not significantly related to extra-pair pater nity. Sperm competition was thus an important component of sexual sele ction by increasing the variance in male mating success, and by being associated with the expression of secondary sexual characters, in part icular in dense and asynchronously breeding populations of birds.