Interdependent effects of male and female body size plasticity on mating behaviour of predatory mites.

The adaptive canalization hypothesis predicts that traits with low phenotypic plasticity are more fitness relevant, because they have been canalized via strong past selection, than traits with high phenotypic plasticity. Based on differing male body size plasticities of the predatory mites (low plasticity) and (high plasticity), we accordingly hypothesized that small male body size entails higher costs in female choice and male-male competition in than . Males of both species are highly polygynous but females differ in the level of polyandry (low level in ; medium level in ). We videotaped the mating interactions in triplets of either or , consisting of a virgin female (small or standard-sized) and a small and a standard-sized male. Mating by both small and standard-sized females was biased towards standard-sized males, resulting from the interplay between female preference for standard-sized males and the inferiority of small males in male-male competition. In contrast, mating by females was equally balanced between small and standard-sized males. Small males were more aggressive ('Napoleon complex') in male-male competition, reducing the likelihood of encounter between the standard-sized male and the female, and thus counterbalancing female preference for standard-sized males. Our results support the hypothesis that male body size is more important to fitness in the low-level polyandrous than in the medium-level polyandrous and provide a key example of the implications of sexually selected body size plasticity on mating behaviour.