Alternative male mating behaviour in the two-spotted spider mite: dependence on age and density

Abstract

Alternative male mating tactics occur in many taxa, and usually include sneaking behaviour. Typically, sneaking behaviour is expressed by less competitive males to avoid fighting over females guarded by stronger males and to increase their mating chances, but in theory it can be expressed by any male as long as it improves his future reproduction. Recently, we characterized fighting and sneaking behaviour in males of the two-spotted spider mite that guard females during their preadult moulting stage. Sneaking behaviour involves not responding to rival males to avoid eliciting attack, whereas fighting behaviour involves challenging all rivals. To understand these alternative precopulatory behaviours from a life history perspective, we investigated (1) their heritability, and how they are affected by (2) male density, (3) male age and (4) competition between males differing in age. We established genetic lines for fighting or sneaking by crossing a male that displayed fighting or sneaking behaviour with one of his daughters. In fourth-generation males from these lines, however, the proportions of fighting and sneaking behaviour were not significantly different. The frequency of sneaking behaviour increased with male density, yet decreased with male age. Old males always displayed fighting and rarely lost females they guarded to young males, whereas young fighting males lost females to old rival males more frequently than young sneaking males. Of the young fighting males that did not lose the female they guarded to old males, about 40% switched to sneaking. We conclude that alternative male mating behaviours in this mite are not maintained as a genetic polymorphism, but arise from phenotypic plasticity in response to the male's environment (male density) and its own condition (age). We hypothesize that young males opt for sneaking behaviour because fighting jeopardises their future reproduction.