Strategies, costs and counter-strategies to sexual coercion.

Sexual conflict, the conflict between the evolutionary interests of females and males over mating, occasionally results in the evolution of traits favourable for one sex and adverse for the other. In this context, males can use sexual coercion to increase their mating success, at the expense of their female targets' mate choice. An increasing number of studies highlight a great diversity of male and female behaviours that serve as strategies and counter-strategies, respectively, to sexual coercion. Previous studies have reviewed the literature on infanticide but not the literature on forced copulation, sexual harassment, intimidation or punishment. This qualitative review synthesises the empirical evidence and draws a unified framework of the ecology of sexual coercion across animals, presenting male sexually coercive strategies and co-evolved female counter-strategies that can reduce coercion and its fitness costs. Using examples from insects to humans, it shows that different strategies of sexual coercion can lead to the evolution of similar counter-strategies. These counter-strategies include female promiscuity, deception of males (e.g. concealed ovulation or pseudo-oestrus), avoidance of certain males and association with others for protection, female aggregation to dilute coercion and ultimately physical resistance by single or allied females. Extending previous work, this review provides compelling evidence of sexually antagonistic coevolution amid sexual coercion. It also calls for future work to clarify, first, which individual traits are linked to greater coercion rates in males and a higher likelihood of receiving coercion in females and, second, any causal relationships between different strategies of sexual coercion and the evolution of different social and mating systems.