Abstract
Sexual conflict over mating frequency has driven the evolution of morphological and behavioural traits across taxa. Interactions may be termed ‘coercive’ and assumed to arise from conflict when male mating behaviours cause physical injury to females and females appear to resist injurious matings.However, coercion per se occurs only if the behaviour reduces female fitness; and such outcomes are rarely measured. Here we show that a damaging mating tactic, apparently adaptive for males, is not coercive for females. Adult male Latrodectus spiders mate with immature females after tearing the exoskeleton covering the female’s recently-developed reproductive tract, which can cause haemolymph bleeding. We show that, relative to pairings with adult females, males use reduced courtship displays when approaching immature females, which in some cases respond with elevated deterrent behavioural responses. Nevertheless, we found no reproductive cost for immature-mated females in terms of longevity, fertility or fecundity. Moreover, most immature-mated females did not produce sex pheromones as adults, so did not seek additional matings. Thus, despite the appearance of conflict there is no evidence that immature-mating is coercive. These results show it is critical to measure fitness outcomes, in addition to behavioural responses, to test for coercion.
Highlights
Sexual selection on males has driven the evolution of a wide range of courtship behaviors that may persuade females to mate[1]
When L. hasselti males mate with adult females (AM), prolonged vibratory courtship is typical, and males that attempt copulation early are killed by females before mating is complete
We randomly assigned laboratory-reared L. hasselti females to one of 3 experimental groups that differed in mating treatment: (1) Immature females mated with adult males during their final immature instar (IM); (2) Adult females mated with adult males after their final adult moult (AM); and (3) Unmated adult females were never exposed to males and never mated (V)
Summary
Sexual selection on males has driven the evolution of a wide range of courtship behaviors that may persuade females to mate[1]. In the Australian redback spider Latrodectus hasselti, at least 1/3 of immature females are mated in this way in the field[15] This behaviour can be staged in the laboratory[15], where we were able to examine male and female behaviour and fitness consequences of mating in detail. There has been no detailed analysis of immature-mating behaviours to date, so it is unclear how males change their behaviour when approaching immature females, or whether females engage in deterrent behaviours (see[16]) It is not clear whether immature mating affects future mate attraction, nor whether the damage caused to immature females (Fig. 1) decreases longevity (an important fitness component). We test a critical prediction of the coercive mating hypothesis; that immature-mated females will have lower fitness (measured as fecundity, fertility, and post-mating survivorship) than females mated as adults
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