Experimental limitation of oviposition sites affects the mating system of an arachnid with resource defence polygyny

The availability and spatial distribution of reproductive resources determine female distribution, thus affecting the organization of mating systems and the intensity of sexual selection. Females of the harvestman Serracutisoma proximum lay eggs on specific plant species, which are defended by males of the large morph (majors). After oviposition, females remain inside the major's territory, forming a harem, which may be invaded by males of the small morph (minors) to sneak copulations. We experimentally manipulated the availability of the plants used as oviposition sites, and investigated how it affected the mating system of S. proximum. Our results show that females and minors moved preferentially to plots with high plant availability. Plots with high plant availability had more harems, which were established on the preferred plants. In plots with low plant availability, many females laid eggs on plants where S. proximum clutches had never been observed previously, rather than choosing to be a part of a harem on a preferred plant. Harem invasions were accomplished by both minors and majors, and the frequency of harem invasions was not influenced by plant availability. Finally, male-male competition for harems and the potential for sexual selection were also not influenced by plant availability. Female plasticity in oviposition site use probably attenuates competition among majors, allowing males with poorly developed weaponry to establish a territory and achieve copulations. Minors apparently adjust their distribution in response to harem density, but the income of minors does not increase the frequency of harem invasions in plots with high plant availability. Plasticity in the major mating tactic probably accounts for the high frequency of harem invasions, regardless of plant availability. We argue that behavioural plasticity may thus profoundly affect our ability to make predictions about the effects of oviposition site limitation on mating systems and the intensity of sexual selection in species exhibiting resource defence polygyny.