Males of the nursery web spider Pisaura mirabilis usually offer an insect prey wrapped in white silk as a nuptial gift to facilitate copulation. Males exploit female foraging preferences in a sexual context as females feed on the gift during copulation. It is possible for males to copulate without a gift, however strong female preference for the gift leads to dramatically higher mating success for gift-giving males. Females are polyandrous, and gift-giving males achieve higher mating success, longer copulations, and increased sperm transfer that confer advantages in sperm competition. Intriguingly, field studies show that approximately one third of males carry a worthless gift consisting of dry and empty insect exoskeletons or plant fragments wrapped in white silk. Silk wrapping disguises gift content and females are able to disclose gift content only after accepting and feeding on the gift, meanwhile males succeed in transferring sperm. The evolution of deceit by worthless gift donation may be favoured by strong intra-sexual competition and costs of gift-construction including prey capture, lost foraging opportunities and investment in silk wrapping. Females that receive empty worthless gifts terminate copulation sooner, which reduces sperm transfer and likely disadvantages males in sperm competition. The gift-giving trait may thus become a target of sexually antagonistic co-evolution, where deceit by worthless gifts leads to female resistance to the trait. We discuss factors such as female mating rate and intensity of sperm competition that may shape the evolution of male deception, and how ecological factors may influence the evolution and maintenance of worthless gifts as an evolutionarily stable alternative mating strategy by frequency dependent selection.

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The expression of alternative reproductive tactics can be plastic and occur simultaneously depending on cues that vary spatially or temporally. For example, variation in resources and sexual selection intensity is expected to influence the pay-off of each tactic and shape the decision of which tactic to employ. Males of the nuptial gift-giving spider Pisaura mirabilis can adopt three tactics: offering a genuine prey gift, a 'worthless' non-nutritious gift or no gift. We hypothesized that resources and/or male body condition, and mating opportunity and sexual selection intensity, vary over the course of the mating season to shape the co-existence of alternative traits. We measured these variables in the field over two seasons, to investigate the predictions that as the mating season progresses, (i) males become more likely to employ a gift-giving tactic, and (ii) the likelihood of switching from worthless to genuine gifts increases. Prey availability increased over the season and co-varied with the propensity of males to employ the gift-giving tactic, but we found no support for condition-dependent gift giving. Males responded to an increase in female availability by increasing their mating effort (gift production). Furthermore, the frequency of genuine gift use increased with sexual selection intensity, consistent with the assumption that sperm competition intensity increases with time. Our results suggest that the frequency of alternative tactics is shaped by seasonal changes in ecological factors and sexual selection. This leads to relaxed selection for the gift-giving tactic early in the season when females are less choosy and resources more scarce, and increased selection for genuine gifts later in the season driven by mating opportunity and risk of sperm competition.

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