Silk wrapping of nuptial gifts aids cheating behaviour in male spiders

摘要

Sexual traits, such as nuptial gifts, are costly and often condition-dependent. Males should be under selection to reduce these costs without impairing their reproductive success. Spider gifts consist of silk-wrapped food, but may also consist of worthless (non-nutritive) donations that successfully lead to mating, despite yielding shorter copulations. Worthless gifts may either represent a cheaper cheating strategy or the inability to produce genuine gifts due to resource limitations (i.e. poor body condition). Unless energetic constraints limit expenditure in silk, males should apply more silk to worthless gifts to compensate for their lower reproductive value. We ask whether in Pisaura mirabilis 1) worthless gifts are condition-dependent and 2) males strategically use silk based on gift type (genuine vs worthless). We tested whether male body condition explains the gift-giving strategy and compared silk amounts covering each gift type, in gifts collected from the field and produced in the laboratory by males given different feeding regimes. Our findings show that worthless gifts are not promoted by poor body condition or limited resources. They rather result from a cheating strategy evolved to opportunistically reduce the costs of genuine gifts while ensuring nutritional advantages, with cheaters gaining body mass. Males applied more silk to worthless gifts regardless of their body condition or feeding state, suggesting they can strategically adjust silk expenditure despite its costs. By masking gift contents and prolonging female feeding, silk is crucial for the maintenance of cheating, likely resulting from an evolutionary arms race between male deception and female assessment.