Enough for all: no mating effort adjustment to varying mate availability in a gift-giving spider

Males of a gift-giving spider do not modify their allocation to reproduction when mating opportunities vary. Due to their costly courtship via provision of food gifts to females, with high female availability males should reduce their reproductive investment per partner to avoid exhausting their energetic budget too early. Our findings suggest instead that males may be able to enlarge their total reproductive budget, possibly drawing resources from their food gifts by partially feeding on them. Reproduction is costly, and because males possess a finite energetic budget, resource allocation to one mating event may constrain investment in subsequent matings. Consequently, males of many species evolved to adjust their reproductive investment in response to mating opportunities. When female availability is high, males are predicted to partition their reproductive effort among multiple partners to avoid resource depletion before mating opportunities have ceased. We tested this prediction in males of the spider Pisaura mirabilis, which mate via costly nuptial gifts consisting of silk-wrapped prey and are known to respond to social cues (rivals’ presence) by adjusting their reproductive investment. We manipulated the number of females exposed to males to induce perception of high mating opportunities versus low mating opportunities, and scored male investment to premating traits (time allocated to gift construction and courtship) and traits at mating (copulation duration) during interactions with a female. We expected males facing higher mating opportunities to reduce their investment in all measured traits, but instead found no differences in male trait expression. These findings indicate lack of resource partitioning in response to variation in female availability, as males may be able to draw resources from nonreproductive traits (growth or immune defense) or may increase their food intake at mating by partially consuming the nuptial gift. Hence, despite their associated costs, by providing a source of nutrition, nuptial prey gifts may weaken selection for adaptive plasticity in male reproductive investment.