Sexual Cannibalism in Orb-Weaving Spiders: An Economic Model

Sexual cannibalism in orb-weaving spiders commonly takes place before copulation, even among virgin females. This behavior is curious because, although the female may obtain a nutritious meal, she risks remaining unmated. We present an economic model that accounts for sexual cannibalism by virgin females using stochastic dynamic programming. The model considers a female's expected reproductive output (which is a function of the number of copulations she has achieved) and her body size, both of which will depend upon her decision to either mate or cannibalize a courting male. The model demonstrates that cannibalism of courting males by virgin females can arise purely through foraging considerations and that the most important factors for the evolution of this behavior are the expected number of males encountered during the season and the distribution of mass gained from other prey items. The model also suggests a novel form of selection pressure on male body size.