Family Affects Sibling Cannibalism in the Black Widow Spider, Latrodectus hesperus

Adaptive foraging tactics are shaped by genes, the environment and gene-environment interactions. Because of relatively high levels of agonism toward conspecifics, spiders have been a popular focus for behavioral-ecological examinations of conspecific predation, or cannibalism. Surprisingly, studies examining the underlying, proximate assumption that cannibalism in spiders is a heritable trait shaped by interactions between genes and the environment are virtually non-existent. Here, we examine the influence of family on the expression of sibling cannibalism in the post-hatching, group-living phase of an otherwise solitary, web-building spider, the North American black widow (Latrodectus hesperus). Our results showed significant levels of variation in cannibalistic propensity among 26 sibships, with some families cannibalizing full sibs within 2 d and other families waiting 3 wk before resorting to cannibalism. A similar family-level effect was evident in measures of sibling cohabitation, voracity toward cricket prey, and development speed. Negative correlations between maternal egg sac investment and offspring cannibalism suggest that this family effect may stem, at least in part, from a maternal effect, although we were not able to directly test the prediction that cannibalism is most common from spiderlings in poor condition. Thus, we present novel data suggesting family effects seem to be responsible for cannibalism in L. hesperus spiderlings; however, future work will be required to disentangle the relative importance of shared genes and shared maternal environment. We discuss several mechanisms that could explain the persistence of family-level variation in cannibalism, a trait that seems likely to be subject to strong directional selection.