Extreme developmental synchrony reduces sibling cannibalism in the black widow spider, Latrodectus hesperus

Cannibalism, intraspecific predation, is a fascinating behaviour that can serve as a model system for researchers interested in learning more about conspecific conflicts ranging from siblicide to sexual cannibalism. Cannibalism is most often studied in terms of the benefits it provides the predator, but the prevalence of such an ecologically significant behaviour is also shaped by the costs inherent in a cannibalistic attack. Perhaps most intuitively, the risk of injury when attacking a conspecific of similar or larger size has been considered, and led to the prediction that cannibalism should be most common by larger individuals among asymmetrically sized pairs. This prediction is commonplace in the cannibalism literature, and leads to the more novel hypothesis that siblicide then may be reduced to the extent that maternal investment yields developmental synchrony and size-matched siblings. Here we test these hypotheses in the western black widow spider, Latrodectus hesperus, by rearing some eggs at 4 °C warmer temperatures for 7 days, and thus experimentally creating size asymmetries between clutchmates. Our temperature manipulation was successful at speeding development (eggs reared at warmer temperatures attained their first moult 2.5 days sooner than their counterparts). This developmental asynchrony accelerated cannibalism by more than 3 days relative to control groups in which eggs and spiderlings were all reared at either consistently colder or warmer temperatures. Lastly, the mass and size of an egg were poor predictors of cannibalism in both treatments that included individuals reared at warm temperatures. In contrast, in our control group where both eggs were reared at cooler temperatures, we found that families typified by heavier and larger eggs were slower to show cannibalistic behaviour. We discuss the implications of these findings, paying particular attention to their relevance to the black widow's emerging role as an urban pest.