When sibling tolerance meets cannibalism of the dead in spiderlings

Social life requires group members to tolerate each other, which implies the presence of effective mechanisms to prevent aggression. The existence of a transient social life brings an additional dimension, as the response to social signals provided by conspecifics varies during ontogenesis. This raises questions about the mechanism driving the switch in the processing of social cues. Spiders are a relevant model for answering these questions, as juveniles are gregarious and tolerant at the start of their development and then behave aggressively towards their conspecifics at more advanced stages. Previous research has shown that spiderlings reared in groups are tolerant of their siblings, but that those maintained alone are cannibalistic. In many taxa, the nutritional status of individuals plays an important role in the expression of intraspecific aggression. We found that in juveniles of the solitary spider Agelena labyrinthica, lipid stores decreased over time when starved, but social context did not have any effect. This suggests that differences in energy stores do not cause differences in how individuals interact with each other. We then examined whether spiderlings that tolerated living siblings cannibalized corpses immediately after death or after a 24 h delay. We found that spiders engage in necrophagy at a similar rate, regardless of the age of the corpse. These results suggest that living spiders produce an as-yet-undetermined signal that protects them from cannibalism but disappears immediately after their death, exposing them to predation by siblings.