Cannibalism in spiderlings is not only about starvation

Many species show a transient group life, and dispersal often coincides with the onset of agonistic behaviors. Changes in the nature of interactions among conspecifics can rely on a variation in the production of communication cues and/or on a switch in the processing of social information. The relative contribution of each process on the initiation of aggression still remains to be investigated. Spiders constitute relevant models to address this issue since all solitary species undergo a transient gregarious phase prior dispersal. In this study, we developed a combination of behavioral and physiological assays to examine the mechanisms accompanying the onset of agonistic interactions in spiderlings of the solitary species Agelena labyrinthica. Juveniles of different developmental stages were supplied with diets differing in prey availability. We showed that unfed spiderlings never molted, retained their cuticular lipid signature, and did not behave aggressively. This contrasted with fed individuals that molted, changed their cuticular profiles, and displayed agonistic interactions and cannibalism. We demonstrated that depletion in lipid stores was not sufficient to elicit aggression or cannibalism. Our analysis also revealed that major shifts in cuticular profiles only occurred after the first molt outside the cocoon. The lack of agonistic interactions in unfed spiderlings suggests an absence of behavioral plasticity in response to food shortage at the earliest developmental stages. We propose that the initiation of aggression relies more on a shift in the production of cuticular cues accompanying molting rather than on changes in information processing depending on the physiological state of individuals. Most species of spiders display a transient gregarious phase preceding the initiation of agonistic interactions and dispersal. We examined what factors are associated with the onset of aggression in gregarious juveniles and we showed that pre-dispersing spiderlings despite low energetic stores never cannibalized conspecifics. This contrasts with later instars that displayed increased levels of aggression. The initiation of agonistic interactions coincided with major changes in the profiles of cuticular lipids, which are used in spiders to regulate social interactions. Our study suggests that depletion of lipid stores is not sufficient to trigger aggression but that it likely requires changes in communication cues associated with molting. We propose that the absence of behavioral plasticity of pre-dispersing juveniles in response to starvation allows spiderlings to maintain social cohesion and to benefit from the potential advantages granted by gregariousness, irrespective of environmental fluctuations.