How non-nestmates affect the cohesion of swarming groups in social spiders

In social biology, it is often considered that an organized society cannot exist without exclusion behaviour towards newcomers from another nest. Unlike most vertebrate and invertebrate social species, social spiders such as Anelosimus eximius accept unrelated migrants without agonistic behaviour. Does it imply that spiders cannot recognize non-nestmates from nestmates or is there any evidence of recognition without aggression ? In order to answer this question, we studied behavioural differences between groups coming from single and mixed-nests in the overall context of swarming. Our study shows that the presence of non-nestmate conspecifics reduces the cohesion of the swarm groups during the settlement process and increases the spatial dispersion of spiders, the asymmetry in the spatial distribution being less pronounced. Individuals belonging to different nests are not as mutually attractive. This paper shows that, during the induced migration, two processes counteract each other: the amplification process resulting from the addition of silk drives individuals to form groups with non-nestmates and the recognition process reduces the cohesion of groups composed of non-nestmates. The collective decision-making during migration results from the balance between these two trends. Received 30 October 2007; revised 3 April and 19 May 2008; accepted 22 May 2008.