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Keywords:Araneae behavioural syndrome collective behaviour keystone individual personality spider Stegodyphus dumicola temperament Behavioural ecologists often note that one or a few group members appear to shape the collective behaviour of social groups differentially. Our understanding of these keystone individuals is largely taken from meticulous field observations and semi-scientific anecdotes. In this study we experimentally test whether the behavioural tendencies of putative keystone individuals shift the collective behaviour of colonies using the social spider Stegodyphus dumicola. Prior studies on Stegodyphus demonstrated that the single best predictor of colonies' collective behaviour is the behaviour of colonies' boldest individual. Here, we probe the causal relationship between the traits of extremely bold individuals and colonies' collective behaviour by experimentally creating colonies of identical size and personality composition in the laboratory and then adding a single individual of varying boldness (the putative keystone individual). Experimentally adding just one extremely bold individual increased the foraging aggressiveness of entire colonies and altered the total mass gained by fellow group members, relative to the addition of a less bold individual. Additionally, our data suggest that bold individuals are capable of such influence because they catalyse variation in the behavioural tendencies of fellow group members.The Association for the Study of Animal Behaviour. Published by Elsevier Ltd.

Describing the factors that shape collective behaviour is central to our understanding of animal societies. Countless studies have demonstrated an effect of group size in the emergence of collective behaviours, but comparatively few have accounted for the composition/diversity of behavioural phenotypes, which is often conflated with group size. Here, we simultaneously examine the effect of personality composition and group size on nest architecture and collective foraging aggressiveness in the social spider Stegodyphus dumicola. We created colonies of two different sizes (10 or 30 individuals) and four compositions of boldness (all bold, all shy, mixed bold and shy, or average individuals) in the field and then measured their collective behaviour. Larger colonies produced bigger capture webs, while colonies containing a higher proportion of bold individuals responded to and attacked prey more rapidly. The number of attackers during collective foraging was determined jointly by composition and size, although composition had an effect size more than twice that of colony size: our results suggest that colonies of just 10 bold spiders would attack prey with as many attackers as colonies of 110 'average' spiders. Thus, personality composition is a more potent (albeit more cryptic) determinant of collective foraging in these societies.

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