Group members often vary in the information that they have about their environment. In this study, we evaluated the relative contribution of information held by the population majority vs. new immigrants to groups in determining group function. To do so we created experimental groups of the social spider Stegodyphus dumicola that were either iteratively exposed to a dangerous predator, the ant Anoplopepis custodiens, or kept in safety. We then seeded these groups (i.e., the population majority) with an “immigrant” individual that either had or did not have prior experience with the predator and was either shy or bold. Bold group members are argued to be particularly influential for group function in S. dumicola. We then evaluated colonies’ response towards predators over multiple trials to determine the effect of the immigrant’s and the majority’s prior experience with the predator and the immigrant’s boldness. We found that groups adopt a “better safe than sorry” strategy, where groups avoided predators when either the group or the immigrant had been previously exposed to risk, regardless of immigrant boldness. These findings suggest that past experience with predators, even if only experienced by a single individual in the group, can alter how groups respond to risk in a potentially advantageous manner.

The ecological impacts of animal groups may be different and predictable depending on their collective behavior. Farmerfish (Stegastes nigricans) live in social groups and collectively defend gardens of palatable algae. These gardens also serve as settlement and nursery habitats for corals because farmerfish mob corallivores that attempt to forage on corals within their gardens. We detected large among-colony differences in farmerfish collective aggression towards intruder fish that persist across years. We further found that the territories of aggressive groups and territories containing larger farmerfish provided greater protection to corals than non-aggressive groups: territories of aggressive groups naturally harbored more branching corals than non-aggressive groups, and experimentally outplanted branching corals experienced 80% less skeletal loss and grew larger over 25 weeks in aggressive territories and in territories guarded by larger fish. These findings hint that factors that increase farmerfish group aggressiveness (e.g., higher temperatures) could enhance the protective value of their territories for the replenishment of coral populations.

Predator-prey interactions often vary on the basis of the traits of the individual predators and prey involved. Here we examine whether the multidimensional behavioral diversity of predator groups shapes prey mortality rates and selection on prey behavior. We ran individual sea stars (Pisaster ochraceus) through three behavioral assays to characterize individuals’ behavioral phenotype along three axes. We then created groups that varied in the volume of behavioral space that they occupied. We further manipulated the ability of predators to interact with one another physically via the addition of barriers. Prey snails (Chlorostome funebralis) were also run through an assay to evaluate their predator avoidance behavior before their use in mesocosm experiments. We then subjected pools of prey to predator groups and recorded the number of prey consumed and their behavioral phenotypes. We found that predator-predator interactions changed survival selection on prey traits: when predators were prevented from interacting, more fearful snails had higher survival rates, whereas prey fearfulness had no effect on survival when predators were free to interact. We also found that groups of predators that occupied a larger volume in behavioral trait space consumed 35% more prey snails than homogeneous predator groups. Finally, we found that behavioral hypervolumes were better predictors of prey survival rates than single behavioral traits or other multivariate statistics (i.e., principal component analysis). Taken together, predator-predator interactions and multidimensional behavioral diversity determine prey survival rates and selection on prey traits in this system.

The collective behaviour of social groups is often strongly influenced by one or few individuals, termed here ‘keystone individuals’. We examined whether the influence of keystone individuals on collective behaviour lingers after their departure and whether these lingering effects scale with their tenure in the group. In the social spider, Stegodyphus dumicola, colonies' boldest individuals wield a disproportionately large influence over colony behaviour. We experimentally manipulated keystones' tenure in laboratory-housed colonies and tracked their legacy effects on collective prey capture following their removal. We found that bolder keystones caused more aggressive collective foraging behaviour and catalysed greater inter-individual variation in boldness within their colonies. The longer keystones remained in a colony, the longer both of these effects lingered after their departure. Our data demonstrate that, long after their disappearance, keystones have large and lasting effects on social dynamics at both the individual and colony levels.