The Achilles' heel hypothesis: misinformed keystone individuals impair collective learning and reduce group success.

Many animal societies rely on highly influential keystone individuals for proper functioning. When information quality is important for group success, such keystone individuals have the potential to diminish group performance if they possess inaccurate information. Here, we test whether information quality (accurate or inaccurate) influences collective outcomes when keystone individuals are the first to acquire it. We trained keystone or generic individuals to attack or avoid novel stimuli and implanted these trained individuals within groups of naive colony-mates. We subsequently tracked how quickly groups learned about their environment in situations that matched (accurate information) or mismatched (inaccurate information) the training of the trained individual. We found that colonies with just one accurately informed individual were quicker to learn to attack a novel prey stimulus than colonies with no informed individuals. However, this effect was no more pronounced when the informed individual was a keystone individual. In contrast, keystones with inaccurate information had larger effects than generic individuals with identical information: groups containing keystones with inaccurate information took longer to learn to attack/avoid prey/predator stimuli and gained less weight than groups harbouring generic individuals with identical information. Our results convey that misinformed keystone individuals can become points of vulnerability for their societies.

Avoidance or escape? Discriminating between two hypotheses for the function of schooling in threespine sticklebacks.

In many species, there are antipredator benefits of grouping with conspecifics. For example, animals often aggregate to better avoid potential predators (the 'avoidance hypothesis'). Animals also often group together in direct response to predators to facilitate escape (the 'escape hypothesis'). The avoidance hypothesis predicts that animals with previous experience with predation risk will aggregate more than animals without experience with predation risk. In contrast, the escape hypothesis predicts that immediate exposure to predation risk causes animals to aggregate. We simultaneously tested these two nonexclusive hypotheses in threespine sticklebacks, Gasterosteus aculeatus. Schooling behaviour (time spent schooling, latency to school and time schooling in the middle of the school) was quantified with a mobile model school. Fish that had been chased by a model predator in the past schooled more, started schooling faster and spent a marginally greater proportion of time schooling in the middle of the school than fish that had not been chased. In contrast, there was no difference in the schooling behaviour of fish that were immediately exposed to either a model pike or a control, stick stimulus. A second experiment confirmed that fish perceived the model pike and stick differently: fish froze more often in the presence of the model pike, oriented to it more often and spent less time with the model pike than they did with the stick. These results provide strong support for the avoidance hypothesis.