Group-enhanced predator detection and quality of vigilance in a social ground squirrel

Animals may form groups for different reasons, and one major benefit of grouping in many species is reduced predation risk. In diurnal species, vigilance is used to detect predators, resulting in a trade-off between feeding activity and predation risk. Species can reduce the cost of this trade-off with low-quality vigilance – performing another behaviour while vigilant – in comparison to high-quality vigilance (only being vigilant). Two nonmutually exclusive hypotheses explaining an inverse relationship between individual vigilance and group size are the dilution effect, where predation risk decreases in larger groups, and collective detection, where larger groups have more individuals that may detect a predator. Two predictions that support collective detection but not the dilution effect are that (1) overall group (collective) vigilance will increase with increasing group size, even while individual vigilance decreases, and (2) at least one group member must be vigilant to detect potential danger and communicate that information to group members. To test these predictions, we recorded behavioural data on low- and high-quality vigilance and alarm calling in the gregarious Barbary ground squirrel, Atlantoxerus getulus . Barbary ground squirrels allocated more time to high-quality vigilance than low-quality vigilance. The collective detection hypothesis was partly supported: as group size increased, individual low- and high-quality vigilance did not decrease, but collective high-quality vigilance did increase. Furthermore, we found that repetitive alarm calling warned group members of terrestrial threats. Our results show that this invasive species displays specific antipredator behaviours to different aerial and terrestrial predators compared to predators in their endemic range. The low level of time allocated to low-quality vigilance indicates that natural selection strongly favours high-quality vigilance in this species despite the trade-off with foraging. Our study broadens our understanding of antipredator and risk-sensitive behaviour.