“Space use and leadership modify dilution effects on optimal vigilance under food/safety trade-offs”

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Rémi Patin, Daniel Fortin, Cédric Sueur, and Simon Chamaillé-Jammes (Jan 2019)

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Decision-making on space use matters! A model of interacting antipredator strategies: vigilance, grouping, and space use

Living in groups sometimes allows animals to be less vigilant – but it could also allow them to use riskier areas, and group leaders would then be safer than followers

A group of plains zebras (Equus quagga) with one vigilant individual.
(Credit: S. Chamaillé-Jammes)

Most animals live under predation risk, and adjust their vigilance to the level of risk they perceive. When living in group, individuals benefit from a “dilution” effect, by which their own risk is reduced by the presence of the other group members that could also be targeted by the predator. Many models of evolution of anti-predator behaviours have focused on the interaction between vigilance and group size, and have always suggested that vigilance should decrease with increasing group size. This is commonly observed, but not always. And animals also adjust predation risk by foraging in areas that they perceive as less risky. In a new article appearing in The American Naturalist, Rémi Patin and colleagues propose a model, designed with social species such as primates or equids in mind, to study the interactions between vigilance, group size, and space use. The model assumes that, within groups, a leader takes all space use decisions for the rest of the group made up of followers. In this situation, the relationship between vigilance and group size becomes complex. Vigilance does not always decrease with group size, because the leader may favor using riskier and richer areas over becoming less vigilant. Followers are less vigilant than the leader, because they need to compensate for uncertainties in future foraging opportunities, and die from predation more often. Overall, followers still benefit from group living, but less than the leader. Leaders might gain more by remaining leaders than by becoming followers in a larger group. These results might therefore explain the apparently paradoxical observation made in some species, such as in equids, that a group leader would reject a potential new group member despite the dilution benefit the addition of the new individual would bring.


Abstract

Dilution of predation risk within groups allows individuals to be less vigilant and forage more while still facing lower risk than if they were alone. How group size influences vigilance when individuals can also adjust their space use, and whether this relationship differs among individuals contributing differently to space use decisions, remain unknown. We present a model-based study on how dilution affects the optimal anti-predator behavior of group members, in groups where all individuals determine their vigilance level, while group leaders also determine space use. We showed that optimal vigilance did not always decrease with group size, as it was sometimes favorable for individuals in larger groups to use riskier patches while remaining vigilant. Followers were also generally less vigilant than leaders. Indeed, followers needed to acquire more resources than leaders as only the latter could decide when to go to richer patches. Followers still benefit from dilution of predation risk compared to solitary individuals. For leaders, keeping their leadership status can be more important than incorporating new group members to increase dilution. We demonstrate that risk dilution impacts both optimal vigilance and space use, with fitness reward being tied to a member’s ability to influence group space use.