When are two heads better than one?: Visual perception and information transfer affect vigilance coordination in foraging groups

Animals frequently raise their heads to check for danger. In a group, individuals generally raise their heads independently. Earlier models suggest that all group members could gain by coordinating their vigilance, i.e., each member raising its head when others are not. We re-examine these suggestions, considering groups of different sizes, in light of empirical findings that: (1) animals can sometimes detect a predator without raising their heads, and (2) when one member of a group detects a predator, the information does not always spread to other members of the group. Including these effects in models generally decreases the value of coordinated vigilance. Coordinated vigilance is highly favored only when animals have a low probability of detecting predators without lifting their heads but a high probability of being warned when another member of the group detects a predator. For other combinations, coordinated vigilance has little value and may have a negative value. Group size has contrasting effects depending on how social information is obtained. Coordination is favored in smaller groups when one or more detecting individuals provide a constant amount of information to individuals unaware of the predator. On the other hand, coordination is favored in larger groups if each detecting individual provides unaware individuals with an independent source of information (i.e., if the amount of information increases as the number of detecting individuals increases). These results depend on the balance of an escape due to social information and dilution of risk in groups with imperfect information spread. This framework could be tested by examining species with different visual fields and in different environments.