Abstract
Vigilance in animals is an important means for predator detection. Animals living in groups reduce their predation risk as more individuals are present. In contrast to most other animals studied, many studies on primates do not support the prediction that individual vigilance will decline as group size increases. For animals to obtain visual information during vigilance behaviour, their eyes must be open. Therefore, if animals are able to perceive differential risk of predation, the inter-blink interval (eye-opening) should increase, and the blink duration (eye-closure) should decrease under higher predation risk. We tested this prediction by measuring inter-blink interval in wild anubis baboons (Papio anubis) in peripheral and centre individuals within a group, and between larger and smaller groups. We found that the inter-blink interval for young males, often located at the front edge of the group, was longer than that of adult males, adult females, and young females, often located in the center of the group, and that the inter-blink interval for adult males was longer when the group was smaller. These results suggest that inter-blink interval can be used as an indicator of primate vigilance toward predators.
Highlights
Predation is one of the most important selective pressures for animal behaviours[1] and group-living is widespread among most taxa of the animal kingdom including mammals, birds, fishes and insects[2]
Individual animals belonging to a large group do not have to be vigilant for the same amount of time as when they live in a small group
Post hoc analyses indicated that young males opened their eyes for longer periods than other age/sex classes
Summary
Predation is one of the most important selective pressures for animal behaviours[1] and group-living is widespread among most taxa of the animal kingdom including mammals, birds, fishes and insects[2]. The latest review on the group size effect between vigilance and group (including subgroup) in non-human primates reported that 6 of 15 studies had no effect, 3 showed a positive effect and 6 showed a negative effect[17]. Previous results in primates were based on different methods and cannot be compared collectively This suggests it is necessary to develop indicators of vigilance behaviours that can be used on both primates and other mammals. The study of non-human primates in zoos compared the blink rate among 71 species, showed that the blink rate was not affect by habitat types (arboreal, semiarboreal, terrestrial) but increased as average group size increased[26]. Adult male peacocks (Pavo cristatus) shortened blinking after a predator was revealed[28] These studies suggest that animals suppress blinking in alert situations and acquire more information by opening their eyes for a longer time. To minimize loss of information, blinks need to be strategically adjusted
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