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ABSTRACT: Animals generally allocate some time during foraging to detecting predators. We used a combination of observations and an experiment to examine how vegetation height and peripheral obstructions influence vigilance by foraging yellow-bellied marmots (Marmota flaviventris). First, we analyzed a large sample of observations of marmots foraging in nature. Marmots increased vigilance with vegetation height and reared on their hind legs when in tall vegetation. Second, we observed that marmots foraged in locations with lower vegetation than randomly selected sites in the same meadow. These observations suggest that marmots account for what they can see while foraging but do not rule out the influence of other factors correlated with vegetation height. Therefore, we experimentally blocked the view for 3 sides for marmots feeding on a controlled food source. When the apparatus blocked their vigilance, marmots were less vigilant when foraging, often moved outside the apparatus, and showed heightened vigilance while outside the apparatus. Peripheral obstructions explained more of the variance in our experimental than in our observational results. Together, our results demonstrate that marmots employ antipredator behavior to compensate for peripheral obstructions. Long-term studies show that marmots go locally extinct more often in areas with more obstructions to vigilance. Thus, marmots likely face greater predation risk in those areas, despite the behavioral responses documented in this study. Copyright 2009, Oxford University Press.
Behavioral Ecology. 01/2009; 20(5):1111-1117.
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ABSTRACT: Animals often feed more quickly when in larger groups. This group-size effect is often explained by safety advantages for groups but an alternative explanation is that animals feed faster in larger groups because of greater scramble competition for limited food. We show that predation risk enhances the group-size effect if groups vary in size. By contrast, competition leads to the group-size effect only when individuals feed in groups of constant size. When individuals feed in groups that vary in size, the best strategy for dealing with competition is to feed intensely when in smaller groups and feed little when in larger (more competitive) groups. In all situations, the effects of competition interact with the effects of predation risk in a simple multiplicative way. Our results suggest that scramble competition is not a general explanation for the group-size effect on vigilance in situations where group size changes relatively rapidly.
Proceedings of the Royal Society B: Biological Sciences 08/2004; 271(1547):1491-6. · 5.41 Impact Factor
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ABSTRACT: 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. Copyright 2004.
Behavioral Ecology. 01/2004; 15(6):898-906.
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ABSTRACT: In socially feeding birds and mammals, as group size increases, individuals devote less time to scanning their environment
and more time to feeding. This vigilance “group size effect” has long been attributed to the anti-predatory benefits of group
living, but many investigators have suggested that this effect may be driven by scramble competition for limited food. We
addressed this issue of causation by focusing on the way in which the scan durations of free-living dark-eyed juncos (Junco hyemalis) decrease with group size. We were particularly interested in vigilance scanning concomitant with the handling of food items,
since a decrease in food handling times (i.e. scan durations) with increasing group size could theoretically be driven by
scramble competition for limited food resources. However, we showed that food-handling scan durations decrease with group
size in an environment with an effectively unlimited food supply. Furthermore, this food-handling effect was qualitatively
similar to that observed in the duration of standard vigilance scans (scanning exclusive of food ingestion), and both responded
to changes in the risk of predation (proximity of a refuge) as one might expect based upon anti-predator considerations. The
group size effects in both food-handling and standard scan durations may reflect a lesser need for personal information about
risk as group size increases. Scramble competition may influence vigilance in some circumstances, but demonstrating an effect
of competition beyond that of predation may prove challenging.
Behavioral Ecology and Sociobiology 01/1999; 46(2):110-116. · 3.18 Impact Factor
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ABSTRACT: Many birds and mammals respond to a heightened risk of predation, especially that associated with smaller group sizes, with an increase in vigilance. All interpretations of the way in which vigilance responds to changes in predation risk assume that animals feeding with their heads down (i.e. animals in a nonvigilant state) cannot detect approaching predators. We provide the first explicit test of this assumption by ‘flying’ a mounted hawk down a 15-m chute towards actively feeding, free-living, dark-eyed juncos, Junco hyemalis. Juncos were targeted individually for simulated attack when they had either a ‘head-down’ view up the chute, or a completely unobstructed view; a junco with a head-down view could see up the chute only when it lowered its head to feed. Juncos with an unobstructed view almost always detected the hawk at the maximum distance of 15 m. Juncos with a head-down view usually detected the attack at a distance of 10–15 m against a grey background, but detection distances were shorter when attacks occurred against a camouflaged background. The results demonstrate that these birds have a considerable ability to detect approaching predators even when not overtly vigilant, although their detection ability is greater when they raise their heads. Vigilance sequences, therefore, probably consist of bouts of low-quality detection (active feeding) interspersed with bouts of higher-quality detection (overt vigilance) that can only be accomplished at the expense of feeding. This realization has major implications for current interpretations of the vigilance group size effect and antipredator vigilance in general.
Animal Behaviour.
