Vigilance, in the field of behavioural ecology, refers to an animal's monitoring of its surroundings in order to heighten awareness of predator presence. Vigilance is an important behaviour during foraging as animals must often venture away from the safety of shelter to find food. However, being vigilant comes at the expense of time spent feeding, so there is a trade-off between the two. The length of time animals devote to vigilance is dependent on many factors including predation risk and hunger.
Vigilance is often observed in animals that forage in groups, such as yellow-eyed juncos (Junco phaeonutus) and meerkats (Suricata suricatta). Foraging in groups dilutes an individual's risk of predation, and allows them to reduce their own vigilance while the vigilance of the group is maintained.Davies, N.B., Krebs, J.R. & West, S.A. (2012) An Introduction to Behavioural Ecology 4th edn, Wiley-Blackwell, Oxford, UK Alarm signals may be used to alert the group to the presence of predators. Groups of some species have at least one individual on sentry duty who looks out for predators on a perch while the rest of the group forages.
Feeding and predation risk trade-off Vigilance and feeding (both searching for and handling food) are generally mutually exclusive activities, leading to foragers facing a trade-off between energy intake and safety from predation. As time allocated to scanning reduces the time spent feeding, vigilant individuals must devote more time to foraging to obtain the required food intake.Illius, A.W. & Fitzgibbon, C. (1994) Costs of vigilance in foraging ungulates. Animal Behaviour 47: 481-484 This impedes other activities, such as mating, and prolongs their exposure to predation as foraging occurs away from shelter. When foraging time is limited, vigilant animals are left with a reduced energy intake.
Optimality models can be used to predict foraging decisions of an animal based on costs (predation risk, starvation) and benefits (safety, food), which are also affected by physiology such as hunger levels.
Grey squirrels (Sciurus carolinensis) alter their behaviour according to the relative costs and benefits when foraging in the open. Small food items are consumed immediately to maximise energy intake, as they require little handling time so the predation risk is low. Large items that require a long handling time, and hence time exposed to predators, are carried back to the safety of a tree to minimise predation risk. Although there is an energetic cost to transporting food, large food items have a high contribution to nutrient intake, so the benefits outweigh the cost.
The overall predation risk is a function of the abundance, activity and ability of predators to detect the forager, as well as the likelihood that the forager can escape the predator if it is not vigilant.Brown, J.S. (1999) "Vigilance, patch use and habitat selection: foraging under predation risk." Evolutionary Ecology Research 1: 49-71 Animals prioritise vigilance over feeding when the predation risk is high. For example, yellow-eyed juncos spend more time scanning for predators when a potential predator, a Harris's hawk (Parabuteo unicinctus), is present compared to when the hawk is absent.Caraco, T., Martindale, S. & Pulliam, H.R. (1980) Avian flocking in the presence of a predator. Nature 285: 400-401
Another factor that influences vigilance is the benefit that is expected from foraging in the absence of predation. This is dependent on the quality of the food as well as the energetic state of the individual. If there is much to be gained from feeding, foragers may forgo vigilance. Similarly, if hungry animals have a higher chance of dying from starvation than from predation, it is more beneficial to sacrifice vigilance to fulfill their energy requirements. When three-spined sticklebacks (Gasterosteus aculeatus) are deprived of food, they prefer to feed in locations with a high density of water fleas.Milinski, M. & Heller, R. (1978) Influence of a predator on the optimal foraging behaviour of sticklebacks (Gasterosteus aculeatus). Nature 275: 642-644 The cost to this choice is that the sticklebacks must concentrate on picking out the prey due to the 'predator confusion effect' where many moving targets make it difficult for predators to pick out individual prey. This choice means that the sticklebacks are less able to scan for predators; however, the risk of starvation is relatively higher than the risk of predation. Similarly, juncos that have been deprived of food exhibit lower levels of vigilance, instead focusing on rapid feeding, which is a behaviour incompatible with scanning.Lima, S.L. (1995) Back to the basics of anti-predatory vigilance: the group-size effect. Animal Behaviour 49: 11-20
Habitat and food choice The state of an animal can change due to its behaviour and vice versa due to the dynamic feedback between foraging, body reserves and predation risk. The feedback can influence an individual's choice of where, when and what to feed on. If the predation risk is so great that an animal must maintain a level of vigilance that drastically inhibits feeding, it may opt for an alternative.
For example, the bluegill sunfish (Lepomis macrochirus) has the choice of foraging on plankton in the safety of reeds or on benthic invertebrates which are a better quality food source.Gilliam, J.F. (1982) Foraging under mortality risk in size-structured populations. PhD thesis. Michigan State University, MI When a predator (the largemouth bass) is present, smaller sunfish spend the majority of their time foraging in the reeds despite this choice reducing their food intake and seasonal growth rate. Sunfish that are too large to be eaten by the bass forage almost entirely on benthos. Although staying in the reeds results in a slower growth rate and a longer period of being a size vulnerable to predators, for maximum survival sunfish choose to remain in the reeds feeding on plankton until they reach a certain size and then leave to feed on benthos.
Nocturnal animals alter the timing of their foraging based on the level of light – avoiding feeding when the moonlight is bright as this is when predation risk is highest.Clarke, J.A. (1983) Moonlight’s influence on predator/prey interactions between short-eyed owls (Asio flammeus) and deermice (Peromyscus maniculatus). Behavioral Ecology and Sociobiology 13: 205-209
A raised head is the most common indicator of vigilance, as many animals require their heads to be lowered to search for and handle food.Makowska, J. & Kramer, D.L. (2007) Vigilance during food handling in grey squirrels, Sciurus carolinensis. Animal Behaviour 74: 153-158 Different foods require different handling that can affect the amount of vigilance an animal can maintain. Seeds without husks, for example, require little handling by birds so are rapidly pecked up with the bird's head down, which is incompatible with vigilance.Lima, S.L. (1988) Vigilance and diet selection: a simple example in the dark-eyed junco. Canadian Journal of Zoology 66: 593-596 In situations of high predation risk, animals may choose foods that can be foraged while maintaining vigilance. When dark-eyed juncos (Junco hymenalis) feed in small flocks, they prefer to feed on larger pieces of food than when they are part of a larger flock. As individuals in smaller flocks have a greater need to be vigilant (see more in Vigilance in groups), large pieces of food are more beneficial as they require a longer handling time that can be simultaneously spent scanning, whereas birds feeding on small pieces must intermittently stop foraging to scan their environment.
Vigilance in groups
Both solitary and group foraging come with a variety of costs and benefits, but for many animals, foraging in a group is the most optimal strategy. Among the many benefits of group foraging, reduction of predation risk by improved vigilance is one. Wood pigeons (Columba palumbus) in large flocks are more able to escape predation by goshawks because they are able to spot them and fly away more quickly than they would individually. This is because in larger flocks, it is more likely that one bird will notice the hawk sooner and alert the group by flying away.
Group foraging
Advantages A group of animals may be better at both finding and capturing food than solitary animals. For species that feed on food that is patchy, the likelihood of locating food is greater if individuals search for food independently and inform the rest of the group when a food patch is found.Clark, C.W. & Mangel, M. (1984) Foraging and flocking strategies: information in an uncertain environment. The American Naturalist 123(5): 626-641 Information sharing in this way has little cost to the individual who locates the food if it occurs in high abundance in the patch.
Group hunting allows predators to take down larger prey, as well as prey that can outrun the predator but can be caught by an ambush. For example, lionesses (Panthera leo) hunt cooperatively for large prey such as zebra (Equus burchelli) and wildebeest (Connochaetes taurinus), with each lioness adopting particular positions in a formation.Stander, P.E. (1992) Cooperative hunting in lions: the role of the individual. Behavioral Ecology and Sociobiology 29: 445-454 The presence of many predators also causes panic in groups of prey, often causing them to flee in different directions making it easier for the predators to single out a target.
Foraging in a group also has some anti-predator benefits. Being part of a group dilutes an individual's risk of being attacked, as the more group members there are, the lower the probability that that individual will be the victim.Foster, W.A. & Treherne, J.E. (1981) Evidence for the dilution effect in the selfish herd from fish predation on a marine insect. Nature 293: 466-467 Grouping may swamp the predator's capacity to capture prey - for example, hawks are unable to capture more than one yellow-eyed junco per attack. Large groups also cause predator confusion as it makes it difficult for prey to focus on one target. Groups of animals may engage in communal defences, such as mobbing, for further protection.
Disadvantages Dilution of predation risk only occurs if groups of animals are not more prone to attack than individuals. Often large groups are more conspicuous to predators so grouping may increase the risk of attack. Blue acara cichlid fish (Aequidens pulcher) choose shoals of guppies (Poecilia reticulate) to attack based on how visually conspicuous they are, preferring shoals that are larger or exhibit more movement.Krause, J. & Godin, J.J. (1995) Predator preferences for attacking particular prey group sizes: consequences for predator hunting success and prey predation risk. Animal Behaviour 50: 465-473 Foraging in groups requires sharing so may also lead to greater competition for food.
Effect of group size