98:
355:
498:
436:) in large groups both spend less time with their head up than individuals in small groups; however, collective vigilance increases with group size in ostriches but not rheas. Therefore, the many eyes hypothesis does not always hold. Although individuals in groups may be able to decrease the time they spend scanning, and hence reach their energy requirements in a shorter period of time, the increased competition for food in groups may mean that animals need to devote more time overall to foraging due to an increased allocation of time for
216:) 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.
120:) 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
571:
detection of the group while being able to feed more than other individuals. However, cheating is not a stable strategy because if all members of the group did it, then there would be no collective vigilance. Adopting a very high level of vigilance in a group is also not a stable strategy as an individual who adopts a lower level of vigilance would have a greater advantage. The evolutionarily stable (
172:. 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,
420:
decrease the time it allocates to vigilance without any increase in its personal risk of being attacked - particularly as large groups already have a diluted predation risk. Lower individual levels of vigilance with increasing group size has been observed in many bird, mammal and fish species. Individual
578:
Maintaining individual vigilance may be a more beneficial strategy if vigilant animals gain some sort of advantage. Non-vigilant individuals are often the last to flee to safety, as groups generally flee in succession from the individual who spots the predator, individuals who were vigilant when the
513:
In some species, individuals in a foraging group take on sentry duty. The sentinels look out for predators (often from a good vantage point) while the rest of the group forages, and sound an alarm when they spot a predator. Sentry duty is particularly important for species whose foraging activity is
474:
of birds often exhibit collective detection. One or more birds initially detect the threat, and other birds that did not perceive the threat detect their departure and also respond by fleeing. The departure of multiple birds simultaneously is likely to be a more effective alarm signal than that of a
147:
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
304:
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. Information
419:
Larger groups may have a greater overall vigilance than small groups due to the ‘many eyes’ hypothesis: more eyes scanning for predators means that the proportion of time that at least one individual is vigilant (collective vigilance) is higher. This improved vigilance may allow an individual to
467:
in successful attacks, early detection of predators reduces predation risk. The detection of a predator by one individual only translates to collective detection if that individual makes some sort of signal to alert the rest of the group. The signal may be a deliberate call made by the vigilant
570:
In a large group, it is possible for an individual to cheat by scanning less than other members of the group without having any negative effect on the collective vigilance of the group. Cheating would seem to be the most beneficial strategy as the individual still benefits from the collective
188:
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.
552:
as an individual on sentry duty is unable to feed, may be more exposed to predators, and may attract the predators' attention when they make an alarm call. However, meerkats on sentry duty are at no greater risk of predation as they are generally the first to detect predators (e.g.
135:
the forager, as well as the likelihood that the forager can escape the predator if it is not vigilant. 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
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species) and flee to safety. Meerkats also only go on guard once they are satiated, so if no other individual is on sentry duty, guarding may be the most beneficial behaviour as the individual has no requirement for feeding and can benefit from early predator detection.
234:
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. In situations of high predation risk, animals may choose foods that can be foraged while maintaining vigilance. When
373:
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
229:
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. Different foods require different handling that can affect the amount of vigilance an animal can maintain.
267:
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.
251:), 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.
489:: individuals at the edge of the flotilla detect a predator and move, bumping into their neighbours, who in turn start moving and bumping into more individuals. This ‘wave’ of alarm has been termed the ‘Trafalgar Effect’.
280:
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.
79:
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. This impedes other activities, such as
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Large groups of animals are able to detect predators earlier because of the higher probability that at least one individual is being vigilant when the predator approaches. As many predators rely on the element of
386:
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.
66:
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.
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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
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first animal departs, and finally the non-vigilant individuals. Predators may also target less vigilant prey as they are likely to detect the predator later and therefore respond more slowly.
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Clutton-Brock, T.H., O’Riain, M.J., Brotherton, P.N.M., Gaynor, D., Kansky, R., Griffin, A.S. & Manser, M. (1999) Selfish sentinels in cooperative mammals. Science 284: 1640-1644
407:) to attack based on how visually conspicuous they are, preferring shoals that are larger or exhibit more movement. Foraging in groups requires sharing so may also lead to greater
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Wickler, W. (1985) Coordination of vigilance in bird groups, the ‘Watchman’s Song’ hypothesis. Zeitschrift fur
Tierphysiologie, Tierernahrung und Futtermittelkunde 69: 250-253
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Often, the sentry makes quiet calls that function as a ‘watchman’s song’ to reassure the rest of the group that an individual is on guard. In response to a vocalising sentry,
247:, 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
84:, 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.
912:
Treherne, J.E. & Foster, W.A. (1981) Group transmission of predator avoidance behaviour in a marine insect: the
Trafalgar Effect. Animal Behaviour 29: 911-917
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Fernandez, G.J., Capurro, A.F. & Reboreda, J.C. (2003) Effect of group size on individual and collective vigilance in greater rheas. Ethology 109: 413-425
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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
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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. Grouping may
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Rasa, O.A.E. (1986) Coordinated vigilance in dwarf mongoose family groups: the ‘Watchman’s Song’ hypothesis and the costs of guarding. Ethology 71: 340-344
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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
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Lima, S.L. & Dill, L.M. (1990) Behavioral decisions made under the risk of predation: a review and prospectus. Canadian
Journal of Zoology 68: 619-640
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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
575:) scanning rate is the rate that if all group members adopted, an individual that scanned more or less frequently would have a lower chance of survival.
75:
Vigilance and feeding (both searching for and handling food) are generally mutually exclusive activities, leading to foragers facing a trade-off between
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Fitzgibbon, C.D. (1989) A cost to individuals with reduced vigilance in groups of
Thomson’s gazelles hunted by cheetahs. Animal Behaviour 37(3): 508-510
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Hollen, L.I., Bell, M.B.V. & Radford, A.N. (2008) Cooperative sentinel calling? Foragers gain increased biomass intake. Current
Biology 18: 576-579
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that have been deprived of food exhibit lower levels of vigilance, instead focusing on rapid feeding, which is a behaviour incompatible with scanning.
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Clark, C.W. & Mangel, M. (1984) Foraging and flocking strategies: information in an uncertain environment. The
American Naturalist 123(5): 626-641
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Lima, S.L., Valone, T.J. & Caraco, T. (1985) Foraging-efficiency – predation-risk trade-off in the grey squirrel. Animal
Behaviour 33: 155-165
545:) decrease their own vigilance, spread out further from the group and forage in more exposed patches resulting in a higher intake of biomass.
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McNamara, J.M. & Houston, A.I. (1992) Evolutionarily stable levels of vigilance as a function of group size. Animal
Behaviour 43: 641-658
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Brown, J.S. (1999) Vigilance, patch use and habitat selection: foraging under predation risk. Evolutionary
Ecology Research 1: 49-71
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in groups of prey, often causing them to flee in different directions making it easier for the predators to single out a target.
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Lima, S.L. (1988) Vigilance and diet selection: a simple example in the dark-eyed junco. Canadian Journal of Zoology 66: 593-596
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Stander, P.E. (1992) Cooperative hunting in lions: the role of the individual. Behavioral Ecology and Sociobiology 29: 445-454
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Davies, N.B., Krebs, J.R. & West, S.A. (2012) An Introduction to Behavioural Ecology 4th edn, Wiley-Blackwell, Oxford, UK
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Pulliam, H.R. (1982) The scanning behavior of juncos: a game-theoretical approach. Journal of Theoretical Biology 95: 89-103
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Gilliam, J.F. (1982) Foraging under mortality risk in size-structured populations. PhD thesis. Michigan State University, MI
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between the two. The length of time animals devote to vigilance is dependent on many factors including predation risk and
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sharing in this way has little cost to the individual who locates the food if it occurs in high abundance in the patch.
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Caraco, T., Martindale, S. & Pulliam, H.R. (1980) Avian flocking in the presence of a predator. Nature 285: 400-401
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Lima, S.L. (1995) Back to the basics of anti-predatory vigilance: the group-size effect. Animal Behaviour 49: 11-20
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individual (as in the case of meerkats) or simply the departure of the individual who has detected the predator.
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Milinski, M. & Heller, R. (1978) Influence of a predator on the optimal foraging behaviour of sticklebacks (
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allows predators to take down larger prey, as well as prey that can outrun the predator but can be caught by an
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Illius, A.W. & Fitzgibbon, C. (1994) Costs of vigilance in foraging ungulates. Animal Behaviour 47: 481-484
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Barnard, C.J. (1980) Factors affecting flock size mean and variance in a winter population of house sparrows (
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than from predation, it is more beneficial to sacrifice vigilance to fulfill their energy requirements. When
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Roberts, G. (1996) Why individual vigilance declines as group size increases. Animal Behaviour 51: 1077-086
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incompatible with vigilance, or who feed in areas where they are highly exposed to predators. For example,
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Lima, S.L. (1994) On the personal benefits of anti-predatory vigilance. Animal Behaviour 48: 734-736
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Clarke, J.A. (1983) Moonlight’s influence on predator/prey interactions between short-eyed owls (
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62:, and allows them to reduce their own vigilance while the vigilance of the group is maintained.
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alter the timing of their foraging based on the level of light – avoiding feeding when the
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Bertram, B.C.R. (1980) Vigilance and group size in ostriches. Animal Behaviour 28: 278-286
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Makowska, J. & Kramer, D.L. (2007) Vigilance during food handling in grey squirrels,
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The state of an animal can change due to its behaviour and vice versa due to the dynamic
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23:, refers to an animal's monitoring of its surroundings in order to heighten awareness of
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single bird as birds regularly depart flocks for reasons other than predator detection.
109:) feeding in the safety of a tree to minimise exposure to predators during food handling
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Foraging in a group also has some anti-predator benefits. Being part of a group
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from the ground to eat; an activity which requires fixation of both their
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Vigilance is often observed in animals that forage in groups, such as
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which are a better quality food source. When a predator (the
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during foraging. For example, the rate of fighting between
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in a formation. The presence of many predators also causes
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595:) before initiating a chase and target them for attack.
276:) in large flocks are more able to escape predation by
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is bright as this is when predation risk is highest.
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presence. Vigilance is an important behaviour during
509:) on sentry duty looks out for predators on a perch
144:), is present compared to when the hawk is absent.
782:). Behavioral Ecology and Sociobiology 13: 205-209
295:Anti-predator adaptation § Safety in numbers
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548:Guarding may appear to be an
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900:) Behaviour 74(1):114-127
625:Elk antipredator behavior
158:three-spined sticklebacks
610:Anti-predator adaptation
605:Optimal foraging theory
587:) select less vigilant
327:for large prey such as
180:Habitat and food choice
94:such as hunger levels.
780:Peromyscus maniculatus
735:). Nature 275: 642-644
733:Gasterosteus aculeatus
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162:Gasterosteus aculeatus
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58:). Foraging in groups
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456:Further information:
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341:Connochaetes taurinus
289:Further information:
210:benthic invertebrates
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898:Passer domesticus L.
793:Sciurus carolinensis
550:altruistic behaviour
438:aggressive behaviour
415:Effect of group size
142:Parabuteo unicinctus
118:Sciurus carolinensis
107:Sciurus carolinensis
405:Poecilia reticulate
255:Vigilance in groups
249:Vigilance in groups
198:Lepomis macrochirus
21:behavioural ecology
1006:Behavioral ecology
593:Eudorcas thomsonii
589:Thomson's gazelles
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507:Suricata suricatta
483:Halobates robustus
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359:Red-billed queleas
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56:Suricata suricatta
44:yellow-eyed juncos
19:, in the field of
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446:Passer domesticus
393:Aequidens pulcher
220:Nocturnal animals
204:in the safety of
192:For example, the
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520:Helogale parvula
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243:) feed in small
237:dark-eyed juncos
194:bluegill sunfish
48:Junco phaeonutus
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631:References
524:arthropods
411:for food.
389:Blue acara
337:wildebeest
300:Advantages
154:starvation
92:physiology
532:olfaction
522:) dig up
493:Sentinels
422:ostriches
395:) choose
345:positions
317:lionesses
224:moonlight
129:abundance
33:trade-off
17:Vigilance
1000:Category
599:See also
581:Cheetahs
566:Cheating
465:surprise
278:goshawks
261:solitary
202:plankton
186:feedback
52:meerkats
29:foraging
25:predator
555:jackals
503:meerkat
401:guppies
376:mobbing
367:dilutes
323:) hunt
166:density
528:vision
472:Flocks
428:) and
397:shoals
335:) and
313:ambush
293:, and
245:flocks
208:or on
174:juncos
150:hungry
133:detect
82:mating
50:) and
37:hunger
559:eagle
487:touch
349:panic
329:zebra
265:group
259:Both
232:Seeds
206:reeds
530:and
263:and
573:ESS
399:of
168:of
1002::
956:^
944:^
905:^
871:^
859:^
800:^
756:^
740:^
722:^
710:^
698:^
682:^
668:^
638:^
557:,
501:A
101:A
39:.
591:(
583:(
541:(
518:(
505:(
481:(
444:(
432:(
424:(
403:(
339:(
331:(
319:(
272:(
239:(
196:(
160:(
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54:(
46:(
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