Elsevier

Behavioural Processes

Volume 63, Issue 3, 31 July 2003, Pages 111-121
Behavioural Processes

Review
Group-size effects on vigilance: a search for mechanisms

https://doi.org/10.1016/S0376-6357(03)00002-0Get rights and content

Introduction

In foraging animals, visual scanning of the surrounding environment is referred to as vigilance and is commonly assumed to be aimed at potential predation threats (Roberts, 1996, Treves, 2000). In several species of birds and mammals, vigilance levels have been shown to decrease with increases in group size (Elgar, 1989, Quenette, 1990). The decline in vigilance is usually interpreted as a response to the lower risk of predation in larger groups. Several mechanisms may be involved in the decline. For instance, more eyes are available to scan the surrounding environment in larger groups thus allowing individuals to detect predators more easily and decrease individual investment in vigilance without increasing predation risk. When predators attack a single prey per group, the presence of companions in a group dilutes individual risk which again may favour a decrease in individual vigilance investment. The time thus saved can be allocated to other activities such as foraging.

Despite the apparent success of behavioural studies in documenting group-size effects on vigilance, two problems have beset interpretation of the results. The first problem is related to the cause of the decline in vigilance since mechanisms unrelated to predation risk may also produce decreases in vigilance with group size. In particular, increased competition in large groups may induce a decrease in individual vigilance levels as foragers scramble for a greater proportion of limited resources (Clark and Mangel, 1986). Although the potential influence of scramble competition on vigilance has long been recognised, theoretical and empirical interest in the matter has only risen recently.

The second problem is related to the nature of the decline in vigilance. Predation factors, such as dilution or detection effects, are expected to produce an asymptotic decrease in vigilance with group size (Pulliam, 1973, Dehn, 1990). Although models have considered the relative contribution of detection and dilution effects to the decline in vigilance, the role of other behavioural mechanisms, which may interact with predation risk, has been neglected. For instance, interference among foragers can reduce individual allocation of time to feeding and vigilance (Slotow and Coumi, 2000, Blumstein et al., 2001a). Here, vigilance levels are expected to reflect a compromise between the pull of predation risk and the push of interference competition. Under these circumstances, vigilance levels have been predicted to decline in a linear, rather than non-linear fashion with group size (Blumstein et al., 2001a) or to fail to decline at all (Slotow and Coumi, 2000). Scrounging is another mechanism that could influence vigilance levels in groups of different sizes. Scanning can serve to detect the discovery of food sources by companions thus allowing foragers to scrounge resources produced by others (Giraldeau and Beauchamp, 1999). The inclusion of scrounging in vigilance models leads to changes in vigilance levels with group size and even calls into question the prediction of a decline in vigilance in large groups (Beauchamp, 2001). Therefore, unless the contribution of these factors is known, the precise pattern of changes in vigilance with group size is not easy to predict.

The purpose of this paper is to review recent theoretical and empirical research to understand the cause and the nature of changes in vigilance levels with group size with the ultimate view to assess the contribution of several mechanisms to the group-size effect.

Section snippets

Detection

Although several researchers postulated that groups could detect predators more easily than solitary animals, the first attempt to model group-size effects on vigilance, and thus to establish relevant assumptions about the predation process, was made by Pulliam (Pulliam, 1973). In Pulliam’s model, predators attack from ambush without regard to the level of vigilance maintained by the prey. The attack will be successful if the predator crosses a threshold distance undetected by any group member.

The nature of changes in vigilance with group size

Predation-related factors, such as dilution or detection effects, are expected to produce non-linear declines in vigilance with group size. What is the relative contribution of each factor and how will the shape of the expected decline in vigilance change as a result? In addition, what happens if other ecological factors interact with predation risk to determine the shape of the group-size effect?

Discussion

With respect to the causes of the decline in vigilance with group size, models indicate clearly that mechanisms based on predation effects or scramble competition effects represent equally viable alternative explanations. Therefore, the challenge for future studies is clearly to establish the relevance of each explanation to the group-size effect on vigilance. In terms of theoretical effort, models that combine the two types of effects appear most promising especially if it could be shown that

First page preview

First page preview
Click to open first page preview

References (53)

  • C.M.S. Plowright et al.

    A direct effect of competition on food choice by pigeons

    Behav. Proc.

    (2000)
  • H. Pöysä

    Group foraging, distance to cover and vigilance in the teal, Anas crecca

    Anim. Behav.

    (1994)
  • H.R. Pulliam

    On the advantages of flocking

    J. Theor. Biol.

    (1973)
  • H.R. Pulliam et al.

    The scanning behavior of juncos: a game-theoretical approach

    J. Theor. Biol.

    (1982)
  • G. Roberts

    Why individual vigilance declines as group size increases

    Anim. Behav.

    (1996)
  • N. Saino

    Time budget variation in relation to flock size in carrion crows, Corvus corone corone

    Anim. Behav.

    (1994)
  • C.W. Tolman et al.

    Social feeding in domestic chicks

    Anim. Behav.

    (1965)
  • A. Treves

    Theory and method in studies of vigilance and aggregation

    Anim. Behav.

    (2000)
  • C.J. Barnard et al.

    Costs and benefits of single and mixed-species flocking in fieldfares (Turdus pilaris) and redwings (T. iliacus)

    Behaviour

    (1983)
  • C.J. Barnard et al.

    Time and energy budgets and competition in the common shrew (Sorex araneus L.)

    Behav. Ecol. Sociobiol.

    (1983)
  • G. Beauchamp

    The effect of group size on mean food intake rate in birds

    Biol. Rev.

    (1998)
  • G. Beauchamp

    Should vigilance always decrease with group size?

    Behav. Ecol. Sociobiol.

    (2001)
  • G. Beauchamp et al.

    The effect of group size on vigilance and feeding rate in spice finches (Lonchura punctulata)

    Can. J. Zool.

    (1997)
  • P.A. Bednekoff et al.

    Re-examining safety in numbers: interactions between risk dilution and collective detection depend upon predator targeting behaviour

    Proc. R. Soc. London B.

    (1998)
  • M. Bekoff

    Vigilance, flock size, and flock geometry—information gathering by western evening grosbeaks (Aves, Fringillidae)

    Ethology

    (1995)
  • D.T. Blumstein et al.

    Yellow-footed rock-wallaby group size effects reflect a trade-off

    Ethology

    (2001)
  • Cited by (97)

    • Prey herding and predators’ feeding satiation induce multiple stability

      2023, Communications in Nonlinear Science and Numerical Simulation
    • Function of meerkats' mobbing-like response to secondary predator cues: recruitment not teaching

      2022, Animal Behaviour
      Citation Excerpt :

      If the type of threat responded to was not known, it was recorded as ‘unknown’ and excluded from our analyses. Group size and composition was included to investigate the effect of pups (Prediction 1) as well as the role of group size on antipredator responses (Beauchamp, 2003; Elgar, 1989). The daily maximum temperature and the total rainfall measured at the study site over the previous 30 days were used to control for current habitat conditions and food abundance, as these may influence investment in costly behaviours (Hodge et al., 2009; Thornton, 2008; Wiley & Ridley, 2016).

    • The effect of age, environment and management on social contact patterns in sheep

      2020, Applied Animal Behaviour Science
      Citation Excerpt :

      that individuals of similar weight (i.e., body mass) will interact more with each other given that body mass is an important determinant of social rank in ungulates (McElligott et al., 2001; Holand et al., 2004); iv. individuals would assort based on lambing date (i.e., reproductive status) due to the potential anti-predator benefits that lactating ewes may gain by associating together (Beauchamp, 2003; Rieucau and Martin, 2008); v. individuals belonging to the same mating group will be more familiar with each other and maintain the social bonds formed in the sub-group (Keller et al., 2011). Lastly, we evaluated the effect of environmental conditions on patterns of social contact.

    View all citing articles on Scopus
    View full text