Why individual vigilance declines as group size increases

  title={Why individual vigilance declines as group size increases},
  author={Gilbert Roberts},
  journal={Animal Behaviour},
  • G. Roberts
  • Published 1 May 1996
  • Psychology
  • Animal Behaviour
A reduction in individual vigilance with an increase in group size is one of the most frequently reported relationships in the study of animal behaviour. It has been argued that this phenomenon may not be a direct consequence of an increase in group size but may be due to other factors relating to increased group size, such as increased foraging competition. However, there is evidence for a direct relationship between group size and vigilance where other variables have been controlled. The aim… 

The contingencies of group size and vigilance

This work uses game theory and the fitness-generating function to determine the ESS level of vigilance of an individual within a group, and finds that vigilance decreases with group size as a consequence of the many-eyes and dilution effects, but increases with group sizes as a result of the attraction effect.

Individual variation in the relationship between vigilance and group size in eastern grey kangaroos

The results suggest that only some prey individuals may gain anti-predator benefits by reducing their time spent scanning when in larger groups, and propose that some females exhibit higher levels of social vigilance than others, and that this social vigilance increases with group size, cancelling out any group-size effect on anti- predator vigilance for those females.

Prey synchronize their vigilant behaviour with other group members

The results confirmed that the proportion of time an individual spent in vigilance decreased with group size, but the time during which at least one individual in the group scanned the environment (collective vigilance) increased, and it was claimed that these waves are triggered by allelomimetic effects i.e. an individual copying its neighbour's behaviour.

Changes in Vigilance with Group Size under Scramble Competition

The role of resource availability on vigilance patterns is explored and it is indicated that when resources are limited, a reduction in vigilance may allow individuals to allocate more time to foraging and so obtain a greater share of the re-foraging time.

The group size effect and synchronization of vigilance in the Tibetan wild ass

There was a positive correlation between group members’ behaviors, indicating that Tibetan wild asses tend to synchronize their vigilance, and many models of vigilance assume that group members scan independently of one another.

Effect of Group Size on Individual and Collective Vigilance in Greater Rheas

The results indicate that rheas foraging in large groups would not receive the benefit of an increase in collective vigilance, although they could still benefit from a reduction of predation risk by the dilution effect.




  • M. Elgar
  • Environmental Science
    Biological reviews of the Cambridge Philosophical Society
  • 1989
Most of the studies fail to adequately demonstrate an unambiguous relationship between vigilance behaviour and group size, but many studies reveal interesting features of the relationship between Vigilance and Group size that should provide fruitful avenues for future research.

Back to the basics of anti-predatory vigilance: the group-size effect

  • S. L. Lima
  • Psychology, Environmental Science
    Animal Behaviour
  • 1995

Vigilance does not covary with group size in an island population of silvereyes (Zosterops lateralis)

There was no relationship between scanning behavior and group size under Heron Island's predator-free environment, and covariation between group size and vigilance is not expected in the absence of predators.

Vigilance while feeding and its relation to the risk of predation

Evolutionarily stable levels of vigilance as a function of group size

From individual to collective vigilance in wild boar (Sus scrofa)

Vigilance behaviour of captive wild boars at feeding points was investigated using films. Results show that individual vigilance decreased with increasing group size, especially between solitary

Vigilance and group size in ostriches

Group Vigilance and Shoal Size in a Small Characin Fish

The importance of the enhanced predator detection ability of fish shoals and the social transmission of alarms within them is discussed in relation to predator avoidance behaviour and other activities of fish in shoals.