Biology Reference
In-Depth Information
Summary
Grouping can reduce individual predation in several ways: by diluting the risk of attack
(water skaters, mayflies), by reducing an individual's domain of danger (selfish herd:
seals attacked by sharks), by confusing predator attacks (fish shoals), by communal
defence (gull and guillemot colonies) and by improved vigilance for predators (bird
flocks). Within groups, individuals behave in their own selfish interests. When hungry,
they may go to the front of the group where they will encounter food first, while when
satiated they may seek safety in the centre (selfish herd effect).
Individual vigilance levels are maintained in larger groups because more vigilant
individuals are less likely to be attacked (gazelles) and they can flee to cover faster,
because they are sooner to spot the predator or their alarmed companions. In some
groups, satiated individuals may benefit from acting as sentinels (meerkats, babblers).
Grouping can also improve foraging through better food finding (information centres;
e.g. raven roosts) or better prey capture, either through coordinated group hunting
(lions) or through scattering prey groups (gulls, some predatory fish). A study of guppies
in Trinidad has revealed the evolution of reduced and then increased shoaling tendency
in response to changing predation pressure over a 30-year period.
In theory, there may be an optimal group size but this may not be stable. There is
often 'skew' in the net benefits to individuals from grouping. Subordinates may restrain
their resource intake to avoid being evicted by dominants (fish size hierarchies).
Dominants may evict subordinates who try to increase their share of resources (banded
mongooses).
Complex group movements emerge from individuals adopting simple local movement
rules in response to neighbours (e.g. traffic lanes in army ants, fish shoaling). Leadership
may emerge if a small proportion of the group has a preferred direction of movement.
A group may move after individuals have voted for their preferred choice (e.g. house
hunting in ants and bees).
Further reading
Krause and Ruxton (2002) is an excellent review of the benefits and costs of group
living. Caro (2005) reviews anti-predator defences in birds and mammals, including
grouping. Cresswell (1994) is a fine field study of raptors hunting flocks of redshank.
Giraldeau and Caraco (2000) review models and empirical studies of social foraging.
Lima (1998) considers individual decision making under the risk of predation.
Packer
et al
. (1990) and Mosser and Packer (2009) show that a major benefit of lion
sociality is through joint defence of a high quality territory versus other lion prides.
Cant (2011) reviews how reproductive conflict and skew in social groups might be
resolved by threats from dominants (attack, eviction), action by dominants (infanticide,
interference with mating, aggression) or by threats or action by subordinates in the
group (departure, interference). Cant and Johnstone (2009) model how these threats to
exercising alternative options (leaving the group or evicting others from the group)
might influence conflict resolution. Hamilton (2000) uses skew theory to predict how
