Group-Level Behaviour
Key References
SELECTIVE PRESSURES FOR GROUP LIVING
Anti-predator
Dilution
DAVIES et al. (2012): if a group of n individuals suffer less than n times as many attacks as an individual,
individuals will be safer in a group (where the risk of being the victim is 1/n).
Predator confusion
NEILL AND CULLEN (1974): tested the hunting success of four aquatic predators in laboratory experiments
which varied prey shoal size
Results: the success of an attack declined with increasing prey shoal size, which NEILL AND CULLEN
(1974) attributed mainly to increasing predator confusion
Example: fish shoals (IOANNOU et al. 2017)
In more cohesive shoals of guppies, predator attack success was lower
Synchronised movement
Prey vigilance
Example: meerkat (Suricata suricatta) sentinel systems (CLUTTON-BROCK et al. 1999).
Members of the group watch for predators from a prominent look-out perch while the rest of the
group forages on the ground below
Meerkats forage for 5 to 8 hours per day in the open, digging up to 20cm below ground to reach
invertebrates and small vertebrates
While digging, individuals are unable to detect predators and rely on the alarm calls of sentinels
Explanations of the evolution of sentinel behaviour have frequently relied on kin selection or
reciprocal altruism, but recent models suggest that guarding may be an individual's optimal activity
once its stomach is full if no other animal is on guard
Based on research in the Kalahari Gemsbok Park, South Africa, CLUTTON-BROCK et al. (1999) found
that although individuals seldom take successive guarding bouts, there is no regular rota, and the
provision of food increases contributions to guarding and reduces the latency between bouts by the
same individual. Furthermore, sentinel behaviour was found to provide a safety advantage to the
individual on guard, as these individuals were the first to detect predators and usually the first
below ground when a predator approached. As a result, acting as sentinel when satiated seems to
be the optimal selfish behaviour
One should note that giving guard calls to the rest of the group is unlikely to incur immediate direct
benefits (though the cost is probably low), making this aspect of sentinel behaviour difficult to
explain without invoking either deferred direct benefits of some kind or kin selection
Prey defence
Example: nesting black-headed gulls (Larus ridibundus) will mob crows who come to their colony in search
of eggs and chicks (KRUUK 1964)
Should a crow attack in the middle of a dense colony and thus encroach on many nests, numerous
gulls will mob the crow at the same time, reducing crow success in proportion to group size
Key References
SELECTIVE PRESSURES FOR GROUP LIVING
Anti-predator
Dilution
DAVIES et al. (2012): if a group of n individuals suffer less than n times as many attacks as an individual,
individuals will be safer in a group (where the risk of being the victim is 1/n).
Predator confusion
NEILL AND CULLEN (1974): tested the hunting success of four aquatic predators in laboratory experiments
which varied prey shoal size
Results: the success of an attack declined with increasing prey shoal size, which NEILL AND CULLEN
(1974) attributed mainly to increasing predator confusion
Example: fish shoals (IOANNOU et al. 2017)
In more cohesive shoals of guppies, predator attack success was lower
Synchronised movement
Prey vigilance
Example: meerkat (Suricata suricatta) sentinel systems (CLUTTON-BROCK et al. 1999).
Members of the group watch for predators from a prominent look-out perch while the rest of the
group forages on the ground below
Meerkats forage for 5 to 8 hours per day in the open, digging up to 20cm below ground to reach
invertebrates and small vertebrates
While digging, individuals are unable to detect predators and rely on the alarm calls of sentinels
Explanations of the evolution of sentinel behaviour have frequently relied on kin selection or
reciprocal altruism, but recent models suggest that guarding may be an individual's optimal activity
once its stomach is full if no other animal is on guard
Based on research in the Kalahari Gemsbok Park, South Africa, CLUTTON-BROCK et al. (1999) found
that although individuals seldom take successive guarding bouts, there is no regular rota, and the
provision of food increases contributions to guarding and reduces the latency between bouts by the
same individual. Furthermore, sentinel behaviour was found to provide a safety advantage to the
individual on guard, as these individuals were the first to detect predators and usually the first
below ground when a predator approached. As a result, acting as sentinel when satiated seems to
be the optimal selfish behaviour
One should note that giving guard calls to the rest of the group is unlikely to incur immediate direct
benefits (though the cost is probably low), making this aspect of sentinel behaviour difficult to
explain without invoking either deferred direct benefits of some kind or kin selection
Prey defence
Example: nesting black-headed gulls (Larus ridibundus) will mob crows who come to their colony in search
of eggs and chicks (KRUUK 1964)
Should a crow attack in the middle of a dense colony and thus encroach on many nests, numerous
gulls will mob the crow at the same time, reducing crow success in proportion to group size
