Living in groups!
• Diversity of social grouping
Spatial distribution of organisms… Pattern of spacing among individuals:
Clumped, e.g. shoals, herds, flocks, colonies
Uniform, e.g. nesting territories (parents attempting to protect their eggs
from predation)
Random, e.g. trees in a forest
Diversity of animal social groups…
Insects: swarms, colonies
Fish: shoals, schools (shoal = any group of fish that form social
aggregations, school = group of fish swimming in the same direction)
Birds: flocks, colonies
Mammals: prides, herds, packs
Group size can vary from a few individuals (e.g. a gorilla troop) to many millions
(e.g locust swarm)
However, an animal group is not merely a spatial position of individuals. It is a
social construct (interaction) between spatially aggregated participants.
• Group movements - coordination matters!
Large shoals of fish and bird flocks have complex patterns of movement (e.g.
murmuration of starling flocks)
What rules govern complex movements?
Individuals watch the whole flock and adjust their behaviour to the whole
flock
Each individual watches 6-7 nearest neighbours and coordinates with
them
Individuals differ: there are leaders and followers
Figures demonstrate that although the full (absolute) velocities
of the birds are relatively similar, the velocity fluctuations
differ significantly between birds depending on their position in
the flock.
Study: Here we suggest that collective response in animal
groups may be achieved through scale-free behavioral
correlations. By reconstructing the 3D position and velocity of
individual birds in large flocks of starlings, we measured to
what extent the velocity fluctuations of different birds are
correlated to each other. We found that the range of such
spatial correlation does not have a constant value, but it scales
with the linear size of the flock. This result indicates that behavioral correlations
are scale free: The change in the behavioral state of one animal affects and is
affected by that of all other animals in the group, no matter how large the group
is. Scale-free correlations provide each animal with an effective perception range
, much larger than the direct interindividual interaction range, thus enhancing
global response to perturbations. Our results suggest that flocks behave as
critical systems, poised to respond maximally to environmental perturbations.
• Why do animals live in groups: when does natural
selection favour group living?
• Benefits
Simple answer (very rare): the benefit is apparent in many groups e.g.
mating balls in anacondas (when a female is fertile, males can sense the
pheromones secreted by the female and congregate in a group around her
in an attempt to mate.
Typically: an economic approach is needed i.e. an analysis of costs &
benefits
Benefit 1: Reduced time spent ‘predator vigilant’
Total group vigilance increases with group size,
AND each individual spends less time being
vigilant (many eyes hypothesis)
More time left for other activities such as
foraging, e.g. ostrich, meerkats
Data in ostriches: percentage of time an
individual in the group has its head up decreases
as number of birds in the group increases – each
individual does not have to spend as much time
looking for predators so can yield benefits of
extra foraging time
: % of time one or more heads
are up positively correlated with number of birds in the group – total group
vigilance is increased
BUT… an alternative reason for this trend in feeding groups may be that
there is increased competition for resources and so individuals spend less
time being vigilant
Assumes:
Individuals in a group ‘monitor’ the alert behaviour of other individuals in
the group
Individuals detecting predator will alert other group
members (actively – e.g. alarm calls given by meerkats
or passively – e.g. if one group member runs away,
other members observe this as a signal of danger)
Benefit 2: Predator detection
Field observation evidence: reduced
probability of redshank capture by
sparrowhawks with a greater group size –
decreases from 0.4 with 10 redshanks in the
group to 0.1 with 50 redshanks (with the sparrowhawk
approaching from 20m away).
• Diversity of social grouping
Spatial distribution of organisms… Pattern of spacing among individuals:
Clumped, e.g. shoals, herds, flocks, colonies
Uniform, e.g. nesting territories (parents attempting to protect their eggs
from predation)
Random, e.g. trees in a forest
Diversity of animal social groups…
Insects: swarms, colonies
Fish: shoals, schools (shoal = any group of fish that form social
aggregations, school = group of fish swimming in the same direction)
Birds: flocks, colonies
Mammals: prides, herds, packs
Group size can vary from a few individuals (e.g. a gorilla troop) to many millions
(e.g locust swarm)
However, an animal group is not merely a spatial position of individuals. It is a
social construct (interaction) between spatially aggregated participants.
• Group movements - coordination matters!
Large shoals of fish and bird flocks have complex patterns of movement (e.g.
murmuration of starling flocks)
What rules govern complex movements?
Individuals watch the whole flock and adjust their behaviour to the whole
flock
Each individual watches 6-7 nearest neighbours and coordinates with
them
Individuals differ: there are leaders and followers
Figures demonstrate that although the full (absolute) velocities
of the birds are relatively similar, the velocity fluctuations
differ significantly between birds depending on their position in
the flock.
Study: Here we suggest that collective response in animal
groups may be achieved through scale-free behavioral
correlations. By reconstructing the 3D position and velocity of
individual birds in large flocks of starlings, we measured to
what extent the velocity fluctuations of different birds are
correlated to each other. We found that the range of such
spatial correlation does not have a constant value, but it scales
with the linear size of the flock. This result indicates that behavioral correlations
are scale free: The change in the behavioral state of one animal affects and is
affected by that of all other animals in the group, no matter how large the group
is. Scale-free correlations provide each animal with an effective perception range
, much larger than the direct interindividual interaction range, thus enhancing
global response to perturbations. Our results suggest that flocks behave as
critical systems, poised to respond maximally to environmental perturbations.
• Why do animals live in groups: when does natural
selection favour group living?
• Benefits
Simple answer (very rare): the benefit is apparent in many groups e.g.
mating balls in anacondas (when a female is fertile, males can sense the
pheromones secreted by the female and congregate in a group around her
in an attempt to mate.
Typically: an economic approach is needed i.e. an analysis of costs &
benefits
Benefit 1: Reduced time spent ‘predator vigilant’
Total group vigilance increases with group size,
AND each individual spends less time being
vigilant (many eyes hypothesis)
More time left for other activities such as
foraging, e.g. ostrich, meerkats
Data in ostriches: percentage of time an
individual in the group has its head up decreases
as number of birds in the group increases – each
individual does not have to spend as much time
looking for predators so can yield benefits of
extra foraging time
: % of time one or more heads
are up positively correlated with number of birds in the group – total group
vigilance is increased
BUT… an alternative reason for this trend in feeding groups may be that
there is increased competition for resources and so individuals spend less
time being vigilant
Assumes:
Individuals in a group ‘monitor’ the alert behaviour of other individuals in
the group
Individuals detecting predator will alert other group
members (actively – e.g. alarm calls given by meerkats
or passively – e.g. if one group member runs away,
other members observe this as a signal of danger)
Benefit 2: Predator detection
Field observation evidence: reduced
probability of redshank capture by
sparrowhawks with a greater group size –
decreases from 0.4 with 10 redshanks in the
group to 0.1 with 50 redshanks (with the sparrowhawk
approaching from 20m away).
