3. Group Living Flashcards
The two selective pressures usually
hypothesised to influence whether animals
live in groups are:
- Predator/Predation avoidance
- Resource (food) acquisition and defence
* They are not mutually exclusive
– i.e. both could be operating simultaneously
There are several ways in which group living
could give individuals an increased chance of
avoiding predation:
- Increased vigilance
- Dilution of risk
- Predator Confusion
- Group Defence
The group size effect is generally
considered a consequence of an increased
number of individuals scanning for predators
As group size increases individuals can decrease their vigilance
There may be direct benefits for vigilant
individuals such as:
-Predators may preferentially attack non-vigilant individuals
-Spotting predator yourself may allow a quicker response
-Being vigilant may allow you to react more quickly to the signal of another group member
How by living in groups animals may dilute their
risk of being killed by a predator
– there is a good chance that another group
member will be attacked instead
– probability of survival increases with group size
When will there be no dilution of risk in groups
– there is a linear relationship between group size
and probability of detection by a predator
– predators can attack all members of a group
Eg. of dilution benefitting a group
Water skaters sitting on water surface are attacked by fish from below
Attack rate by fish is similar for groups of different sizes
Decline in attack rate with group size is due to dilution
When would predation confusion be most effective
Predator confusion will be most effective when all members of the group are alike
– Individuals that stand out may suffer more
predation (the oddity effect)
Eg. of how group living can avoid predation
Many bird species mob predators
* Each bird may be responding individually to
the threat to its own nest
– but the combined response of group members
increases the effectiveness of the behaviour
Benefits of group living
Resource Acquisition and Defence
–Finding food
– Co-operative hunting
– Defending food
How does group living benefit in finding food
Individuals that feed in groups suffer a
disadvantage through competition with other
group members
– The main benefit is the decrease in time taken to
find a rich patch of food
How does group living benefit in co-operative hunting
– Catch prey that is too large for one animal alone
– Catch prey that is too elusive for one animal alone
– Separate an individual from a group
Hunting success in a predatory fish is related to size of the hunting group
How does group living benefit in defending food
Just because animals hunt together doesn’t
mean that they are co-operating to catch prey
Advantage of group hunting in lions may be
that after the kill there are more individuals to
keep scavengers away from the carcass
Some Other Benefits of Group Living
- Conservation of heat and water
– Through a reduction in exposed surface area - Reducing energetic costs of moving
– In water and air - Finding a mate
– Leks are aggregations of males
– Females choose mates - Defence against parasites
– Horses in large herds suffer fewer attacks from
biting flies
What are some costs of group living
–Predation: Large groups more conspicuous
– Resources: Greater competition for food
Another potential cost of group living is an
increase in parasite burden (or disease
transmission)
Some examples of costs of grouping in predation are:
Cost of shoaling in oceanic fish
– Sonar used by predatory cetaceans cannot detect
single fish but can easily detect aggregations
Predation can also be intra-specific
– Infanticide
– Larger swallow colonies suffer more heavily from
infanticide by unmated males
Some examples of costs of grouping in competition are:
Kleptoparasitism: Food discovered by one
animal is stolen by another
When a food patch is found, the more individuals that exploit it the smaller the individual share
Some examples of costs of grouping in parasitic burden are:
Cliff swallows nest colonially
Nestlings often attacked by an ectoparasite
– Swallow bug (spend most of
their lives in swallow nests)
– Larger colonies have more bugs
per nestling
The bugs have a negative effect on nestling growth
Another study quantified a fitness cost of shoaling
behaviour
– Increased likelihood of parasite transmission with
increased shoaling
Some Other Costs of Group Living
There may be reproductive competition between group members for mates.
Cuckoldry
– Potential cost to males that provide parental care
– Females may be fertilised by males other than
their mates
Example of cost of group living in schooling
Guppies that school are less successful at
competing for food.
A study found:
An initial decrease in schooling behaviour in
the absence of predators
A subsequent increase in schooling
behaviour due to predation pressure
What are optimum group sizes
Optimal group size is where the net benefit is maximised.
As group size increases both costs and benefits increase
Increase in benefits is a decelerating function
Increase in costs is an accelerating function
Costs will eventually outweigh benefit
Are Optimal Group Sizes Likely?
Groups of greater than optimal size expected
* But close to optimal group sizes could form if:
* Subgroups can form
* Group members can exclude outsiders
Active Recruitment to Foraging Group
When a sparrow discovers food it may call to attract others
More likely to call if food is divisible
More likely to feed if joined by another individual
Calls less as group size increases
Costs and Benefits within Groups
Fitness consequences of grouping do not
apply equally to all members of a group
Spatial positioning within a group can be
important for individual fitness
How can spatial positioning in groups benefit animals
Food availability and food quality may be higher
for individuals in peripheral and front positions
Energy expenditure of individuals may be lower
at the rear of the group
Peripheral group members may be at higher risk
of predation than those in centre of group
Breeding success may be higher for those in
centre of group
