Living in Groups
Living in Groups
• By living in groups, many animals incur the
cost of increased competition with their
conspecifics
• And theory predicts that animals should
distribute themselves to maximize resources
and reduce agonistic competitive interactions
Herds, Flocks, and Schools
• But yet many animals occur in groups
So why live in a group?
• Predator Avoidance
• Foraging Advantage
Predator Avoidance
• Dilution Theory
• You don’t have to out swim the shark, you just have to
out swim your friend!
Dilution Effect
• This advantage is true so long as
– Attack rate does not increase proportionately with
group size
• Do you think this is true?
• No, attack rate usually increases with group size
Dilution Effect
• This advantage is true so long as
– Attack rate does not increase proportionately
with group size
Attacks per individual
In a group of 100
individuals, an individual
suffers 1/100th the attack
rate compared to being
alone
Predicted based on dilution
Observed
Number of water skaters in group
Predator Avoidance: Swamping
• Predator Swamping is another example of the
dilution effect
Predator Avoidance
• Reproductive Synchrony
Predator Avoidance: Reproductive
Synchrony
• Some estuarine crab
species release their
larvae at night time
high tides to avoid
planktivorous fishes
Predator Avoidance
• Animals maintain groups to benefit
themselves as individuals
– Position within the group is also important and
animals often vie for the safest position
• Selfish herd theory
• Sheep with backpacks
Selfish herd: domains of danger
Predator Avoidance
• Selfish herd theory
– Cape fur seals and Great White Sharks
– Air Jaws
• Towed different arrangements of 4-5 decoys behind a
boat
– Presented the prey in different domains of danger
• Examined predation on seal position
Selfish Herd Theory
Positive correlation
between the size of
the domain of danger
and the relative
predation risk
Domain of danger was
higher for attacked
decoys
Selfish Herd
• Do alarmed individuals seek safety amongst
companions?
– Dace and Minnows
• Shoals of dace were habituated to the scent of injured
conspecifics, naïve minnows added to shoal
Selfish Herd
• But group position is also likely a trade-off
– Inside the group is the safest from predators
• But also least likely to encounter food first
• Or to intercept any information from outside the group
Predator Avoidance
• Confusion Effect
– Groups of prey confuse predators because
predators cannot focus on an individual
– Schools of Fish in the Andaman Sea
Confusion Effect
• Neil and Cullen (1974)
– Tested hunting behavior of 4 different types of
animals with increasing fish shoal size
Predator Defense
• Communal Defense- prey will group together
to mob and attack predators and enhance
their defense
Predator Defense
• Black headed Gulls- Larus ridibundus
• Mobbing behavior
Improved Vigilance
• Animals also can spend less time being vigilant
(searching for predators) when in groups
Improved Vigilance
• Do minnows respond to another minnow’s
alarm response?
Vigilance and Cheating
• This group safety effect may also promote
cheating
• Why bother looking up, your buddies have got it right?
Vigilance and Cheating
• But if everyone cheats, there is little vigilance
• So a stable strategy must be achieved to
balance the vigilance of the group
Vigilance and Cheating
• But, it’s also important to pay attention
– Predators may be less likely to attack a vigilant
individual
– Noticing the danger first gives you a few extra
seconds to flee or avoid predators
Groups and Foraging
• Predators can improve their prey capture rate
by hunting in groups
• Dolphins in Savannah, Ga
• Dolphins corralling fish
Foraging in Groups
How does group living evolve?
• C.P Haskins moved 200 guppies from a
predator rich system to almost predator free
head waters in another river system
– 30 years after, guppies were collected from several
streams (including the original and head water
introduction site)
How does group living evolve?
• Shoaling in guppies reduced predation at a
cost
• Selection for increased shoaling decreases
competitiveness for food
Optimality and Group Size
Benefits – Costs (Gain)
• The costs and benefits of a group are likely to
vary with its size
Group Size
• But even if there is an optimum size is it
stable?
Optimality and Group Size
• Imagine a group
whose optimum
size is 6, but since
groups can be
more
advantageous
than being alone,
they can keep
joining
– At some point, it
may be better to
be alone
Optimality and Group Size
Individual Net Benefit
• As long as the benefit of joining a group >
being alone, animals will join a group
1
6
Group Size
12
Groups and Skew Theory
• Competitive differences in animals are likely to
affect their benefits derived from a group
• Skew Theory- subordinate individuals will put
up with lower pay offs so long as they do not
increase their net benefits by going elsewhere
– Examined through individual reproductive success
Groups and Skew Theory
• Dominant individuals may control group
reproduction
– Or subordinates may get a share
• How is group size maintained despite a skew
in benefits?
Skew Theory and Coral Reef Fish
• Many shoaling coral reef fish form dominance
hierarchies within their groups
– Only the largest individuals breed
• Observation in gobies that subordinates
tended to be 90-95% of the lengths of the
next sized fish in the hierarchy
– Results in a stepped size gradient
Groups and Skew Theory
• As gobies reach the next step in length
(dominance), their growth rate slows
– Cause: to prevent eviction!
Schooling in Fishes
• Shoals or schools of fish are made up of
various individuals that are all consistently
shifting position
• If fishes are all acting independently, how do
they decide which way to go as a group?
– Simple decision rules
Collective Behaviors
• Zone of repulsion
• Zone of orientation
• Zone of attraction
Couzin et al. 2002
Collective Behaviors
Collective Behaviors
individual
individual
Couzin et al. 2002
Collective Behaviors
individual
Couzin et al. 2002
Collective Behaviors
individual
individual
Couzin et al. 2002
Collective Behaviors
parameters
Couzin et al. 2002
Collective Behaviors
• Modeling fish movements
Couzin et al. 2002
Collective Behaviors
ZOO Weakened
ZOO
Strengthened
Couzin et al. 2002
Collective Behaviors
• Waves of prey moving away from a predator
are caused by a few prey sighting the predator
and then moving into other fishes ZOR- results
in ripple of movement across the shoal
• Predators attack a bait ball
• Simulation
Couzin et al. 2002
Collective Behaviors
• Predatory fish have eyes
that face more forward
creating a blind spot
• As a result, fish follow
each other resulting in a
torus shoal or milling
Couzin et al. 2002
Collective Behaviors
Couzin et al. 2002