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

Number of water skaters in group

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Predicted based on dilution

ObservedIn a group of 100 individuals, an individual suffers 1/100th the attack rate compared to being alone

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

Domain of danger was higher for attacked decoys

Positive correlation between the size of the domain of danger and the relative predation risk

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)

• Shoaling in guppies reduced predation at a cost

• Selection for increased shoaling decreases competitiveness for food

How does group living evolve?

Optimality and Group Size

• The costs and benefits of a group are likely to vary with its size

• But even if there is an optimum size is it stable?

Group Size

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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

• As long as the benefit of joining a group > being alone, animals will join a group

Group Size1 6 12

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Optimality and Group Size

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

• Dominant individuals may control group reproduction– Or subordinates may get a share

• How is group size maintained despite a skew in benefits?

Groups and Skew Theory

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

• As gobies reach the next step in length (dominance), their growth rate slows– Cause: to prevent eviction!

Groups and Skew Theory

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

• Zone of repulsion

• Zone of orientation

• Zone of attraction

Collective Behaviors

Couzin et al. 2002

Collective Behaviors

Collective Behaviors

Couzin et al. 2002

individual

individual

Collective Behaviors

Couzin et al. 2002

individual

Couzin et al. 2002

Collective Behaviors

individual

individual

parameters

Couzin et al. 2002

Collective Behaviors

• Modeling fish movements

Collective Behaviors

Couzin et al. 2002

ZOO Weakened ZOO Strengthened

Collective Behaviors

Couzin et al. 2002

• 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

Collective Behaviors

Couzin et al. 2002

• 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

Collective Behaviors

Couzin et al. 2002

Collective Behaviors

Couzin et al. 2002