living in groups. by living in groups, many animals incur the cost of increased competition with...
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Living in Groups
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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
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Herds, Flocks, and Schools
• But yet many animals occur in groups
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So why live in a group?
• Predator Avoidance
• Foraging Advantage
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Predator Avoidance
• Dilution Theory • You don’t have to out swim the shark, you just have to
out swim your friend!
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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
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Dilution Effect
• This advantage is true so long as– Attack rate does not increase proportionately with
group size
Number of water skaters in group
Attac
ks p
er in
divi
dual
Predicted based on dilution
ObservedIn a group of 100 individuals, an individual suffers 1/100th the attack rate compared to being alone
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Predator Avoidance: Swamping
• Predator Swamping is another example of the dilution effect
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Predator Avoidance
• Reproductive Synchrony
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Predator Avoidance: Reproductive Synchrony
• Some estuarine crab species release their larvae at night time high tides to avoid planktivorous fishes
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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
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Selfish herd: domains of danger
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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
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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
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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
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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
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Predator Avoidance
• Confusion Effect– Groups of prey confuse predators because
predators cannot focus on an individual– Schools of Fish in the Andaman Sea
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Confusion Effect
• Neil and Cullen (1974)– Tested hunting behavior of 4 different types of
animals with increasing fish shoal size
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Predator Defense
• Communal Defense- prey will group together to mob and attack predators and enhance their defense
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Predator Defense
• Black headed Gulls- Larus ridibundus• Mobbing behavior
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Improved Vigilance
• Animals also can spend less time being vigilant (searching for predators) when in groups
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Improved Vigilance
• Do minnows respond to another minnow’s alarm response?
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Vigilance and Cheating
• This group safety effect may also promote cheating
• Why bother looking up, your buddies have got it right?
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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
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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
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Groups and Foraging
• Predators can improve their prey capture rate by hunting in groups
• Dolphins in Savannah, Ga• Dolphins corralling fish
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Foraging in Groups
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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)
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• Shoaling in guppies reduced predation at a cost
• Selection for increased shoaling decreases competitiveness for food
How does group living evolve?
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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
Bene
fits
– Co
sts
(Gai
n)
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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
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• As long as the benefit of joining a group > being alone, animals will join a group
Group Size1 6 12
Indi
vidu
al N
et B
enefi
t
Optimality and Group Size
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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
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• 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
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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
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• As gobies reach the next step in length (dominance), their growth rate slows– Cause: to prevent eviction!
Groups and Skew Theory
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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
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• Zone of repulsion
• Zone of orientation
• Zone of attraction
Collective Behaviors
Couzin et al. 2002
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Collective Behaviors
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Collective Behaviors
Couzin et al. 2002
individual
individual
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Collective Behaviors
Couzin et al. 2002
individual
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Couzin et al. 2002
Collective Behaviors
individual
individual
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parameters
Couzin et al. 2002
Collective Behaviors
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• Modeling fish movements
Collective Behaviors
Couzin et al. 2002
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ZOO Weakened ZOO Strengthened
Collective Behaviors
Couzin et al. 2002
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• 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
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• 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
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Collective Behaviors
Couzin et al. 2002