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Living in Groups

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.

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Living in Groups

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  1. Living in Groups

  2. 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

  3. Herds, Flocks, and Schools • But yet many animals occur in groups

  4. So why live in a group? • Predator Avoidance • Foraging Advantage

  5. Predator Avoidance • Dilution Theory • You don’t have to out swim the shark, you just have to out swim your friend!

  6. 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

  7. Dilution Effect • This advantage is true so long as • Attack rate does not increase proportionately with group size Predicted based on dilution In a group of 100 individuals, an individual suffers 1/100th the attack rate compared to being alone Observed Attacks per individual Number of water skaters in group

  8. Predator Avoidance: Swamping • Predator Swamping is another example of the dilution effect

  9. Predator Avoidance • Reproductive Synchrony

  10. Predator Avoidance: Reproductive Synchrony • Some estuarine crab species release their larvae at night time high tides to avoid planktivorous fishes

  11. 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

  12. Selfish herd: domains of danger

  13. 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

  14. 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

  15. 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

  16. 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

  17. Predator Avoidance • Confusion Effect • Groups of prey confuse predators because predators cannot focus on an individual • Schools of Fish in the Andaman Sea

  18. Confusion Effect • Neil and Cullen (1974) • Tested hunting behavior of 4 different types of animals with increasing fish shoal size

  19. Predator Defense • Communal Defense- prey will group together to mob and attack predators and enhance their defense

  20. Predator Defense • Black headed Gulls- Larusridibundus • Mobbing behavior

  21. Improved Vigilance • Animals also can spend less time being vigilant (searching for predators) when in groups

  22. Improved Vigilance • Do minnows respond to another minnow’s alarm response?

  23. Vigilance and Cheating • This group safety effect may also promote cheating • Why bother looking up, your buddies have got it right?

  24. 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

  25. 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

  26. Groups and Foraging • Predators can improve their prey capture rate by hunting in groups • Dolphins in Savannah, Ga • Dolphins corralling fish

  27. Foraging in Groups

  28. 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)

  29. How does group living evolve? • Shoaling in guppies reduced predation at a cost • Selection for increased shoaling decreases competitiveness for food

  30. 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? Benefits – Costs (Gain) Group Size

  31. 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

  32. Optimality and Group Size • As long as the benefit of joining a group > being alone, animals will join a group Individual Net Benefit 1 6 12 Group Size

  33. 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

  34. 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?

  35. 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

  36. Groups and Skew Theory • As gobies reach the next step in length (dominance), their growth rate slows • Cause: to prevent eviction!

  37. 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

  38. Collective Behaviors • Zone of repulsion • Zone of orientation • Zone of attraction Couzin et al. 2002

  39. Collective Behaviors

  40. Collective Behaviors individual individual Couzin et al. 2002

  41. Collective Behaviors individual Couzin et al. 2002

  42. Collective Behaviors individual individual Couzin et al. 2002

  43. Collective Behaviors parameters Couzin et al. 2002

  44. Collective Behaviors • Modeling fish movements Couzin et al. 2002

  45. Collective Behaviors ZOO Weakened ZOO Strengthened Couzin et al. 2002

  46. 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

  47. 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

  48. Collective Behaviors Couzin et al. 2002

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