Chapter 13: Evolution of Social Behavior

1. Chapter 13: Evolution of Social Behavior • Costs and benefits of Social Life. • There are a large number of possible costs and benefits associated with social behavior.

2. Potential costs and benefits of sociality • Greater conspicuousness to predators, but also better defense against predators. • Many social behavior e.g. schooling by fish seem to be primarily anti-predator behaviors.

3. Fig 13.6 Schooling catfish

4. Potential costs and benefits of sociality • There have been numerous studies that have documented the anti-predator benefits of social behavior. • Groups detect predators sooner and there is also a dilution effect by being a member of a group.

5. Potential costs and benefits of sociality • Group defense also is a benefit. • Colonial nesting gulls deter predatory birds. • Males in colonies of bluegill sunfish collaborate to drive away egg-eating predators.

6. Male bluegill sunfish nest colonially as defensive adaptation against egg-eating predators.

7. Potential costs and benefits of sociality • If bluegill sunfish have evolved colonial nesting to deter predators, then we would expect solitarily nesting related species to suffer less from predators. • As predicted, solitarily nesting pumpkinseed sunfish has powerful jaws with which it can deter predators and so does not need to group nests for protection.

8. Potential costs and benefits of sociality • More rapid disease transmission is likely among social organisms and parasites can spread more readily. • There may be some advantage to sociality in that more grooming assistance may be available, but on balance disease transmission appears to be a clear cost of sociality.

9. Potential costs and benefits of sociality • For example, colonial cliff swallow nestlings are much more affected by swallow bugs than solitarily nesting birds. • Nestlings parasitized by bugs were significantly smaller and less likely to survive than unparasitized individuals.

10. Fig 13.5 Cliff swallow young exposed to parasites (left) and unexposed (right).

11. Potential costs and benefits of sociality • More competition for food is a likely cost of sociality. • Among lions females are forced to wait until the males have fed before having a chance to eat (not always). • In fieldfares (a European thrush) the larger the colony, the lower the survival rate of nestlings because starvation rates increase.

12. Fig 13.4

13. Potential costs and benefits of sociality • However, for other birds (and bees), which feed on spatially clumped, but unpredictable food supplies, colonial breeding appears to significantly improve foraging success. • This can occur through the use of information centers and more effective use of local enhancement information because colonies clumps foragers in space.

14. Potential costs and benefits of sociality • Sociality also increases opportunities for reproductive interference, which can be positive or negative depending on an individual’s success.

15. Living in Groups

16. Altruism • Altruisitic behavior is puzzling as it is behavior that imposes a cost on the actor for the benefit of another individual. • It should thus be selected against. • However, kin selection and the possibility of reciprocal altruism can favor altruistic behavior.

17. Coefficient of relatedness • A key parameter in understanding kin selection is the coefficient of relatedness: r. • r is the probability that the homologous alleles in two individuals are identical by descent.

18. Calculating r • Need a pedigree to calculate r that includes both the actor and recipient and that shows all possible direct routes of connection between the two. • Because parents contribute half their genes to each offspring, the probability that genes are identical by descent for each step is 50% or 0.5.

19. Calculating r • To calculate r one should trace each path between the two individuals and count the number of steps needed. Then for this path r = 0.5 (number of steps) • Thus, if two steps r for this path = 0.5 (2) = 0.25. • To calculate final value of r one adds together the r values calculated from each path.

23. Hamilton’s rule • Given r the coefficient of relatedness between the actor and the recipient, Hamilton’s rule states that an allele for altruistic behavior will be favored and spread if; • Br/C > 1/r • Where Br is benefit to recipient and C is the cost to the actor. Unit of measurement for B and C is surviving offspring.

24. Hamilton’s rule • Altruistic behaviors are most likely to spread when costs are low, benefits to recipient are high, and the participants are closely related.

25. Inclusive fitness • Hamilton invented the idea of inclusive fitness. Fitness can be divided into two components: • Direct fitness results from personal reproduction • Indirect fitness results from additional reproduction by relatives, that is made possible by an individual’s actions.

26. Kin selection • Natural selection favoring the spread of alleles that increase the indirect component of fitness is called kin selection.

27. Kin selection in Belding’s Ground Squirrels • Giving alarm calls alerts other individuals but may attract a predator’s attention. • Belding’s Ground Squirrels give two different calls depending on whether predator is a predatory mammal (trill) or a hawk (whistle; Sherman 1985).

28. Is alarm calling altruistic? • Sherman and colleagues observed 256 natural predator attacks. • In hawk attacks whistling squirrel is killed 2% of the time whereas non-whistling squirrels are killed 28% of the time. • Calling squirrel appears to reduce its chance of being killed.

29. Kin selection in Belding’s Ground Squirrels • In predatory mammal attacks trilling squirrel is killed 8% of the time and a non-trilling squirrel is killed 4% of the time. • Calling squirrel thus appears to increase its risk of predation. • Whistling appears to be selfish, but trilling altruistic.

30. Kin selection in Belding’s Ground Squirrels • Belding’s Ground Squirrels breed in colonies in Alpine meadows. • Males disperse, but female offspring tend to remain and breed close by. Thus, females in colony tend to be related.

31. Kin selection in Belding’s Ground Squirrels • Sherman had marked animals and had pedigrees that showed relatedness among study animals. • Analysis of who called showed that females were much more likely to call than males.

33. Kin selection in Belding’s Ground Squirrels • In addition, females were more likely to call when they had relatives within earshot.

35. Kin selection in Belding’s Ground Squirrels • Relatives also cooperated in behaviors besides alarm calling. • Females were much more likely to join close relatives in chasing away trespassing ground squirrels than less closely related kin and non-kin.

37. Kin selection in Belding’s Ground Squirrels • Overall, data show that altruistic behavior is not randomly directed. It is focused on close relatives and should result in indirect fitness gains.

38. Reciprocal Altruism • The second major way in which altruism can be favored is if recipients repay altruistic behavior in the future.

39. Reciprocal Altruism • Some animals occasionally behave altruistically towards non-relatives. • Such behavior is adaptive if the recipient is likely to return the favor in the future.

40. Reciprocal altruism • Reciprocal altruism most likely in social animals where individuals interact repeatedly because they are long-lived and form groups, and also when individuals have good memories.

41. Reciprocal altruism in Vampire bats • E.g. Vampire Bats. Feed on blood and share communal roosts. • Bats may starve if they fail to feed several nights in a row. • However, bats who have fed successfully often regurgitate blood meals for unsuccessful bats.

42. Reciprocal altruism in Vampire bats • Cost of sharing some blood is relatively low for donor bat but very valuable for recipient. • Research shows that Vampire bats share with relatives, but also share with individuals who have shared with them previously and with whom they usually share a roost.

43. Association is measure of how frequently two individuals associate socially. Regurgitators regurgitate to individuals they associate with regularly.

44. Helpers at the nest. White-fronted Bee-eaters • In a large number of birds young that are old enough to breed on their own instead help their parents rear siblings. • Helpers assist in nest building, nest defense and food delivery.

45. Helpers at the nest. White-fronted Bee-eaters • Helping usually occurs in species where breeding opportunities are limited: territories or nest sites are hard to acquire. • Young make the best of a bad job by remaining home to assist their parents.

46. Helpers at the nest. White-fronted Bee-eaters • Steve Emlen et al. studied white-fronted bee-eaters intensively in Kenya. • Nest in colonies of 40-450 individuals. Groups of relatives (clans) defend feeding territories in vicinity of colony.

47. Helpers at the nest. White-fronted Bee-eaters • First year birds that opt to help can choose among many relatives when deciding whom to help. • Bee-eaters conform to predictions of Hamilton’s rule.

48. Helpers at the nest. White-fronted Bee-eaters • Coefficient of relatedness determines whether a bee-eater helps or not. • Also, bee-eaters choose to help their closest relatives.

49. Helpers at the nest. White-fronted Bee-eaters • Nonbreeders in clan that are not relatives (birds that have paired with members of the clan) are not related to offspring being reared and are much less likely to help than relatives.