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.  Colonially 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 is 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.  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 coloniality 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. Potential costs and benefits of sociality  For social behavior to evolve the balance of costs and benefits should overall favor the behavior.  A major component of social behavior is positive interactions with other individuals. Two major factors will dictate whether individuals are likely to behave altruistically towards others.

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 >0  Where B 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.

51. Helpers at the nest. White-fronted Bee-eaters  Assistance of helpers is of enormous benefit to parents. More than 50% of bee- eater young starve before leaving the nest.  On average, presence of each helper increases number of offspring successfully reared to fledging by 0.47. Thus, there is a clear inclusive fitness benefit.

53. Inclusive fitness and Pied Kingfishers  Another example of a species with helpers at the nest is the Pied Kingfisher.  Pied Kingfishers nest colonially in tunnels.  Some one-year old males may not be able to find a mate and so become primary helpers assisting their mother to feed young and deter predators.

55. Inclusive fitness and Pied Kingfishers  Primary helpers have alternatives. They could choose not to help and delay breeding until next year (delayer) or assist at another unrelated nest (secondary helper).  What are costs and benefits of being a primary helper?

56. Inclusive fitness and Pied Kingfishers  Primary helpers work harder than delayers and secondary helpers so they have a lower chance of surviving to breed the next year (54%) than secondary helpers (74%) or delayers (70%).  Also only 66% of primary helpers attract mates, but 91% of secondary helpers do (in 10 of 27 cases with the female they helped the previous year). Delayers have only a 33% chance.

57. Inclusive fitness and Pied Kingfishers  To determine payoffs need to add the reproductive success of each approach over the two years.  Calculate payoffs by multiplying probability of survival times number of offspring produced times probability of survival times probability finding a mate times relatedness to offspring.

58. Inclusive fitness and Pied Kingfishers  Primary helpers gain reproductive benefit in both years (0.58 [indirect fitness]+ 0.41 young [direct fitness]= 0.99).  Secondary helpers obtain a second year payoff of 0.84 young and delayers only 0.29.  Primary helpers have lower RS is year 2, but this is more than compensated for by indirect fitness benefit from year 1.

59. Evolution of Eusociality  Eusociality (true sociality).  Many eusocial insects (bees, ants, termites) do not reproduce.  Instead they act as helpers at parents nests for their entire life.

60. Evolution of Eusociality  Sterility and obligate helping is an extreme type of altruism that goes far beyond the helpers at the nest behavior seen in birds.  Suicidal behavior in defense of the group is quite common.  E.g. honey bee stings are barbed so that when a bee stings it leaves its poison sac behind and fatally injures itself. One species of ant has grenade soldiers that burst an abdominal gland and spray glue on enemies.

62. Evolution of Eusociality  Eusociality describes social systems with three characteristics: Overlap in generations between parents and offspring. Overlap in generations between parents and offspring. Cooperative brood care. Cooperative brood care. Specialist castes of non-reproductive individuals. Specialist castes of non-reproductive individuals.

63. Haplodiploidy and eusocial Hymenoptera  One idea advanced to explain eusociality is the unusual genetic system (Haplodiploidy) of the Hymenoptera (ants, wasps, bees, etc.).  Males are haploid and females diploid.  Males develop from unfertilized eggs and females from fertilized eggs.

64. Haplodiploidy and eusocial Hymenoptera  Daughters receive all of their fathers genes and half of their mothers genes. Thus, daughters share ¾ of their genes.  This suggests females would be better off if they favored the production of reproductive sisters rather than their own offspring.

65. Haplodiploidy and eusocial Hymenoptera  Queens are equally related to all offspring and so should prefer a 1:1 ratio of sons to daughters among reproductives.  Females workers however should prefer a 1:3 ratio of brothers to sisters among reproductives.

66. Haplodiploidy and eusocial Hymenoptera  It has been shown in wood ants that queens produce equal numbers of male and female eggs, but the hatching ratio is heavily female biased.  Workers apparently selectively destroy male eggs.

67. Haplodiploidy and eusocial Hymenoptera  Further evidence that workers manipulate sex ratios in their favor comes from studies in which queen and worker relatedness is altered.

68. Haplodiploidy and eusocial Hymenoptera  Mueller studying a eusocial bee removed the foundress queen from some nests, but not others. When a queen is removed a daughter takes over as queen.  Workers whose queen was removed are now helping queen produce nieces and nephews (r = 0.375 for both) and are equally related to both. Colonies where queens replaced produced far more males than colonies where original queen was left in place.

69. Haplodiploidy and eusocial Hymenoptera  In a species of Formica ant colonies queens may be monogamous or polyandrous.  Daughters of single-mating mothers heavily biased investment towards daughters, but daughters of polyandrous queens did not bias reproduction towards females.

70. Haplodiploidy and eusocial Hymenoptera  Haplodiploidy appears to influence worker behavior, but consensus today is that it alone cannot explain evolution of eusocial behavior in Hymenoptera.  There are several reasons why.

71. Haplodiploidy and eusociality  First, haplodiploid explanation assumes all workers have the same father. However, honeybee queens mate with more than 17 males on average.  As a result relatedness between worker honeybees often below 1/3.

72. Haplodiploidy and eusociality  Second, in many species, more than one female founds a nest. In this case workers may be completely unrelated.

73. Haplodiploidy and eusociality  Third, many eusocial species are not haploid (e.g. termites) and many haplodiploid species are not eusocial.

74. Haplodiploidy and eusociality  Phylogenetic analysis of Hymenoptera by Hunt (1999) emphasizes that eusociality relatively rare even though haplodiploidy occurs in all groups.  Eusociality occurs in only a few families which are scattered around the tree, which suggests eusociality has evolved independently multiple times.

76. Haplodiploidy and eusociality  Hunt also points out that eusociality has only evolved in groups that build complex nests, and care for young for a long time.  Association between nest building, long term care and eusociality suggests main driving force for eusociality is ecological not genetic.

77. Haplodiploidy and eusociality  Nest building and need to supply offspring with a steady stream of food make it impossible or very difficult for a female to breed alone.  Also, if predation rates are high, solitary breeding individuals may not live long enough to raise their young.

78. Facultative strategies in paper wasps.  Paper wasps (Polistes) are not sterile (unlike ant and bee workers). Females can remain at a nest with their mother, nest with other females or establish their own nest.  Paper wasp queens produce daughters early in the reproduction cycle because many stay at home to help rear siblings.

79. Facultative strategies in paper wasps.  Daughters who help their mother could lay unfertilized eggs to produce male offspring, but usually don’t (when they do the queen [the eggs grandmother] often eats them).  Benefits to daughters are in indirect fitness. They enhance the success of nests by deterring nest predators. Nests that have helpers removed are more likely to fail.

80. Facultative strategies in paper wasps.  Not all species of paper wasp follow the mother-daughter model and some nests may be made up of a mixture of relatives and non-relatives or entirely of non- relatives.

81. Facultative strategies in paper wasps.  Nonacs and Reeve (1995) found in Polistes dominulus that females of this species follow one of three strategies.  Initiate own nest  Join nest as a helper  Wait for a nest to become available

82. Facultative strategies in paper wasps.  Individuals founding their own nest are very likely to fail because adult mortality is high.  Multiple foundresses, however, can keep the nest going.

83. Facultative strategies in paper wasps.  However, in multifoundress nests there may be frequent conflict.  The nests that did best were those where one female was markedly bigger than the others, which reduced fighting.

85. Facultative strategies in paper wasps.  Usually, the queen dominates egg laying (95% of eggs being hers: she generally eats other females’ eggs).  Helpers who are relatives gain indirect fitness benefits, but why do non-relatives help?

86. Facultative strategies in paper wasps.  Often, they can inherit the role of queen if the queen dies.  In one study of 28 nests there were 13 changes of ownership in a season and 10 were achieved by resident helpers.

87. Facultative strategies in paper wasps.  In some paper wasps, the queen cedes some reproduction to non-relatives to persuade them to stay as helpers.  For example, in Polistes fuscatus there is a clear correlation between the proportion of reproduction by the queen and her relatedness to other foundresses.

89. Facultative strategies in paper wasps.  Some individuals in Polistes dominulus choose not to join an established nest as a helper.  This “sit-and-wait” strategy also can pay off because a female often can adopt an orphaned nest or take one over late in the season.

90. Facultative strategies in paper wasps.  Overall, in paper wasps an individual’s decision whether to be a helper or not is affected by her relative size, relatedness to other females, and the availability of unoccupied nests.

91. Naked Mole-rats  Naked mole-rats are highly unusual mammals.  They are nearly hairless and ectothermic. They are eusocial and, like termites, can digest cellulose with the help of bacteria in their gut.

92. Fig 51.33 Naked Mole Rats

93. Naked Mole-rats  The behavior of naked mole-rats is similar to that of termites.  Like termites both males and females are diploid (unlike the Hymenoptera).

94. Naked Mole-rats  Colony may include as many as 200 individuals but there is only a single reproductive female (queen) and 1-3 reproductive males.  Remaining individuals act as workers. They dig tunnels to find food, defend the tunnel system from other mole-rats, and tend the young.

95. Naked Mole-rats  Leading hypothesis for why naked mole- rats are eusocial is inbreeding.  Average coefficient of relatedness is 0.81 and about 85% of matings are between parents and offspring or between full siblings.

96. Naked Mole-rats  Despite high level of relatedness conflicts still occur because reproductive interests of workers and reproductives are not identical.

97. Naked Mole-rats  Queens maintain control through physical dominance.  Queen aggressively shoves workers who do not work hard enough and shoves are mainly directly towards less closely related individuals.  Workers double their work rate after being shoved.

98. Naked Mole-rats  In addition to inbreeding, ecological factors such as severely limited alternative breeding opportunities and group defense appear to contribute to eusociality in naked mole-rats.

99. Naked Mole-rats  In related Damaraland mole rat there does not appear to be inbreeding and reproductives have a mean r of 0.02.  Mean relatedness of colony members is close to 0.5 so in this species ecological factors may be main driver of eusociality.