Types of Social Interactions

1. Types of Social Interactions • Cooperation = mutualism • Fitness gains for both participants • Altruism • Fitness gain for recipient • Cost for actor • Selfishness • Actor gains • Recipient loses • Spite • Fitness loss for both participants

3. Prevalence of Altruism • Appears to be common in nature • Young macaws help parents raise their siblings instead of reproducing themselves • Human runs into a burning house to save a child • Darwin mentioned that altruism was a “special difficulty” for his theory • How can an allele that codes for altruism survive in the face of natural selection?

4. Kin Selection and the Evolution of Altruism • William Hamilton developed a genetic model showing how an allele for altruistic behavior could persist • Coefficient of relationship, r • Probability that homozygous alleles in two individuals are identical by descent • Hamilton’s Rule • Altruistic behavior will spread if • Br – C > 0 • B = benefit to recipient • C = cost to actor

5. Kin Selection and the Evolution of Altruism • Altruism will spread when benefits to recipient are great, cost to actor is small, and participants are closely related • Inclusive Fitness • Direct fitness results from personal reproduction • Indirect fitness results from reproduction by relatives

6. Kin Selection and the Evolution of Altruism • Indirect fitness accrues when relatives reproduce more than they would have without aid by actor • When natural selection favors the spread of alleles that increase indirect fitness, Kin Selection occurs • Kin selection explains many cases of apparent altruism • True altruism does not exist in nature

7. Calculating Coefficients of Relatedness • Haldane was quoted in a bar: “I would lay down my life for the sake of two brothers or eight cousins” • Perform path analysis to assess relatedness • Parents related to offspring 1/2 • Full siblings related 1/2 • Half siblings related 1/4 • Cousins related 1/8

9. Alarm Calling in Belding’s Ground Squirrels • Females are more likely to give alarm calls than males • Mothers, daughters, and sisters were more likely to assist each other chasing trespassers off their territories than unrelated individuals

11. White-Fronted Bee Eaters • Young adults forgo breeding to help their parents raise their siblings • Nest building, nest defense, food delivery, incubation • Helping at the nest usually found in species where breeding opportunities are limited • Best of a bad job strategy • Bee eater coefficient of relatedness determines if they will help

12. White-Fronted Bee Eaters

13. White-Fronted Bee Eaters • Presence of helpers increases parental success by 0.47 fledglings

14. Evolution of Eusociality • True sociality describes social systems with three characteristics • Overlap in generations • Cooperative brood care • Specialized castes of nonreproductive individuals • Will examine two groups • Hymenoptera • Naked mole rats

16. Haplodiploidy and Eusocial Hymenoptera • Hymenopterans exhibit most extreme form of eusociality • Millions of individuals per colony • Very few reproduce • How can this persist? • Hamilton proposed that haplodiploidy may be the reason • Females grow from fertilized eggs • Males grow from unfertilized eggs

17. Haplodiploidy and Eusocial Hymenoptera • Because of disparity in chromosome number, sisters share 75% of genes • Parents and offspring share 50% • Females are better off rearing sisters than offspring • Testing haplodiploidy hypothesis • Workers should prefer to invest in sisters over brothers • Related to sisters 3/4 • Related to brothers 1/4

19. Haplodiploidy and Eusocial Hymenoptera • Testing haplodiploidy hypothesis • Workers should favor 3:1 sex ratio • Queens are equally related to sons and daughters and should favor 1:1 sex ratio • Conflict of interest between queens and workers • Sudstrom found that wood ant queens lays equal numbers of male and female eggs • Workers selectively killed male eggs prior to hatching • Workers win sex ratio battle

20. Haplodiploidy and Eusocial Hymenoptera • Does haplodiploidy explain eusociality? • Workers should favor production of sisters if all have the same father • Honeybee queens mate over 17 times when founding a colony • Average r for workers is 1/3 • In this case, workers are not more closely related to sisters than offspring • Sometimes more than one queen founds nest • Some workers may not be related at all • Many eusocial species are not haplodiploid

21. Using Phylogenies to Analyze Social Evolution • Hunt reconstructed a phylogeny of hymenopterans • All are haplodiploid • Few families are eusocial • Eusocial families not closely related • Eusociality must have evolved multiple times • Evolved in groups that build complex nests and have extended care for larvae

23. Using Phylogenies to Analyze Social Evolution • Phylogeny suggests that the primary agent favoring eusociality is not genetic • Best of a bad job hypothesis • Building a complex nest and caring for many larvae would be impossible for a female to do by herself • Must examine factors that affect B and C as well as r

24. Facultative Strategies in Paper Wasps • Polistes paper wasps are not completely eusocial • Workers are not sterile • Females may reproduce on their own • Females pursue one of three strategies • Initiate own nest • Join a nest as a helper • Wait for a breeding opportunity

25. Facultative Strategies in Paper Wasps • Examine costs and benefits of each strategy • Nests founded by single females or multifemale groups • Single foundress nests less successful • Multifoundress nests more likely to be rebuilt if destroyed • Fights among foundresses determined by body size • Multifoundress nests grew fastest if large size difference of dominant female and subordinate helpers

27. Facultative Strategies in Paper Wasps • Why would females join a coalition and help rear offspring that are not theirs? • Indirect fitness gains because usually related to foundress • Direct fitness gains if foundress dies and a subordinate inherits the nest • Costs and benefits of this strategy depend on female’s body size and coefficient of relatedness

28. Facultative Strategies in Paper Wasps • If females do not help found nests and wait, they may be able to adopt an already-constructed nest • Sit-and-wait tactic • They leave their nest in spring and enter a dormant state until the following season to try to take over a new nest

29. Facultative Strategies in Paper Wasps • Sociality is facultative in Polistes • Adaptive response to environmental conditions • Important conditions are female body size relative to competitors, coefficient of relatedness, and availability of nest sites • Genetic, social, and ecological factors important

30. Naked Mole Rats • Live underground in huge nests in Africa • Colonies of 70-80 members • Hairless, ectothermic, digest cellulose • All species are eusocial • Single queen • 2-3 reproductive males • Workers are males and females • Castes change with age • First they tend young • Later they excavate tunnels • The oldest defend the nest

32. Naked Mole Rats • Not haplodiploid • Why are they eusocial? • Highly inbred • Average r for siblings is 0.81 • Highest coefficient of relatedness ever recorded in mammals • Still conflict among group members • Workers more closely related to offspring than half-siblings

33. Naked Mole Rats • Conflict among group members • Queens maintain control by physical dominance • If the workers slow their work pace the queen shoves them with her head • Afterwards the workers increase their work pace • Shoves are directed more often towards more distant relatives • Queen maintains eusociality by intimidation

35. Parent-Offspring Conflict • Parental care is a special case of kin selection • Even parents and offspring can have conflicts in costs and benefits • Weaning Conflict • Mothers begin to ignore or push young away near end of weaning period • Offspring will scream or attack mother • Fitness interests are not symmetrical

36. Parent-Offspring Conflict • Weaning Conflict • Offspring are related to themselves r = 1 • Parents are related to offspring r = 0.5 • Parents are equally related to all offspring and should optimize their investment in each • Offspring demand unequal amount of parental investment

37. Parent-Offspring Conflict • Weaning Conflict • At start of nursing benefit of offspring relative to cost of parent • Ratio declines with time • Young demand more milk which increases parental cost • Young can start finding own food which decreases benefit • Mothers should stop producing milk when benefit to cost ratio reaches 1

38. Parent-Offspring Conflict • Weaning Conflict • By continuing to nurse, offspring devalue mother’s cost of care • Offspring should continue to try to nurse until benefit-cost ratio is 1/2 • Period between these stages is weaning conflict • Avoidance and aggressive behavior throughout this period • If half-siblings are produced then ratio should be extended to 1/4

40. Parent-Offspring Conflict • White-fronted Bee Eaters • Sons may set up territory or may help at their parental nest • Fathers coerce sons into helping by harassing sons as they attempt to set up territories • Fathers prevent courtship feeding • Harassment is preferentially directed at sons to prevent them from breeding and coerce them to help at the nest

41. Parent-Offspring Conflict • White-fronted Bee Eaters • 16 of 47 observed harassment events resulted in successful recruitment to help • Why don’t sons resist more effectively? • Sons are equally related to siblings and own offspring • Parents are more closely related to each offspring than to grandchildren • Helpers add 0.47 offspring to parental success • Nearly same as own offspring • Worth it to save harassment from father

42. Parent-Offspring Conflict • Siblicide • In birds and mammals it is common to kill siblings • Seems maladaptive since r = 1/2 • Lougheed and Anderson studied boobies in Galápagos Islands • Lay clutch of two eggs separated by 2-10 days • First chick often pushes younger from nest

44. Parent-Offspring Conflict • Siblicide • Masked boobies push second egg from nest immediately • Blue-footed booby older chicks may reduce food intake during short food shortages to provide extra for sibling • During long food shortages they kill their siblings • What role do parents play? • Parents should intervene to prevent death of any of their chicks

45. Parent-Offspring Conflict • Siblicide • Reciprocal transplant experiment • Chicks more likely to die with masked booby nestmate • Chicks more likely to die with masked booby parents (surrogate or real) • Blue-footed booby parents intervene but masked do not • Why there is a difference among species is not known

47. Reciprocal Altruism • What about cooperation among unrelated individuals? • Trivers proposed individuals will act altruistically if favor is later returned • Two conditions for reciprocal alturism to evolve: • Cost to actor must be smaller than or equal to benefit to recipient • Individuals that fail to reciprocate must be punished

48. Reciprocal Altruism • Most likely to evolve when: • Each individual repeatedly interacts with same set of individuals • Many opportunities for altruism in an individual’s lifetime • Individuals have good memories • Potential altruists interact in symmetrical situations • Roughly equal benefits and costs

49. Reciprocal Altruism • Will evolve in long-lived, intelligent, social species with small group size, low dispersal rates, and mutual dependence in activities • Less likely to evolve in species with dominance hierarchies • Difficult to observe and quantify in nature

50. Reciprocal Altruism • Blood-Sharing in Vampire Bats • Social group of 8-12 females and their dependent offspring • Roost together and associate with each other daily • Average r between individuals in study group in Costa Rica was 0.11 • Vampire bats share blood meals • Hunting is difficult and individuals are only successful 67-93% of time • Prey are wary