Social Behaviour

1. Social Behaviour Chapter 19 (& 15: p. 275-276)

3. Advantages of Group-living Protection from predators Improved food search/hunting Easier location of mates Physical warmth Resource defense Richer learning environment Increased ability to modify environment

4. Costs of Group-living • Increased competition for resources • Increased exposure to disease/parasites • Increased conspicuousness to predators/ prey • Interference with reproduction

5. Types of Groups • Social vs. Non-Social (may be a continuum) • Evolution of simple aggregations • How might non-social groups have evolved? • Hamilton’s “Selfish Herd” hypothesis (Ch.15) • Complex social behaviours (altruism)

6. The Evolution of Complex Social Behaviours • Evolution of Altruism • Altruism can increase fitness if individuals helped are kin (relatives) = kin selection • Degree of relatedness (r) • r is the probability that alleles sampled from 2 individuals are identical by descent • Hamilton’s rule: rB>C • Kin Selection Theory • If you help your relatives to survive and reproduce, than you are helping to pass on the genes that both you and your relatives have in common.

7. Gr M Gr F Gr M Gr F r=1/2 r=1/2 r=1/2 mom dad r=1/2 aunt uncle uncle r=1/4 r=1/2 r=1/2 r=1/2 r=1/4 r=1/2 you sib cous r=1/8 Degrees of relatedness between: You and mom, dad, or sib = ½ = 0.5 You and uncle = ½ * ½ = ¼ = 0.25 You and grandparents = ½ * ½ = ¼ = 0.25 You and cousin = ½ * ½ * ½ = 1/8 = 0.125

8. The Evolution of Complex Social Behaviours • Kin Selection Theory • If you help your relatives to survive and reproduce, than you are helping to pass on the genes that both you and your relatives have in common. • e.g. Alarm calls, Eusociality

9. Eusociality • Eusociality describes social systems with 3 key characteristics: (1) overlap in generations between parents and offspring (2) cooperative brood care (3) non-reproductive worker caste

10. Eusociality in Social Insects • Haplodiploidy results in unusual coefficients of relatedness(r) • males develop from unfertilized eggs, • haploid • do not have fathers, only mothers • females develop from fertilized eggs, are diploid • Sisters get the same set of chromosomes from their father, (daughter to father r = 1) • but have a 50% chance of getting the same allele from their mother (daughter to mother r = 1/2)

11. Sex determination in social insects • The unusual genetic system of social insects makes eusociality a likely consequence of kin selection • In haplodiploidy, sex is determined by chromosome number • -males develop from unfertilized eggs, are haploid • -females develop from fertilized eggs, are diploid • Sons do not have fathers, only mothers • Sisters get the same set of chromosomes from their father, but have a 50% chance of getting the same allele from their mother

12. Sisters are highly related to each other in haplodiploidy Path #2 . Path #1. mother (diploid) father (haploid) mother (diploid) father (haploid) 1/2 1/2 1 1/2 X sister sister sister A sister B Sex determination in social insects Odds that one of sister A’s alleles came from mom = 1/2 Odds that mom gave the same allele to sister B = 1/2 -odds of identical-by-descent allele Path 1 =(½) (½)= ¼

13. Path #2. Path #1 . mother (diploid) father (haploid) mother (diploid) father (haploid) 1/2 1/2 1 1/2 X sister A sister B sister sister Sex determination in social insects Sisters are highly related to each other in haplodiploidy Odds that one of sister A’s alleles came from dad = 1/2 Odds that dad gave same allele to sister B = 1 (his complete haploid genome) -odds of identical-by-descent allele Path 2 = (½) (1)= ½

14. mother (diploid) father (haploid) mother (diploid) father (haploid) 1/2 1/2 1 1/2 X X sister sister sister sister Sex determination in social insects Combined odds of sisters sharing identical alleles: (Path #1 odds) + (Path #2 odds) = (1/4) + (1/2) = 3/4 Because of this system, females are more related to their sisters(r = ¾) than they are to their ownoffspring(r = ½)

15. Sex determination in social insects mother (diploid) father (haploid) mother (diploid) father (haploid) 1/2 1/2 1/2 1 1/2 1/2 X X sister brother sister sister Sisters are only distantly related to their brothers: (1/2)(1/2) = 1/4 (only one path links sisters and brothers) In haplodiploidy, females maximize their inclusive fitness by investing in the production of reproductive sisters, who are closer relatives than their own offspring or brothers.

16. Haplodiploidy • females are more related to their sisters (r = ¾) than they are to their own offspring (r = ½) • females are only distantly related to their brothers (r = 1/4) [only one path links brothers and sisters] • In haplodiploidy, females maximize their inclusive fitness by investing in the production of reproductive sisters, who are closer relatives than their own offspring or brothers.

17. Haplodiploidy predisposes Hymenoptera to become eusocial But… haplodiploidy does not cause eusociality Because… • Not all haplodiploid species have sterile castes e.g. honeybees • Some diploid species have sterile castes e.g. diploid termites

18. Honeybee • Queen mates with up to 20 males • Queens mate multiple times • reduces relatedness among sisters –do not share father • r no longer significant • workers able to discriminate between sister that are more or less related

19. Conflict between queen and workers Conflict of interest between queen and non-reproductive workers Queen is equally related to sons and daughters (r = 1/2) -she will favor a 1:1 sex ratio (equal # of daughters and sons) Workers have r = 3/4 with sisters, but only r = 1/4 with brothers -their fitness will be maximized when the queen produces a 3:1 sex ratio (more daughters than sons) Who wins the conflict? -in one species of ant, the queen laid eggs in a 1:1 ratio, but at hatching the sex ratio was biased towards many more females -workers selectively destroyed male larvae -assert their own reproductive agenda over the queen’s

20. Bumble Bee • Queen controls sex of egg (fertilize or not) • Workers control sex by • provisioning • lay male eggs

21. Eusociality without Haplodiploidy • The naked mole rat - 2 castes: • “workers” Non-reproductive adults: dig tunnels, find food • “non-workers” Reproductive female (queen) and several reproductive males (breed, keep young warm) • Native to Africa, droughts common, live in underground tunnels and eat tubers, can only dig when wet, need many individuals to dig to find enough food – ECOLOGY promotes eusociality (Fig. 19.18)

22. Helpers at the Nest • In some bird species (e.g., Florida scrub jays, pied kingfishers), offspring from previous years help their parents – feed and protect younger siblings instead of reproducing • Beneficial: Number of young fledged drops if helpers removed • Altruism through kin selection? • Or, beneficial to individual (selfish)?

23. Helping at the Nest • Why help vs. have your own offspring? • Habitat “saturated” with breeders (no room) • May be better to wait for a high quality territory (inherit it), than leave for a low quality one • May not be able to leave group location (e.g., limited food resources elsewhere) • No mates available • Life history characteristics: Small clutch size and low adult mortality

24. How Do Helpers Benefit? • Increase inclusive fitness if helping kin • Enhance likelihood of future breeding (gain mate if primary male dies - unrelated males) • May increase own survivorship (access to resources, lower risk of predation in a group) • May gain useful reproductive experience (care of young) & may be reciprocated in future • So, kin selection may explain helping through increased inclusive fitness, but many other factors

25. Evolution of Human Social Systems • Read Pgs. 348 – 349 • Possible evidence for kin selection

26. Cooperation in Non-Kin • Types: • Reciprocal altruism (reciprocity) • Recipient benefits, donor’s fitness decreased; later, roles reversed • e.g., vampire bats • Model: Prisoner’s Dilemma • Mutualism (and/or symbiosis) • Mutually beneficial • e.g., symbiotic fish

27. Reciprocal Altruism (Wilkinson, G.W. (1984) Reciprocal food sharing in the vampire bat. Nature. 308:181-184) • 33% of young bats (<2 yrs) fail to get blood on any particular evening versus just 7% of adults • chronic threat of starvation among vampire bats can only survive 3 days without a meal • successful bats regurgitate part of their blood meal for group members that were not successful – but they do not do this randomly, they only give to those from whom they have received blood in the past

28. Altruistic acts dispensed primarily to relatives and frequent roostmates • Bats more likely to regurgitate blood meals to other bats they frequently roost with • Bats more likely to regurgitate blood meals to close relatives

29. Why is this altruistic behaviour favored? Benefits of act to recipient (R) exceed cost of act to donor (D)

30. Primates Alliances in primates seen in: • Grooming behaviour • Fighting behaviour • most grooming and fighting alliances between close relatives but not always

31. Rhesus macaques: • Kin intervene more, both to help if kin are the recipient or if kin are the the aggressors. • Grooming rates highest between kin. Japanese macaques: • Agonistic aid was 81% between kin, primarily mothers and grandmothers. • Kin spend more time together (in proximity) than expected by chance. • Grooming higher between kin than non-kin. • Severe aggression only occurred between non-kin (19 incidents). (Kurland)

32. Grooming and food sharing in chimpanzees • Chimpanzees in captive colony more likely to share food with individuals who had groomed them in previous 2 hours • Resisted approaches by individuals who had not groomed them • Reciprocity depends on history of interactions between two individuals

33. Reciprocal Altruism • Kin selection cannot account for cooperation in non-kin… • How could RA in non-kin have evolved? • A “cheater” could beg blood, then refuse to return the favour = donor may die (unless it finds another donor), cheater lives • Why don’t individuals evolve to act selfishly?

34. Reciprocal Altruism (RA) • Axelrod & Hamilton (1981), others… • “Game theory” – two players interact with goal of maximum individual gains • Model for evolution of reciprocal altruism = “Prisoner’s Dilemma” • “Tit-for-tat” strategy READ 338 -339

35. Evolution of Reciprocal Altruism • Requires these conditions: • Longer-lived animals, such that future opportunity for repayment likely (multiple encounters) • Altruist and recipient must be able to recognize one another; identify and refrain from helping cheaters • Benefit to recipient greater than cost to altruist (but both individuals benefit in the long run)

36. Mutualism (a form of symbiosis) • Cooperation between 2 different species • Both benefit, neither harmed, therefore, not considered altruism (fitness of both organisms is increased)

37. Clownfish (genus Amphiprion) dwell among the tentacles of tropical sea anemones. • Protects anemone from anemone-eating fish • In turn, stinging tentacles of anemone protect fish from its predators (a special mucus on fish protects it from getting stung). • Honeypot ants feed and care for aphids, “milk” them for their honeydew secretions (by stroking them with antennae) • Ants protect aphids, aphids feed ants • Mutualism: • Goby Fish + Shrimp • Plover + Crocodile