Kin Selection and Social Behavior

1. Kin Selection and Social Behavior • Interactions between individuals can have 4 possible outcomes in terms of fitness gains for the participants.

2. Kin Selection and Social Behavior • Cooperation (mutualism): fitness gains for both participants. • Altruism: instigator pays fitness cost, recipient benefits. • Selfishness: instigator gains benefit, other individual pays cost. • Spite: both individuals suffer a fitness cost.

3. Kin Selection and Social Behavior • No clear cut cases of spite documented. • Selfish and cooperative behaviors easily explained by selection theory because they benefit the instigator.

4. The puzzle of altruism • Altruism is the difficult one to explain because the instigator pays a cost and another individual benefits. • Hard to see how selection could favor an allele that produces behavior benefiting another individual at the expense of the individuals bearing the allele.

5. The puzzle of altruism • For Darwin altruism presented a “special difficulty, which at first appears to me insuperable, and actually fatal to my whole theory.” • Darwin suggested however that if a behavior benefited relatives, it might be favored by selection.

6. The puzzle of altruism • W.D. Hamilton (1964) developed a model that showed an allele that favored altruistic behavior could spread under certain conditions.

7. Coefficient of relatedness • Key parameter is the coefficient of relatedness: r. • r is the probability that the homologous alleles in two individuals are identical by descent.

8. 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.

9. 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.

13. 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 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.

14. 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.

15. 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.

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

17. Alarm calling 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).

18. 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.

19. 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.

20. 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.

21. 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.

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

25. 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.

27. 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.

28. Kin selection and cannibalism in tadpoles • Spadefoot toad tadpoles come in two morphs. • Typical morph is omnivorous mainly eats decaying plant material. • Cannibalistic morph has bigger jaws and catches prey including other spadefoot tadpoles.

29. Kin selection and cannibalism in tadpoles • Pfennig (1999) tested whether cannibals discriminate between kin and non-kin. • Placed 28 cannibalistic tadpoles in individual containers. Added two omnivorous tadpoles (tadpole had never seen before) to each container. One was a sibling, the other non-kin.

30. Kin selection and cannibalism in tadpoles • Pfenning waited until cannibal ate one tadpole, then determined which had been eaten. • Found that kin were significantly less likely to be eaten. Only 6 of 28 kin were eaten, but 22 of 28 non-kin.

32. Kin selection and cannibalism in tadpoles • Pfennig also studied tiger salamanders whose tadpoles also develop into cannibalistic morphs. • Kept 18 cannibals in separate enclosures in natural pond. To each enclosure added 6 siblings and 18 non-kin typical morph tadpoles.

33. Kin selection and cannibalism in tadpoles • Some cannibals discriminated between kin and non-kin. Others did not. • Degree of relatedness to siblings = 1/2

34. Kin selection and cannibalism in tadpoles • Thus, by Hamilton’s rule discrimination in favor of kin favored if B(r) - C > 0 • Benefit estimated by counting number of siblings that survived. Siblings of discriminating cannibals twice as likely to survive as siblings of non-discriminating cannibals.

35. Kin selection and cannibalism in tadpoles • Benefit thus approximately 2. • Cost assessed by evaluating effect of not eating siblings by comparing growth of discriminating and non-discriminating cannibals. No difference in growth rates. Cost then estimated as close to 0.

37. Kin selection and cannibalism in tadpoles • By Hamilton’s rule discrimination should be favored because 2(1/2) - 0 = 1 which is >0.

38. Altruistic sperm in wood mice • Moore et al. have demonstrated altruistic behavior by sperm of European wood mice. • Females highly promiscuous. Males have large testes and engage in intense sperm competition with other males.

39. Altruistic sperm in wood mice • Wood mice sperm have hooks on their heads. And connect together to form long trains of sperm that can include thousands of sperm. • Swimming together sperm travel twice as fast as if they swam separately.

41. Altruistic sperm in wood mice • To fertilize egg, train must break up. • To break up train many sperm have to undergo acrosome reaction releasing enzymes that usually help fertilize an egg.

42. Altruistic sperm in wood mice • Releasing these enzymes before reaching an egg means these sperm cannot fertilize the egg. These sperm sacrifice themselves. • Because other sperm carry half of the same alleles, sacrifice makes sense in terms of kin selection.

43. Discrimination against non-kin eggs by coots • Important to avoid paying costs on behalf of non-kin. • Lyon (2003) studied defense against nest parasitism in American coots. • Coots often lay eggs in other coot’s nests in hopes of having them reared.

44. Discrimination against non-kin eggs by coots • Accepting parasitic eggs is costly because half of all chicks starve and same number reared in parasitized and non-parasitized nests. • Thus, host parent loses one offspring for every successful parasite.

45. Discrimination against non-kin eggs by coots • Because of high cost of being parasitized and lack of benefit (assuming parasites are non-kin) Hamilton’s rule predicts coots should discriminate against parasitic eggs. • Coot eggs very variable in appearance. If 2 eggs laid within 24 hours Lyon knew one was a parasite.

46. Discrimination against non-kin eggs by coots • Among 133 hosts 43% rejected one or more parasitic eggs. Rejected eggs differed from hosts eggs significantly more than did accepted eggs.

48. Discrimination against non-kin eggs by coots • Females who accepted eggs laid one fewer egg of their own for each parasitic egg they accepted. Average total clutch (including parasites) 8 eggs,

50. Discrimination against non-kin eggs by coots • Females who rejected eggs laid an average of 8 of their own eggs even though they waited to finish laying before disposing off eggs they were rejecting. Coots can count! • By counting eggs and rejecting extras that do not look right coots prevent themselves from being parasitized.