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.

52. Parent-offspring conflict.
Parental care is an obvious form of altruism. In many species parents invest huge quantities of resources in their offspring.
Initially, parent and offspring agree that investment in the offspring is worthwhile because it enhances the offspring’s prospects of survival and reproduction.

53. Parent-offspring conflict.
However, a parent shares only 50% of its genes with the offspring and is equally related to all of its offspring, whereas offspring is 100% related to itself, but only shares 50% of genes with its siblings.
As a result, at some point a parent will prefer to reserve investment for future offspring rather than investing in the current one, while the current offspring will disagree. This leads to a period of conflict called weaning.

54. Parent-offspring conflict.
The period of weaning conflict ends when both offspring and parent agree that future investment by the parent would be better directed at future offspring. This is when the benefit to cost ratio drops below ½.

55. in reduction in future offspring.
Fig 11.18
Figure shows B/C benefit to cost ratio of investing in the current offspring.
Benefit is measured in benefit to current offspring and cost is measured
in reduction in future offspring.

56. Parent-offspring conflict
In instances where parents produce only half siblings we should expect weaning conflict to last longer because the current offspring is les closely related to future offspring.
This has been confirmed in various field studies.

58. Siblicide
In many species there is intense conflict between siblings for food that may result in younger weaker chicks starving to death.
In other species regardless of food supplies first hatched offspring routinely kill their siblings.

59. Siblicide
For example, in Black Eagles the first hatched chick hatches several days before its sibling. When the younger chick hatches its older sibling attacks and kills it.

60. Siblicide
In species such as Black Eagles siblicide is obligate in that the younger offspring is always killed. Black Eagles are only capable of rearing one young.
The most likely explanation for the later hatched young is that for the parents it serves as an “insurance offspring” in case the first offspring fails to hatch or develop.

61. Siblicide
In other species such as Cattle Egrets there is intense conflict that establishes a clear age-based hierarchy in the brood that determines how food is divided among the brood members.
In cattle egrets, younger chicks usually starve, but if it is a good food year they often fledge.

62. Siblicide
Siblicide is thus facultative in cattle egrets because restraint by the older chicks in not killing the younger siblings can be rewarded in good years.
In Black Eagles there is no prospect of two young being reared, so the older chick ensures its own survival by eliminating its sibling.

63. Siblicide
Siblicide shows that relatedness does not necessarily lead to altruistic behavior. For Cattle Egrets and Black Eagles selfishness is better because the costs of altruism are too high.

64. 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.

65. 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.

66. 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.

67. 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.

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