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Kin Selection and Social Behavior

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Title: 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. The
    behavior clearly harms the instigator for no
    benefit so difficult to see how it could be
    favored by selection.
  • Selfish and cooperative behaviors easily
    explained by selection theory because they
    benefit the instigator.

4
The puzzle of altruism
  • Altruism is hard to explain because the
    instigator pays a cost and another individual
    benefits.
  • How can selection favor the spread of an
    altruistic allele that produces a behavior that
    benefits other individuals at the expense of
    individuals bearing the altruistic allele?

5
  • BTW when I say an allele that produces a
    behavior I dont mean that the allele literally
    directly codes for the behavior.
  • An allele directly code for the structure of
    proteins.
  • However, proteins interact with other proteins
    and the result of all these biochemical
    interactions in building a brain can result in an
    organism with one version of an allele behaving
    somewhat differently from an individual with a
    different version.

6
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.

7
The puzzle of altruism
  • W.D. Hamilton (1964) developed a model that
    showed how an allele that favored altruistic
    behavior could spread under certain conditions.

8
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 (see
    earlier notes on inbreeding for the concept of
    identical by descentbasically copies of an
    allele being inherited from a particular
    individual).

9
Calculating r
  • Use a pedigree to calculate r.
  • Pedigree shows all possible direct routes of
    hereditary connection between the two
    individuals.
  • Because parents contribute half their genes to
    each offspring, the probability that alleles are
    identical by descent for each step is 50 or 0.5.

10
Calculating r
  • To calculate r
  • (i) Trace each unique path between the two
    individuals via common ancestors and count the
    number of steps needed.
  • (ii) For this path r 0.5 (number of steps).
    Thus, if two steps r for this path 0.5 (2)
    0.25.
  • (iii) To calculate final value of r you add
    together the r values calculated from each path.

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14
Hamiltons rule
  • Given r the coefficient of relatedness between
    the actor and the recipient, Hamiltons rule
    states that an allele for altruistic behavior
    will spread if
  • Br - C gt0
  • Where B is benefit to recipient and C is the cost
    to the actor. Unit of measurement for B and C is
    surviving offspring.

15
Hamiltons rule
  • Altruistic behaviors are most likely to spread
    when costs (C) are low, benefits (B) to recipient
    are high, and the participants are closely
    related (r is large).

16
Applying Hamiltons rule
  • You have a food item that is worth 2 units of
    benefit to you.
  • You have a tiny nephew for whom the food would be
    worth 10 units of benefit.
  • Should you eat the food or give it to your nephew?

17
Applying Hamiltons rule
  • The value of r for a nephew is ¼.
  • Cost to you would be 2 (as youre giving up 2
    units of benefit).
  • Benefit to nephew is 10.
  • Is Br - C gt0?
  • 10(¼) 2 ?
  • 2.5 - 2 0.5. This is gt 0 so you should give the
    food to your nephew.
  • Should you share with a cousin? r 1/8 for a
    cousin.

18
  • Share with a cousin?
  • No because Br - C is not gt 0.
  • 10 1/8 2 1.25 2 -0.75

19
Inclusive fitness
  • Hamilton invented the idea of inclusive fitness
    which divides an individuals fitness into two
    components
  • Direct fitness results from an individuals
    personal reproduction (the babies it produces)
  • Indirect fitness results from additional
    reproduction by relatives, that is made possible
    by an individuals actions. For example an
    individual might help feed its sisters offspring
    or guard them from predators.

20
Kin selection
  • Natural selection favoring the spread of alleles
    that increase the indirect component of fitness
    is called kin selection.

21
Alarm calling in Beldings Ground Squirrels
  • Giving alarm calls alerts other individuals but
    may attract a predators attention.
  • Beldings Ground Squirrels give two different
    calls depending on whether predator is a
    predatory mammal (trill) or a hawk (whistle
    Sherman 1985).

22
http//michaelfrye.com/yosemite-journal/?p375
23
Is alarm calling altruistic?
  • Sherman and colleagues observed 256 natural
    predator attacks.

https//www.flickr.com/photos/charlespan/652746514
5/
24
Beldings Ground Squirrels
  • 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.

25
Beldings 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.

http//captainkimo.com/coyote-on-the-hunt-pouncing
-for-prey-at-yellowstone-national-park/
26
Beldings Ground Squirrels
  • Calling squirrel thus appears to increase its
    risk of predation.
  • Whistling appears to be selfish, but trilling
    altruistic.

27
Beldings Ground Squirrels
  • Beldings 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.

28
Beldings Ground Squirrels
  • Sherman had pedigrees that showed relatedness
    among his study animals.
  • Analysis of who called showed that females were
    much more likely to call than males.

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30
Beldings Ground Squirrels
  • Females were also more likely than males to call
    when they had relatives within earshot.

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Beldings 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.

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34
Beldings 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 by increasing
    the survival prospects of these relatives and
    hence their future reproduction.

35
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.

36
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
    (tadpoles had never seen before) to each
    container. One was a sibling, the other non-kin.

37
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.

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39
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.

40
Kin selection and cannibalism in tadpoles
  • Some cannibals discriminated between kin and
    non-kin. Others did not.
  • Degree of relatedness to siblings 1/2

41
Kin selection and cannibalism in tadpoles
  • Thus, by Hamiltons rule discrimination in favor
    of kin favored if B(r) - C gt 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.

42
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.

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44
Kin selection and cannibalism in tadpoles
  • By Hamiltons rule discrimination should be
    favored because 2(1/2) - 0 1 which is gt0.

45
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.

46
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.

47
http//phenomena.nationalgeographic.com/2014/07/23
/these-mice-excel-at-assembling-the-ideal-sperm-sw
im-teams/
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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.

50
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.

51
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 coots nests in
    hopes of having them reared.

52
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.

53
Discrimination against non-kin eggs by coots
  • Because of high cost of being parasitized and
    lack of benefit (assuming parasites are non-kin)
    Hamiltons 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.

54
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.

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56
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,

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58
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 of eggs they were
    rejecting. Coots can count!
  • By counting eggs and rejecting extras that do not
    look right coots prevent themselves from being
    parasitized.

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60
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 offsprings prospects of survival
    and reproduction.

61
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.

62
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.

63
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.
64
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.

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66
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.

67
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.

68
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.

69
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.

70
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.

71
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.

72
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.

73
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.

74
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.

75
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.

76
Association is measure of how frequently two
individuals associate socially.
Regurgitators regurgitate to individuals
they associate with regularly.
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