Kin selection and social behavior can be explained through Hamilton's rule of kin selection (Br-C>0). Altruistic behaviors that decrease an individual's direct fitness can increase if they provide sufficient benefits to the reproductive success of genetic relatives. This explains phenomena like alarm calling in ground squirrels, where females are more likely to call when close kin are present. Haplodiploidy in hymenoptera may have predisposed the evolution of eusociality as workers are more closely related to sisters (r=3/4) than to their own offspring (r=1/2). Parent-offspring conflict can arise when parents and offspring differ in how they value the costs and benefits of continued parental care

1. 1
Kin Selection and Social
Behavior
By
Mr. M. Irfan
Visiting Lecturer Zoology
University of Narowal

2. 2
Types of social interactions among members of the
same species (Table 11.1)
• The actor in any social interaction affects the recipient of the action as well as
himself. The costs and benefits of interactions are measured in units of
surviving offspring (fitness).
Actor benefits Actor is harmed
Recipient benefits Cooperative Altruistic
Recipient is harmed Selfish Spiteful

3. 3
Kin selection and altruistic behavior
• For Darwin, the apparent existence of altruism
presented a “special difficulty, which at first
appeared to me insuperable, and actually fatal to
my whole theory.”
• However, he also suggested a solution — selection
might favor traits that decreased the fitness of the
actor if they increased the reproductive success of
close relatives
• This form of selection, which takes into account
the fitness benefits to relatives is kin selection

4. 4
Hamilton’s Rule
•William Hamilton (1964) developed a genetic
model showing that an altruistic allele could increase
in frequency if the following condition is satisfied:
Br - C > 0
Where B is the benefit to the recipient and C is the cost to the
actor, both being measured in units of surviving offspring,
and r is the coefficient of relatedness (or relationship)
between actor and recipient

5. 5
Inclusive fitness
• Hamilton introduced the concept of
inclusive fitness, which includes direct
fitness + indirect fitness
• Direct fitness is personal reproduction
• Indirect fitness is the additional
reproduction of relatives that is made
possible by an individual’s actions

6. 6
The coefficient of relatedness, r
• The proportion of alleles in two individuals
that are identical by descent (ibd)
• The coefficient of relatedness of full sibs is
r = 0.5
• To see why this is so, we can use the
following example, in which we give each
of the four alleles at a locus in the two
parents a unique label

7. 7
Proportion of alleles shared ibd by full sibs
• P: A1A2 x A3A4
• O:
• The average proportion of alleles shared ibd by
pairs of full sibs is 0.5
Sib 1
A1A3 A2A3 A1A4 A2A4
Sib 2
A1A3 1.0 0.5 0.5 0
A2A3 0.5 1.0 0 0.5
A1A4 0.5 0 1.0 0.5
A2A4 0 0.5 0.5 1.0

8. 8
Some coefficients of relatedness
• Parent to offspring, r = 1/2
• Full sibs, r = 1/2
• Half sibs, r = 1/4
• First cousins, r = 1/8
• Grandparent to grandchild, r = 1/4
• Aunt or uncle to niece or nephew, r = 1/4

9. 9
Altruistic behavior in Belding’s ground squirrels
• Breed in colonies
• Male offspring disperse far from native burrow
• Female offspring tend to remain and breed close by.
Therefore, females in proximity tend to be closely related
• Squirrels give alarm calls when predators are spotted
(different calls for mammalian predators vs. birds of prey)
• Is alarm calling altruistic and can it be understood as a
result of kin selection?

10. 10
In ground squirrels most alarm calling is done by
females (Sherman 1977) (Fig. 11.2 b)
• Based on 102 encounters
with predatory mammals
• Blue line is expected
frequency if each type of
individual called in
proportion to the number
of times it was present
when a predator appeared
• Mortality is 8% for
calling individual vs. 4%
for non-callers when
predator is a mammal

11. 11
Female ground squirrels are more likely to give
alarm calls when close kin are nearby (Sherman
1977) (Fig. 11.3)
• Based on 119 encounters with predatory mammals
• Blue line is expected frequency if each type of pairing produced calls in proportion to the
number of times it was present when a predator appeared

12. 12
Nest helping in white-fronted bee-eaters
• In white-fronted bee-eaters (and some other birds
where breeding opportunities are extremely
restricted), young adults often forego their own
reproduction to help at the nests of other
individuals.
• This is clearly altruistic. Evidence suggests that
nest helping can be explained by kin selection

13. 13
White-fronted bee-eater (Merops bullockoides)

14. 14
In bee-eaters, helpers assist close relatives (Emlen
and Wrege 1988) (Fig.
a) Among non-breeders, those born in a clan are much more likely to be nest helpers than those
who enter a clan from outside and are unrelated to offspring being raised in that season
b) Nest helping is disproportionately directed toward close relatives

15. 15
Nest
helpers
increase
the
number of
young
birds
fledged
(Emlen
and Wrege
1991)
(Fig. 11.7)
• A group size of
2 represents a
pair without
helpers. The
average
number of
young fledged
= 0.51 for pairs
• Each
additional
helper at the
nest increases
the number of
young fledged
by 0.47 on
average

16. 16
Kin-selected discrimination in cannibalistic
spadefoot toad tadpoles (Pfennig 1999) (Fig. 11.8 a, b)
• Tadpoles develop into typical morphs that eat mostly decaying plant matter or into
carnivores that eat other tadpoles.
• Carnivores are more likely to eat non-sibs than sibs when given a choice between
one of each kind

17. 17
Kin selection can explain the presence of discrimination
between sibs and non-sibs in cannibalistic tiger salamander
larvae (Pfennig et al. 1999) (Fig. 11.8c)
Benefit: B ≈ 2 Cost: C ≈ 0
Experiment consisted of placing 1 predatory morph + 6 sibling typical morphs + 18 non-sibling
typical morphs in each of 18 enclosures in natural pond.

18. 18
Coots can avoid parasitic altruism (helping non-kin)
(Lyon 2003) (Fig. 11.10 c, d)
• If selection can favor helping kin, it should also favor avoiding sacrifices for non-kin
• Coots that accept eggs from other birds lose 1 offspring of their own for each
parasitic offspring
• Coots that reject parasitic eggs have the same number of offspring as unparasitized
birds (dashed line)

19. 19
Eusociality: the ultimate in reproductive
altruism
• Characteristics of eusociality
1. Overlap in generations between parents and
offspring
2. Cooperative brood care
3. Specialized castes of non-reproductive
individuals
• Insects (termites, hymenoptera), snapping
shrimp, naked mole rats

20. 20
Haplodiploidy and eusociality in hymenoptera
(bees, wasps, ants)
• Males are haploid (develop from unfertilized eggs) and
females are diploid
• Hamilton (1972) proposed that haplodiploidy predisposes
hymenoptera to eusociality because females are more
closely related to one another (r = 3/4) than they are to
their own offspring (r = 1/2)
• Females may maximize inclusive fitness by being sterile
workers and helping to produce reproductive sisters (rather
than by being reproductives themselves)

21. 21
Proportion of alleles shared ibd by sisters in a
haplodiploid species
• P: A1A2 x A3
• O:
• The average proportion of alleles shared ibd by
pairs of full sibs is 0.75
Sister 1
A1A3 A2A3
Sister 2
A1A3 1.0 0.5
A2A3 0.5 1.0

22. 22
Is haplodiploidy the explanation of eusociality
in hymenoptera?
• Probably not
– The preceding analysis assumed only 1 male fertilizes a queen —
this is not true in honeybees, for example
– In some species, colonies may be founded by more than 1 queen
– Many eusocial non-hymenoptera are diploid (e.g., termites)
– Many hymenoptera are not eusocial (eusociality may have three
independent origins associated with nest-building and the need to
supply larvae with food)
• Haplodiploidy may facilitate the evolution of eusociality
but a more important factor may be the need for help in
rearing young

23. 23
Sociality and
nesting
behavior in
hymenoptera
(Hunt 1999)
(Fig. 11.13
• Families that
include eusocial
species are
indicated in
boldface type

24. 24
Naked mole rats
• All young in a colony produced by a single
queen and 2 – 3 reproductive males
• Not haplodiploid, but colony members are
highly inbred (average r = 0.81)
• 85% of matings are between full-sibs or
parents and offspring
• Queens use physical dominance to coerce
help from less closely related individuals

25. 25
Naked mole rat queens preferentially shove
nonrelatives (Reeve and Sherman 1991) (Fig. 11.16)

26. 26
Parent – offspring conflict
• Parental care is a special case of providing fitness
benefits for close relatives
• The offspring is the fitness of the parent (this
means that benefits and costs of parental care
both accrue to the parent)
• In species that provide extensive parental care, the
fitness benefit to the parent of providing additional
care to current offspring needs to be weighed
against the fitness cost of that additional care in
terms of lost future offspring

27. 27
Parent – offspring conflict occurs because parents and offspring value the
costs of parental care differently (Trivers 1985) (Fig 11.18)
• As offspring grow, the benefit/cost ratio for the parent declines. Benefit (B) is
measured in terms of increased survival of offspring receiving parental care; cost (C) is
measured in terms of lost future offspring due to continued parental care. From
parent’s point of view, it should stop giving parental care when B/C declines to 1.
• But, the offspring devalues the cost to the parent by 1/2 because lost future full-sibs are
related to the offspring by r = 1/2. Therefore, the offspring wants parental care to
continue until the B/C ratio for the parent is 1/2 - fig. (a) [(or 1/4 if future offspring are
half-sibs - fig. (b)]

28. 28
Harassment in white-fronted bee-eaters can also be
analyzed in the context of parent-offspring conflict
and kin selection
• Fathers occasionally coerce sons into helping to raise their
siblings by harassing sons who are trying to raise their own
young
• Sons are as closely related to their full sibs as they are to
their own offspring (r = 1/2 in both cases)
• Furthermore the average number of offspring in nests
without helpers is 0.51, whereas every helper increases the
number of surviving nestlings by 0.47 on average
• Therefore, the direct fitness lost by a son who is coerced
into helping his parents is balanced by the increase in
indirect fitness that results from helping his parents
(provided that the son would not have had helpers)

29. 29
Bee-eaters recruit helpers who are younger and closely
related (Emlen and Wrege 1992) (Fig. 11.19 b)

30. 30
Reciprocal altruism
• Reciprocation is offered to explain altruism between unrelated
individuals
• The necessary conditions for reciprocal altruism to evolve are:
– The fitness cost to the actor must be ≤ the fitness benefit to the recipient
– Non-reciprocators must be punished in some way (otherwise alleles that
caused “cheating” would displace alleles for altruism)
• Conditions that favor the evolution of reciprocal altruism are:
– Stable social groups (so that individuals are involved in repeated
interactions with one another)
– Lots of opportunities for altruistic interactions during an individual’s
lifetime
– Good memory
– Symmetry of interactions between potential altruists

31. 31
Blood-sharing in vampire bats — an example of
reciprocal altruism? (Wilkinson 1984) — 1
• Reciprocal altruism has been difficult to document
• In vampire bats (Desmodus rotundus) the basic social unit
is 8 – 12 females and their dependent offspring that
frequently roost together
• Many individuals preferentially associate with one another
when roosting
• The altruistic act is sharing blood meals by regurgitation
• 33% of young and 7% of adults fail to get a blood meal on
any given night
• Bats are likely to starve to death if they go three
consecutive nights without a meal

32. 32
Blood-sharing in vampire bats — an example of
reciprocal altruism? (Wilkinson 1984) — 2
• Bats are more likely to share blood with relatives and with
associates. Both effects (relationship and association)
were statistically significant. (These data do not include
regurgitation by mother to child because that is parental
care.)
• Blood sharing was not random in a group of captive
individuals. Bats were more likely to receive blood from
and individual that they had fed previously.

33. 33
Association,
relatedness
and altruism
in vampire
bats
(Wilkinson
1984)