Kin Selection and Social
Behavior
 Interactions
between individuals can have
4 possible outcomes in terms of fitness
gains for the participants.
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.
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.
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.
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.
The puzzle of altruism
 W.D.
Hamilton (1964) developed a model
that showed an allele that favored altruistic
behavior could spread under certain
conditions.
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.
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.
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.
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.
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.
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.
Kin selection
 Natural
selection favoring the spread of
alleles that increase the indirect
component of fitness is called kin
selection.
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).
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.
Belding’s Ground Squirrels
 In
predatory mammal attacks trilling
squirrel is killed 8% of the time and a nontrilling 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.
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.
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.
Belding’s Ground Squirrels
 In
addition, females were more likely to
call when they had relatives within
earshot.
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.
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.
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.
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.
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.
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.
Kin selection and cannibalism in
tadpoles
 Some
cannibals discriminated between kin
and non-kin. Others did not.
 Degree
of relatedness to siblings = 1/2
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.
Kin selection and cannibalism in
tadpoles
 Benefit
 Cost
thus approximately 2.
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.
Kin selection and cannibalism in
tadpoles
 By
Hamilton’s rule discrimination should
be favored because 2(1/2) - 0 = 1 which is
>0.
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.
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.
Altruistic sperm in wood mice
 To
 To
fertilize egg, train must break up.
break up train many sperm have to
undergo acrosome reaction releasing
enzymes that usually help fertilize an egg.
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.
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.
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.
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.
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.
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,
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Association is measure
of how frequently two
individuals associate
socially. Regurgitators
regurgitate to
individuals they
associate with regularly.