Kin Selection and Social Behavior
Interactions between individuals can have 4 possible outcomes in terms of fitness gains for the
participants.
 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.
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 are easily
explained by selection theory because they benefit the instigator.
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?
BTW when I say “an allele that ‘produces a behavior’ I don’t mean that the allele literally directly codes
for the behavior. An allele directly codes 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.
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.
W.D. Hamilton (1964) developed a model that showed how an allele that favored altruistic behavior
could spread under certain conditions.
Key parameter is the coefficient of relatedness: r.
r is the probability that the homologous alleles in two different individuals are identical by descent (see
earlier notes on inbreeding for the concept of identical by descent—basically copies of the same allele
being inherited from a particular individual).
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.
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.
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.
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).
Applying Hamilton’s rule
You have a food item that is worth 2 units of benefit to you. You also 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?
The value of r for a nephew is ¼.
Cost to you would be 2 (as you’re giving up 2 units of benefit).
Benefit to nephew is 10.
Is Br - C >0?
10(¼) – 2 = ?
2.5 - 2 = 0.5. This is > 0 so you should give the food to your nephew.
Should you share with a cousin? r = 1/8 for a cousin.
No because Br - C is not > 0. 10 *1/8 – 2 = 1.25 – 2 = -0.75
Inclusive fitness
Hamilton invented the idea of inclusive fitness which divides an individual’s fitness into two
components:
Direct fitness results from an individual’s personal reproduction (the babies it produces)
Indirect fitness results from additional reproduction by relatives, that is made possible by an individual’s
actions. For example an individual might help feed its sisters offspring or guard them from predators.
Kin selection
Natural selection favoring the spread of alleles that increase the indirect component of fitness is called
kin selection.
An example of 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 the whistling squirrel is
killed 2% of the time whereas non-whistling squirrels are killed 28% of the time. The calling squirrel
appears to reduce its chance of being killed.
In predatory mammal attacks the trilling squirrel is killed 8% of the time and a non-trilling squirrel is
killed 4% of the time. The calling squirrel thus appears to increase its risk of predation. Thus, whistling
appears to be selfish, but trilling altruistic.
Belding’s Ground Squirrels breed in colonies in Alpine meadows. Males disperse from their natal area to
join other colonies, but female offspring tend to remain and breed close by. Thus, the females in a
colony tend to be related. Sherman had pedigrees that showed relatedness among his study animals. His
analysis of who called showed that females were much more likely to call than males.
Females were also more likely than males to call when they had relatives within earshot.
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.
Overall, the 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.
Altruistic sperm in wood mice
Moore et al. have demonstrated altruistic behavior by sperm of European wood mice. Females mate
with multiple males. Males have large testes and engage in intense sperm competition with other
males. Wood mice sperm have hooks on their heads. Individual sperm frequently 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.
To fertilize an egg, the train must break up. To break up the train many sperm have to undergo
acrosome reaction releasing enzymes that usually help fertilize an egg. 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
It is just as important to avoid paying costs on behalf of non-kin as it is to behave altruistically towards
ones own 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. 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.
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.
Among 133 hosts 43% rejected one or more parasitic eggs. Rejected eggs differed from hosts eggs
significantly more than did accepted eggs.
Females who accepted eggs laid one fewer egg of their own for each parasitic egg they accepted.
Average total clutch size (including parasites) is 8 eggs.
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.
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.
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.
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.
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. P represents the
parental optimum and O represents the offspring’s optimum. 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.
For example, in Black Eagles (an African species)the parents lay two eggs and the first hatched chick
hatches several days before its sibling. When the younger chick hatches, its older sibling attacks and kills
it. 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 chick.
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.
In Black Eagles there is no chance of more than one chick being reared. In other species there is
uncertainty about future conditions. If it is a good food year then they can rear more chicks than they
could in a bad food year. Because the parents don’t know in advance whether a year will be good or not
they lay an optimistic clutch. If it is a good year then they may rear all the chicks, but if it is a bad year
then the oldest chicks outcompete the youngest chicks.
For example, Cattle Egrets lay up to 4 eggs, and as is the case in Black Eagles stagger their hatching.
There is in a cattle egret nest intense conflict that establishes a clear age-based hierarchy in the brood.
The advantage of hatching early gives the older chicks a big edge in competing for food and this
determines how food is divided among the brood members. The oldest chicks feed first and once they
are full any food left is taken by the younger chicks. In cattle egrets, the younger chicks usually starve,
but if it is a good food year they can fledge. 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.
Siblicidal behavior 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 is most likely to occur 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. A bat is likely to starve if it fails to feed
several nights in a row. However, bats who have fed successfully often regurgitate blood meals for
unsuccessful bats.
Cost of sharing some blood is relatively low for donor bat but very valuable for recipient.
Research shows that Vampire bats unsurprisingly share with relatives, but they also share with
individuals who have shared with them previously and with whom they usually share a roost.
Association is a measure of how frequently two individuals associate socially. Regurgitators regurgitate
to individuals they associate with regularly.