Kin Selection and Social Behavior

Kin Selection and Social Kin Selection and Social BehaviorBehavior

Interactions between individuals can have Interactions between individuals can have 4 possible outcomes in terms of fitness 4 possible outcomes in terms of fitness gains for the participants.gains for the participants.


CooperationCooperation ( (mutualismmutualism): fitness gains ): fitness gains for both participants.for both participants.

AltruismAltruism: instigator pays fitness cost, : instigator pays fitness cost, recipient benefits.recipient benefits.

SelfishnessSelfishness: instigator gains benefit, other : instigator gains benefit, other individual pays cost.individual pays cost.

SpiteSpite: both individuals suffer a fitness : both individuals suffer a fitness cost.cost.


No clear cut cases of spite documented. No clear cut cases of spite documented.

Selfish and cooperative behaviors easily Selfish and cooperative behaviors easily explained by selection theory because explained by selection theory because they benefit the instigator.they benefit the instigator.

The puzzle of altruismThe puzzle of altruism

AltruismAltruism is the difficult one to explain is the difficult one to explain because the instigator pays a cost and because the instigator pays a cost and another individual benefits.another individual benefits.

Hard to see how selection could favor an Hard to see how selection could favor an allele that produces behavior benefiting allele that produces behavior benefiting another individual at the expense of the another individual at the expense of the individuals bearing the allele.individuals bearing the allele.


For Darwin altruism presented a “special For Darwin altruism presented a “special difficulty, which at first appears to me difficulty, which at first appears to me insuperable, and actually fatal to my whole insuperable, and actually fatal to my whole theory.”theory.”

Darwin suggested however that if a Darwin suggested however that if a behavior benefited relatives, it might be behavior benefited relatives, it might be favored by selection.favored by selection.


W.D. Hamilton (1964) developed a model W.D. Hamilton (1964) developed a model that showed an allele that favored altruistic that showed an allele that favored altruistic behavior could spread under certain behavior could spread under certain conditions.conditions.

Coefficient of relatednessCoefficient of relatedness

Key parameter is the Key parameter is the coefficient of coefficient of relatedness: r.relatedness: r.

r is the probability that the homologous r is the probability that the homologous alleles in two individuals are identical by alleles in two individuals are identical by descent.descent.

Calculating rCalculating r

Need a pedigree to calculate r that Need a pedigree to calculate r that includes both the actor and recipient and includes both the actor and recipient and that shows all possible direct routes of that shows all possible direct routes of connection between the two. connection between the two.

Because parents contribute half their Because parents contribute half their genes to each offspring, the probability genes to each offspring, the probability that genes are identical by descent for that genes are identical by descent for each step is 50% or 0.5.each step is 50% or 0.5.

Calculating rCalculating r

To calculate r one should trace each path To calculate r one should trace each path between the two individuals and count the between the two individuals and count the number of steps needed. Then for this number of steps needed. Then for this path r = 0.5 path r = 0.5 (number of steps)(number of steps)

Thus, if two steps r for this path = 0.5 Thus, if two steps r for this path = 0.5 (2)(2) = = 0.25.0.25.

To calculate final value of r one adds To calculate final value of r one adds together the r values calculated from each together the r values calculated from each path.path.

Hamilton’s ruleHamilton’s rule

Given r the coefficient of relatedness Given r the coefficient of relatedness between the actor and the recipient, between the actor and the recipient, Hamilton’s ruleHamilton’s rule states that an allele for states that an allele for altruistic behavior will spread ifaltruistic behavior will spread if

Br - C >0Br - C >0 Where B is benefit to recipient and C is the Where B is benefit to recipient and C is the

cost to the actor. Unit of measurement for cost to the actor. Unit of measurement for B and C is surviving offspring.B and C is surviving offspring.

Hamilton’s ruleHamilton’s rule

Altruistic behaviors are most likely to Altruistic behaviors are most likely to spread when costs are low, benefits to spread when costs are low, benefits to recipient are high, and the participants are recipient are high, and the participants are closely related.closely related.

Inclusive fitnessInclusive fitness

Hamilton invented the idea of inclusive Hamilton invented the idea of inclusive fitness. Fitness can be divided into two fitness. Fitness can be divided into two components: components:

Direct fitness results from personal Direct fitness results from personal reproductionreproduction

Indirect fitness results from additional Indirect fitness results from additional reproduction by relatives, reproduction by relatives, that is made that is made possible by an individual’s actions.possible by an individual’s actions.

Kin selectionKin selection

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

Alarm calling in Belding’s Alarm calling in Belding’s Ground SquirrelsGround Squirrels

Giving alarm calls alerts other individuals Giving alarm calls alerts other individuals but may attract a predator’s attention.but may attract a predator’s attention.

Belding’s Ground Squirrels give two Belding’s Ground Squirrels give two different calls depending on whether different calls depending on whether predator is a predatory mammal (trill) or a predator is a predatory mammal (trill) or a hawk (whistle; Sherman 1985).hawk (whistle; Sherman 1985).

Is alarm calling altruistic? Is alarm calling altruistic?

Sherman and colleagues observed 256 Sherman and colleagues observed 256 natural predator attacks.natural predator attacks.

In hawk attacks whistling squirrel is killed In hawk attacks whistling squirrel is killed 2% of the time whereas non-whistling 2% of the time whereas non-whistling squirrels are killed 28% of the time.squirrels are killed 28% of the time.

Calling squirrel appears to reduce its Calling squirrel appears to reduce its chance of being killed.chance of being killed.

Belding’s Ground SquirrelsBelding’s Ground Squirrels

In predatory mammal attacks trilling In predatory mammal attacks trilling squirrel is killed 8% of the time and a non-squirrel is killed 8% of the time and a non-trilling squirrel is killed 4% of the time.trilling squirrel is killed 4% of the time.

Calling squirrel thus appears to increase Calling squirrel thus appears to increase its risk of predation. its risk of predation.

Whistling appears to be selfish, but trilling Whistling appears to be selfish, but trilling altruistic.altruistic.


Belding’s Ground Squirrels breed in Belding’s Ground Squirrels breed in colonies in Alpine meadows.colonies in Alpine meadows.

Males disperse, but female offspring tend Males disperse, but female offspring tend to remain and breed close by. Thus, to remain and breed close by. Thus, females in colony tend to be related.females in colony tend to be related.


Sherman had marked animals and had Sherman had marked animals and had pedigrees that showed relatedness among pedigrees that showed relatedness among study animals.study animals.

Analysis of who called showed that Analysis of who called showed that females were much more likely to call than females were much more likely to call than males.males.


In addition, females were more likely to In addition, females were more likely to call when they had relatives within call when they had relatives within earshot.earshot.


Relatives also cooperated in behaviors Relatives also cooperated in behaviors besides alarm calling.besides alarm calling.

Females were much more likely to join Females were much more likely to join close relatives in chasing away close relatives in chasing away trespassing ground squirrels than less trespassing ground squirrels than less closely related kin and non-kin.closely related kin and non-kin.


Overall, data show that altruistic behavior Overall, data show that altruistic behavior is not randomly directed. It is focused on is not randomly directed. It is focused on close relatives and should result in indirect close relatives and should result in indirect fitness gains.fitness gains.

Kin selection and cannibalism in Kin selection and cannibalism in tadpolestadpoles

Spadefoot toad tadpoles come in two Spadefoot toad tadpoles come in two morphs.morphs.

Typical morph is omnivorous mainly eats Typical morph is omnivorous mainly eats decaying plant material.decaying plant material.

Cannibalistic morph has bigger jaws and Cannibalistic morph has bigger jaws and catches prey including other spadefoot catches prey including other spadefoot tadpoles.tadpoles.


Pfennig (1999) tested whether cannibals Pfennig (1999) tested whether cannibals discriminate between kin and non-kin.discriminate between kin and non-kin.

Placed 28 cannibalistic tadpoles in Placed 28 cannibalistic tadpoles in individual containers. Added two individual containers. Added two omnivorous tadpoles (tadpole had never omnivorous tadpoles (tadpole had never seen before) to each container. One was seen before) to each container. One was a sibling, the other non-kin.a sibling, the other non-kin.


Pfenning waited until cannibal ate one Pfenning waited until cannibal ate one tadpole, then determined which had been tadpole, then determined which had been eaten.eaten.

Found that kin were significantly less likely Found that kin were significantly less likely to be eaten. Only 6 of 28 kin were eaten, to be eaten. Only 6 of 28 kin were eaten, but 22 of 28 non-kin.but 22 of 28 non-kin.


Pfennig also studied tiger salamanders Pfennig also studied tiger salamanders whose tadpoles also develop into whose tadpoles also develop into cannibalistic morphs.cannibalistic morphs.

Kept 18 cannibals in separate enclosures Kept 18 cannibals in separate enclosures in natural pond. To each enclosure added in natural pond. To each enclosure added 6 siblings and 18 non-kin typical morph 6 siblings and 18 non-kin typical morph tadpoles.tadpoles.


Some cannibals discriminated between kin Some cannibals discriminated between kin and non-kin. Others did not.and non-kin. Others did not.

Degree of relatedness to siblings = 1/2Degree of relatedness to siblings = 1/2


Thus, by Hamilton’s rule discrimination in Thus, by Hamilton’s rule discrimination in favor of kin favored if B(r) - C > 0favor of kin favored if B(r) - C > 0

Benefit estimated by counting number of Benefit estimated by counting number of siblings that survived. Siblings of siblings that survived. Siblings of discriminating cannibals twice as likely to discriminating cannibals twice as likely to survive as siblings of non-discriminating survive as siblings of non-discriminating cannibals.cannibals.


Benefit thus approximately 2.Benefit thus approximately 2.

Cost assessed by evaluating effect of not Cost assessed by evaluating effect of not eating siblings by comparing growth of eating siblings by comparing growth of discriminating and non-discriminating discriminating and non-discriminating cannibals. No difference in growth rates. cannibals. No difference in growth rates. Cost then estimated as close to 0.Cost then estimated as close to 0.


By Hamilton’s rule discrimination should By Hamilton’s rule discrimination should be favored because 2(1/2) - 0 = 1 which is be favored because 2(1/2) - 0 = 1 which is >0.>0.

Altruistic sperm in wood miceAltruistic sperm in wood mice

Moore et al. have demonstrated altruistic Moore et al. have demonstrated altruistic behavior by sperm of European wood behavior by sperm of European wood mice.mice.

Females highly promiscuous. Males have Females highly promiscuous. Males have large testes and engage in intense sperm large testes and engage in intense sperm competition with other males.competition with other males.


Wood mice sperm have hooks on their Wood mice sperm have hooks on their heads. And connect together to form long heads. And connect together to form long trains of sperm that can include thousands trains of sperm that can include thousands of sperm.of sperm.

Swimming together sperm travel twice as Swimming together sperm travel twice as fast as if they swam separately.fast as if they swam separately.


To fertilize egg, train must break up.To fertilize egg, train must break up.

To break up train many sperm have to To break up train many sperm have to undergo acrosome reaction releasing undergo acrosome reaction releasing enzymes that usually help fertilize an egg.enzymes that usually help fertilize an egg.


Releasing these enzymes before reaching Releasing these enzymes before reaching an egg means these sperm cannot fertilize an egg means these sperm cannot fertilize the egg. These sperm sacrifice the egg. These sperm sacrifice themselves. themselves.

Because other sperm carry half of the Because other sperm carry half of the same alleles, sacrifice makes sense in same alleles, sacrifice makes sense in terms of kin selection.terms of kin selection.

Discrimination against non-kin Discrimination against non-kin eggs by cootseggs by coots

Important to avoid paying costs on behalf Important to avoid paying costs on behalf of non-kin.of non-kin.

Lyon (2003) studied defense against nest Lyon (2003) studied defense against nest parasitism in American coots.parasitism in American coots.

Coots often lay eggs in other coot’s nests Coots often lay eggs in other coot’s nests in hopes of having them reared.in hopes of having them reared.


Accepting parasitic eggs is costly Accepting parasitic eggs is costly because half of all chicks starve and because half of all chicks starve and same number reared in parasitized and same number reared in parasitized and non-parasitized nests.non-parasitized nests.

Thus, host parent loses one offspring Thus, host parent loses one offspring for every successful parasite.for every successful parasite.


Because of high cost of being parasitized Because of high cost of being parasitized and lack of benefit (assuming parasites and lack of benefit (assuming parasites are non-kin) Hamilton’s rule predicts coots are non-kin) Hamilton’s rule predicts coots should discriminate against parasitic eggs.should discriminate against parasitic eggs.

Coot eggs very variable in appearance. If Coot eggs very variable in appearance. If 2 eggs laid within 24 hours Lyon knew one 2 eggs laid within 24 hours Lyon knew one was a parasite.was a parasite.


Among 133 hosts 43% rejected one or Among 133 hosts 43% rejected one or more parasitic eggs. Rejected eggs more parasitic eggs. Rejected eggs differed from hosts eggs significantly more differed from hosts eggs significantly more than did accepted eggs.than did accepted eggs.


Females who accepted eggs laid one Females who accepted eggs laid one fewer egg of their own for each parasitic fewer egg of their own for each parasitic egg they accepted. Average total clutch egg they accepted. Average total clutch (including parasites) 8 eggs,(including parasites) 8 eggs,


Females who rejected eggs laid an Females who rejected eggs laid an average of 8 of their own eggs even average of 8 of their own eggs even though they waited to finish laying before though they waited to finish laying before disposing off eggs they were rejecting. disposing off eggs they were rejecting. Coots can count!Coots can count!

By counting eggs and rejecting extras that By counting eggs and rejecting extras that do not look right coots prevent themselves do not look right coots prevent themselves from being parasitized.from being parasitized.

Parent-offspring conflict.Parent-offspring conflict.

Parental care is an obvious form of Parental care is an obvious form of altruism. In many species parents invest altruism. In many species parents invest huge quantities of resources in their huge quantities of resources in their offspring.offspring.

Initially, parent and offspring agree that Initially, parent and offspring agree that investment in the offspring is worthwhile investment in the offspring is worthwhile because it enhances the offspring’s because it enhances the offspring’s prospects of survival and reproduction.prospects of survival and reproduction.


However, a parent shares only 50% of its genes However, a parent shares only 50% of its genes with the offspring and is equally related to all of with the offspring and is equally related to all of its offspring, whereas offspring is 100% related its offspring, whereas offspring is 100% related to itself, but only shares 50% of genes with its to itself, but only shares 50% of genes with its siblings.siblings.

As a result, at some point a parent will prefer to As a result, at some point a parent will prefer to reserve investment for future offspring rather reserve investment for future offspring rather than investing in the current one, while the than investing in the current one, while the current offspring will disagree. This leads to a current offspring will disagree. This leads to a period of conflict called weaning.period of conflict called weaning.


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

In instances where parents produce only In instances where parents produce only half siblings we should expect weaning half siblings we should expect weaning conflict to last longer because the current conflict to last longer because the current offspring is les closely related to future offspring is les closely related to future offspring.offspring.

This has been confirmed in various field This has been confirmed in various field studies.studies.

SiblicideSiblicide

In many species there is intense conflict In many species there is intense conflict between siblings for food that may result in between siblings for food that may result in younger weaker chicks starving to death.younger weaker chicks starving to death.

In other species regardless of food In other species regardless of food supplies first hatched offspring routinely supplies first hatched offspring routinely kill their siblings. kill their siblings.

SiblicideSiblicide

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

SiblicideSiblicide

In species such as Black Eagles siblicide In species such as Black Eagles siblicide is obligate in that the younger offspring is is obligate in that the younger offspring is always killed. Black Eagles are only always killed. Black Eagles are only capable of rearing one young.capable of rearing one young.

The most likely explanation for the later The most likely explanation for the later hatched young is that for the parents it hatched young is that for the parents it serves as an “insurance offspring” in case serves as an “insurance offspring” in case the first offspring fails to hatch or develop. the first offspring fails to hatch or develop.

SiblicideSiblicide

In other species such as Cattle Egrets In other species such as Cattle Egrets there is intense conflict that establishes a there is intense conflict that establishes a clear age-based hierarchy in the brood clear age-based hierarchy in the brood that determines how food is divided that determines how food is divided among the brood members.among the brood members.

In cattle egrets, younger chicks usually In cattle egrets, younger chicks usually starve, but if it is a good food year they starve, but if it is a good food year they often fledge. often fledge.

SiblicideSiblicide

Siblicide is thus facultative in cattle egrets Siblicide is thus facultative in cattle egrets because restraint by the older chicks in not because restraint by the older chicks in not killing the younger siblings can be killing the younger siblings can be rewarded in good years.rewarded in good years.

In Black Eagles there is no prospect of two In Black Eagles there is no prospect of two young being reared, so the older chick young being reared, so the older chick ensures its own survival by eliminating its ensures its own survival by eliminating its sibling.sibling.

SiblicideSiblicide

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

Reciprocal AltruismReciprocal Altruism

Some animals occasionally behave Some animals occasionally behave altruistically towards non-relatives.altruistically towards non-relatives.

Such behavior is adaptive if the recipient is Such behavior is adaptive if the recipient is likely to return the favor in the future.likely to return the favor in the future.

Reciprocal altruismReciprocal altruism

Reciprocal altruism most likely in social Reciprocal altruism most likely in social animals where individuals interact animals where individuals interact repeatedly because they are long-lived repeatedly because they are long-lived and form groups, and also when and form groups, and also when individuals have good memories.individuals have good memories.

Reciprocal altruism in Vampire batsReciprocal altruism in Vampire bats

E.g. Vampire Bats. Feed on blood and E.g. Vampire Bats. Feed on blood and share communal roosts.share communal roosts.

Bats may starve if they fail to feed several Bats may starve if they fail to feed several nights in a row. nights in a row.

However, bats who have fed successfully However, bats who have fed successfully often regurgitate blood meals for often regurgitate blood meals for unsuccessful bats. unsuccessful bats.

Reciprocal altruism in Vampire batsReciprocal altruism in Vampire bats

Cost of sharing some blood is relatively Cost of sharing some blood is relatively low for donor bat but very valuable for low for donor bat but very valuable for recipient. recipient.

Research shows that Vampire bats share Research shows that Vampire bats share with relatives, but also share with with relatives, but also share with individuals who have shared with them individuals who have shared with them previously and with whom they usually previously and with whom they usually share a roost. share a roost.

Association is measure of how frequently two individuals associate socially. Regurgitatorsregurgitate toindividuals theyassociate with regularly.

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Kin Selection and Social Behavior