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Chapter 11. 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.

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chapter 11 kin selection and social behavior
Chapter 11. 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
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 behavior3
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
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 altruism5
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 altruism6
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
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
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 r9
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
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 rule14
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
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
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
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
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
Belding’s 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.
  • Calling squirrel thus appears to increase its risk of predation.
  • Whistling appears to be selfish, but trilling altruistic.
belding s ground squirrels20
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 squirrels21
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 squirrels23
Belding’s Ground Squirrels
  • In addition, females were more likely to call when they had relatives within earshot.
belding s ground squirrels25
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 squirrels27
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.
helping behavior in birds white fronted bee eaters
Helping behavior in birds:White-fronted Bee-eaters
  • In a large number of birds young that are old enough to breed on their own instead help their parents rear siblings.
  • Helpers assist in nest building, nest defense and food delivery.
helping behavior in birds white fronted bee eaters29
Helping behavior in birds:White-fronted Bee-eaters
  • Helping usually occurs in species where breeding opportunities are limited: territories or nest sites are hard to acquire.
  • Young make the best of a bad job by remaining home to assist their parents.
helping behavior in birds white fronted bee eaters30
Helping behavior in birds:White-fronted Bee-eaters
  • Steve Emlen et al. studied white-fronted bee-eaters intensively in Kenya.
  • Nest in colonies of 40-450 individuals. Groups of relatives (clans) defend feeding territories in vicinity of colony.
helping behavior in birds white fronted bee eaters31
Helping behavior in birds:White-fronted Bee-eaters
  • First year birds that opt to help can choose among many relatives when deciding whom to help.
  • Bee-eaters conform to predictions of Hamilton’s rule.
Coefficient of relatedness determines whether a bee-eater helps or not.
  • Also, bee-eaters choose to help their closest relatives.
Nonbreeders in clan that are not relatives (birds that have paired with members of the clan) are not related to offspring being reared and are much less likely to help than relatives.
Assistance of helpers is of enormous benefit to parents. More than 50% of bee-eater young starve before leaving the nest.
  • On average, presence of each helper increases number of offspring successfully reared to fledging by 0.47. Thus, there is a clear inclusive fitness benefit.
kin selection and cannibalism in tadpoles
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 tadpoles38
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 tadpoles39
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 tadpoles41
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 tadpoles42
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 tadpoles43
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 tadpoles44
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.
kin selection and cannibalism in tadpoles46
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
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 mice48
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 mice50
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.
altruistic sperm in wood mice51
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
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 coots53
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 coots54
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 coots55
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 coots57
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 coots59
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.
the greenbeard effect
The greenbeard effect
  • Sometimes altruistic alleles help different alleles inadvertently when they help kin. However, behavior is still favored because it assists identical alleles half of the time.
the greenbeard effect62
The greenbeard effect
  • If alleles could recognize which individuals carried other copies of them then they could selectively act altruistically towards those individuals.
  • Dawkins (1976) called this the greenbeard effect.
the greenbeard effect63
The greenbeard effect
  • Dawkins imagined an allele that caused its carriers to grow green beards, to recognize green beards in others and act altruistically towards them.
  • Hard to imagine in wild because single allele must cause three different effects.
the greenbeard effect64
The greenbeard effect
  • However, Quellar et al. (2003) have described greenbeard effect in slime molds.
  • Slime molds live in soil. Germinate from spores and spend most of life as independent, single-celled amoebae.
slime molds
Slime molds
  • When food scarce, individuals signal each other chemically and aggregate together to form a slug-like mass.
  • Slug travels some distance, then transforms into a tall, thin stalk with fruiting body on top.
  • Cells in fruiting body form spores which disperse and begin cycle again.
slime molds67
Slime molds
  • Cells in stalk (20% of the individuals) sacrifice themselves.
slime molds68
Slime molds
  • Quellar et al. studied wild-type allele csA
  • Allele codes for protein on cell surface of amoeba and that protein sticks to same protein on other amoebae. Allele thus codes for both trait and recognition (adhesion).
slime molds69
Slime molds
  • Remaining greenbeard trait is discriminating altruism.
  • Quellar et al. mixed wild-type amoebae and amoebae carrying a knocked-out version of the csA allele and grew them on agar plates.
  • Starved amoebae to induce slime molds to form fruiting bodies.
slime molds70
Slime molds
  • Quellar et al. found that wild-type cells were disproportionately represented in the stalk (suckers!) and knock-out type in the fruiting body.
  • Wild-type apparently ended up in stalk because they stuck together better.
slime molds72
Slime molds
  • Situation reversed when slime molds grown on soil, their natural environment.
  • More difficult for amoebae to stream on soil and wild-type can stick together and pull each other along.
slime molds73
Slime molds
  • Wild-type cells disproportionately represented in fruiting body as well as stalk.
  • Less adhesive knockout cells tend to get left out of aggregations altogether.
slime molds75
Slime molds
  • Thus, in natural conditions wild-type allele of csA makes its carriers altruistic towards other wild-type cells.
  • Kin selection thus works at level of individual alleles, not just individual organisms.
evolution of eusociality
Evolution of Eusociality
  • Eusociality (true sociality).
  • Many eusocial insects (bees, ants, termites) do not reproduce. Instead they act as helpers at parents nests for their entire life. This is an extreme type of altruism.
evolution of eusociality77
Evolution of Eusociality
  • Eusociality describes social systems with three characteristics:
    • Overlap in generations between parents and offspring.
    • Cooperative brood care.
    • Specialist castes of non-reproductive individuals.
haplodiploidy and eusocial hymenoptera
Haplodiploidy and eusocial Hymenoptera
  • One idea advanced to explain eusociality is the unusual genetic system (Haplodiploidy) of the Hymenoptera (ants, wasps, bees, etc.).
  • Males are haploid and females diploid.
  • Males develop from unfertilized eggs and females from fertilized eggs.
haplodiploidy and eusocial hymenoptera79
Haplodiploidy and eusocial Hymenoptera
  • Daughters receive all of their fathers genes and half of their mothers genes. Thus, daughters share ¾ of their genes.
  • This suggests females would be better off if they favored the production of reproductive sisters rather than their own offspring.
haplodiploidy and eusocial hymenoptera80
Haplodiploidy and eusocial Hymenoptera
  • Queens are equally related to all offspring and so should prefer a 1:1 ratio of sons to daughters among reproductives.
  • Females workers however should prefer a 1:3 ratio of brothers to sisters among reproductives.
haplodiploidy and eusocial hymenoptera81
Haplodiploidy and eusocial Hymenoptera
  • It has been shown in wood ants that queens produce equal numbers of male and female eggs, but the hatching ratio is heavily female biased. Workers apparently selectively destroy male eggs.
haplodiploidy and eusocial hymenoptera82
Haplodiploidy and eusocial Hymenoptera
  • Haplodiploidy appears to influence worker behavior, but consensus today is that it does not explain evolution of eusocial behavior in Hymenoptera.
  • There are several reasons why.
haplodiploidy and eusociality
Haplodiploidy and eusociality
  • First, haplodiploid explanation assumes all workers have the same father. However, honeybee queens mate with more than 17 males on average.
  • As a result relatedness between worker honeybees often below 1/3.
haplodiploidy and eusociality84
Haplodiploidy and eusociality
  • Second, in many species, more than one female founds a nest. In this case workers may be completely unrelated.
haplodiploidy and eusociality85
Haplodiploidy and eusociality
  • Third, many eusocial species are not haploid (e.g. termites) and many haplodiploid species are not eusocial.
haplodiploidy and eusociality86
Haplodiploidy and eusociality
  • Phylogenetic analysis of Hymenoptera by Hunt (1999) emphasizes that eusociality relatively rare even though haplodiploidy occurs in all groups.
  • Eusociality occurs in only a few families which are scattered around the tree, which suggests eusociality has evolved independently multiple times.
haplodiploidy and eusociality89
Haplodiploidy and eusociality
  • Hunt also points out that eusociality has only evolved in groups that build complex nests, and care for young for a long time.
  • Association between nest building, long term care and eusociality suggests main driving force for eusociality is ecological not genetic.
haplodiploidy and eusociality90
Haplodiploidy and eusociality
  • Nest building and need to supply offspring with a steady stream of food make it impossible or very difficult for a female to breed alone.
  • Also, if predation rates are high, solitary breeding individuals may not live long enough to raise their young.
facultative strategies in paper wasps
Facultative strategies in paper wasps.
  • Paper wasps (Polistes) are not sterile (unlike ant and bee workers). Females can nest with other females or establish their own nest.
  • Nonacs and Reeve (1995) found in Polistes dominulus that females follow one of three strategies.
facultative strategies in paper wasps92
Facultative strategies in paper wasps.
  • Initiate own nest
  • Join nest as a helper
  • Wait for a nest to become available
facultative strategies in paper wasps93
Facultative strategies in paper wasps.
  • Individuals founding their own nest are very likely to fail because adult mortality is high and nests with multiple foundresses can keep the nest going.
  • However in multifoundress nests there may be frequent conflict. The nests that did best were those where one female was markedly bigger than the others, which reduced fighting.
facultative strategies in paper wasps95
Facultative strategies in paper wasps.
  • The “sit-and-wait” strategy also can pay off because females often can adopt an orphaned nest or take one over late in the season.
facultative strategies in paper wasps96
Facultative strategies in paper wasps.
  • Overall, in paper wasps an individual’s decision is affected by her relative size, relatedness to other females, and availability of unoccupied nests.
naked mole rats
Naked Mole-rats
  • Naked mole-rats are highly unusual mammals.
  • They are nearly hairless and ectothermic. They are eusocial and, like termites, can digest cellulose with the help of bacteria in their gut.
naked mole rats99
Naked Mole-rats
  • The behavior of naked mole-rats is similar to that of colonial insects.
  • There is a single reproductive female (queen) and 1-3 reproductive males. The remaining individuals act as workers. They dig tunnels to find food, defend the tunnel system from other mole-rats, and tend the young.
naked mole rats100
Naked Mole-rats
  • Leading hypothesis for why naked mole-rats are eusocial is inbreeding.
  • Average coefficient of relatedness is 0.81 and about 85% of matings are between parents and offspring or between full siblings.
naked mole rats101
Naked Mole-rats
  • Despite high level of relatedness conflicts still occur because reproductive interests of workers and reproductives are not identical.
naked mole rats102
Naked Mole-rats
  • Queens maintain control through physical dominance.
  • Queen aggressively shoves workers who do not work hard enough and shoves are mainly directly towards less closely related individuals.
  • Workers double their work rate after being shoved.
naked mole rats103
Naked Mole-rats
  • In addition to inbreeding, ecological factors such as severely limited breeding opportunities and group defense appear to contribute to eusociality in naked mole-rats.
parent offspring conflict
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 conflict105
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 conflict106
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 conflict108
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.
  • 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 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 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.
  • 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 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 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
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 altruism117
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
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 bats119
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.