chapter 13: evolution of social behavior costs and benefits of social life. there are a large...

Chapter 13: Evolution of Social Chapter 13: Evolution of Social BehaviorBehavior

Costs and benefits of Social Life.Costs and benefits of Social Life.

There are a large number of possible There are a large number of possible costs and benefits associated with social costs and benefits associated with social behavior.behavior.

Potential costs and benefits of Potential costs and benefits of socialitysociality

Greater conspicuousness to predators, but Greater conspicuousness to predators, but also better defense against predators. also better defense against predators.

Many social behavior e.g. schooling by Many social behavior e.g. schooling by fish seem to be primarily anti-predator fish seem to be primarily anti-predator behaviors.behaviors.

Fig 13.6

Schooling catfish


There have been numerous studies that There have been numerous studies that have documented the anti-predator have documented the anti-predator benefits of social behavior.benefits of social behavior.

Groups detect predators sooner and there Groups detect predators sooner and there is also a dilution effect by being a member is also a dilution effect by being a member of a group.of a group.


Group defense also is a benefit. Group defense also is a benefit. Colonially nesting gulls deter predatory Colonially nesting gulls deter predatory

birds.birds. Males in colonies of bluegill sunfish Males in colonies of bluegill sunfish

collaborate to drive away egg-eating collaborate to drive away egg-eating predators.predators.

Male bluegill sunfish nest colonially as defensive adaptation against egg-eating predators.


If bluegill sunfish have evolved colonial nesting If bluegill sunfish have evolved colonial nesting to deter predators, then we would expect to deter predators, then we would expect solitarily nesting related species to suffer less solitarily nesting related species to suffer less from predators.from predators.

As predicted, solitarily nesting pumpkinseed As predicted, solitarily nesting pumpkinseed sunfish has powerful jaws with which it can deter sunfish has powerful jaws with which it can deter predators and so does not need to group nests predators and so does not need to group nests for protection.for protection.


More rapid disease transmission is likely More rapid disease transmission is likely among social organisms and parasites can among social organisms and parasites can spread more readily. spread more readily.

There may be some advantage is sociality There may be some advantage is sociality in that more grooming assistance may be in that more grooming assistance may be available, but on balance disease available, but on balance disease transmission appears to be a clear cost of transmission appears to be a clear cost of sociality.sociality.


For example, colonial cliff swallow For example, colonial cliff swallow nestlings are much more affected by nestlings are much more affected by swallow bugs than solitarily nesting birds.swallow bugs than solitarily nesting birds.

Nestlings parasitized by bugs were Nestlings parasitized by bugs were significantly smaller and less likely to significantly smaller and less likely to survive than unparasitized individuals.survive than unparasitized individuals.

Fig 13.5

Cliff swallow young exposed to parasites (left) and unexposed (right).


More competition for food is a likely cost of More competition for food is a likely cost of sociality.sociality.

Among lions females are forced to wait until the Among lions females are forced to wait until the males have fed before having a chance to eat.males have fed before having a chance to eat.

In fieldfares (a European thrush) the larger the In fieldfares (a European thrush) the larger the colony, the lower the survival rate of nestlings colony, the lower the survival rate of nestlings because starvation rates increase. because starvation rates increase.

Fig 13.4


However, for other birds (and bees), which feed However, for other birds (and bees), which feed on spatially clumped, but unpredictable food on spatially clumped, but unpredictable food supplies, colonial breeding appears to supplies, colonial breeding appears to significantly improve foraging success.significantly improve foraging success.

This can occur through the use of information This can occur through the use of information centers and more effective use of local centers and more effective use of local enhancement information because coloniality enhancement information because coloniality clumps foragers in space.clumps foragers in space.


Sociality also increases opportunities for Sociality also increases opportunities for reproductive interference, which can be reproductive interference, which can be positive or negative depending on an positive or negative depending on an individual’s success.individual’s success.


For social behavior to evolve the balance For social behavior to evolve the balance of costs and benefits should overall favor of costs and benefits should overall favor the behavior.the behavior.

A major component of social behavior is A major component of social behavior is positive interactions with other individuals. positive interactions with other individuals. Two major factors will dictate whether Two major factors will dictate whether individuals are likely to behave individuals are likely to behave altruistically towards others. altruistically towards others.

AltruismAltruism

Altruisitic behavior is puzzling as it is Altruisitic behavior is puzzling as it is behavior that imposes a cost on the actor behavior that imposes a cost on the actor for the benefit of another individual.for the benefit of another individual.

It should thus be selected against.It should thus be selected against.

However, However, kin selectionkin selection and the possibility and the possibility of of reciprocal altruismreciprocal altruism can favor altruistic can favor altruistic behavior.behavior.

Coefficient of relatednessCoefficient of relatedness

A key parameter in understanding kin A key parameter in understanding kin selection is the selection 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 be favored and altruistic behavior will be favored and spread ifspread 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 Hamilton invented the idea of inclusive inclusive fitnessfitness. Fitness can be divided into two . Fitness can be divided into two components: components:

Direct fitnessDirect fitness results from personal results from personal reproductionreproduction

Indirect fitnessIndirect fitness results from additional 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..

Kin selection in Belding’s Kin selection 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.


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.

Reciprocal AltruismReciprocal Altruism

The second major way in which altruism The second major way in which altruism can be favored is if recipients repay can be favored is if recipients repay altruistic behavior in the future.altruistic behavior in the future.

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.

Helpers at the nest. Helpers at the nest. White-fronted Bee-eatersWhite-fronted Bee-eaters

In a large number of birds young that are In a large number of birds young that are old enough to breed on their own instead old enough to breed on their own instead help their parents rear siblings.help their parents rear siblings.

Helpers assist in nest building, nest Helpers assist in nest building, nest defense and food delivery.defense and food delivery.


Helping usually occurs in species where Helping usually occurs in species where breeding opportunities are limited: breeding opportunities are limited: territories or nest sites are hard to acquire.territories or nest sites are hard to acquire.

Young make the best of a bad job by Young make the best of a bad job by remaining home to assist their parents.remaining home to assist their parents.


Steve Emlen et al. studied white-fronted Steve Emlen et al. studied white-fronted bee-eaters intensively in Kenya.bee-eaters intensively in Kenya.

Nest in colonies of 40-450 individuals. Nest in colonies of 40-450 individuals. Groups of relatives (clans) defend feeding Groups of relatives (clans) defend feeding territories in vicinity of colony.territories in vicinity of colony.


First year birds that opt to help can choose First year birds that opt to help can choose among many relatives when deciding among many relatives when deciding whom to help.whom to help.

Bee-eaters conform to predictions of Bee-eaters conform to predictions of Hamilton’s rule.Hamilton’s rule.


Coefficient of relatedness determines Coefficient of relatedness determines whether a bee-eater helps or not.whether a bee-eater helps or not.

Also, bee-eaters choose to help their Also, bee-eaters choose to help their closest relatives.closest relatives.


Nonbreeders in clan that are not relatives Nonbreeders in clan that are not relatives (birds that have paired with members of (birds that have paired with members of the clan) are not related to offspring being the clan) are not related to offspring being reared and are much less likely to help reared and are much less likely to help than relatives.than relatives.


Assistance of helpers is of enormous Assistance of helpers is of enormous benefit to parents. More than 50% of bee-benefit to parents. More than 50% of bee-eater young starve before leaving the nest.eater young starve before leaving the nest.

On average, presence of each helper On average, presence of each helper increases number of offspring successfully increases number of offspring successfully reared to fledging by 0.47. Thus, there is reared to fledging by 0.47. Thus, there is a clear inclusive fitness benefit.a clear inclusive fitness benefit.

Inclusive fitness and Pied Inclusive fitness and Pied KingfishersKingfishers

Another example of a species with helpers Another example of a species with helpers at the nest is the Pied Kingfisher.at the nest is the Pied Kingfisher.

Pied Kingfishers nest colonially in tunnels.Pied Kingfishers nest colonially in tunnels.

Some one-year old males may not be able Some one-year old males may not be able to find a mate and so become to find a mate and so become primaryprimary helpershelpers assisting their mother to feed assisting their mother to feed young and deter predators.young and deter predators.


Primary helpers have alternatives. They Primary helpers have alternatives. They could choose not to help and delay could choose not to help and delay breeding until next year (breeding until next year (delayerdelayer) or assist ) or assist at another unrelated nest (at another unrelated nest (secondary secondary helperhelper). ).

What are costs and benefits of being a What are costs and benefits of being a primary helper?primary helper?


Primary helpers work harder than delayers and Primary helpers work harder than delayers and secondary helpers so they have a lower chance secondary helpers so they have a lower chance of surviving to breed the next year (54%) than of surviving to breed the next year (54%) than secondary helpers (74%) or delayers (70%).secondary helpers (74%) or delayers (70%).

Also only 66% of primary helpers attract mates, Also only 66% of primary helpers attract mates, but 91% of secondary helpers do (in 10 of 27 but 91% of secondary helpers do (in 10 of 27 cases with the female they helped the previous cases with the female they helped the previous year). Delayers have only a 33% chance.year). Delayers have only a 33% chance.


To determine payoffs need to add the To determine payoffs need to add the reproductive success of each approach reproductive success of each approach over the two years.over the two years.

Calculate payoffs by multiplying probability Calculate payoffs by multiplying probability of survival times number of offspring of survival times number of offspring produced times probability of survival produced times probability of survival times probability finding a mate times times probability finding a mate times relatedness to offspring.relatedness to offspring.


Primary helpers gain reproductive benefit Primary helpers gain reproductive benefit in both years (0.58 [indirect fitness]+ 0.41 in both years (0.58 [indirect fitness]+ 0.41 young [direct fitness]= 0.99).young [direct fitness]= 0.99).

Secondary helpers obtain a second year Secondary helpers obtain a second year payoff of 0.84 young and delayers only payoff of 0.84 young and delayers only 0.29.0.29.

Primary helpers have lower RS is year 2, Primary helpers have lower RS is year 2, but this is more than compensated for by but this is more than compensated for by indirect fitness benefit from year 1.indirect fitness benefit from year 1.

Evolution of EusocialityEvolution of Eusociality

Eusociality (true sociality).Eusociality (true sociality).

Many eusocial insects (bees, ants, Many eusocial insects (bees, ants, termites) do not reproduce. termites) do not reproduce.

Instead they act as helpers at parents Instead they act as helpers at parents nests for their entire life. nests for their entire life.


Sterility and obligate helping is an extreme type of Sterility and obligate helping is an extreme type of altruism that goes far beyond the helpers at the nest altruism that goes far beyond the helpers at the nest behavior seen in birds.behavior seen in birds.

Suicidal behavior in defense of the group is quite Suicidal behavior in defense of the group is quite common. common.

E.g. honey bee stings are barbed so that when a bee E.g. honey bee stings are barbed so that when a bee stings it leaves its poison sac behind and fatally stings it leaves its poison sac behind and fatally injures itself. One species of ant has grenade injures itself. One species of ant has grenade soldiers that burst an abdominal gland and spray soldiers that burst an abdominal gland and spray glue on enemies.glue on enemies.


Eusociality describes social systems with Eusociality describes social systems with three characteristics:three characteristics: Overlap in generations between parents and Overlap in generations between parents and

offspring.offspring. Cooperative brood care.Cooperative brood care. Specialist castes of non-reproductive Specialist castes of non-reproductive

individuals.individuals.

Haplodiploidy and eusocial Haplodiploidy and eusocial HymenopteraHymenoptera

One idea advanced to explain eusociality One idea advanced to explain eusociality is the unusual genetic system is the unusual genetic system (Haplodiploidy) of the Hymenoptera (ants, (Haplodiploidy) of the Hymenoptera (ants, wasps, bees, etc.).wasps, bees, etc.).

Males are haploid and females diploid.Males are haploid and females diploid.

Males develop from unfertilized eggs and Males develop from unfertilized eggs and females from fertilized eggs.females from fertilized eggs.


Daughters receive all of their fathers Daughters receive all of their fathers genes and half of their mothers genes. genes and half of their mothers genes. Thus, daughters share ¾ of their genes.Thus, daughters share ¾ of their genes.

This suggests females would be better off This suggests females would be better off if they favored the production of if they favored the production of reproductivereproductive sisters rather than their own sisters rather than their own offspring.offspring.


Queens are equally related to all offspring Queens are equally related to all offspring and so should prefer a 1:1 ratio of sons to and so should prefer a 1:1 ratio of sons to daughters daughters amongamong reproductivesreproductives..

Females workers however should prefer a Females workers however should prefer a 1:3 ratio of brothers to sisters among 1:3 ratio of brothers to sisters among reproductives.reproductives.


It has been shown in wood ants that It has been shown in wood ants that queens produce equal numbers of male queens produce equal numbers of male and female eggs, but the hatching ratio is and female eggs, but the hatching ratio is heavily female biased. heavily female biased.

Workers apparently selectively destroy Workers apparently selectively destroy male eggs.male eggs.


Further evidence that workers manipulate Further evidence that workers manipulate sex ratios in their favor comes from sex ratios in their favor comes from studies in which queen and worker studies in which queen and worker relatedness is altered.relatedness is altered.


Mueller studying a eusocial bee removed the Mueller studying a eusocial bee removed the foundress queen from some nests, but not foundress queen from some nests, but not others. When a queen is removed a daughter others. When a queen is removed a daughter takes over as queen.takes over as queen.

Workers whose queen was removed are now Workers whose queen was removed are now helping queen produce nieces and nephews (r = helping queen produce nieces and nephews (r = 0.375 for both) and are equally related to both. 0.375 for both) and are equally related to both. Colonies where queens replaced produced far Colonies where queens replaced produced far more males than colonies where original queen more males than colonies where original queen was left in place.was left in place.


In a species of In a species of FormicaFormica ant colonies ant colonies queens may be monogamous or queens may be monogamous or polyandrous. polyandrous.

Daughters of single-mating mothers Daughters of single-mating mothers heavily biased investment towards heavily biased investment towards daughters, but daughters of polyandrous daughters, but daughters of polyandrous queens did not bias reproduction towards queens did not bias reproduction towards females.females.


Haplodiploidy appears to influence worker Haplodiploidy appears to influence worker behavior, but consensus today is that it behavior, but consensus today is that it alone cannot explain evolution of eusocial alone cannot explain evolution of eusocial behavior in Hymenoptera. behavior in Hymenoptera.

There are several reasons why.There are several reasons why.

Haplodiploidy and eusocialityHaplodiploidy and eusociality

First, haplodiploid explanation assumes all First, haplodiploid explanation assumes all workers have the same father. However, workers have the same father. However, honeybee queens mate with more than 17 honeybee queens mate with more than 17 males on average. males on average.

As a result relatedness between worker As a result relatedness between worker honeybees often below 1/3.honeybees often below 1/3.


Second, in many species, more than one Second, in many species, more than one female founds a nest. In this case workers female founds a nest. In this case workers may be completely unrelated.may be completely unrelated.


Third, many eusocial species are not Third, many eusocial species are not haploid (e.g. termites) and many haploid (e.g. termites) and many haplodiploid species are not eusocial.haplodiploid species are not eusocial.


Phylogenetic analysis of Hymenoptera by Phylogenetic analysis of Hymenoptera by Hunt (1999) emphasizes that eusociality Hunt (1999) emphasizes that eusociality relatively rare even though haplodiploidy relatively rare even though haplodiploidy occurs in all groups.occurs in all groups.

Eusociality occurs in only a few families Eusociality occurs in only a few families which are scattered around the tree, which which are scattered around the tree, which suggests eusociality has evolved suggests eusociality has evolved independently multiple times.independently multiple times.


Hunt also points out that eusociality has Hunt also points out that eusociality has only evolved in groups that build complex only evolved in groups that build complex nests, and care for young for a long time.nests, and care for young for a long time.

Association between nest building, long Association between nest building, long term care and eusociality suggests main term care and eusociality suggests main driving force for eusociality is ecological driving force for eusociality is ecological not genetic.not genetic.


Nest building and need to supply offspring Nest building and need to supply offspring with a steady stream of food make it with a steady stream of food make it impossible or very difficult for a female to impossible or very difficult for a female to breed alone.breed alone.

Also, if predation rates are high, solitary Also, if predation rates are high, solitary breeding individuals may not live long breeding individuals may not live long enough to raise their young.enough to raise their young.

Facultative strategies in paper Facultative strategies in paper wasps.wasps.

Paper wasps (Paper wasps (PolistesPolistes) are not sterile ) are not sterile (unlike ant and bee workers). Females (unlike ant and bee workers). Females can remain at a nest with their mother, can remain at a nest with their mother, nest with other females or establish their nest with other females or establish their own nest.own nest.

Paper wasp queens produce daughters Paper wasp queens produce daughters early in the reproduction cycle because early in the reproduction cycle because many stay at home to help rear siblings. many stay at home to help rear siblings.


Daughters who help their mother could lay Daughters who help their mother could lay unfertilized eggs to produce male offspring, but unfertilized eggs to produce male offspring, but usually don’t (when they do the queen [the eggs usually don’t (when they do the queen [the eggs grandmother] often eats them).grandmother] often eats them).

Benefits to daughters are in indirect fitness. Benefits to daughters are in indirect fitness. They enhance the success of nests by deterring They enhance the success of nests by deterring nest predators . Nests that have helpers nest predators . Nests that have helpers removed are more likely to fail.removed are more likely to fail.


Not all species of paper wasp follow the Not all species of paper wasp follow the mother-daughter model and some nests mother-daughter model and some nests may be made up of a mixture of relatives may be made up of a mixture of relatives and non-relatives or entirely of non-and non-relatives or entirely of non-relatives.relatives.


Nonacs and Reeve (1995) found in Nonacs and Reeve (1995) found in Polistes dominulusPolistes dominulus that females of this that females of this species follow one of three strategies.species follow one of three strategies.

Initiate own nestInitiate own nest Join nest as a helperJoin nest as a helper Wait for a nest to become availableWait for a nest to become available


Individuals founding their own nest are Individuals founding their own nest are very likely to fail because adult mortality is very likely to fail because adult mortality is high.high.

Multiple foundresses, however, can keep Multiple foundresses, however, can keep the nest going.the nest going.


However, in multifoundress nests there However, in multifoundress nests there may be frequent conflict. may be frequent conflict.

The nests that did best were those where The nests that did best were those where one female was markedly bigger than the one female was markedly bigger than the others, which reduced fighting.others, which reduced fighting.


Usually, the queen dominates egg laying Usually, the queen dominates egg laying (95% of eggs being hers: she generally (95% of eggs being hers: she generally eats other females’ eggs). eats other females’ eggs).

Helpers who are relatives gain indirect Helpers who are relatives gain indirect fitness benefits, but why do non-relatives fitness benefits, but why do non-relatives help? help?


Often, they can inherit the role of queen if Often, they can inherit the role of queen if the queen dies.the queen dies.

In one study of 28 nests there were 13 In one study of 28 nests there were 13 changes of ownership in a season and 10 changes of ownership in a season and 10 were achieved by resident helpers.were achieved by resident helpers.


In some paper wasps, the queen cedes In some paper wasps, the queen cedes some reproduction to non-relatives to some reproduction to non-relatives to persuade them to stay as helpers.persuade them to stay as helpers.

For example, in For example, in PolistesPolistes fuscatusfuscatus there is there is a clear correlation between the proportion a clear correlation between the proportion of reproduction by the queen and her of reproduction by the queen and her relatedness to other foundresses.relatedness to other foundresses.


Some individuals in Some individuals in Polistes dominulusPolistes dominulus choose not to join an established nest as a choose not to join an established nest as a helper. helper.

This “sit-and-wait” strategy also can pay This “sit-and-wait” strategy also can pay off because a female often can adopt an off because a female often can adopt an orphaned nest or take one over late in the orphaned nest or take one over late in the season.season.


Overall, in paper wasps an individual’s Overall, in paper wasps an individual’s decision whether to be a helper or not is decision whether to be a helper or not is affected by her relative size, relatedness affected by her relative size, relatedness to other females, and the availability of to other females, and the availability of unoccupied nests.unoccupied nests.

Naked Mole-ratsNaked Mole-rats

Naked mole-rats are highly unusual Naked mole-rats are highly unusual mammals.mammals.

They are nearly hairless and ectothermic. They are nearly hairless and ectothermic. They are eusocial and, like termites, can They are eusocial and, like termites, can digest cellulose with the help of bacteria in digest cellulose with the help of bacteria in their gut.their gut.

Fig 51.33

Naked Mole Rats


The behavior of naked mole-rats is similar The behavior of naked mole-rats is similar to that of termites. to that of termites.

Like termites both males and females are Like termites both males and females are diploid (unlike the Hymenoptera). diploid (unlike the Hymenoptera).


Colony may include as many as 200 Colony may include as many as 200 individuals but there is only a single individuals but there is only a single reproductive female (queen) and 1-3 reproductive female (queen) and 1-3 reproductive males. reproductive males.

Remaining individuals act as workers. Remaining individuals act as workers. They dig tunnels to find food, defend the They dig tunnels to find food, defend the tunnel system from other mole-rats, and tunnel system from other mole-rats, and tend the young.tend the young.


Leading hypothesis for why naked mole-Leading hypothesis for why naked mole-rats are eusocial is inbreeding.rats are eusocial is inbreeding.

Average coefficient of relatedness is 0.81 Average coefficient of relatedness is 0.81 and about 85% of matings are between and about 85% of matings are between parents and offspring or between full parents and offspring or between full siblings. siblings.


Despite high level of relatedness conflicts Despite high level of relatedness conflicts still occur because reproductive interests still occur because reproductive interests of workers and reproductives are not of workers and reproductives are not identical.identical.


Queens maintain control through physical Queens maintain control through physical dominance. dominance.

Queen aggressively shoves workers who Queen aggressively shoves workers who do not work hard enough and shoves are do not work hard enough and shoves are mainly directly towards less closely related mainly directly towards less closely related individuals. individuals.

Workers double their work rate after being Workers double their work rate after being shoved.shoved.


In addition to inbreeding, ecological factors In addition to inbreeding, ecological factors such as severely limited alternative such as severely limited alternative breeding opportunities and group defense breeding opportunities and group defense appear to contribute to eusociality in appear to contribute to eusociality in naked mole-rats.naked mole-rats.


In related Damaraland mole rat there does In related Damaraland mole rat there does not appear to be inbreeding and not appear to be inbreeding and reproductives have a mean r of 0.02.reproductives have a mean r of 0.02.

Mean relatedness of colony members is Mean relatedness of colony members is close to 0.5 so in this species ecological close to 0.5 so in this species ecological factors may be main driver of eusociality.factors may be main driver of eusociality.

chapter 13: evolution of social behavior costs and benefits of social life. there are a large...

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