Chapter 13: Evolution of Social Behavior

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Chapter 13 Evolution of Social Behavior

• Costs and benefits of Social Life.
• There are a large number of possible costs and
  benefits associated with social behavior.

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Potential costs and benefits of sociality

• Greater conspicuousness to predators, but also
  better defense against predators.
• Many social behavior e.g. schooling by fish seem
  to be primarily anti-predator behaviors.

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Fig 13.6
Schooling catfish
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Potential costs and benefits of sociality

• There have been numerous studies that have
  documented the anti-predator benefits of social
  behavior.
• Groups detect predators sooner and there is also
  a dilution effect by being a member of a group.

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Potential costs and benefits of sociality

• Group defense also is a benefit.
• Colonial nesting gulls deter predatory birds.
• Males in colonies of bluegill sunfish collaborate
  to drive away egg-eating predators.

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Male bluegill sunfish nest colonially as
defensive adaptation against egg-eating
predators.
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Potential costs and benefits of sociality

• If bluegill sunfish have evolved colonial nesting
  to deter predators, then we would expect
  solitarily nesting related species to suffer less
  from predators.
• As predicted, solitarily nesting pumpkinseed
  sunfish has powerful jaws with which it can deter
  predators and so does not need to group nests for
  protection.

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Potential costs and benefits of sociality

• More rapid disease transmission is likely among
  social organisms and parasites can spread more
  readily.
• There may be some advantage to sociality in that
  more grooming assistance may be available, but on
  balance disease transmission appears to be a
  clear cost of sociality.

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Potential costs and benefits of sociality

• For example, colonial cliff swallow nestlings are
  much more affected by swallow bugs than
  solitarily nesting birds.
• Nestlings parasitized by bugs were significantly
  smaller and less likely to survive than
  unparasitized individuals.

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Fig 13.5
Cliff swallow young exposed to parasites (left)
and unexposed (right).
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Potential costs and benefits of sociality

• More competition for food is a likely cost of
  sociality.
• Among lions females are forced to wait until the
  males have fed before having a chance to eat (not
  always).
• In fieldfares (a European thrush) the larger the
  colony, the lower the survival rate of nestlings
  because starvation rates increase.

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Fig 13.4
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Potential costs and benefits of sociality

• However, for other birds (and bees), which feed
  on spatially clumped, but unpredictable food
  supplies, colonial breeding appears to
  significantly improve foraging success.
• This can occur through the use of information
  centers and more effective use of local
  enhancement information because colonies clumps
  foragers in space.

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Potential costs and benefits of sociality

• Sociality also increases opportunities for
  reproductive interference, which can be positive
  or negative depending on an individuals success.

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Living in Groups
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Altruism

• Altruisitic behavior is puzzling as it is
  behavior that imposes a cost on the actor for the
  benefit of another individual.
• It should thus be selected against.
• However, kin selection and the possibility of
  reciprocal altruism can favor altruistic behavior.

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Coefficient of relatedness

• A key parameter in understanding kin selection is
  the coefficient of relatedness r.
• r is the probability that the homologous alleles
  in two individuals are identical by descent.

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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.

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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.

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Hamiltons rule

• Given r the coefficient of relatedness between
  the actor and the recipient, Hamiltons rule
  states that an allele for altruistic behavior
  will be favored and spread if
• Br/C gt 1/r
• Where Br is benefit to recipient and C is the
  cost to the actor. Unit of measurement for B and
  C is surviving offspring.

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Hamiltons rule

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

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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 individuals actions.

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Kin selection

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

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Kin selection in Beldings Ground Squirrels

• Giving alarm calls alerts other individuals but
  may attract a predators attention.
• Beldings Ground Squirrels give two different
  calls depending on whether predator is a
  predatory mammal (trill) or a hawk (whistle
  Sherman 1985).

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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.

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Kin selection in Beldings 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.

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Kin selection in Beldings Ground Squirrels

• Beldings 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.

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Kin selection in Beldings 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.

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Kin selection in Beldings Ground Squirrels

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

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Kin selection in Beldings 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.

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Kin selection in Beldings 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.

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Reciprocal Altruism

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

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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.

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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.

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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.

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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.

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Association is measure of how frequently two
individuals associate socially.
Regurgitators regurgitate to individuals
they associate with regularly.
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Helpers at the nest. 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.

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Helpers at the nest. 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.

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Helpers at the nest. 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.

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Helpers at the nest. 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 Hamiltons
  rule.

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Helpers at the nest. White-fronted Bee-eaters

• Coefficient of relatedness determines whether a
  bee-eater helps or not.
• Also, bee-eaters choose to help their closest
  relatives.

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Helpers at the nest. White-fronted Bee-eaters

• 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.

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Helpers at the nest. White-fronted Bee-eaters

• 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.

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Inclusive fitness and Pied Kingfishers

• Another example of a species with helpers at the
  nest is the Pied Kingfisher.
• Pied Kingfishers nest colonially in tunnels.
• Some one-year old males may not be able to find a
  mate and so become primary helpers assisting
  their mother to feed young and deter predators.

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Inclusive fitness and Pied Kingfishers

• Primary helpers have alternatives. They could
  choose not to help and delay breeding until next
  year (delayer) or assist at another unrelated
  nest (secondary helper).
• What are costs and benefits of being a primary
  helper?

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Inclusive fitness and Pied Kingfishers

• Primary helpers work harder than delayers and
  secondary helpers so they have a lower chance of
  surviving to breed the next year (54) than
  secondary helpers (74) or delayers (70).
• Also only 66 of primary helpers attract mates,
  but 91 of secondary helpers do (in 10 of 27
  cases with the female they helped the previous
  year). Delayers have only a 33 chance.

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Inclusive fitness and Pied Kingfishers

• To determine payoffs need to add the reproductive
  success of each approach over the two years.
• Calculate payoffs by multiplying probability of
  survival times number of offspring produced times
  probability of survival times probability finding
  a mate times relatedness to offspring.

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Inclusive fitness and Pied Kingfishers

• Primary helpers gain reproductive benefit in both
  years (0.58 indirect fitness 0.41 young
  direct fitness 0.99).
• Secondary helpers obtain a second year payoff of
  0.84 young and delayers only 0.29.
• Primary helpers have lower RS is year 2, but this
  is more than compensated for by indirect fitness
  benefit from year 1.

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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.

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Evolution of Eusociality

• Sterility and obligate helping is an extreme type
  of altruism that goes far beyond the helpers at
  the nest behavior seen in birds.
• Suicidal behavior in defense of the group is
  quite common.
• E.g. honey bee stings are barbed so that when a
  bee stings it leaves its poison sac behind and
  fatally injures itself. One species of ant has
  grenade soldiers that burst an abdominal gland
  and spray glue on enemies.

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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.

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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.

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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.

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Haplodiploidy and eusocial Hymenoptera

• Queens are equally related to all offspring and
  so should prefer a 11 ratio of sons to daughters
  among reproductives.
• Females workers however should prefer a 13 ratio
  of brothers to sisters among reproductives.

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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.

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Haplodiploidy and eusocial Hymenoptera

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

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Haplodiploidy and eusocial Hymenoptera

• Mueller studying a eusocial bee removed the
  foundress queen from some nests, but not others.
  When a queen is removed a daughter takes over as
  queen.
• Workers whose queen was removed are now helping
  queen produce nieces and nephews (r 0.375 for
  both) and are equally related to both. Colonies
  where queens replaced produced far more males
  than colonies where original queen was left in
  place.

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Haplodiploidy and eusocial Hymenoptera

• In a species of Formica ant colonies queens may
  be monogamous or polyandrous.
• Daughters of single-mating mothers heavily biased
  investment towards daughters, but daughters of
  polyandrous queens did not bias reproduction
  towards females.

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Haplodiploidy and eusocial Hymenoptera

• Haplodiploidy appears to influence worker
  behavior, but consensus today is that it alone
  cannot explain evolution of eusocial behavior in
  Hymenoptera.
• There are several reasons why.

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Haplodiploidy and eusociality

• First, haplodiploid explanation assumes all
  workers have the same father. However, honeybee
  queens mate with more than 3 to 4 males on
  average.
• As a result relatedness between worker honeybees
  often below 1/3.

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Haplodiploidy and eusociality

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

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Haplodiploidy and eusociality

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

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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.

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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.

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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.

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Facultative strategies in paper wasps.

• Paper wasps (Polistes) are not sterile (unlike
  ant and bee workers). Females can remain at a
  nest with their mother, nest with other females
  or establish their own nest.
• Paper wasp queens produce daughters early in the
  reproduction cycle because many stay at home to
  help rear siblings.

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Facultative strategies in paper wasps.

• Daughters who help their mother could lay
  unfertilized eggs to produce male offspring, but
  usually dont (when they do the queen the eggs
  grandmother often eats them).
• Benefits to daughters are in indirect fitness.
  They enhance the success of nests by deterring
  nest predators . Nests that have helpers removed
  are more likely to fail.

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Facultative strategies in paper wasps.

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

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Facultative strategies in paper wasps.

• Nonacs and Reeve (1995) found in Polistes
  dominulus that females of this species follow one
  of three strategies.
• Initiate own nest
• Join nest as a helper
• Wait for a nest to become available

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Facultative strategies in paper wasps.

• Individuals founding their own nest are very
  likely to fail because adult mortality is high.
• Multiple foundresses, however, can keep the nest
  going.

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Facultative strategies in paper wasps.

• 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.

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Facultative strategies in paper wasps.

• Usually, the queen dominates egg laying (95 of
  eggs being hers she generally eats other
  females eggs).
• Helpers who are relatives gain indirect fitness
  benefits, but why do non-relatives help?

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Facultative strategies in paper wasps.

• Often, they can inherit the role of queen if the
  queen dies.
• In one study of 28 nests there were 13 changes of
  ownership in a season and 10 were achieved by
  resident helpers.

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Facultative strategies in paper wasps.

• In some paper wasps, the queen cedes some
  reproduction to non-relatives to persuade them to
  stay as helpers.
• For example, in Polistes fuscatus there is a
  clear correlation between the proportion of
  reproduction by the queen and her relatedness to
  other foundresses.

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Facultative strategies in paper wasps.

• Some individuals in Polistes dominulus choose not
  to join an established nest as a helper.
• This sit-and-wait strategy also can pay off
  because a female often can adopt an orphaned nest
  or take one over late in the season.

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Facultative strategies in paper wasps.

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

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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.

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Naked Mole Rats
Fig 51.33
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Naked Mole-rats

• The behavior of naked mole-rats is similar to
  that of termites.
• Like termites both males and females are diploid
  (unlike the Hymenoptera).

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Naked Mole-rats

• Colony may include as many as 200 individuals but
  there is only a single reproductive female
  (queen) and 1-3 reproductive males.
• Remaining individuals act as workers. They dig
  tunnels to find food, defend the tunnel system
  from other mole-rats, and tend the young.

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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.

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Naked Mole-rats

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

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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.

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Naked Mole-rats

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

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Naked Mole-rats

• In related Damaraland mole rat there does not
  appear to be inbreeding and reproductives have a
  mean r of 0.02.
• Mean relatedness of colony members is close to
  0.5 so in this species ecological factors may be
  main driver of eusociality.