Types of Social Interactions Cooperation = mutualism Cooperation = mutualism – Fitness gains for both participants Altruism Altruism – Fitness gain for

Types of Social InteractionsTypes of Social Interactions

• Cooperation = mutualismCooperation = mutualism– Fitness gains for both participantsFitness gains for both participants

• AltruismAltruism– Fitness gain for recipientFitness gain for recipient– Cost for actorCost for actor

• SelfishnessSelfishness– Actor gainsActor gains– Recipient losesRecipient loses

• SpiteSpite– Fitness loss for both participantsFitness loss for both participants

• Cooperation = mutualismCooperation = mutualism– Fitness gains for both participantsFitness gains for both participants

• AltruismAltruism– Fitness gain for recipientFitness gain for recipient– Cost for actorCost for actor

• SelfishnessSelfishness– Actor gainsActor gains– Recipient losesRecipient loses

• SpiteSpite– Fitness loss for both participantsFitness loss for both participants

Prevalence of AltruismPrevalence of Altruism

• Appears to be common in natureAppears to be common in nature– Young macaws help parents raise their siblings Young macaws help parents raise their siblings

instead of reproducing themselvesinstead of reproducing themselves– Human runs into a burning house to save a childHuman runs into a burning house to save a child

• Darwin mentioned that altruism was a Darwin mentioned that altruism was a “special difficulty” for his theory“special difficulty” for his theory

• How can an allele that codes for altruism How can an allele that codes for altruism survive in the face of natural selection?survive in the face of natural selection?

• Appears to be common in natureAppears to be common in nature– Young macaws help parents raise their siblings Young macaws help parents raise their siblings

instead of reproducing themselvesinstead of reproducing themselves– Human runs into a burning house to save a childHuman runs into a burning house to save a child

• Darwin mentioned that altruism was a Darwin mentioned that altruism was a “special difficulty” for his theory“special difficulty” for his theory

• How can an allele that codes for altruism How can an allele that codes for altruism survive in the face of natural selection?survive in the face of natural selection?

Kin Selection and the Evolution of Altruism


• William Hamilton developed a genetic model William Hamilton developed a genetic model showing how an allele for altruistic behavior showing how an allele for altruistic behavior could persistcould persist

• Coefficient of relationship, Coefficient of relationship, rr– Probability that homozygous alleles in two Probability that homozygous alleles in two

individuals are identical by descentindividuals are identical by descent

• Hamilton’s RuleHamilton’s Rule– Altruistic behavior will spread ifAltruistic behavior will spread if• Br – CBr – C > 0 > 0

– BB = benefit to recipient = benefit to recipient– CC = cost to actor = cost to actor

• William Hamilton developed a genetic model William Hamilton developed a genetic model showing how an allele for altruistic behavior showing how an allele for altruistic behavior could persistcould persist

• Coefficient of relationship, Coefficient of relationship, rr– Probability that homozygous alleles in two Probability that homozygous alleles in two

individuals are identical by descentindividuals are identical by descent

• Hamilton’s RuleHamilton’s Rule– Altruistic behavior will spread ifAltruistic behavior will spread if• Br – CBr – C > 0 > 0

– BB = benefit to recipient = benefit to recipient– CC = cost to actor = cost to actor



• Altruism will spread when benefits to Altruism will spread when benefits to recipient are great, cost to actor is recipient are great, cost to actor is small, and participants are closely small, and participants are closely relatedrelated

• Inclusive FitnessInclusive Fitness– Direct fitness results from personal Direct fitness results from personal

reproductionreproduction– Indirect fitness results from reproduction Indirect fitness results from reproduction

by relativesby relatives

• Altruism will spread when benefits to Altruism will spread when benefits to recipient are great, cost to actor is recipient are great, cost to actor is small, and participants are closely small, and participants are closely relatedrelated

• Inclusive FitnessInclusive Fitness– Direct fitness results from personal Direct fitness results from personal

reproductionreproduction– Indirect fitness results from reproduction Indirect fitness results from reproduction

by relativesby relatives



• Indirect fitness accrues when relatives Indirect fitness accrues when relatives reproduce more than they would have reproduce more than they would have without aid by actorwithout aid by actor

• When natural selection favors the When natural selection favors the spread of alleles that increase indirect spread of alleles that increase indirect fitness, fitness, Kin SelectionKin Selection occurs occurs

• Kin selection explains many cases of Kin selection explains many cases of apparent altruismapparent altruism– True altruism does not exist in natureTrue altruism does not exist in nature

• Indirect fitness accrues when relatives Indirect fitness accrues when relatives reproduce more than they would have reproduce more than they would have without aid by actorwithout aid by actor

• When natural selection favors the When natural selection favors the spread of alleles that increase indirect spread of alleles that increase indirect fitness, fitness, Kin SelectionKin Selection occurs occurs

• Kin selection explains many cases of Kin selection explains many cases of apparent altruismapparent altruism– True altruism does not exist in natureTrue altruism does not exist in nature

Calculating Coefficients of Relatedness

Calculating Coefficients of Relatedness

• Haldane was quoted in a bar: “I would Haldane was quoted in a bar: “I would lay down my life for the sake of two lay down my life for the sake of two brothers or eight cousins”brothers or eight cousins”

• Perform path analysis to assess Perform path analysis to assess relatednessrelatedness– Parents related to offspring 1/2Parents related to offspring 1/2– Full siblings related 1/2Full siblings related 1/2– Half siblings related 1/4Half siblings related 1/4– Cousins related 1/8Cousins related 1/8

• Haldane was quoted in a bar: “I would Haldane was quoted in a bar: “I would lay down my life for the sake of two lay down my life for the sake of two brothers or eight cousins”brothers or eight cousins”

• Perform path analysis to assess Perform path analysis to assess relatednessrelatedness– Parents related to offspring 1/2Parents related to offspring 1/2– Full siblings related 1/2Full siblings related 1/2– Half siblings related 1/4Half siblings related 1/4– Cousins related 1/8Cousins related 1/8

Alarm Calling in Belding’s Ground Squirrels

Alarm Calling in Belding’s Ground Squirrels

• Females are more likely to give alarm Females are more likely to give alarm calls than malescalls than males

• Mothers, daughters, and sisters were Mothers, daughters, and sisters were more likely to assist each other chasing more likely to assist each other chasing trespassers off their territories than trespassers off their territories than unrelated individualsunrelated individuals

• Females are more likely to give alarm Females are more likely to give alarm calls than malescalls than males

• Mothers, daughters, and sisters were Mothers, daughters, and sisters were more likely to assist each other chasing more likely to assist each other chasing trespassers off their territories than trespassers off their territories than unrelated individualsunrelated individuals

White-Fronted Bee EatersWhite-Fronted Bee Eaters

• Young adults forgo breeding to help Young adults forgo breeding to help their parents raise their siblingstheir parents raise their siblings– Nest building, nest defense, food delivery, Nest building, nest defense, food delivery,

incubationincubation

• Helping at the nest usually found in Helping at the nest usually found in species where breeding opportunities species where breeding opportunities are limitedare limited– Best of a bad job strategyBest of a bad job strategy

• Bee eater coefficient of relatedness Bee eater coefficient of relatedness determines if they will helpdetermines if they will help

• Young adults forgo breeding to help Young adults forgo breeding to help their parents raise their siblingstheir parents raise their siblings– Nest building, nest defense, food delivery, Nest building, nest defense, food delivery,

incubationincubation

• Helping at the nest usually found in Helping at the nest usually found in species where breeding opportunities species where breeding opportunities are limitedare limited– Best of a bad job strategyBest of a bad job strategy

• Bee eater coefficient of relatedness Bee eater coefficient of relatedness determines if they will helpdetermines if they will help



• Presence of helpers increases parental Presence of helpers increases parental success by 0.47 fledglingssuccess by 0.47 fledglings

• Presence of helpers increases parental Presence of helpers increases parental success by 0.47 fledglingssuccess by 0.47 fledglings

Evolution of EusocialityEvolution of Eusociality

• True sociality describes social systems True sociality describes social systems with three characteristicswith three characteristics– Overlap in generationsOverlap in generations– Cooperative brood careCooperative brood care– Specialized castes of nonreproductive Specialized castes of nonreproductive

individualsindividuals

• Will examine two groupsWill examine two groups– HymenopteraHymenoptera– Naked mole ratsNaked mole rats

• True sociality describes social systems True sociality describes social systems with three characteristicswith three characteristics– Overlap in generationsOverlap in generations– Cooperative brood careCooperative brood care– Specialized castes of nonreproductive Specialized castes of nonreproductive

individualsindividuals

• Will examine two groupsWill examine two groups– HymenopteraHymenoptera– Naked mole ratsNaked mole rats

Haplodiploidy and Eusocial Hymenoptera


• Hymenopterans exhibit most extreme Hymenopterans exhibit most extreme form of eusocialityform of eusociality• Millions of individuals per colonyMillions of individuals per colony

– Very few reproduceVery few reproduce

• How can this persist?How can this persist?• Hamilton proposed that haplodiploidy Hamilton proposed that haplodiploidy

may be the reasonmay be the reason– Females grow from fertilized eggsFemales grow from fertilized eggs– Males grow from unfertilized eggsMales grow from unfertilized eggs

• Hymenopterans exhibit most extreme Hymenopterans exhibit most extreme form of eusocialityform of eusociality• Millions of individuals per colonyMillions of individuals per colony

– Very few reproduceVery few reproduce

• How can this persist?How can this persist?• Hamilton proposed that haplodiploidy Hamilton proposed that haplodiploidy

may be the reasonmay be the reason– Females grow from fertilized eggsFemales grow from fertilized eggs– Males grow from unfertilized eggsMales grow from unfertilized eggs



• Because of disparity in chromosome Because of disparity in chromosome number, sisters share 75% of genesnumber, sisters share 75% of genes– Parents and offspring share 50%Parents and offspring share 50%– Females are better off rearing sisters than Females are better off rearing sisters than

offspringoffspring

• Testing haplodiploidy hypothesisTesting haplodiploidy hypothesis– Workers should prefer to invest in sisters Workers should prefer to invest in sisters

over brothersover brothers• Related to sisters 3/4Related to sisters 3/4• Related to brothers 1/4 Related to brothers 1/4

• Because of disparity in chromosome Because of disparity in chromosome number, sisters share 75% of genesnumber, sisters share 75% of genes– Parents and offspring share 50%Parents and offspring share 50%– Females are better off rearing sisters than Females are better off rearing sisters than

offspringoffspring

• Testing haplodiploidy hypothesisTesting haplodiploidy hypothesis– Workers should prefer to invest in sisters Workers should prefer to invest in sisters

over brothersover brothers• Related to sisters 3/4Related to sisters 3/4• Related to brothers 1/4 Related to brothers 1/4



• Testing haplodiploidy hypothesisTesting haplodiploidy hypothesis– Workers should favor 3:1 sex ratioWorkers should favor 3:1 sex ratio– Queens are equally related to sons and Queens are equally related to sons and

daughters and should favor 1:1 sex ratiodaughters and should favor 1:1 sex ratio– Conflict of interest between queens and Conflict of interest between queens and

workersworkers– Sudstrom found that wood ant queens lays Sudstrom found that wood ant queens lays

equal numbers of male and female eggsequal numbers of male and female eggs– Workers selectively killed male eggs prior Workers selectively killed male eggs prior

to hatchingto hatching– Workers win sex ratio battle Workers win sex ratio battle

• Testing haplodiploidy hypothesisTesting haplodiploidy hypothesis– Workers should favor 3:1 sex ratioWorkers should favor 3:1 sex ratio– Queens are equally related to sons and Queens are equally related to sons and

daughters and should favor 1:1 sex ratiodaughters and should favor 1:1 sex ratio– Conflict of interest between queens and Conflict of interest between queens and

workersworkers– Sudstrom found that wood ant queens lays Sudstrom found that wood ant queens lays

equal numbers of male and female eggsequal numbers of male and female eggs– Workers selectively killed male eggs prior Workers selectively killed male eggs prior

to hatchingto hatching– Workers win sex ratio battle Workers win sex ratio battle



• Does haplodiploidy explain eusociality?Does haplodiploidy explain eusociality?– Workers should favor production of sisters if Workers should favor production of sisters if

all have the same fatherall have the same father• Honeybee queens mate over 17 times when Honeybee queens mate over 17 times when

founding a colonyfounding a colony

• Average Average rr for workers is 1/3 for workers is 1/3

• In this case, workers are not more closely In this case, workers are not more closely related to sisters than offspringrelated to sisters than offspring

• Sometimes more than one queen founds nestSometimes more than one queen founds nest

• Some workers may not be related at allSome workers may not be related at all

• Many eusocial species are not haplodiploidMany eusocial species are not haplodiploid

• Does haplodiploidy explain eusociality?Does haplodiploidy explain eusociality?– Workers should favor production of sisters if Workers should favor production of sisters if

all have the same fatherall have the same father• Honeybee queens mate over 17 times when Honeybee queens mate over 17 times when

founding a colonyfounding a colony

• Average Average rr for workers is 1/3 for workers is 1/3

• In this case, workers are not more closely In this case, workers are not more closely related to sisters than offspringrelated to sisters than offspring

• Sometimes more than one queen founds nestSometimes more than one queen founds nest

• Some workers may not be related at allSome workers may not be related at all

• Many eusocial species are not haplodiploidMany eusocial species are not haplodiploid

Using Phylogenies to Analyze Social Evolution


• Hunt reconstructed a phylogeny of Hunt reconstructed a phylogeny of hymenopteranshymenopterans– All are haplodiploidAll are haplodiploid– Few families are eusocialFew families are eusocial– Eusocial families not closely relatedEusocial families not closely related– Eusociality must have evolved multiple Eusociality must have evolved multiple

timestimes– Evolved in groups that build complex Evolved in groups that build complex

nests and have extended care for larvaenests and have extended care for larvae

• Hunt reconstructed a phylogeny of Hunt reconstructed a phylogeny of hymenopteranshymenopterans– All are haplodiploidAll are haplodiploid– Few families are eusocialFew families are eusocial– Eusocial families not closely relatedEusocial families not closely related– Eusociality must have evolved multiple Eusociality must have evolved multiple

timestimes– Evolved in groups that build complex Evolved in groups that build complex

nests and have extended care for larvaenests and have extended care for larvae



• Phylogeny suggests that the primary Phylogeny suggests that the primary agent favoring eusociality is not agent favoring eusociality is not geneticgenetic

• Best of a bad job hypothesisBest of a bad job hypothesis– Building a complex nest and caring for Building a complex nest and caring for

many larvae would be impossible for a many larvae would be impossible for a female to do by herselffemale to do by herself

– Must examine factors that affect Must examine factors that affect BB and and CC as well as as well as rr

• Phylogeny suggests that the primary Phylogeny suggests that the primary agent favoring eusociality is not agent favoring eusociality is not geneticgenetic

• Best of a bad job hypothesisBest of a bad job hypothesis– Building a complex nest and caring for Building a complex nest and caring for

many larvae would be impossible for a many larvae would be impossible for a female to do by herselffemale to do by herself

– Must examine factors that affect Must examine factors that affect BB and and CC as well as as well as rr

Facultative Strategies in Paper Wasps


• Polistes Polistes paper wasps are not paper wasps are not completely eusocialcompletely eusocial

• Workers are not sterileWorkers are not sterile

• Females may reproduce on their ownFemales may reproduce on their own

• Females pursue one of three strategiesFemales pursue one of three strategies– Initiate own nestInitiate own nest– Join a nest as a helperJoin a nest as a helper– Wait for a breeding opportunityWait for a breeding opportunity

• Polistes Polistes paper wasps are not paper wasps are not completely eusocialcompletely eusocial

• Workers are not sterileWorkers are not sterile

• Females may reproduce on their ownFemales may reproduce on their own

• Females pursue one of three strategiesFemales pursue one of three strategies– Initiate own nestInitiate own nest– Join a nest as a helperJoin a nest as a helper– Wait for a breeding opportunityWait for a breeding opportunity



• Examine costs and benefits of each strategyExamine costs and benefits of each strategy• Nests founded by single females or Nests founded by single females or

multifemale groupsmultifemale groups– Single foundress nests less successfulSingle foundress nests less successful– Multifoundress nests more likely to be rebuilt if Multifoundress nests more likely to be rebuilt if

destroyeddestroyed– Fights among foundresses determined by body Fights among foundresses determined by body

sizesize– Multifoundress nests grew fastest if large size Multifoundress nests grew fastest if large size

difference of dominant female and subordinate difference of dominant female and subordinate helpershelpers

• Examine costs and benefits of each strategyExamine costs and benefits of each strategy• Nests founded by single females or Nests founded by single females or

multifemale groupsmultifemale groups– Single foundress nests less successfulSingle foundress nests less successful– Multifoundress nests more likely to be rebuilt if Multifoundress nests more likely to be rebuilt if

destroyeddestroyed– Fights among foundresses determined by body Fights among foundresses determined by body

sizesize– Multifoundress nests grew fastest if large size Multifoundress nests grew fastest if large size

difference of dominant female and subordinate difference of dominant female and subordinate helpershelpers



• Why would females join a coalition and Why would females join a coalition and help rear offspring that are not theirs?help rear offspring that are not theirs?– Indirect fitness gains because usually Indirect fitness gains because usually

related to foundressrelated to foundress– Direct fitness gains if foundress dies and a Direct fitness gains if foundress dies and a

subordinate inherits the nestsubordinate inherits the nest– Costs and benefits of this strategy depend Costs and benefits of this strategy depend

on female’s body size and coefficient of on female’s body size and coefficient of relatednessrelatedness

• Why would females join a coalition and Why would females join a coalition and help rear offspring that are not theirs?help rear offspring that are not theirs?– Indirect fitness gains because usually Indirect fitness gains because usually

related to foundressrelated to foundress– Direct fitness gains if foundress dies and a Direct fitness gains if foundress dies and a

subordinate inherits the nestsubordinate inherits the nest– Costs and benefits of this strategy depend Costs and benefits of this strategy depend

on female’s body size and coefficient of on female’s body size and coefficient of relatednessrelatedness



• If females do not help found nests and If females do not help found nests and wait, they may be able to adopt an wait, they may be able to adopt an already-constructed nestalready-constructed nest– Sit-and-wait tacticSit-and-wait tactic– They leave their nest in spring and enter a They leave their nest in spring and enter a

dormant state until the following season to dormant state until the following season to try to take over a new nesttry to take over a new nest

• If females do not help found nests and If females do not help found nests and wait, they may be able to adopt an wait, they may be able to adopt an already-constructed nestalready-constructed nest– Sit-and-wait tacticSit-and-wait tactic– They leave their nest in spring and enter a They leave their nest in spring and enter a

dormant state until the following season to dormant state until the following season to try to take over a new nesttry to take over a new nest



• Sociality is facultative in Sociality is facultative in PolistesPolistes• Adaptive response to environmental Adaptive response to environmental

conditionsconditions• Important conditions are female body Important conditions are female body

size relative to competitors, coefficient size relative to competitors, coefficient of relatedness, and availability of nest of relatedness, and availability of nest sitessites• Genetic, social, and ecological factors Genetic, social, and ecological factors

importantimportant

• Sociality is facultative in Sociality is facultative in PolistesPolistes• Adaptive response to environmental Adaptive response to environmental

conditionsconditions• Important conditions are female body Important conditions are female body

size relative to competitors, coefficient size relative to competitors, coefficient of relatedness, and availability of nest of relatedness, and availability of nest sitessites• Genetic, social, and ecological factors Genetic, social, and ecological factors

importantimportant

Naked Mole RatsNaked Mole Rats

• Live underground in huge nests in AfricaLive underground in huge nests in Africa• Colonies of 70-80 membersColonies of 70-80 members• Hairless, ectothermic, digest celluloseHairless, ectothermic, digest cellulose• All species are eusocialAll species are eusocial

– Single queenSingle queen– 2-3 reproductive males2-3 reproductive males– Workers are males and femalesWorkers are males and females• Castes change with ageCastes change with age

– First they tend youngFirst they tend young– Later they excavate tunnelsLater they excavate tunnels– The oldest defend the nestThe oldest defend the nest

• Live underground in huge nests in AfricaLive underground in huge nests in Africa• Colonies of 70-80 membersColonies of 70-80 members• Hairless, ectothermic, digest celluloseHairless, ectothermic, digest cellulose• All species are eusocialAll species are eusocial

– Single queenSingle queen– 2-3 reproductive males2-3 reproductive males– Workers are males and femalesWorkers are males and females• Castes change with ageCastes change with age

– First they tend youngFirst they tend young– Later they excavate tunnelsLater they excavate tunnels– The oldest defend the nestThe oldest defend the nest


• Not haplodiploidNot haplodiploid

• Why are they eusocial?Why are they eusocial?– Highly inbredHighly inbred• Average Average rr for siblings is 0.81 for siblings is 0.81

• Highest coefficient of relatedness ever Highest coefficient of relatedness ever recorded in mammalsrecorded in mammals

– Still conflict among group membersStill conflict among group members• Workers more closely related to offspring Workers more closely related to offspring

than half-siblingsthan half-siblings

• Not haplodiploidNot haplodiploid

• Why are they eusocial?Why are they eusocial?– Highly inbredHighly inbred• Average Average rr for siblings is 0.81 for siblings is 0.81

• Highest coefficient of relatedness ever Highest coefficient of relatedness ever recorded in mammalsrecorded in mammals

– Still conflict among group membersStill conflict among group members• Workers more closely related to offspring Workers more closely related to offspring

than half-siblingsthan half-siblings


• Conflict among group membersConflict among group members– Queens maintain control by physical dominanceQueens maintain control by physical dominance– If the workers slow their work pace the queen If the workers slow their work pace the queen

shoves them with her headshoves them with her head– Afterwards the workers increase their work paceAfterwards the workers increase their work pace– Shoves are directed more often towards more Shoves are directed more often towards more

distant relativesdistant relatives– Queen maintains eusociality by intimidationQueen maintains eusociality by intimidation

• Conflict among group membersConflict among group members– Queens maintain control by physical dominanceQueens maintain control by physical dominance– If the workers slow their work pace the queen If the workers slow their work pace the queen

shoves them with her headshoves them with her head– Afterwards the workers increase their work paceAfterwards the workers increase their work pace– Shoves are directed more often towards more Shoves are directed more often towards more

distant relativesdistant relatives– Queen maintains eusociality by intimidationQueen maintains eusociality by intimidation

Parent-Offspring ConflictParent-Offspring Conflict

• Parental care is a special case of kin Parental care is a special case of kin selectionselection

• Even parents and offspring can have Even parents and offspring can have conflicts in costs and benefitsconflicts in costs and benefits

• Weaning ConflictWeaning Conflict– Mothers begin to ignore or push young Mothers begin to ignore or push young

away near end of weaning periodaway near end of weaning period– Offspring will scream or attack motherOffspring will scream or attack mother– Fitness interests are not symmetricalFitness interests are not symmetrical

• Parental care is a special case of kin Parental care is a special case of kin selectionselection

• Even parents and offspring can have Even parents and offspring can have conflicts in costs and benefitsconflicts in costs and benefits

• Weaning ConflictWeaning Conflict– Mothers begin to ignore or push young Mothers begin to ignore or push young

away near end of weaning periodaway near end of weaning period– Offspring will scream or attack motherOffspring will scream or attack mother– Fitness interests are not symmetricalFitness interests are not symmetrical


• Weaning ConflictWeaning Conflict– Offspring are related to themselves Offspring are related to themselves rr = 1 = 1– Parents are related to offspring Parents are related to offspring rr = 0.5 = 0.5– Parents are equally related to all offspring Parents are equally related to all offspring

and should optimize their investment in and should optimize their investment in eacheach

– Offspring demand unequal amount of Offspring demand unequal amount of parental investmentparental investment

• Weaning ConflictWeaning Conflict– Offspring are related to themselves Offspring are related to themselves rr = 1 = 1– Parents are related to offspring Parents are related to offspring rr = 0.5 = 0.5– Parents are equally related to all offspring Parents are equally related to all offspring

and should optimize their investment in and should optimize their investment in eacheach

– Offspring demand unequal amount of Offspring demand unequal amount of parental investmentparental investment


• Weaning ConflictWeaning Conflict– At start of nursing benefit of offspring At start of nursing benefit of offspring

relative to cost of parentrelative to cost of parent– Ratio declines with timeRatio declines with time– Young demand more milk which increases Young demand more milk which increases

parental costparental cost– Young can start finding own food which Young can start finding own food which

decreases benefitdecreases benefit– Mothers should stop producing milk when Mothers should stop producing milk when

benefit to cost ratio reaches 1benefit to cost ratio reaches 1

• Weaning ConflictWeaning Conflict– At start of nursing benefit of offspring At start of nursing benefit of offspring

relative to cost of parentrelative to cost of parent– Ratio declines with timeRatio declines with time– Young demand more milk which increases Young demand more milk which increases

parental costparental cost– Young can start finding own food which Young can start finding own food which

decreases benefitdecreases benefit– Mothers should stop producing milk when Mothers should stop producing milk when

benefit to cost ratio reaches 1benefit to cost ratio reaches 1


• Weaning ConflictWeaning Conflict– By continuing to nurse, offspring devalue By continuing to nurse, offspring devalue

mother’s cost of caremother’s cost of care– Offspring should continue to try to nurse until Offspring should continue to try to nurse until

benefit-cost ratio is 1/2benefit-cost ratio is 1/2– Period between these stages is weaning Period between these stages is weaning

conflictconflict– Avoidance and aggressive behavior Avoidance and aggressive behavior

throughout this periodthroughout this period– If half-siblings are produced then ratio should If half-siblings are produced then ratio should

be extended to 1/4be extended to 1/4

• Weaning ConflictWeaning Conflict– By continuing to nurse, offspring devalue By continuing to nurse, offspring devalue

mother’s cost of caremother’s cost of care– Offspring should continue to try to nurse until Offspring should continue to try to nurse until

benefit-cost ratio is 1/2benefit-cost ratio is 1/2– Period between these stages is weaning Period between these stages is weaning

conflictconflict– Avoidance and aggressive behavior Avoidance and aggressive behavior

throughout this periodthroughout this period– If half-siblings are produced then ratio should If half-siblings are produced then ratio should

be extended to 1/4be extended to 1/4


• White-fronted Bee EatersWhite-fronted Bee Eaters– Sons may set up territory or may help at Sons may set up territory or may help at

their parental nesttheir parental nest– Fathers coerce sons into helping by Fathers coerce sons into helping by

harassing sons as they attempt to set up harassing sons as they attempt to set up territoriesterritories• Fathers prevent courtship feedingFathers prevent courtship feeding

• Harassment is preferentially directed at Harassment is preferentially directed at sons to prevent them from breeding and sons to prevent them from breeding and coerce them to help at the nestcoerce them to help at the nest

• White-fronted Bee EatersWhite-fronted Bee Eaters– Sons may set up territory or may help at Sons may set up territory or may help at

their parental nesttheir parental nest– Fathers coerce sons into helping by Fathers coerce sons into helping by

harassing sons as they attempt to set up harassing sons as they attempt to set up territoriesterritories• Fathers prevent courtship feedingFathers prevent courtship feeding

• Harassment is preferentially directed at Harassment is preferentially directed at sons to prevent them from breeding and sons to prevent them from breeding and coerce them to help at the nestcoerce them to help at the nest


• White-fronted Bee EatersWhite-fronted Bee Eaters– 16 of 47 observed harassment events resulted 16 of 47 observed harassment events resulted

in successful recruitment to helpin successful recruitment to help– Why don’t sons resist more effectively?Why don’t sons resist more effectively?• Sons are equally related to siblings and own Sons are equally related to siblings and own

offspringoffspring• Parents are more closely related to each Parents are more closely related to each

offspring than to grandchildrenoffspring than to grandchildren• Helpers add 0.47 offspring to parental successHelpers add 0.47 offspring to parental success• Nearly same as own offspringNearly same as own offspring

– Worth it to save harassment from fatherWorth it to save harassment from father

• White-fronted Bee EatersWhite-fronted Bee Eaters– 16 of 47 observed harassment events resulted 16 of 47 observed harassment events resulted

in successful recruitment to helpin successful recruitment to help– Why don’t sons resist more effectively?Why don’t sons resist more effectively?• Sons are equally related to siblings and own Sons are equally related to siblings and own

offspringoffspring• Parents are more closely related to each Parents are more closely related to each

offspring than to grandchildrenoffspring than to grandchildren• Helpers add 0.47 offspring to parental successHelpers add 0.47 offspring to parental success• Nearly same as own offspringNearly same as own offspring

– Worth it to save harassment from fatherWorth it to save harassment from father


• SiblicideSiblicide– In birds and mammals it is common to kill In birds and mammals it is common to kill

siblingssiblings– Seems maladaptive since Seems maladaptive since r r = 1/2= 1/2– Lougheed and Anderson studied boobies Lougheed and Anderson studied boobies

in Galin Galáápagos Islandspagos Islands– Lay clutch of two eggs separated by 2-10 Lay clutch of two eggs separated by 2-10

daysdays– First chick often pushes younger from First chick often pushes younger from

nestnest

• SiblicideSiblicide– In birds and mammals it is common to kill In birds and mammals it is common to kill

siblingssiblings– Seems maladaptive since Seems maladaptive since r r = 1/2= 1/2– Lougheed and Anderson studied boobies Lougheed and Anderson studied boobies

in Galin Galáápagos Islandspagos Islands– Lay clutch of two eggs separated by 2-10 Lay clutch of two eggs separated by 2-10

daysdays– First chick often pushes younger from First chick often pushes younger from

nestnest


• SiblicideSiblicide– Masked boobies push second egg from Masked boobies push second egg from

nest immediatelynest immediately– Blue-footed booby older chicks may Blue-footed booby older chicks may

reduce food intake during short food reduce food intake during short food shortages to provide extra for siblingshortages to provide extra for sibling• During long food shortages they kill their During long food shortages they kill their

siblingssiblings– What role do parents play?What role do parents play?• Parents should intervene to prevent death Parents should intervene to prevent death

of any of their chicksof any of their chicks

• SiblicideSiblicide– Masked boobies push second egg from Masked boobies push second egg from

nest immediatelynest immediately– Blue-footed booby older chicks may Blue-footed booby older chicks may

reduce food intake during short food reduce food intake during short food shortages to provide extra for siblingshortages to provide extra for sibling• During long food shortages they kill their During long food shortages they kill their

siblingssiblings– What role do parents play?What role do parents play?• Parents should intervene to prevent death Parents should intervene to prevent death

of any of their chicksof any of their chicks


• SiblicideSiblicide– Reciprocal transplant experimentReciprocal transplant experiment– Chicks more likely to die with masked Chicks more likely to die with masked

booby nestmatebooby nestmate– Chicks more likely to die with masked Chicks more likely to die with masked

booby parents (surrogate or real)booby parents (surrogate or real)– Blue-footed booby parents intervene but Blue-footed booby parents intervene but

masked do notmasked do not– Why there is a difference among species is Why there is a difference among species is

not knownnot known

• SiblicideSiblicide– Reciprocal transplant experimentReciprocal transplant experiment– Chicks more likely to die with masked Chicks more likely to die with masked

booby nestmatebooby nestmate– Chicks more likely to die with masked Chicks more likely to die with masked

booby parents (surrogate or real)booby parents (surrogate or real)– Blue-footed booby parents intervene but Blue-footed booby parents intervene but

masked do notmasked do not– Why there is a difference among species is Why there is a difference among species is

not knownnot known

Reciprocal AltruismReciprocal Altruism

• What about cooperation among What about cooperation among unrelated individuals?unrelated individuals?• Trivers proposed individuals will act Trivers proposed individuals will act

altruistically if favor is later returnedaltruistically if favor is later returned• Two conditions for reciprocal alturism Two conditions for reciprocal alturism

to evolve:to evolve:– Cost to actor must be smaller than or Cost to actor must be smaller than or

equal to benefit to recipientequal to benefit to recipient– Individuals that fail to reciprocate must be Individuals that fail to reciprocate must be

punishedpunished

• What about cooperation among What about cooperation among unrelated individuals?unrelated individuals?• Trivers proposed individuals will act Trivers proposed individuals will act

altruistically if favor is later returnedaltruistically if favor is later returned• Two conditions for reciprocal alturism Two conditions for reciprocal alturism

to evolve:to evolve:– Cost to actor must be smaller than or Cost to actor must be smaller than or

equal to benefit to recipientequal to benefit to recipient– Individuals that fail to reciprocate must be Individuals that fail to reciprocate must be

punishedpunished


• Most likely to evolve when:Most likely to evolve when:– Each individual repeatedly interacts with Each individual repeatedly interacts with

same set of individualssame set of individuals– Many opportunities for altruism in an Many opportunities for altruism in an

individual’s lifetimeindividual’s lifetime– Individuals have good memoriesIndividuals have good memories– Potential altruists interact in symmetrical Potential altruists interact in symmetrical

situationssituations

• Roughly equal benefits and costsRoughly equal benefits and costs

• Most likely to evolve when:Most likely to evolve when:– Each individual repeatedly interacts with Each individual repeatedly interacts with

same set of individualssame set of individuals– Many opportunities for altruism in an Many opportunities for altruism in an

individual’s lifetimeindividual’s lifetime– Individuals have good memoriesIndividuals have good memories– Potential altruists interact in symmetrical Potential altruists interact in symmetrical

situationssituations

• Roughly equal benefits and costsRoughly equal benefits and costs


• Will evolve in long-lived, intelligent, Will evolve in long-lived, intelligent, social species with small group size, social species with small group size, low dispersal rates, and mutual low dispersal rates, and mutual dependence in activitiesdependence in activities

• Less likely to evolve in species with Less likely to evolve in species with dominance hierarchiesdominance hierarchies

• Difficult to observe and quantify in Difficult to observe and quantify in naturenature

• Will evolve in long-lived, intelligent, Will evolve in long-lived, intelligent, social species with small group size, social species with small group size, low dispersal rates, and mutual low dispersal rates, and mutual dependence in activitiesdependence in activities

• Less likely to evolve in species with Less likely to evolve in species with dominance hierarchiesdominance hierarchies

• Difficult to observe and quantify in Difficult to observe and quantify in naturenature


• Blood-Sharing in Vampire BatsBlood-Sharing in Vampire Bats– Social group of 8-12 females and their dependent Social group of 8-12 females and their dependent

offspringoffspring– Roost together and associate with each other Roost together and associate with each other

dailydaily– Average Average rr between individuals in study group in between individuals in study group in

Costa Rica was 0.11Costa Rica was 0.11– Vampire bats share blood mealsVampire bats share blood meals– Hunting is difficult and individuals are only Hunting is difficult and individuals are only

successful 67-93% of timesuccessful 67-93% of time• Prey are waryPrey are wary

• Blood-Sharing in Vampire BatsBlood-Sharing in Vampire Bats– Social group of 8-12 females and their dependent Social group of 8-12 females and their dependent

offspringoffspring– Roost together and associate with each other Roost together and associate with each other

dailydaily– Average Average rr between individuals in study group in between individuals in study group in

Costa Rica was 0.11Costa Rica was 0.11– Vampire bats share blood mealsVampire bats share blood meals– Hunting is difficult and individuals are only Hunting is difficult and individuals are only

successful 67-93% of timesuccessful 67-93% of time• Prey are waryPrey are wary


• Blood-Sharing in Vampire BatsBlood-Sharing in Vampire Bats– Without eating for three nights, a bat will Without eating for three nights, a bat will

starve to deathstarve to death– Both degree of relatedness and degree of Both degree of relatedness and degree of

association were significantly related to association were significantly related to probability of regurgitating bloodprobability of regurgitating blood

– Blood-sharing is not random but based on Blood-sharing is not random but based on relatedness and hope of future reciprocityrelatedness and hope of future reciprocity

• Blood-Sharing in Vampire BatsBlood-Sharing in Vampire Bats– Without eating for three nights, a bat will Without eating for three nights, a bat will

starve to deathstarve to death– Both degree of relatedness and degree of Both degree of relatedness and degree of

association were significantly related to association were significantly related to probability of regurgitating bloodprobability of regurgitating blood

– Blood-sharing is not random but based on Blood-sharing is not random but based on relatedness and hope of future reciprocityrelatedness and hope of future reciprocity


• Territory Defense in LionsTerritory Defense in Lions– Females cooperate in defending young Females cooperate in defending young

against infanticidal males, hunting prey, against infanticidal males, hunting prey, and defending pride’s territoryand defending pride’s territory

– When females hear the roar of a female of When females hear the roar of a female of another territory, they head to the area to another territory, they head to the area to attackattack

– Some individuals always tend to lead the Some individuals always tend to lead the attack while others always seem to lag attack while others always seem to lag behindbehind

• Territory Defense in LionsTerritory Defense in Lions– Females cooperate in defending young Females cooperate in defending young

against infanticidal males, hunting prey, against infanticidal males, hunting prey, and defending pride’s territoryand defending pride’s territory

– When females hear the roar of a female of When females hear the roar of a female of another territory, they head to the area to another territory, they head to the area to attackattack

– Some individuals always tend to lead the Some individuals always tend to lead the attack while others always seem to lag attack while others always seem to lag behindbehind


• Territory Defense in LionsTerritory Defense in Lions– Why are laggards tolerated?Why are laggards tolerated?– It is not known but perhaps the laggards It is not known but perhaps the laggards

make up later by being exceptional make up later by being exceptional hunters or good milk producershunters or good milk producers

• Social interactions are very complex Social interactions are very complex and much needs to be learned about and much needs to be learned about them to fully understand their natural them to fully understand their natural selectionselection

• Territory Defense in LionsTerritory Defense in Lions– Why are laggards tolerated?Why are laggards tolerated?– It is not known but perhaps the laggards It is not known but perhaps the laggards

make up later by being exceptional make up later by being exceptional hunters or good milk producershunters or good milk producers

• Social interactions are very complex Social interactions are very complex and much needs to be learned about and much needs to be learned about them to fully understand their natural them to fully understand their natural selectionselection

Documents

Types of Social Interactions Cooperation = mutualism Cooperation = mutualism – Fitness gains for both participants Altruism Altruism – Fitness gain for