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Categories of Social Behavior. Actor. Cooperative behaviors can be favored when advantage to actor > receiver. +. Selfish behaviors are always favorable. Selfish. Cooperative. +. -. Recipient. Spiteful. Altruism.

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Categories of Social Behavior

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Categories of social behavior

Categories of Social Behavior


Cooperative behaviors can be favored when advantage to actor > receiver


Selfishbehaviors are always favorable








Spiteful behaviors are theoretically possible if the harm to receiver >harm to actor

Altruistic behaviors are NEVER (by definition) favored through individual RS


Evolution of social behavior

Evolution of Social Behavior

Altruism presents a paradox for natural selection:

If natural selection favors traits that increase individual fitness, how can we explain behaviors that cost an individual fitness while helping another?



Darwin hinted at an answer:

Selection could favor traits that result in a decrease of individual fitness if they INCREASE a RELATIVE’S fitness

r = coefficient of relatedness

r = the probability that two alleles in two individuals are identical by descent

By helping a relative--> more of “your” genes are passed to the next generation

Inclusive fitness

Inclusive Fitness

An individual's fitness can be partitioned into direct and indirect components:

Inclusive fitness = Direct fitness + Indirect fitness


Direct W = personal reproductive success

Indirect W = RS of individuals that share alleles (weighted by the probability of sharing alleles, that is, relatedness)

Hamilton s rule

Hamilton’s Rule

  • A gene for altruistic behavior would be favored by natural selection if:

    Br> C

  • In other words, altruism may evolve when:

    • The cost to the actor (C) is low

    • The benefit to the recipient (B) is high

    • The action is between close relatives (high r)



r  relationship

  1/2  parents-offspring; full-sibs

  1/4  grandparents-grandkids; half-sibs

  1/8  cousins (first cousins), uncle/aunt/niece/nephew

  • “I wouldn’t save a man from drowning but I would save two brothers or eight cousins,” - JBS Haldane

Haldane s rule predicts that altruistic behavior should be directed toward relatives

Haldane’s Rule predicts that Altruistic Behavior Should be Directed Toward Relatives

Belding’s Ground Squirrels (work by Paul Sherman)

Provide alarm calls to warn others of approaching predators.

Cost to actor -higher predation risk-->Benefit to receiver, reduced predation risk

males disperse from the natal territory, so that females in a colony tend to be closely related whereas males are unrelated to the females

Who screams?

Females more than expected

Males less than expected

When are alarm calls given

When are alarm calls given?

Females are more likely to give alarm calls when close relatives are close by

Females are more likely to help close relatives chase away trespassers than nonrelatives


Kin recognition

Kin Recognition

In order to direct altruistic behavior towards relatives, individuals must be able to recognize relatives

Cannibalistic amphibians

Cannibalistic Amphibians

  • David Pfennig polymorphic salamanders and tadpoles that occur in either omnivorous or carnivorous morphs

  • Cannibals actively avoid eating relatives (they suck them in, but spit them out). Experiments have shown that plugging the nares prevents recognition and cannibals will happily eat anyone.

  • Avoiding eating a relative improves one's inclusive fitness.Discrimination leads to > 2x siblings surviving at virtually 0 cost

Major histocompatibility loci mhc

Major histocompatibility Loci (MHC)

genes that code for membrane proteins that display antigens. It is thought that a greater diverstiy of MHC types allows more proteins to be recognized and therefore resistance to diseases is higher

  • both humans and mice avoid mating with individuals of similar MHC type. Mice can detect MHC similarity in the urine, while humans can (at least) detect it in sweat (The T-shirt box)

  • Female humans who are pregnant or taking oral contraceptives prefer males with similar MHC genotypes, Females not taking contraceptives prefer males with unrelated MHC genotypes - ie associate with relatives during child bearing and rearing, but not during mating

  • Recent evidence suggests perfume preferences are correlated with MHC genotypes - perfume might function to broadcast MHC type

Categories of social behavior


MHC is a great example of how many processes work together:

  • sexual selection (MHC type affects mating success)

  • inbreeding (avoided to maintain high MHC diversity)

  • evolution of sex (maintains variance in MHC)

  • * coevolution with pathogens (avoiding specialization by one pathogen type)

Eusocilaity ultimate example of altruism

Eusocilaity: “ultimate” example of altruism

True eusociality:

Overlapping generations

Cooperation among individuals in raising young

Specialized castes of individuals that are nonreproductive

Found in many insects (hymenoptera, termites, thrips), one group of mammals, and snapping shrimp

Reproductive female

Pheidole ants

Haplo diploidy


Why would sterile castes give up all direct reproduction?

In Hymenoptera:

Males - 1n, develop from unfertilized eggs

Females - 2n develop from fertilized eggs











females are more closely related to their sisters than their own offspring(3/4 vs 1/2, assuming the same dad)!

an allele spreads faster by helping mom reproduce than by reproducing itself!

Categories of social behavior


• Some haplo-diploid species are not eusocial

• Some eusocial species are not haplo-diploid


Haplo-diploidy may allow eusociality to evolve more easily, but it neither necessary nor sufficient for eusociality to evolve

Another case of eusociality

Another case of Eusociality

Naked Mole Rats!

• Reproduction is by a single queen and 2-3 males

• Most matings are between parents & offspring or full-sibs--> r = 0.81

• Workers care for young, dig tunnels, defend colony

But workers would still be more related to their own offspring, so why don’t they reproduce?

Naked mole rats

Naked Mole Rats

Queens beat workers into submission

• Queens shove non-relatives more than relatives

Shoving by the queen increases effort by workers

Reciprocal altruism

Reciprocal Altruism

Can altruistic behavior to evolve even when directed to nonrelatives?



repeated interactions with other individuals

many opportunities (and an unpredictable number) to be altruistic

symmetrical costs and benefits among the interactants

When these conditions exist, what type of actions will natural selection favor?

Game theory

Game Theory

  • Invented in 1940’s to analyze contrasting strategies in games (like poker, blackjack) --> later applied to economics, biology, etc.

  • Goal is to determine which strategy will give the largest average payoff over multiple repetitions

The prisoner s dilemma

The Prisoner’s Dilemma

  • 2 prisoners charged as accomplices are locked in separate cells

  • The punishment they receive depends on whether they cooperate with each other or defect and turn against each other

The payoff

The Payoff

  • For individual A: T>R>P>S and R>(S+T)/2

  • For one play, the highest payoff for A would be T

  • But, if they play again, what’s the probability B will play the sucker again?

Over the long term what s the best strategy

Over the long term, what’s the best strategy?

  • Can be shown through economic analysis (game theory)

  • “tit-for-tat” = an individual starts by cooperating and then simply does whatever the opponent did in the previous round

  • This is an ESS (evolutionary stable strategy) --> cannot be invaded by mutant strategy

  • Another? “pavlov” = win-stay, lose-switch

  • Play on-line for yourself!

Vampire bats

Vampire Bats

Vampires - forage at night for blood meals on large mammals

33% of young bats and 7% of adults fail to feed on any given night - 3 consecutive bloodless nights and a bat dies

vampires roost in small groups, and membership in groups changes, some members are regular associates and others aren’t

vampires will share bloodmeals each other, preferentially to related bats, but also to those with whom they have some experience

Bloodmeal sharing in vampire bats

Bloodmeal Sharing in Vampire Bats

Bats preferentially share with nonrelatives that they are frequent roostmates with

Bats preferentially share with relatives, especially those related by more than 1/4

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