categories of social behavior

Categories of Social BehaviorActor

Recipient

+

+-

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Selfish

Altruism

Cooperative

Spiteful

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

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

Selfish behaviors are always favorable

Cooperative behaviors can be favored when advantage to actor > receiver

Evolution of Social Behavior

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

Altruism presents a paradox for natural selection:

Altruism

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

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

Inclusive fitness = Direct fitness + Indirect fitness

where 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

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

Br> CBr> C

• In other words, altruism may evolve when:In other words, altruism may evolve when:

– The cost to the actor (C) is lowThe cost to the actor (C) is low– The benefit to the recipient (B) is highThe benefit to the recipient (B) is high– The action is between close relatives (high r)The action is between close relatives (high r)

RelatednessRelatedness

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

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 riskmales 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 expectedMales less than expected

When are alarm calls given?

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

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

Kin Recognition

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

• 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

Cannibalistic Amphibians

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

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

Sooo

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

True eusociality:

1) Overlapping generations2) Cooperation among

individuals in raising young

3) Specialized castes of individuals that are nonreproductive

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

Pheidole antsReproductive female

Haplo-diploidy

Why would sterile castes give up all direct reproduction?

In Hymenoptera:

Males - 1n, develop from unfertilized eggsFemales - 2n develop from fertilized eggs 2n 1n

2n 2n1n

1/21/2

1/211

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!

But…

• Some haplo-diploid species are not eusocial

• Some eusocial species are not haplo-diploid

Conclusion:

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

Another case of EusocialityNaked 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

Queens beat workers into submission

• Queens shove non-relatives more than relatives

Work level Before shove

After shove

All shoves 0.14 0.25

Tunnel shoves

0.34 0.58Shoving by the queen increases effort by workers

Reciprocal Altruism

Can altruistic behavior to evolve even when directed to nonrelatives?

Yes!

Requires:1) repeated interactions with other individuals 2) many opportunities (and an unpredictable number) to be

altruistic 3) symmetrical costs and benefits among the interactants

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

• 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

Game Theory

• 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 Prisoner’s Dilemma

B’s action

Cooperate with A

Defect against A

A’s actio

n

Cooperate with B

R (reward for cooperation)

S (sucker’s payoff)

Defects against B

T (temptation to cheat)

P (punishment for mutual defection)

• 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?

The PayoffB’s action

Cooperate with A

Defect against A

A’s actio

n

Cooperate with B

R--> A gets 3

B gets 3

S--> A gets 0

B gets 5

Defects against B

T --> A gets 5

B gets 0

P--> A gets 1

B gets 1

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! http://serendip.brynmawr.edu/bb/pd.html

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

Bats preferentially share with nonrelatives that they are frequent roostmates with





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

Bloodmeal Sharing in Vampire Bats

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