Categories of Social Behavior

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Categories of Social Behavior
Actor
Cooperative behaviors can be favored when
advantage to actor gt receiver

Selfish behaviors are always favorable
Selfish
Cooperative

Recipient
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Altruism
Spiteful
Altruistic behaviors are NEVER (by definition)
favored through individual RS
Spiteful behaviors are theoretically possible if
the harm to receiver gtharm to actor
-
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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?
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Altruism
Darwin hinted at an answer Selection could
favor traits that result in a decrease of
individual fitness if they INCREASE a RELATIVES
fitness
r coefficient of relatedness r the
probability that two alleles in two individuals
are identical by descent
By helping a relative--gt more of your genes are
passed to the next generation
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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)
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Hamiltons Rule

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

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Relatedness
  r   relationship   1/2   parents-offspring
full-sibs   1/4   grandparents-grandkids
half-sibs   1/8   cousins (first cousins),
uncle/aunt/niece/nephew

• I wouldnt save a man from drowning but I would
  save two brothers or eight cousins, - JBS
  Haldane

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Haldanes Rule predicts that Altruistic Behavior
Should be Directed Toward Relatives
Beldings Ground Squirrels (work by Paul
Sherman) Provide alarm calls to warn others of
approaching predators. Cost to actor -higher
predation risk--gtBenefit 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
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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
gt
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Kin Recognition
In order to direct altruistic behavior towards
relatives, individuals must be able to recognize
relatives
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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 gt 2x
  siblings surviving at virtually 0 cost

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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

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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)

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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
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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
2n
1n
1/2
1/2
1
1/2
1
2n
2n
1n
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!
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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
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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--gt r 0.81  Workers
care for young, dig tunnels, defend colony
But workers would still be more related to their
own offspring, so why dont they reproduce?
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Naked Mole Rats
Queens beat workers into submission
 Queens shove non-relatives more than relatives
Shoving by the queen increases effort by workers
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Reciprocal Altruism

• Can altruistic behavior to evolve even when
  directed to nonrelatives?
• Yes!
• Requires
• 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?

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Game Theory

• Invented in 1940s to analyze contrasting
  strategies in games (like poker, blackjack) --gt
  later applied to economics, biology, etc.
• Goal is to determine which strategy will give the
  largest average payoff over multiple repetitions

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The Prisoners 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

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The Payoff

• For individual A TgtRgtPgtS and Rgt(ST)/2
• For one play, the highest payoff for A would be T
• But, if they play again, whats the probability B
  will play the sucker again?

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Over the long term, whats 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) --gt
  cannot be invaded by mutant strategy
• Another? pavlov win-stay, lose-switch
• Play on-line for yourself! http//serendip.brynmaw
  r.edu/bb/pd.html

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