Download Social Behavior

Categories of Social Behavior
Actor
Selfish
behaviors are
always
Selfish
favorable
Cooperative
behaviors can be
favored when
Cooperative advantage to actor >
receiver
+
+
Spiteful
Spiteful behaviors are
theoretically possible if
the harm to receiver
>harm to actor
Recipient
Altruism
-
Altruistic behaviors are
NEVER (by definition)
favored through
individual RS
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?
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
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)
Relatedness
r
1/2
1/4
1/8
relationship
parents-offspring; full-sibs
grandparents-grandkids; half-sibs
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
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?
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
>
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Kin Recognition
In order to direct altruistic behavior towards
relatives, individuals must be able to
recognize relatives
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)
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 generations
2) 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 ants
Reproductive
female
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
1/2
2n
1/2
1n
2n
1n
1/2
1
1
2n
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 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
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
0.58
Shoving 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?
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
• 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
B’s action
A’s action
Cooperate
with A
Defect against
A
Cooperate
with B
R (reward for
cooperation)
S (sucker’s
payoff)
Defects
against B
T (temptation
to cheat)
P (punishment
for mutual
defection)
The Payoff
B’s action
A’s action
Cooperate
with B
Defects
against B
Cooperate
with A
Defect against
A
R--> A gets 3
B gets 3
T --> A gets 5
B gets 0
S--> A gets 0
B gets 5
P--> A gets 1
B gets 1
• 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?
• 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
Bloodmeal Sharing in Vampire Bats
Bats preferentially share with
nonrelatives that they are
frequent roostmates with
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Bats preferentially share with
relatives, especially those related
by more than 1/4
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.