Social Behaviors
Animal Behavior 2011
Effect on Reproductive Success of Various
Social Interactions
Different categories of
“mutualism”
• Both benefit, but benefits delayed
• Reciprocity
• Reciprocal altruism
• Problems with cheaters
• However, if each animal must perform a necessary
minimum itself that may benefit another individual as a
byproduct
• By-Product Mutualism
• Cheating is not a problem
• Altruistic Behaviors
• Do they exist?
• Possible Examples:
• helpers at the nest in birds?
• social insects – some individuals incapable of
reproducing
Why???
• Would a ground squirrel give an alarm call when it sees a
predator??
Theory
• “Kinship Theory” explained several
puzzles
• The earliest expressions of the basic
concepts were by R.A. Fisher (1930), J.
B. S. Haldane (1955) “I would risk my
life to save two of my brothers and seven
of my cousins”
• But it was W. D. Hamilton who truly
formalized the concept (1964).
1936-2000
Hamilton’s Rule
Hamilton (1964a, b) The genetic evolution of social behavior I,
II J. Theor. Biol. 7, 1-52.
• Cooperative behavior will evolve when the average
inclusive fitness benefits exceed the direct fitness costs to
the actor
• Simple version of rule: rb - c > 0
• where r is the relatedness of the altruist to the beneficiary
• b is the extra offspring altruist helps beneficiary have
• c is the cost in number of offspring to altruist
• Helpers at the Nest
• Florida Scrub Jay
• Individuals other than parents feed young and protect them from
predators
• True Altruistic Behavior?
• Helpers are related to parents!
Indirect versus Direct Selection
• Direct selection - acts on variation in individual reproductive success
• Indirect selection – (indirect benefits) acts on variation in the effects
individuals have on their relatives’ reproductive success
Can cooperation evolve in
non-kin groups?
Three areas of cooperation
research - not mutually exclusive
• Reciprocity
• By-product mutualism
• Group selection
Example: Prisoners Dilemma
PLAYER 2
Silent
Confess
R
Silent
-0.5,
-0.5
S
-3.0,
0.0
PLAYER 1
Confess
T
0.0,
-3.0
P
-1.0,
-1.0 P
• A two person game,
imperfect information,
one time or repeated,
normal form
• The dilemma: the
outcome is not
efficient.
• Efficient = there is no
other outcome that
pays all players more.
Payoffs for Player 1 are listed first
T (temptation) > R (reward) > P (punishment) > S (sucker)
Use arrows to determine ESS
• 1. Arrow along left side…. what
player 1 should do if player 2
remains silent.
PLAYER 2
Silent
Silent
-0.5, R
-0.5
Confess
-3.0,
S
0.0
PLAYER 1
Confess
0.0, T
-3.0
-1.0, P
-1.0
• 2. Arrow along right side….what
player 1 should do if player 2
confesses.
• 3. Arrow along top…what player
2 should do if player 1 remains
silent?
• 4. Arrow along bottom…what
player 2 should do if player 1
confesses?
Prisoner’s Dilemma
• The dominant
strategy for both
players is to
CONFESS.
PLAYER 2
Silent
Silent
Confess
-0.5,
-0.5
-3.0,
0.0,
-3.0
-1.0,
-1.0
0.0
PLAYER 1
Confess
*
• But both could do
better if they
cooperated and stayed
silent.
Axelrod and Hamilton
(1981. Science 211,1390)
• Ran computer tournaments, had 14 entries, played in
round robin
• Winner was Tit-for-Tat
• nice (starts by cooperating)
• retaliates (defects in response to defections)
• forgives (does not hold grudges…only remembers one play
back)
• Modeled Tit-for-Tat – Cooperation through Reciprocity
• stays dominate if chances of running into same player are high
Evidence for TFT (Reciprocity) in nature??
• Egg swapping in hermaphroditic fish
• (Nowak and Sigmund 1992 Nature 355, 250)
• Chalk bass retaliate against cheaters - wait longer to parcel out eggs
• Reciprocal grooming in Impala
•
•
(Hart and Hart 1992 Anim. Behav. 44, 1073-1083)
Blood sharing in Vampire Bats
•
(Desmodus rotundus, Wilkinson 1988. Etho. Sociobiology 9:85-100)
Blood-sharing in Vampire Bats
•
Who do vampire bats share
blood with??
•
More often with relatives as
well as individuals who are
frequent roostmates (not
random).
•
Experiment - Held unrelated
bats for nine days (hungry)
and released a different one
each night
•
More likely to be fed by
individuals they had fed
before.
Prisoner’s Dilemma as model of
non-kin cooperation
• Limitations to model
• can’t switch partners
• iterated and don’t know when it will end
• Many extensions have been suggested ---Parcelling
model (Connor 1995 Animal Behavior 49, 528)
• Claims of model too general (Clements and Stephens
1995)
C
• Conditions for Reciprocity (P Matrix)
• T > R, P > S
• Conditions for By-product Mutualism
(M Matrix)
D
C
R= 3
S=1
D
T=4
P=2
C
D
• R > T and S > P
• Each animal must perform a necessary
minimum itself that may benefit another
individual as a by-product.
• Payoffs such that it pays little to nothing to
cheat…lowers the value of temptation (T).
C
D
R= 4
S=1
T=1
P=0
Examples of By-Product Mutualism
• Territorial defense in Pied Wagtails (Davies and
Houston 1981 J. Anim. Ecol. 50, 157-180)
• Blue jays in a “skinner box” (Clements and Stephens
1995 Anim. Behav. 50, 527)
• Blue jays had two keys of different colors to peck that would
deliver food
• Payoffs (food pellets) were either as a By-product Mutualism or
Prisoner’s Dilemma (depended on key other bird in pair
pecked).
• Blue jays only cooperated with the By-product Mutualism
payoffs.
Difficulties in Telling Reciprocity from
Mutualism (R > T or T > R)
• Cooperation in African
Lions
• Group Hunting
• Hunting together better than
hunting alone, so R > P
• Success alone already high,
second hunter does not add
much, then T > R
• Two much better than one,
then R > T
C
D
C
R= 4
S=1
D
T=5
P=0
C
C
D
D
R= 4
S=1
T=3
P=0
Group Selection - History
• Wynne-Edwards (1962)
• Idea that populations self regulate. Realized would
not work under individual selection. Suggested
group selection.
• Critique by George Williams (1966).
• Biotic (for Group) versus Organic (for Individual)
Adaptations
• Argued biotic adaptations could not exist. Either it
is not an adaptation, or it evolved by individual
selection.
Group Selection versus
Individual Selection
• Individuals are more numerous than the populations
and they turn over much more rapidly than
populations.
• The rate of replacement of less fit by more fit
individuals is potentially much greater than the rate
of replacement of less fit by more fit populations.
• Therefore, if individual selection is opposed to group
selection, individual selection will prevail.
NEW View on Group Selection
• Spearheaded by D.S. Wilson (1975, 1976, 1977) and the
empirical and theoretical work of Wade (1977, 1978, 1979).
• Differs from Wynne-Edwards (1962)
• genetic models that partition variance into within- and
between-group components
• definition of group no longer limited to reproductively
isolated deme (trait group)
• cooperation evolves when within group cost is offset by
between group benefit
Altruistic allele (black) increases in frequency overall, even
though its frequency decreases within each group, as group with
higher frequency of altruistic alleles produces more offspring.
Evidence
• Many theoretical studies since have demonstrated that the
conditions for group selection to cause the evolution of
altruistic behaviors are realistic (Slatkin and Wade 1978,
Crow and Aoki 1982, Leigh 1983, Wilson 1983).
• ‘Rethinking the Theoretical Foundation of Sociobiology’
(Wilson and Wilson 2007) and ‘Evolution “For the Good of
the Group”’ (Wilson and Wilson 2008)
Conditions for Group
Selection
• Altruistic allele must not be too deleterious
to individuals, but very advantageous to
population
• Very little gene flow (5%)
• Population small
Maynard Smith 1975. The Theory
of Evolution, Cambridge U. Press
• Clearly an entire group of altruists will do
better than a group consisting of all selfish
individuals.
• Problem - explain how a group comes to consist
wholly of altruists in the first place (since altruism
will be eliminated in a mixed group).
• Answers • 1) Cultural Transmission
• 2) Group Selection
Difference in cannibalism rates (%)
Evidence: Wade (1977 Evolution 33, 749) showed
group selection could override individual in
Tribolium in the lab
Within-group mating
Random Mating
Generations
Black = full sibs, Yellow = half sibs
Larvae benefit from cannibalizing eggs
Altruism (refraining from cannibalism) evolved only in those
populations that were divided in to inbreeding subpopulations.
Evidence
Rainey and Rainey 2003. Nature 425, 72-74
•
The “wrinkly spreader (WS)” strain of Pseudomonas fluorescens evolves in
response to anoxic conditions in unmixed liquid medium, by producing a
cellulosic polymer that forms a mat on the surface.
•
The polymer is expensive to produce, which means that non- producing
“cheaters” have the highest relative fitness within the group.
•
However, as the cheaters spread, the mat deteriorates and eventually
sinks to the bottom.
•
WS is maintained in the total population by between-group selection,
despite its selective disadvantage within groups, exactly as envisioned by
multilevel selection theory.
Evidence
Packer and Heinsohn 1996. Science, 271, 1215–1216
•
Female lions share a common resource, the territory; but only a proportion of females pay the full costs
of territorial defense.
•
If too few females accept the responsibilities of leadership, the territory will be lost. If enough females
cooperate to defend the range, their territory is maintained.
•
Collective effort is vulnerable to abuse by their companions.
•
Leaders do not gain `additional benefits' from leading.
•
By failing to find a within-group advantage for territorial defense, between-group selection left as the
most likely— and fully plausible—alternative.