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Recipient’s fitness

Act

or’s

fitn

ess

increases

increases

decreases

decreases

Cooperation

Altruism Spite

Selfishness

Types of social action

Altruism

An act that increases the fitness of another individual at a cost to the actor (individuals not related).

33% of young bats fail to feed each nightOnly 7% of adults fail to feed.

Chronic threat of starvation among vampire bats since they can only survive 3 days without a meal!

Successful bats will regurgitate part of their blood meal for group members that were not successful – but not randomly!

Bats only give to those from whom they have received blood in the past.

Wilkinson, G.W. 1984. Reciprocal food sharing in the vampire bat. Nature. 308:181-184)

Reciprocal Altruism?

Prisoner’s Dilemma

Player A / Player B outcomes = D/C > C/C > D/D > C/D

Prisoner’s Dilemma

Player A best strategy is to defect but only if B cooperates, therefore cooperation becomes stable.

Prisoner’s Dilemma - modified for previous experience.

Tit for tat - remember previous outcome and adjust your behavior accordingly.

Nice - both players cooperate on the first moveRetaliatory - a player defects if an individual defected beforeForgiving - a player cooperates with a past defector that has now chosen to cooperate.

A key component for this to work isindividual recognition!

NYTimesStudy of Social Interactions Starts With a Test of TrustApril 1, 2005In a finding that could help explain why a sucker never gets an even break, scientists are reporting today that they have succeeded in visualizing feelings of trust developing in a specific region of the brain. In the study, pairs of anonymous subjects were strapped into magnetic resonance imaging scanners 1,500 miles apart. The participants played 10 consecutive rounds of a risk-taking game that involved balancing monetary profit and trust. While they played, the scanners, synchronized through the Internet, measured how the subjects' brains reacted

Camponotus hyatti; Alex Wild

Kin Selection

Kin Selection & Hamilton’s Rule

Altruism is favored when:

rB - C > 0

r = relatedness between altruist and recipient

B = fitness benefit to recipient

C = fitness cost to the altruist

Relatedness

A measure of genetic similarity that arises fromshared ancestry.

Pedigree relatedness - direct links via bloodlines- produces genetic similarity due to common ancestry- difficult for organisms to assess history

Relatedness: the proportion of alleles, on average, that two individuals share.

Relationship rIdentical (monozyotic) twins 1Parent-offspring 0.5Fraternal (dizygotic) twins 0.5Full siblings 0.5Half siblings 0.25Step siblings 0non-relatives 0

In pied kingfisher, primary helpers deliver more help (fish) and are more closely related to breeders than secondary helpers.

Relatedness within a population

When individuals share alleles at a higher frequencythan randomly selected individuals, r is positive.

When individuals share alleles at the same frequencya randomly selected individuals, r = 0.

When individuals share alleles at a lower frequencythan randomly selected individuals, r is negative.

Important to consider effective population size…

Other factors that reduce selfish behavior

Kin selection - increases indirect fitness of individuals.

Repression of competition - the success of the group isincreased with competition occurs between groups,not among group members.

Synergistic mutualism/obligate symbiosis

Delayed benefits - give up something now for laterfitness benefits (e.g. reciprocal altruism).

7. Eusociality

Reproductive division of labor (sterile workers).

Overlapping generations.

Cooperative brood care.

Examples include: Hymenoptera

(all ants, some bees & wasps; >12 origins) Isoptera (termites; 1 or 2 origins) aphids ambrosia beetles some naked mole rats

How can worker sterility spread, or be maintained, when w=0?

How can altruism evolve?

“But with the working ant we have an insect differinggreatly from its parents, yet absolutely sterile; so thatit could never have transmitted successively acquiredmodifications of structure or instinct to its progeny.It may well be asked how is it possible to reconcile thiscase with the theory of natural selection?” C.D.

The haplo-diploid Hymenoptera

A B C

B CA B A C

ABCBCA BACr between sisters = 0.75r between mother & daughters = 0.50

Workers are more closely related to sisters than daughters (when singly

mated, single queen colony).

The haplo-diploid Hymenoptera

Potentially provides an explanation for:

Kin selection within social groups

• why eusociality arose >12 times in Hymenoptera, but only 1-2 times in Isoptera.

• why workers are female in Hymenoptera, but both sexes in Isoptera.

…but the principles of kin selection remain important and instructive.

Potential problems: r among sisters often below 0.75 (multiple

mating, multiple queens) reproductive value, sex ratios B can be very large

ABCBCA BACr between sisters = 0.75r between brothers & sisters = 0.25For workers 3:1 sex ratio is optimal

r between mothers & daughters = 0.5r between mothers & sons = 0.5For queens, 1:1 sex ratio is optimal.

Conflict between workers & queensover the optimal sex ratio.

Kin selection can also cause conflictwithin colonies

Sex ratio skew

Is the ratio 1:3 or 1:1? (are workers in control, or are queens?)

1. Because males are haploid, queens control the primary sex ratio.

2. Because workers rear the young, they control the secondary sex ratio.

Sex ratio skew

Ratios are female-biased in single-queen ant colonies.

No relatedness asymmetry = less sex ratio bias:multiple-queen ant colonies (low r)solitary bees & wasps (only primary sex ratio)termites (diploid)

Sex ratio skew

Within-species variation also matches expected patterns.

Queller & Strassmann 1998

How direct altruism toward relatives?

Many transitions were promoted by mechanismsthat isolate particular groups.

Social Insect Recognition Systems

Label

Template AB EF

A E

Label

Template AB EF

A E

AB

AB

Social Insect Recognition Systems

Social Insect Recognition Systems

Problem: imprinting decouples template from kinship.

In cross-fostering experiments, callows imprint on the wrong odors.

In some cases, things can go really awry…

Cal. Acad. Sci.

When recognition systems break down:

Polyergus breviceps

Specialized mandibles: cannot hunt, eat, etc.

Polyergus breviceps; Alex Wild

When recognition systems break down:

Many forms of social parasitism within ant colonies:reproductive skewslavemakinginquilinismcuckoos

When recognition systems break down: