chapter 8 reciprocity and group behaviour. reciprocal altruism non-kin sacrifice now for possible...

Chapter 8

Reciprocity and Group Behaviour

Reciprocal Altruism

• Non-kin

• Sacrifice now for possible future return

• Requirements for behaviour’s evolution– Cost to donor less than benefit to recipient– Recognition of individuals– Long(ish) life span

Non-human Examples

• Thwarting predators (alarm calls)

• Cooperative breeding

• Food sharing

• Dominance (ritualized aggression)

• Apes, monkeys, wolves, African wild dogs, vampire bats...

Baboon Mating (Packer 1977)

• Males solicit help in conflict situations– Establish eye contact with “helper”– Gaze at antagonist– 20 cases of conflict between males over

females– 16 helpers; helper doesn’t directly benefit– Males who give help are more likely to get help

• “Friendless” males rarely mate

Vampire Bats• Up to 12 mothers with offspring

– Males leave early

– Associated together 60% of the time

• Successful feeding learned over time– 33% yearlings failed vs. 7% of 2+ years

• Can go 3 days without feeding

• Regurgitate blood for “friends”– Friends = bats who have given food in the past

• Friends regurgitate for friends in need– e.g., within 13 hours of starvation

Chimpanzee Politics

• Chimps in Arnhem (Netherlands) zoo• Yeroen: dominant male

– 75% of matings

• Luit: younger, subordinate male– 25% of matings; challenger

• Females gradually defect to Luit– Yeroen’s matings dropped to 0%– Luit’s matings increased to >50%

• Yeroen makes alliance with Nikkie– Nikkie’s matings: 0% --> 50%– Yeroren’s matings: 0% --> 25%

• Male-male alliances and male-female support– Can’t be dominant without female support

Risk-reduction Hypothesis

• Reciprocal relationships as insurance– Safety in uncertain conditions– Survival

• Behavioural risk– Positive correlation with reciprocal altruism

Failure of Reciprocal Altruism

• What would cause it to fail?

• Cheating– Societal knowledge– Recognition of individuals– Selection against cheaters

Issues to Consider

• Resources being shared– Food, shelter, labour, etc.

• Size of population– Chance of kinship, risk of extinction, etc.

• Environmental conditions– Variability/predictiveness, carrying capacity

• Benefit to altruist?

Competitive Altruism

• Desire to be known as an altruist– Advertising

• Reputation and social standing

Show-off Hypothesis• Take risks for increased status

• Results in subsequent “reciprocity”

• Competitive altruism

• Mating opportunities– Intersexual and intrasexual selection– More common in males than females– Conflict between mating and parenting

strategies

• Audience

Social Incentives

• Hawkes (1993)• “Safe” game provides more food than “risky”

– Hunting may not only be about food provisioning

• “Risky” resources shared by men– Provides social attention --> more helpers and mates

• Controversial hypothesis– May be delayed reciprocity

Wall of Heroes

• Postman’s Park, London

• Mr. G.F. Watts’ memorial, started 1900

War• Until recently, many

argued humans and ants were the only species to wage war.

• This is not true.• Other species wage

war, albeit differently from the modern human style.

Dyer (2004) p. vi

Origins

• Organized war throughout recorded history

• Artifact of civilization?

• Innate behaviour?

• Nurture or nature?

Thomas Hobbes

• Leviathan (1651)

• Pre-civilized man:– “No arts; no letters; no society; and which is

worst of all, continual fear, and danger of violent death; and the life of man, solitary, poor, nasty, brutish and short.”

• Political tract favouring centralized state

Jean-Jacques Rousseau

• Discourses on the Origin of Inequality (1755)

• “Noble savage”

• Freedom and equality

Bias• Political theorists

– No actual knowledge of the lifestyle of “pre-civilized” people

• Assumption: wars caused by inequality– French Revolutionaries,

Marxist, most 20th century anthropologists

• Konrad Lorenz (1966) On Aggression– “Malfunctions of

aggression”

• Quincy Wright (1965) A Study of War– Data from 633 primitive

cultures

– “the collectors, lower hunters and lower agriculturalists are the least warlike… while the highest agriculturalists… are the most warlike of all.” (p. 63)

Hunter Gatherers• Really very rare now

– Remaining groups in highly marginalized environments

• 20-80 adults• Not much in the way of social hierarchy• Often collective decision making

– Easy to leave one group and join another

• Sharp division of labour• Men: more “political” power (women out-

marry)

Fatal Encounter• “One year later, a gang from Kasekela found their third victim. This time the

target was Goliath, now well past his prime, with a bald head, very worn teeth, protruding ribs and spine… He had been a well-integrated member of the Kasekela community only five years before, and now (though he had since joined the Kahama group) he was little threat to anyone. It began as a border patrol…. The raiders rushed madly down the slope to their target. While Goliath screamed…he was held and beaten and kicked and lifted and dropped and bitten and jumped on. At first he tried to protect his head, but soon he gave up and lay stretched out and still…. They kept up the attack for 18 minutes, then turned for home…. Bleeding freely from his head, gashed on his back, Goliath tried to sit up but fell back shivering. He too was never seen again.”

• Wrangham & Peterson (1996), p. 17.

Opportunistic Conflict

• Small group attacks

• Ambush

• Only if distinct advantage to one side

Hunter-Gatherer War

• “Choreographed”

• Distinct advantage?

• Few die in each battle

• Many battles each year = lot of dead

Dani formal battle, New Guinea highlands (Dyer 2004, p. 78)

The Mae Enga• 25% of men and 5% of women died from warfare• Staged confrontations a way of measuring

opposition’s strength• When strength imbalance, daytime choreographed

battles go to nighttime or dawn raids• Aim is to exterminate rival group• 30% of villages destroyed each century

Pre-history

• Homo erectus– 750,000 years ago– Depression fractures in skulls consistent with clubs– Cut marks on bones suggesting de-fleshing;

cannibalism

• Homo sapiens neanderthalis– 100,000 - 40,000 years ago– Death by weapons, spear wounds, mass burial– 5+% of Neanderthal burials show marks of violence

Warfare and the Predator

• If you’re not equipped to kill interspecifically, you probably aren’t going to be very good at intraspecific killing

• But not all predators wage “war”– e.g., tigers, eagles, weasels rarely fight to death– Too much risk; no advantage worth 50%

chance of death

Social Structure

• Loosely related group (kinship)

• Variable size

• Leads to opportunities to eliminate individuals from rival groups

• Group kills individual with little risk

• Lions, hyenas, wolves, chimpanzees, human hunter-gatherers

Selfish Genes and Fitness• Competition for resources• Reducing membership of another group gives your

group an advantage• Kinship• Adults who do the killing are probably related; if

the group does well, so do the individuals’ genes• Selective pressure for behaviour

– Kill if opportunity arises and risk is low

Dominance Systems• Within-group regulation• Despotism or hierarchical• Vertical and horizontal• Absolute and relative dominance hierarchies• Dominants and subordinates• Reduces conflict in long run• Evolved rules, behaviours