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
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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
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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
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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 “precivilized” 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 outmarry)
Fatal Encounter
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“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
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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