This document discusses several studies on primate aggression. It finds that primate aggression is mainly driven by competition for resources like food and territories, as well as competition for mates. Studies show chimpanzees and other primates exhibit aggression between groups to defend resources and mates. High testosterone levels and stress are also found to correlate with increased aggression in some primates. While human aggression may have similar evolutionary drivers, humans have developed ways to negotiate cooperation and reduce aggression through trade, alliances and language.

1. Nathaniel AllenHevenstone
ProfessorDetwiler
Primate Behavior
Spring2013
Primate Aggression
1
Throughout the animal kingdom, aggression seems to be a dominating factor of the
competition for survival. Numerous hypotheses have been offered to explain why this is, but
nearly all of them focus on food and mating. Aggression seems to be mainly about gaining
access to territories with abundant food and mates. However, is that the only part of it, or is there
more to aggression and what determines whether or not an aggressive encounter will occur? I
want to look at primate aggression and the causes, and hope to find analogies to certain forms of
human aggression.
Pan troglodytes, common chimpanzees, are our most closely related evolutionary
cousins. We share around 98.4% of our DNA with chimpanzees, along with a number of social
behaviors, including very similar forms of aggression. In 1991, Joseph H. Manson and Richard
W. Wrangham published a paper comparing intergroup aggression amongst Chimpanzees and
humans. Their hypothesis was that the inalienability of females and of resources important to
female reproduction is extremely important to the development of the sometimes fatal
competition between males in both chimpanzees and humans. They note that with Chimpanzees,
as with humans, males tend to be the more aggressive sex, while females usually avoid
aggressive encounters. They also note that this is rare in primate aggression; in rhesus macaques
at Cayo Santiago, for example, females tend to be the more aggressive sex, along with younger
males, while older males try to stay out of harm’s way (Manson & Wrangham, 1991).
In 1999, Ned H. Kalin published a paper attempting to determine different models of
aggression in primates and how those models might be used to understand human aggression.
Kalin and his colleagues focused mainly on Rhesus monkeys because they “show behavioral and

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emotional responses that resemble those in humans”. Kalin looked at three categories of
aggression: defensive, offensive, and self-injurious. Kalin didn’t seem to focus on the third one
at all, but did spend a long time dealing with the defensive and offensive models. Both models
are relatively self-explanatory. The defensive model looks at responses to perceived threats,
while the offensive model looks at monkeys actually making threats. Kalin and his colleagues
studied both environmental factors and biological factors. The latter entailed reading a monkey’s
brain waves to see what happens when the monkey creates or responds to a threat. They found
that, in Rhesus macaques, defensive (fear-based) responses correlated with “increased electrical
activity in the right frontal region of the brain and that rhesus monkeys with extreme right frontal
activity have higher plasma cortisol concentrations”, while offensive aggression correlated with
“decreased serotonin and increased testosterone”. They concluded that further study is needed
(Kalin, 1999).
In December 2007, the journal Behaviour published an entire issue (Volume 144, Issue
12) devoted to Intergroup Aggression in Wild Primates. With one introduction and eight articles,
this issue focused on non-human primates in general, but also had one article devoted to white-
faced capuchin monkeys (“Mating and feeding competition in white-faced capuchins (Cebus
capucinus): the importance of short and long-term strategies”), one article devoted to gorillas
(“Intergroup encounters in mountain gorillas of Bwindi Impenetrable National Park, Uganda”),
one article devoted to blue monkeys (“Variable participation in the defense of communal feeding
0territories by blue monkeys in the Kakamega Forest, Kenya”), one article devoted to the
Thomas langur (“Familiarity and threat of opponents determine variation in Thomas langur
(Presbytis thomasi) male behaviour during between-group encounters”), and one article devoted
to chimpanzees (“Chimpanzees (Pan troglodytes) modify grouping and vocal behaviour in

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response to location-specific risk”). Aside from a small mention in the introduction, human
aggression is not considered in this issue. However, it does provide many fascinating insights
into the nature of primate aggression ("Intergroup aggression in," 2007).
The first article examines mating and feeding competition in white-faced capuchin
monkeys. In the study, Margaret C. Crofoot suggests that intergroup aggression was not
motivated by short-term reproductive strategies, but by long-term reproductive strategies. She
also suggests that a good percentage of the intergroup aggression observed in these capuchins
could be males of one group attempting to overthrow the alpha male of the targeted group, while
a smaller percentage were mainly just “scope-outs” (the males of one group assessing the gender
make-up and size of another group). However, neither explanation accounted for all observed
periods of intergroup aggression. She concludes that there may be more explanations than mating
and even access to resources for intergroup competition (Crofoot, 2007).
The next article, written by Martha M. Robbins and Sarah C. Sawyer, looks at the
intergroup encounters of gorillas at Bwindi Impenetrable National Park in Uganda. Robbins and
Sawyer looked at “the influence of frugivory and social factors on behaviour during intergroup
encounters”. They both suggest that while encounters themselves did occur at limited fruit
patches, frugivory had no impact on the length of the encounter, or on the type of behaviour
exhibited, suggesting that while neighbouring groups may be attracted to limited fruit patches,
there was no indication of between group feeding competition.” The number of migrant females
and the presence of silverbacks also had no impact on this. How a group behaved depended on
the make-up of the second group in the encounter. If one group of gorillas encountered a solitary
male, they were more likely to herd females away from the male, and practice avoidance at all
costs, suggesting that solitary males were more likely to take more risks in a takeover than

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groups including females. Their conclusion is that “between group competition is linked more to
mate defence and acquisition than resource defence” (Robbins & Sawyer, 2007).
The third article in the issue, written by Tara R. Harris, is a much more generalized study
of primate aggression. Harris looks at three proposed functions of intergroup aggression: mate
defense, infant defense, and resource defense. Her goal here is to raise some issues with the
predictions and methods used to study these and make suggestions that she feels could improve
future studies (Harris, 2007). The fourth article, written by Marina Cords, looks at the defense of
communal feeding territories by blue monkeys. She is questioning why group members did not
participate equally in the defense. Using data spanning five years and five wild groups, Cords
found that adult females participated the most, while males participated less and less as they
matured and female participation increased with maturation. She also found that females with
juveniles participated less, while higher-ranking females participated more. She was surprised at
these last two, though especially the latter, having expected the opposite (Cords, 2007).
The fifth article in the issue, written by Dawn M. Kitchen and Jacinta C. Beehner, looks
at individual participation amongst non-human primate aggression. They suggest that benefits
such as group defense are not monopolizable, and as such, there is an economic problem: “who
should contribute to public goods and who should freeload?” This is mainly a review paper, in
which Kitchen and Beehner looked at over 20 years of data and focused on individual
participation in intergroup encounters. They found that between-sex variation is explainable
mainly by food-defense and mate-defense, while within-sex variation is explainable mainly by
rank and reproductive success. They also suggest that some individuals within a primate group
may use intergroup encounters to see if there are any possibilities for transfer and mating

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success. Overall, they find that the question of who participates in intergroup encounters has a lot
of answers and depends largely on individual factors (Kitchen & Beehner, 2007).
The sixth article in the issue, written by Serge A.Wich and Elisabeth H.M. Sterck, looks
at whether or not familiarity and the kind of threat opponents present determine the intensity of a
between-group encounter in Thomas langurs. Perhaps not surprisingly, they found that less
familiar groups posing greater threats caused more intense encounters. Thomas langurs use both
familiarity and the level of a threat to determine the intensity of their responses, which were
measured in the number and type of calls given towards a possible threat (Wich & Sterck, 2007).
The seventh article in the issue, written by Richard Wrangham, Meg Crofoot, Rochelle
Lundy, and Ian Gilby, tests the risk hypothesis. Specifically, they look at ranging data from three
species of group-living primates (redtail monkeys, white-faced capuchins, and chimpanzees) in
order to see if “overlap zones are smaller or used less in species that are subject to a higher risk
of lethal aggression in intergroup encounters.” They assessed how high the risk of encounters
was for each species: redtail monkeys have a low risk of encounters, white-faced capuchins have
an intermediate risk, and chimpanzees have a very high risk. Rather surprisingly, these risks did
not seem to be a factor in the low use or small size of overlap zones, since the overlaps zones
were small and little-used for all three species (Wrangham, Crofoot, Lundy & Gilby, 2007).
The eighth and final article in the issue, written by Michael L. Wilson, Marc D. Hauser,
and Richard W. Wrangham, looks at whether or not chimpanzees they modify their vocal
responses and group behavior based on the different risks associated with different locations.
They looked at the Kanyawara chimpanzees, comparing behavior in the territory core with
behavior in two different contexts: the periphery and the cropland; two potentially dangerous
areas of the range. They found that when male chimpanzees visited the periphery, their groups

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had nearly twice as many males as they did in the core, and when they visited the croplands, they
were much more silent than in the core (as an aside, it’d be interesting to see what they did do
individuals who broke that silence in the croplands, alerting any potential dangers to their
presence, and whether this reaction would be different or the same if the individual causing the
disturbance was part of the group or an outsider, and a chimpanzee or not a chimpanzee). They
could be more vocal in the periphery, however, perhaps in order to advertise (and maybe
exaggerate?) coalition strength and their ownership of the territory (Wilson, Hauser &
Wrangham, 2007).
Other factors besides defense of mates, juveniles, and resources might contribute to
primate aggression, including high levels of testosterone and stress. In a test of the former,
Martin N. Muller and Richard W. Wrangham submitted a paper to Animal Behaviour in 2004
called Dominance, aggression and testosterone in wild chimpanzees: a test of the ‘challenge
hypothesis’. The challenge hypothesis basically says that “variation in male testosterone levels is
more closely associated with aggression in reproductive contexts than it is with changes in
reproductive physiology.” They tested this by conducting behavioral observations and
noninvasive hormone sampling of eleven male Kanyawara chimpanzees. They found that male
testosterone level increases when parous females show maximally tumescent sexual swellings,
and aggressive behavior amongst males also increases during this time. Although male
testosterone levels do not increase in the presence of nulliparous females when they were
maximally tumescent, they do mate with both parous and nulliparous males equally, “suggesting
that testosterone increases in the presence of cycling parous females are associated with
aggression rather than sexual behaviour.” Therefore, the challenge hypothesis was largely

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upheld, and might actually have greater implications for aggressive behavior in primates,
including humans (Muller & Wrangham, 2004).
In 2006, P.E. Honess and C.M. Marin submitted a paper to the Neuroscience and
Biobehavioral Reviews journal called Behavioural and physiological aspects of stress and
aggression in nonhuman primates. In it, they study the effects of stress on aggression in non-
human primates. Although it is a generalized review paper, they focus primarily on macaques
since they are held in biomedical research facilities (this actually suggests, to me, a problem with
this paper, as I’m not sure how much studies on captive animals can say about their wild
counterparts). They mainly found that “the relationships between stress and aggression and in
particular the hormonal changes that are associated with them are highly complex in primates.”
Their study was frequently confounded by “variation in these between studies”, including
numerous sources of variation and inconsistencies in definitions used by different authors.
Honess and Marin do suggest that, despite this, a number of relationships are already very clear,
not least the consequences for animals of elevated stress levels in terms of their health,
development and reproductive success and the need to manage this stress is important in
determining social and reproductive strategies.” The role that stress plays in intergroup
aggression is still a question, and one that needs to be answered. However, perhaps studies of
wild primates would yield better data and results (Honess & Marin, 2006).
So why are intergroup encounters so prevalent? A paper published in 2012 by Richard
W. Wrangham and Luke Glowacki, comparing intergroup aggression in Chimpanzees with war
in Nomadic Hunter-Gatherers, suggests that individual natural selection may be the explanation.
In the discussion, Wrangham and Glowacki ask if the chimpanzee model is compatible with
evolutionary theory. They discuss three different adaptive explanations: individual selection,

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genetic group selection, and cultural group selection. After ruling out both genetic and cultural
group selection, they suggest that “[n]atural selection acting at the individual level is… the only
adaptive process that can explain the evolution of the predisposition for intergroup killing in
chimpanzees. This might seem surprising given that individuals apparently do not receive
benefits in proportion to their investment. Furthermore the costs of territorial defense and
intergroup aggression, while low, are positive, including not only the marginal risk of being
wounded but also the energetic costs of patrolling the territorial boundary, which can be high
(Amsler 2010). However direct individual benefits could come from an increased effectiveness
of attack when more aggressors cooperate: for example, a greater likelihood of damaging or
killing the opponent and a reduced likelihood of any members of the killing party suffering
injury, with the concomitant benefits of reduced enemy threat and increased access to resources
(Clutton-Brock 2009).” After determining this, they ask whether the chimpanzee model of
intergroup aggression can apply to nomadic hunter-gatherer warfare. They aren’t looking at
variations in war practices, but whether or not “nomadic hunter-gatherers neighboring other
nomadic hunter-gatherer societies tend to show the three key elements of the chimpanzee model:
consistent intergroup hostility, safe killing, and benefits from intergroup dominance” (Wrangham
& Glowacki, 2012).
Wrangham and Glowacki do find a context that fits the chimpanzee model; in the
Andaman Islands. “Because multiple societies were isolated in the Andaman Islands in the
absence of agriculture, Kelly (2000:96) regarded them as an ideal natural experiment.” However,
they suggest that the Andamanese were unusual in their warfare. Not because of limited access to
resources, but because of their failure to negotiate intersocietal cooperation. They found that
hunter-gather intrasocietal warfare “oscillate[s] with periods of relaxed peace and therefore do

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not conform to the chimpanzee model. Peacemaking is readily explained by bands being
members of the same society, with opportunities for linguistically negotiated intermarriage,
trade, alliances, and migration between bands (Chapais 2008; Rodseth et al. 1991).” However,
they did find that warfare between societies did conform to the chimpanzee model because the
aggression did, in fact, have the three key elements of the chimpanzee model (Wrangham &
Glowacki, 2012).
So now a picture of primate aggression begins to form. Rather unsurprisingly, it is mainly
the way primates, like all other life on this planet, survive; through competition. That is, the
aggression seems to be largely about obtaining greater access to resources and mates, and then
defending those resources and mates from others. This suggests that some form of competition
drives survival almost entirely. But what does this say for humans?
Do we still have to compete in order to survive? Do we still have to fight for mates and
resources? If we do, then human aggression may be explainable mainly by the same mechanisms
that explain non-human animal aggression. But even if we don’t need to compete anymore in
order to survive, that drive still exists within us, which means our aggression can still be
explained that way. We may not have to, but we still do compete, and we compete for survival…
for resources and, perhaps, mates. However, there is one form of human aggression that remains
mysterious: fanaticism.
Fanaticism is a unique form of human aggression. It is mainly associated with religion.
We recognize fanatics as those who stand on street corners and knock on doors, intent on
“spreading the good news”. They believe that the world is only between 5500 and 10,000 years
old. We also look at them as a “fringe element”. However, fanaticism is much more ubiquitous
than that. For starters, fully 46% of US-Americans believe that the world is young instead of old

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(Newport, 2012). There’s nothing “fringe” about 46% (in fact, only 15% actually accept the
scientific explanation for the origin of species; so one could argue, at least in terms of numbers
and percentages, those of us who do accept the science as is are the “fringe”). For another thing,
fanaticism reaches far beyond religion, and is not necessarily overtly aggressive. Nearly any
social gathering around a common theme can have its fanatics. Some would argue that most
politicians are fanatics. Patriotism itself is, in fact, a form of fanaticism. Fanaticism even shows
up in a much more benign way: social groupings/gatherings around celebrities, musicians, bands,
artists, movies, books, TV shows, and games (including sports); after all, the word “fan” is short
for “fanatic”. Though even in this case it is not always benign, as the infamous rivalry between
Yankees fans and Red Sox fans can show.
So what causes fanaticism? As I said above, survival seems to mainly be a… I guess…
“competitive sport”. One could suggest that even plants compete to survive. However, for the
human species, we may no longer need to compete. Despite this, that drive will not have gone
anywhere. We seem to still be driven to compete, as the drive for survival still seems to exist for
us. I suggest that, if we really do not need to compete with each other and with non-human
animals in order to survive, yet that drive still exists for us, then perhaps fanaticism may be at
least partially explainable as a manifestation of that drive. It has to manifest somehow, and
perhaps defending our beliefs and ideas, even in the face of contradictory evidence, is one way
that competitive drive manifests itself. Which, sadly, means that fanaticism may be impossible to
get rid of.

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