Part Of: Anthopogeny sequence
Content Summary: 1000 words, 5 min read. 

Territoriality in Mammals

Many mammals exhibit territoriality. A community will occupy a fixed piece of real estate, and defend it from conspecifics. Once an intrusion is detected, boundaries are defended by group members (often the females) coming together to chase the outsider away. The goal of these fights over land is simply the opponents’ defeat.

In general, mammals do not kill their conspecifics, despite the xenophobic emotions inherent in matters of territory defense. To be clear, infanticide is extremely pervasive in the animal kingdom. But territoriality is a defensive posture, and the killing of adult members of one’s own species is virtually unknown.

Which brings me to chimpanzees.

The Raid Adaptation

Chimpanzees also inhabit demarcated territories, and neighboring communities are treated with hostility, so much so that up to 75% of the time is spent in the central 35% of the range. Border patrols are conducted by groups of male chimps moving stealthily to enforce their territory’s boundaries.

But chimpanzees also engage in raids with large groups of males penetrating deep into enemy territory, stalking and killing members of competing troops.

https://www.youtube.com/watch?v=a7XuXi3mqYM

The ape raiders are quiet, alert to enemies… The raiding chimps appear to assess tactical risk by locating and observing their enemy before attacking, they make sure they have a clear numerical advantage, and they try as well to gain the advantage of surprise. In addition, such attacks typically immobilize the victim, so that the attackers themselves are barely injured. The victims may be either male or female; but the aggression usually focuses more severely on adult males, less severely on obviously fertile females. Young females at the start of their breeding careers (nulliparous females) are most likely to escape injury and can be forced to travel back with the raiding party into their home territory.

In comparison to the vast majority of mammals, chimpanzee raids feature a deliberate search for victims, their killing and mutilation of a helpless neighbor despite his appeals for mercy. 

Why then has such behavior been selected? Killing opens the door to territorial expansion (Mitani et al 2010), by weakening the other groups’ overall fighting power. In Gombe, a series of these raids ultimately led to the collapse of the targeted group.  In turn, territory size directly correlates with resource and mate availability.

So why haven’t other animals evolved to kill?

The Imbalance of Power Hypothesis

A handful of other exceptional species also manifest lethal territoriality. But none of these involve raids. Hyena patrols do often kill rival gang members in gang-like warfare, but these occur in patrols, there are no secret incursions in enemy territory. And because hyenas are female-bonded species, the warfighters are female. Hyenas do not “take captives” of either sex; but they do plunder resources (e.g meat). Finally, lions kill lions from other groups; but this occurs exclusively in the context of takeover events.

What do lions, hyenas, and chimpanzees have in common? All are fission-fusion groups, with a stable residential group, but with foraging group size varying with seasonal fluctuations in resources. 

We’ve already discussed how raids are governed by the logic of a local imbalance of power. Raids preferentially occur when the attacking party has gathered significantly more fighting power than the defender (Wrangham 1999). 

Perhaps these are linked! Variable-size foraging groups  Encounters in fission-fusion species are often catastrophically imbalanced – this should provide the selective pressure towards the evolution of lethal territoriality. 

The Lowly Origin of Human War

Modern warfare doesn’t resemble chimpanzee raids. But primitive warfare is heavily reliant on raids! Let’s dive into one example from Wrangham ():

War among the Yanomamö is an overtly acknowledged relationship, part of an escalating tension between villages, possessing a history that men and women discuss. It can be provoked by sorcery. It can be motivated by revenge. The combatants prepare ceremonially. They use hand-held weapons instead of teeth, and their poisoned arrows can pierce the body of an individual or be fired in a volley against a whole village. Their war can include dastardly tricks. It sometimes has a plan. It targets specific enemies. A raid often takes days, not hours. Abduction and rape are common. Retaliation is expected. And so on. When Gombe and Yanomamö are compared, the gulf that divides our two species is unmistakable. Because language makes discussion and meaning possible, the cultural dimensions to human war will always make it richer, more complicated, more exciting, as well as more self-deceiving and confused, than chimpanzee intercommunity violence.

But the similarities are also clear. All seven features of chimpanzee raids discussed above manifest in human raids. Humans engage not only in lethal territoriality, but we also share a particular style of warmaking with  our closest ancestor.

Indeed, even the rate at which foraging humans and chimpanzees engage in between-group violence is quite similar:

These data suggest a common mechanism. It is not that humans evolved a unique thirst for warfare. Rather, this instinct likely derives from our common ancestor with chimpanzees.

Implications

Why should a hairless ape behave so strangely? From the perspective of an alien scientist, the imbalance of power hypothesis makes human warmaking behavior less surprising.  

It appears that warfare is an adaptation. That is not to say that war is good (the naturalistic fallacy). Neither is war inevitable (biological determinism).  

There is a live debate on how prevalent warmaking was in foraging communities. We have strong evidence of high homicide rates in such bands. But it is difficult to say how much stems from between-group raids, versus within-group feuds. I have not yet looked at this data thoroughly.

The fact that humans live in multilevel societies makes us somewhat more xenophilic than our chimpanzee relatives. Total war, with weaker peacemaking affordances, is more prevalent on meta-ethnic frontiers between very culturally dissimilar groups. 

There is also a large amount of variation in warmaking behavior across groups. We can plumb Fry et al (2021) work on peace systems to improve our understanding on how to engineer a world system with less war. There is also variation across time; the transition from achievement-based societies and chiefdoms in particular marked an intensification of war. Thus, cultural group selection might also inform the search for peace technologies.

Until next time. 

References

• Fry et al (2021). Societies within peace systems avoid war and build positive intergroup relationships
• Keeley (1996). War before civilization: the myth of the peaceful savage.
• Wrangham & Peterson (1996). Demonic Males: apes and the origins of human violence.
• Manson et al (1991). Intergroup aggression in chimpanzees and humans
• Mitani et al (2010). Lethal intergroup aggression leads to territorial expansion in chimpanzees
• Wrangham (1999). Evolution of Coalitionary Killing