Intergroup conflicts among chimpanzees in Taï National Park: lethal violence and the female perspective

Abstract

Intercommunity aggression in chimpanzees and primitive warfare in humans possess striking similarities, such as the common occurrence of large male coalitions, systematic control of territory boundaries, and lethal attacks on isolated individuals from neighboring groups. However, an important apparent contrast is the absence of recurrent peaceful interactions between neighboring groups of chimpanzees. We observed a remarkable range of behavior in intergroup encounters among three habituated communities of chimpanzees in Taï National Park, Côte d'Ivoire. Lethal attacks are documented in these study groups for the first time, as well as year-long exchanges of parous adult females and peaceful intergroup visits of mothers with infants. Demographic factors, including group size and number of adult males, are shown to affect the nature of intergroup interactions in ways not considered previously. A reconsideration of the difference in intergroup interactions between eastern and western chimpanzees is proposed including a more important consideration of the female's perspective. The inclusion of the new complexities in intergroup interactions in chimpanzees allows new parallels to be drawn with the evolution of primitive warfare in humans.

Oxytocin reactivity during intergroup conflict in wild chimpanzees

“…As with humans, chimpanzee group defense requires coordination and coalitionary support to be effective (11,17), and when such cooperative behavior is maintained, access to benefits is more likely. For instance, in chimpanzees, intergroup lethal violence occurs predominantly at times of power imbalance in favor of attackers (10), and thus group cohesion may reduce the likelihood of suffering costs.…”

“…Wild chimpanzees in almost all long-term field sites engage in competitive intergroup conflicts (10,11), which are characterized by two sets of behavior (Movie S1), intergroup encounters and border patrols. Intergroup encounters (direct out-group contact) are characterized by coordinated attacks, with synchronous vocalizations and charges toward and combat against chimpanzees from rival groups (12,13).…”

“…Access to benefits of intergroup conflict can be maintained through cooperation, providing benefits to both actor and recipient regardless of potential short-term costs to the actor (16). Chimpanzee in-group behavior during intergroup conflict is considered to be cooperative (11,17), as it encompasses prolonged coordination and cohesion, as well as coalitionary support, which can involve individuals safe from the out-group running forward to defend an in-group member under attack from the out-group (11). In humans, cohesion and support also intensify under life-threatening situations, such as war (18).…”

Primate archaeology reveals cultural transmission in wild chimpanzees (Pan troglodytes verus)

“…However, the following lines of argument provide supporting evidence that all these females originated from the chimpanzee community directly neighbouring the south of habituated South Group [42]. Longterm data of inter-group encounters of the Taï Chimpanzee Project reveal frequent encounters of the South Group with a large community in the south of their territory [51,53] (South of South, or SoS territory, figure 1). In early 2012, no more inter-group encounters were observed with the southern community of the South Group, while at the same time the males of South Group were observed exploring the SoS territory up to 5 km outside their own home range, without encountering any members of another chimpanzee community.…”

“…South Group, the community where the influx of immigration was observed, has been fully habituated since 2000 [42,50]. Based on the average home range size of our study communities according to group size, we estimated the potential home range of the SoS Group to have been approximately 36 km 2 [42,53].…”