Between-group peace and cooperation are critical for navigating pressing societal challenges such as security threats and climate change, and as attested by the timely target article by Glowacki, humans have an arsenal of strategies at their disposal to establish, maintain, and restore peace. I generally concur with Glowacki that stable interdependent relationships crossing group boundaries are the province of humans. However, are they really exceedingly rare among nonhuman mammals, including nonhuman primates? Here, I argue that they are not, and that by examining potential parallels between humans and nonhuman primates with multilevel societies we can better characterize the frequency and predictors of peace in primates.

As pointed out by Glowacki, some primate taxa exhibit peaceful encounters between groups (e.g., western gorillas, Forcina et al., Reference Forcina, Vallet, Le Gouar, Bernardo-Madrid, Illera, Molina-Vacas and Bermejo2019; white-handed gibbons, Reichard & Sommer, Reference Reichard and Sommer1997) and even cooperative interactions (bonobos, Tokuyama, Sakamaki, & Furuichi Reference Tokuyama, Sakamaki and Furuichi2019). However, as Glowacki notes, these tend to be transient (Grueter & Wilson, Reference Grueter and Wilson2021). Rudimentary forms of interdependence resembling durable positive-sum relationships do seem to occur in species that feature multilevel societies. Multilevel societies, that is, where multiple core social units coalesce in a semi-interactive manner to form higher-level “clans,” bands, and communities, are best known from snub-nosed monkeys, papionins, and humans (Grueter, Chapais, & Zinner, Reference Grueter, Chapais and Zinner2012; Grueter et al., Reference Grueter, Qi, Zinner, Bergman, Li, Li and Swedell2020; Rodseth, Wrangham, Harrigan, & Smuts, Reference Rodseth, Wrangham, Harrigan and Smuts1991). Mechanistically, a multilevel system emerges through coordinated behaviour among the core units and persistent mutual tolerance. The adaptive utility of this complex nexus of social interactions lies in its members being able to concurrently reap the benefits of multiple social levels, for example, access to cooperation partners at the band level and access to social services such as grooming or allocare at the core unit level (Grueter et al., Reference Grueter, Qi, Zinner, Bergman, Li, Li and Swedell2020).

The between-unit tolerance underlying multilevel societies is evolutionarily stabilized by several interactive forces, for example, the need for collective defence against external adversaries, the need to access patchily distributed resources and the existence of supragroup kin networks. Similar forms of social and economic interdependence can also explain peaceful intergroup relationships in humans.

In humans, between-group competition represents a major selective force for the evolution of intergroup peace and suprafamilial groupings (Alexander, Reference Alexander1990). The need for cooperative defence (or aggression) against shared external threats may suppress in-group favouritism and elicit amicable sentiments to neighbouring communities. A contemporary real-life example includes Russia's invasion of Ukraine which improved Ukraine's relationship with its allies, and the relationships among its allies (especially among European Nation states). Exposure to a common threat (such as a terrorist attack) can also cause strangers to identify themselves as belonging to a uniform social entity (Drury, Cocking, & Reicher, Reference Drury, Cocking and Reicher2009). In primate/animal multilevel systems, a similar catalyst for the emergence of tolerance between members of different units is an external threat which can originate from conspecifics or predators. Through aggregation for common defence, different groups can pool the risks of foreign threats (Camerlenghi, Nolazco, Farine, Magrath, & Peters, Reference Camerlenghi, Nolazco, Farine, Magrath and Peters2023; Grueter & van Schaik, Reference Grueter and van Schaik2010; Kummer, Reference Kummer1968; Xiang et al., Reference Xiang, Yang, Yu, Yao, Grueter, Garber and Li2014).

Economic and ecological interdependence such as trade of subsistence goods or mutually agreed-upon permission to use shared territories can also function against the waging of war in humans (Fry, Reference Fry2012; target article). Such symbiotic exchange networks between groups are critical in environments in which variance in resource access among groups is high because they can act as a buffer against resource failure in one's area (Kelly, Reference Kelly2013; Pisor & Gurven, Reference Pisor and Gurven2016; Wiessner, Reference Wiessner2002). Evidence from both small- and large-scale societies shows that a densification of trade alliances can effectively prevent intersociety and interstate war (Grueter & White, Reference Grueter and White2014; Jackson & Nei, Reference Jackson and Nei2015). Ecological interdependence may also explain intergroup tolerance in some primates, in particular those with multilevel societies. When critical resources across neighbouring ranges are heterogeneously distributed, gains to exclusive access could be reduced. The resulting relaxation in intergroup relations provides scope for intergroup tolerance (Jaeggi, Boose, White, & Gurven, Reference Jaeggi, Boose, White and Gurven2016; Robinson & Barker, Reference Robinson and Barker2017) and higher-level social structures such as multilevel systems, as long as these do not impose unsustainable costs on its constituents (Grueter & White, Reference Grueter and White2014; see also Macdonald & Johnson, Reference Macdonald and Johnson2015).

In humans, reciprocal exogamy, whereby individual dispersal for marriage suppresses latent violent tendencies and perpetuates alliances based on consanguineal and affinal kinship between groups, has been argued to be a strong determinant of nonwarring relations in small-scale societies (Chapais, Reference Chapais2008; Lévi-Strauss, Reference Lévi-Strauss1949; Rodseth et al., Reference Rodseth, Wrangham, Harrigan and Smuts1991; but see Grueter & White, Reference Grueter and White2014; Kang, Reference Kang1979). In primates, the buildup of kin networks encompassing multiple units and the opportunity for members of different units to interact regularly may also foster between-group tolerance. Shared descendance resulting from localized dispersal, social viscosity, and extragroup matings may be the source of reduced aggression between neighbouring groups and facilitate the formation of affiliative intergroup associations (Mirville et al., Reference Mirville, Ridley, Samedi, Vecellio, Ndagijimana, Stoinski and Grueter2018; Reichard & Sommer, Reference Reichard and Sommer1997; Rodrigues, Barker, & Robinson, Reference Rodrigues, Barker and Robinson2023; Snyder-Mackler, Alberts, & Bergman, Reference Snyder-Mackler, Alberts and Bergman2014; see also Camerlenghi et al., Reference Camerlenghi, McQueen, Delhey, Cook, Kingma, Farine and Peters2022).

A comparison between nonhuman primates and humans will help us understand if the prevalence of peace in human intergroup encounters can be explained with the canonical factors proposed for primates. It does seem that some of the same drivers can generate and stabilize between-group peace in humans and primates (i.e., mutual access to resources in ecologically complementary environments, collective group defence, and “exogamy”) although these take more elaborate forms in humans. However, what is unique to human peace systems is a cultural overlay that prescribes the adherence to peace norms (target article). This narrower definition of peace does not apply to nonhuman multilevel societies. However, if we use the less restrictive definition of tolerance and motivation to interact across group boundaries, then peace is present in nonhuman multilevel societies, albeit in more embryonic forms.