Groups work together via a model of parochial cooperation, which is the tendency for individuals within a group non-related group to favour the in-group over the out-group. This cultural trait spreads as larger groups' tendency towards cooperation have competitive advantage in inter-group conflicts where the out-group is less co-operative. This importance of parochial cooperation is paramount to effective resource and territory defence, however maintaining the social bonds and groups necessary for this cohesion may become strained under rising temperatures and reduced water availability.
An example of how seasonal variation can significantly impact chimpanzee social structures has been seen in western chimpanzees at Fongoli. Fongoli, in southeastern Senegal, is the hottest and driest site where habituated chimpanzees are studied, and during the 7 month long dry season all but a few water sources dry up. Soaring temperatures results in chimpanzees spending most of their activities budgets resting, which may impact non-kin social bonding behaviour used to maintain social ties. Despite this, chimpanzees at Fongoli are significantly more cohesive than chimpanzees studied elsewhere, with an average party size of 15. When the party size is taken in the context of the community size, following Boesch’s (1996) calculation of relative mean party size, Fongoli chimpanzees also form significantly larger parties. Fongoli chimpanzees exhibit an average party size of close to 50% of their community size, compared to the average of 15% seen at other sites \cite{Pruetz_2009}. This cohesion does not make the group to immune to intra-group violence however as the group has been recorded killing the previous alpha male for approaching females in estrus \cite{Pruetz_2017}. Whether these social structures are changing as temperatures continue to rise and water availability becomes lower generally has not been explored, however increasing group size and/or territory size would be expected under the ecological constraints model.
Additionally how decreased water availability and predictability impacts food availability has not been explored. Increased seasonality in food availability in chimpanzees has been associated with increased levels on infanticide and decreased female sociality \cite{Doran_2002}, which can impact group cohesion and increase the likelihood of intra-group conflict.
Ultimately in the face of ongoing climate catastrophe the chimpanzee communities that are able to maintain group cohesion, size and number of males will be more successful compared to others, and strategies used to reduce infanticide, intra-group conflict and . Whether other chimpanzee communities' social structures will start to resemble those seen in more resource limited areas such as Fongoli is an area of debate, and would require projections of how current supplies of water and food will become limited in existing areas.
Intergroup violence
Compared to all species, chimpanzees show high levels of territoriality however levels of intergroup violence vary across chimpanzee populations \cite{G_mez_2016}. Inter-group conflict is regular and expressed mostly as vocal exchanges, which can escalate to chasing and attacking the out-group, occasionally ending in lethal violence \cite{Watts_2001}. Some chimpanzee communities also conduct lethal raids on other groups, killing males and juveniles \cite{G_mez_2016}.
Population density is correlated with the frequency of inter-group conflict in chimpanzee species. West African chimpanzees (Pan troglodytes verus) are the most sparsely populated, and fewer intergroup killings are observed in this sub-species compared to all others we have data on \cite{Wessling_2020}. Notably for 2 studied west African populations (at Bompusa and Bossou), no intergroup killing has been observed at all, suggested to be due to comparatively large home ranges and reduced border patrols (\cite{Lemoine_2022}). In stark comparison to east African chimpanzee species (Pan troglodytes schweinfurthii), where the ecology of their habitats can support much population densities and interaction rates increase accordingly \cite{Wilson_2014}.
The intensity of intergroup conflict also varies between species and sites, notably the lethality of intergroup encounters. Killing is most common in eastern chimpanzees and least common in bonobos, and the most common victims are adult males and unweaned infants. Previous studies have shown lethality of intergroup encounters generally increases with population density, and the number of adult males in the attacking community \cite{G_mez_2016,Southern_2021,Wilson_2014}.