>>13706970Evolutionary models of in-group favoritism (2015)
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4371377/noted that “[between-group] interactions often involve hostile competition, such as red fire ants that kill others with different odor cues, and competitive mating of side-blotched lizards with different morph colors. In contrast, within-group interactions are more likely to be cooperative with (sub) group members in the same territory/nest/site.”
Perhaps the most renowned example of between-group competition among primates is the Gombe Chimpanzee War (1974–1978)
https://en.wikipedia.org/wiki/Gombe_Chimpanzee_War, a four-year conflict between two neighboring chimp tribes, dubbed Kasakela and Kahama. The victorious Kasakela tribe killed all of the males of the Kahama, expanding into their territory. This brought them into contact with another tribe, the Kalande, who attacked them in border skirmishes, forcing the Kasakela to retreat. The conflict finally ended when each chimp tribe settled into their own territorial regions. Jane Goodall, the primatologist who studied the conflict, was most astonished by the brutality of the attacks between the tribes; she previously believed that chimps were “rather nicer” than humans.
