After the Ngogo #chimpanzee group killed 21 members of neighboring groups and expanded their territory by 22%, female #birth rates more than doubled and #infant survival increased sharply—showing clear fitness benefits from intergroup killing.
Beware that this will be taken to back up #warfare models for early human societies. But this is a rather different story because female chimps get no help feeding their infants, rarely help each other and are constrained to great ape brain sizes. Chimps are way more territorial than hunter-gatherers in general. It's very doubtful if warfare would help Homo mothers of extremely large-brained offspring.