Chimpanzee gangs fight for territory
According to the evolutionary teaching of Darwin, the struggle for existence is manifested in three forms. Internal struggle occurs between individuals of one species, interspecific struggle - between individuals of different types. The third form is the struggle against unfavorable environmental conditions.
Different species compete with their own kind for resources and reproductive opportunities in different ways, some more aggressively while others in a more ritual way, like for example nightingales, who compete by singing. Although other animals apart from humans are known to kill one of their kind, it's not usually thought of as a planned murder but rather as an instinct.
It turns out that we are not alone, chimpanzees show more or less same behavior as humans when it comes to killing other chimpanzees. They have a strategy to form groups or gangs of 5 - 8 males and seek out lonely males from rival groups. They would then sneak up on the victim, ambush and kill him. It is also common to feast on the dead enemies corpse.
This behavior was first documented by Jane Goodall in what is now known as "Gombe Chimpanzee War". During this 4 year conflict a once-unified chimp community disintegrated into two rival factions. Males from a bigger fraction used the above mentioned tactic to take out all the males from othe fraction.
It was debated for some time that the feeding station installed by the researchers brought two separate communities together for a short period of time and later was the reason for the violence spree. Later research of this conflict allowed to conclude that the probable cause of the conflict was a power struggle between three high-ranking males. Community split up during the rising tension between a recently crowned alpha male and two other guys who were constantly confronting him.
Later studies were also made to prove that human interaction was not the cause for cruelty inside chimp comunities. This documentary on youtube shows how Ngogo community fight with other chimpanzees to expand their territory
How infighting turns toxic for chimpanzees https://www.sciencedaily.com/releases/2018/03/180326140214.htm