"Sonso" chimps hunt in small groups for colobus monkeys, while those from the "Waibira" troop hunt solo and catch "whatever they can get their hands on".
Biologists who have followed and studied these animals for years think that work may have disturbed the group hunting that seems key to chasing and catching colobus monkeys.
I trust nobody is surprised. You'd probably change your hunting methods if you were being watched by chimps. And I've a strong suspicion that the spear-wielding chimps learned from watching people.
Research can be tough sometimes. And that's when you're not dealing with people. (The author of the latter was one of the Unibomber's targets. "McConnell originally published satirical articles alongside serious scientific articles in the Journal of Biological Psychology but received complaints that it was difficult if not impossible to tell which was which.")