Chimps mostly eat fruits, leaves, and other wholesome vegetarian foods. However, they also hunt monkeys for ~2% of their diet. Sometimes - scarily enough - with spears. In particular, their favourite snack is colobus monkeys. The fact that colobus monkeys make up such a small part of their diet has led many to suggest that it might be a social activity for bonding. Or maybe just a way to top up some nutrients, like a furry vitamin pill. But despite the fact that it is so unimportant to their diet, it seems that chimpanzees are hunting their prey to extinction.
But what's perhaps more interesting is why this is happening and how the chimps are responding to it.
The fact that chimpanzees are hunting these animals into decline has been known for a while, with several studies documenting a relatively dramatic drop in the colobus population size. Other similar monkeys in the region which aren't hunted by chimps haven't seen such a drop, helping rule out other factors like deforestation etc. Additionally, in regions chimps aren't out for their blood the colobus monkeys seem to be doing alright despite things like deforestation and a drop in food quality. They're hardy little primates.
So why are chimps making such an impact on these monkeys when it doesn't seem to be necessary for their diet? I mean, they're driving this species towards extinction simply as a pair-bonding exercise. They don't need to eat them.
And this is the crucial issue. In most predator-prey relationships a sort of equilibrium develops (albeit one based on constant death and struggle). If the predator population gets too large then the prey can't support it and many die off. If they prey population gets too large then this allows the predator population to increase, reducing the prey population back down. But because chimps don't need to hunt these monkeys their population can grow even if the colobus population decreases. It's a no-win scenario for the monkeys.
So it seems chimpanzees are hunting themselves into a bit of a corner, snack wise. How are they reacting to this situation? Well, they seem to be aware they're creating an issue. Remember how I mentioned that there are other monkeys living in the region that the chimps typically ignore? Well, in response to the decrease of colobus monkeys chimpanzees are hunting these alternatives a lot more frequently. They seem to be aware that their preferred choice is on the decrease and are trying to make do.
Perhaps the fact that chimpanzees are hunting these alternatives might just give the colobus monkeys a chance to recover.
Reference
Watts, D. P., & Mitani, J. C. (2015). Hunting and Prey Switching by Chimpanzees (Pan troglodytes schweinfurthii) at Ngogo. International Journal of Primatology, 1-21.