Humans have made a nasty habit of killing off entire species of animal.
Passenger pigeons once formed huge flocks in North America, but humans hunted them on an industrial scale. Largely as a result of this, the species was utterly wiped out by the early 20th century.
But it seems we're not the only animal that can't stop ourselves over-hunting our prey.
In a sinister echo of the passenger pigeon story, chimpanzees in one area of Africa have over-hunted the monkeys they prey on. As a result, the monkey population has been pushed close to local extinction.
David Watts of Yale University in New Haven, Connecticut and John Mitani of the University of Michigan in Ann Arbor have both spent years studying a single population of chimpanzees. They live in an area called Ngogo in Kibale National Park, Uganda.
They have been so effective as predators that they really have knocked down the local population
Although the chimpanzees mostly eat fruit, they also regularly hunt for meat. Their main prey are red colobus monkeys.
Between 1995 and 2014, Watts and Mitani observed 556 hunts, 356 of which targeted red colobus. These hunts were very successful: 912 colobus were killed, an average of 3 per hunt.
It has been clear for several years that the red colobus population has declined as a result. A 2011 study found that the population fell by 89% between 1975 and 2007. In the late 1990s the chimps were killing up to half the red colobus population every year.
"They have been so effective as predators that they really have knocked down the local population," says Watts.
"Red colobus are easier to catch than other monkeys," says Watts. That might be partly because they tend to stand their ground against chimps, rather than running away. "That can work, but not if there are 20 chimps trying to catch them."
The chimpanzee population in Ngogo is unusually large, hovering around 190, which means the chimps hunt in large groups and can overwhelm the colobus.
Watts and Mitani have now found that the chimps are starting to switch to alternative prey. Red colobus hunting reached a peak in 2002, and since then they have been hunting red colobus less often, and hunting other species more.
The new results are published in the International Journal of Primatology.
The second-most-hunted species are monkeys called mantled guerezas. Watts and Mitani have found that these seem to be becoming less common, suggesting that the chimps are causing their population to decline as well.
Black-and-white colobus are also in the chimps' firing line.
If they kill 2 monkeys out of a group that only has 10 or 12, that's a big impact
"I've been here a little over a month," says Watts. "The chimpanzees have hunted quite often in that time. They've hunted black-and-white colobus eight times. I've never seen anything like that before."
Black-and-white colobus are relatively uncommon compared to red colobus, and live in smaller groups. So the chimps' decision to hunt them more is bad news for them.
"One group they've targeted at least twice," says Watts. "If they kill 2 monkeys out of a group that only has 10 or 12, that's a big impact."
The good news is, the chimps probably won't wipe out the populations entirely.
"I doubt that it will happen, because as the red colobus becomes scarcer, then it requires more energy to find them," says Watts. It's not worth the chimps' effort to seek out the red colobus, so the population might stabilise – albeit at a much lower level.
It does require a lot of skill to catch the monkeys
A lack of prey might also change the chimpanzees' behaviour.
"One possibility is, if the chimpanzees hunt less often, then young chimpanzees as they grow up get less experienced trying to hunt, and they don't learn to be as good as their predecessors," says Watts. "It does require a lot of skill to catch the monkeys, and it's interesting to see that some chimpanzees are better at doing it than others. They need to learn."
That said, Watts hasn't seen any evidence of this yet. "My subjective impression this summer is they are getting better at hunting black-and-white colobus."
In their tendency to blindly over-hunt their prey, chimpanzees are rather similar to humans. Perhaps that's not too surprising, as they are our closest living relatives.
I don't think chimpanzees are capable of thinking about a long-term future
"People are fond of making claims about similarities between chimpanzees and humans," says Watts. "It's possible to push that too far. But one characteristic we have in common is we over-harvest resources. We're not good conservationists."
That said, Watts says there is also a crucial difference between chimps and humans: we are supposed to be smarter than they are.
"I don't think chimpanzees are capable of thinking about a long-term future," says Watts. If they do switch to a different prey, it's not because they are trying to conserve the red colobus. "They're just responding to what they encounter and what they see."
By contrast, humans understand that we can drive a species to extinction. Unlike the chimps, we really can think about a long-term future. But as Watts puts it, "we are too prone to devalue it and not care about it."