Tool using monkeys driving prey extinct

Macaques use rocks as hammers and anvils to crack open shellfish. But they’re so good at it these tool using monkeys are driving their prey extinct


There’s this classic image of the hunter-gatherer living in balance with their environment. It’s a romantic idea for sure, but it’s also wrong. As we learn more about our ancestors’ spread out of Africa, we’re also learning how they devastated the environments they arrived in. But it turns out we weren’t the only ones causing a problem. New research has revealed how tool using monkeys are also driving their prey extinct.

They join the illustrious ranks of primates using their smarts to overly exploit their local environment. Previous winners of the title include chimps, ancient humans, and of course, modern humans. But this also raises the question. If a pack of monkeys armed with rocks can cause this sort of environmental impact, perhaps we’ve been underestimating ancient humans’ role too.

Tool using monkeys

The monkeys in question are macaques in Thailand. This species, in this country, is known to be very adaptable and innovative. In fact, researchers recently documented them inventing new tools to deal with human-made changes to their environment. They adapted their hammer and anvil technology to be used on food from human farms.

Tool using monkey using his tools to harvest shellfish

This trick of using rocks as a hammer and anvil to smash stuff actually has its origins on the coast. Many of the macaque groups studied by scientists live on islands in Thai National Parks. There, the macaques use their tools to crack open a variety of seafood that would otherwise be inaccessible to them. It’s this behaviour that seems to be having an impact on the local environment.

Researchers discovered this because the island-dwelling nature of these tool-using monkeys creates a natural experiment. Some islands are close together but can have very different numbers of macaque inhabitants. This makes it fairly easy to track how more macaques impact an island. In this case, the scientists studied monkeys on  Koram Island and NomSao Island, in Khao Sam Roi Yot National Park.

Despite being quite close together, Koram island has more than 6 times the macaques of NomSao Island. A whopping 24 macaques versus 4. When the islands’ size differences are taken into account, this amounts to roughly 3 times more macaques per square metre.

Notably, this didn’t manifest as fewer shellfish at Koram island. Rather, shellfish there were 60% smaller than their NomSao counterparts. This means that the tool using monkeys were killing off most of the adult shellfish. This is a sure sign the population is being put under severe pressure. Producing the next generation is hard when all the adults have been eaten.

Tool using humans

We’re an egocentric species. So, of course, we want to know what these tool using monkeys say about us. Ostensibly there’s not much new here. In fact, we’ve seen this same sort of thing in ancient human populations many times.  Many stone age sites show human prey decreasing in size, documenting this same trend as the macaques.

The classic example of this comes from tortoises, who were hunted by both humans and Neanderthal around the Mediterranean. Over time the body size of hunted tortoises declines, showing human straining their population. Interestingly, the hunters appear to have recognised this, with many sites documenting a shift towards hares and other fast-reproducing animals that can survive under the strain of human hunting.

Decreasing size of tortoises hunted by ancient humans (left) and the decreasing percent of their diet they made up (right)

But whilst we talk about examples of these “ancient” humans impacting the environment, it’s worth remembering they aren’t that ancient. Those charts above only go back to around 200,000 years ago. For reference, we started using tools closer to 3 million years ago. But it would be absurd to suggest those early tool users had the smarts (or the technology) do impact their environment. Wouldn’t it?

Well, that’s exactly what these results from the macaques suggest could happen. All they have is a hammer and anvil. The earliest tool users had the same sort of thing. In fact, that ancient site I just linked to actually features signs of stones being used as hammers and anvils.

So tool using monkeys are driving their prey extinct. But it means our ancestors may have been doing the same for even longer.

References

Perry, G.H. and Codding, B.F., 2017. Stone Tool Use: Monkeys overharvest shellfish. eLife6, p.e30865.

Speth, J.D. and Tchernov, E., 2002. Middle Paleolithic tortoise use at Kebara cave (Israel). Journal of Archaeological Science29(5), pp.471-483.

Stiner, M.C., Munro, N.D., Surovell, T.A., Tchernov, E. and Bar-Yosef, O., 1999. Paleolithic population growth pulses evidenced by small animal exploitation. Science283(5399), pp.190-194.

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