Tooled-up monkeys drive shellfish to extinction
Humans aren’t the only animals to use tools. And nor, it would appear, are we the only group that risks, through the use of such tools, driving other species to extinction. Speaking with Chris Smith, Lydia Luncz has been looking at a population of macaques that use stones to harvest shellfish. As they deplete their stocks of prey, they’re changing their tool choices. And when the food runs out, the knowledge of that tool use will also most likely vanish…
Lydia: We were looking at stone tool use of macaques so we went to the Gulf of Thailand. There's a National Park called Sam Roi Yot National Park and the two islands are called Koram and Nam Sao. There are long tail macaques that live on both of those islands. One island, on Koram, it’s a bit of a larger island. There are a lot of monkeys. We have about 80 monkeys right now on this island and on the other island, there's just 8 of them.
Chris: What sorts of tools do these monkeys use?
Lydia: Basically, they forage for seafood. They love seafood just like us. So, when they crack open oysters for example, they use something that we call an axe hammer where they use the pointy tip of an elongated stone but when they crack open for example sea snails or crabs, then they use bigger stones where they used the flat surface of a stone. So, we can see by the tools that we find, the use that we find on the stone, what they actually have been eating.
Chris: And the overriding question you want to answer was what?
Lydia: So we went out there to those both islands. The first thing that became very apparent was that those monkeys on the different islands used very different sizes of stone tools and we wanted to know the underlying reason for this. Why would they select different stone tools for the exactly the same task? So we observed the monkeys and we collected our stone tools and then we explored their home range. So we collected information on stone availability. Is the same raw material available to them? And then we looked a bit closer at the prey that they target with those stone tools.
Chris: And what did you find?
Lydia: So we found that actually, the prey species on both islands were very different in size and that the monkeys were actually matching the stones that they selected to the prey size that was found on their island. So on the island where we had a lot of monkeys, we found that the prey species there was very small and also, reduced in number. So, they weren’t so abundant. And on the island where there was little amount of monkeys – just a few of them – we found that the prey population was flourishing. They were large. There were a lot of them there. so that was the main difference that we found between those islands.
Chris: And it’s your interpretation that on the island with lots of monkeys, there's a lot of pressure being brought to bear on what they're eating. And so therefore, you’ve driven down the average population size because your monkey is going to go for a big treat preferentially over a small one, so they picked off all the big ones. On the smaller island with fewer monkeys, there's less pressure. Therefore, there are more bigger specimens there. And to get the big ones off, they're using bigger tools.
Lydia: That is exactly what we found, yes. That was right. So, we look closer on the sexual organs of the prey species and wanted to know whether we find smaller prey, whether this had like an evolutionary reason. Because we see this, when humans exploit snails a lot, they become quicker in reproducing. So even when they're smaller, they're already able to reproduce. So we wanted to test that on our islands as well. And it turns out that they are not. So, the small individuals that we found on Koram island are not able to reproduce yet which means that the overharvesting of those shellfish must have happened quite quickly in the last decades. That means that the monkeys have harvested all the large individuals already and only leave the young ones that aren't able to reproduce yet.
Chris: If your intuition is correct then, the animals that are now exploiting the small prey would have in the past, had much richer pickings and big prey to play with and feed upon. So, were you to go digging on the beach for example, do you think you would find stones which were more in keeping with the other island where the prey is still larger and it’s a shift in tool use that’s happening, the animals are adapting their tool-use to the prey that’s available to them?
Lydia: Absolutely. that’s what we would expect if we would dig down to see what happened like maybe 20, 30, 40 years ago. We would expect that we also see larger stone tools with larger prey.
Chris: So, what are you concluding from this then? It’s an interesting observation. It’s a predictable observation borne out by some nice facts and evidence that you’ve been able to demonstrate here. But what are the implications of this?
Lydia: Well, maybe that every tool invention isn’t always going to lead to better and more advanced tools. That something maybe just ends in a dead end like those macaques. They're heading towards an extinction of prey on their island and which might have a detrimental effect to their survival, to their fitness. We think that stone tool use is socially learned in macaques. So we think that once their prey might be gone, that they will be left without the knowledge of how to use stone tools. And they're losing cultural behaviour that’s been around for hundreds of years.