- [Narrator] This is Koram Island off Thailand's east coast, home to beach combing long-tailed macaques.
They've mastered the art of a particular type of tool use.
Using rocks to crack open shellfish.
Amanda Tan has been observing this unusual behavior for seven years.
- The tool use in macaques is really, really rare.
You find macaques all over Southeast Asia, but it's really only a few populations living out on island that we see tool-use behaviors.
It's just a culture that's really unique only to some groups of macaques.
- [Narrator] For the macaque groups that can use tools, life is easier.
(macaques squeaking) But there's a twist to this tale On Koram Island, Amanda has discovered that their highly developed skills are having some surprising consequences.
By using tools, these macaques are able to target the largest, juiciest oysters and they can devour as many as 40 a day.
- We do see the evidence that they are depleting the shellfish on the island.
So when we compare the shellfish here versus an island, just next to us, we see that the shellfish here are less abundant and they're also smaller in size.
- [Narrator] This is the first reported case of a tool using animal, other than ourselves, over exploiting a natural resource.
- We know for sure that humans are depleting the natural resources on the planet, and we never really thought that any other animal was doing it as well.
- [Narrator] If these monkeys continue to over harvest their environment, the shellfish could disappear.
Ironically, this extraordinary tool using behavior would then also disappear.