A recently discovered characteristic of one population of this species is that it uses stones as a tool to open hard nuts. First it chooses ripe nuts from a nut palm. It uses its teeth to strip off the nut's fibrous husk. Then it leaves the nut to dry for about a week. When the nut is dry, the monkey lays the nut on a large, flat rock or fallen tree, hammering the nut with a suitable stone until the nut cracks. The hammerstones are often large enough to require lifting with both hands. The anvil rock is often pockmarked with hollows as a result of repeated use.