Monkey skills are diminishing in the wake of human expansion into the macaques' habitat or, more specifically the expansion of humans' dogs into the habitat.
Talking to the BBC, Dr. Michael Gumert of Singapore's Nanyang Technological University explains the problem:
"What's been happening is that over the past six years on the island, we've just seen more palm oil and rubber farms being developed in the forest. I've begun to notice that the groups that are closest to human activities, just aren't having kids anymore... The monkeys come down to the big rocky coasts and pick up rocks and crack things like oysters and crabs. But if the dogs repel them, the monkeys use the shore less and less and they will stop using tools as much. What we're looking at with these stone-tool-using monkeys is a rare case of truly wild long-tail macaques doing their original wild behavior, unlike most of the other macaques that have had their behavior destroyed by human development. If we develop right next to them, they will stop going to the coast to feed and go to the local rubbish bin and find food there."