In sweet-potato washing, dirty tubers were dumped by the researchers on the beach for the monkeys to eat, and Imo, a 1.5-year- old female, invented a technique of cleaning the potatoes by washing them in the stream nearby. The food-processing technique spread to her peers, their siblings and their mothers, but not to adult males, who ignored these (childish?) acts. As the spud-scrubbers matured and had offspring, the mothers passed on the habit to their youngsters, in orthodox downward vertical transmission.
Similarly, 4 years later, Imo invented wheat sluicing, in which mixed handfuls of wheat and sand were thrown into the sea. These cereals, too, had been provided by the researchers. The sand sank, the grain floated, and the wheat could easily be skimmed off the surface, this was much more efficient and less gritty than picking up grains of wheat one by one from the beach...
Western comparative psychologists and other critics continue to be dismissive of the cultural achievements of Japanese macaques.
from The Cultured Chimpanzee by William Clement McGrew