Shadows of Forgotten Ancestors

As anyone knows who’s ever been to a picnic at the beach, sand sticks to food and makes it unpleasantly gritty. In September 1953 a one-and-a-half-year-old female named Imo figured out that she could rinse the sand off her sweet potatoes by dunking them in a nearby brook.After Imo, the next individual to learn potato washing was Imo’s playmate, who did so in October. Imo’s mother and another male peer began to wash in January 1954. In subsequent years (1955 and 1956), three of Imo’s lineage (younger brother, elder sister, and niece) and four animals from other lineages (two were a year younger and two were a year older than Imo) started to do so. Thus, with the exception of her mother, all the individuals that learned potato washing quickly were either peers or young close relatives of Imo …After 1959, features of information transfer changed. Sweet potato washing was no longer a new mode of behavior: when infants were born, they found most of their mothers and elders washing potatoes and learned this behavior from them as they learned the group’s usual food repertoire. Infants are taken to the edge of the water during the period when they are dependent on mothers’ milk. While their mothers wash potatoes, infants watch carefully and put into their mouths pieces of potatoes that mothers drop in the water. Most of the infants acquire potato washing around 1 to 2.5 years old …[I]n the second period (1959-present, the period of “precultural propagation”), acquisition of potato washing occurred independent of sex and age. During the second period, virtually all individuals … acquired this habit through their mothers or playmates when they were infants or juveniles.

But there was still the problem of sandy wheat—until Imo’s second epiphany:In 1956, when Imo was 4 years old, she took a handful of mixed wheat and sand to the brook. When it was dropped on the water, the sand sank and the floating wheat could be skimmed off the water’s surface, now clean again. This “placer-mining” technique* was also adopted by some of the other monkeys, and soon more and more animals learned it …Compared to potato washing, placer mining was quite slow to propagate …Placer mining appears to require more understanding of complex relations between objects and may be particularly difficult to learn because a monkey must “discard” his food first, while in potato washing he can keep the potato from the beginning to the end.¹³

Imo was a primate genius, an Archimedes or an Edison among the macaques. Her inventions spread slowly; macaque society, like traditional human societies, is very conservative. Perhaps the fact that she came from a high-ranking family in a species given to hereditary matriarchy aided acceptance. As is usually true, adult males were the slowest to catch on, obstinate to the last; a female invented the process, other females copied her, and then it was taken up by youngsters of both sexes. Eventually, infants learned it at their mother’s knee. The reluctance of the adult males must tell us something. They are fiercely competitive and hierarchy-ridden. They are not much given to friendships or even to alliances. Perhaps they felt impending humiliation—if they were to imitate Imo, they would be following her lead, becoming in some sense subservient to her, and thereby losing dominance status. They would rather eat sand.

No other group of macaques anywhere in the world is known to have made such inventions. By 1962, it is true, macaques on other islands and the mainland, recently provisioned with potatoes, began washing their food before eating it. But it is unclear whether this was due to independent invention or to cultural diffusion: In 1960, for example, Jugo—a macaque who had become adept at washing potatoes—swam from Koshima to a nearby island where he stayed for four years and may have trained the resident macaques.¹⁴ Perhaps there were other macaque Archimedes; perhaps not. Imo is the only one we know for certain.

It took a generation for these two obviously useful inventions to become widely accepted.¹⁵ The conservative, near immobility of popular prejudice, the reluctance to adopt a new practice even if its advantages are clear, is a tendency not restricted to Japanese macaques.

¹⁶ Perhaps the stolidity of the adult males is partly a matter of learning abilities declining with age. Human teenagers seem so much more adept than their parents at, say, operating a personal computer or programming a videocassette recorder. But this doesn’t explain why adult female macaques learned so much more readily than their male counterparts.

We can see how such inventions made in different, nearly isolated, groups can lead to cultural differentiation even in monkeys. A much more innovative species of primate, in which various groups are in occasional contact, conflict, or competition, might, we would guess, devise spectacular new forms of culture and technology.

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