It's thought of as a sexual stereotype: boys tend to play with toy cars and diggers, while girls like dolls. But male monkeys, suggests research, are no different.
This could mean that males, whether human or monkey, have a biological predisposition to certain toys, says Kim Wallen, a psychologist at Yerkes National Primate Research Center in Atlanta, Georgia.
Wallen's team looked at 11 male and 23 female rhesus monkeys. In general the males preferred to play with wheeled toys, such as dumper trucks, over plush dolls, while female monkeys played with both kinds of toys.
This conclusion may upset those psychologists who insist that sex differences – for example the tendency of boys to favour toy soldiers and girls to prefer dolls – depend on social factors, not innate differences.
"A five-year-old boy whose compatriots discover has a collection of Barbies is likely to take a lot of flak," Wallen says.
Social factors undoubtedly influence children's preferences, he says, but in general boys tend to be pickier with toys than girls.
To try and tease out the effects of nature over those of nurture, Wallen and his colleagues studied a group of captive rhesus monkeys. His team reasoned that the choices of the monkeys wouldn't be determined by social pressures. Most of the study animals were juvenile (age one to four years), but some sub-adult and adult monkeys were included.
"They are not subject to advertising. They are not subject to parental encouragement, they are not subject to peer chastisement," Wallen says.
Wallen's team offered the monkeys two categories of toys: "wheeled" and "plush". The wheeled toys, intended to be masculine, included wagons and vehicles. The more feminine plush toys included Winnie the Pooh and Raggedy-Ann dolls.
Two toys, one wheeled and one plush, were placed 10 metres apart. At first the monkeys formed a circle around a toy, but eventually one would snatch the toy and run off. Other monkeys soon joined in the fun, Wallen says.
The researchers captured play sessions on video and measured how long each monkey spent with plush versus wheeled toys. The team found that the males spent more time playing with wheeled toys, while the females played with both plush and wheeled toys equally.
Wallen cautions against over-interpreting the results. The plush and wheeled categories served as proxies for feminine and masculine, but other toy characteristics, such as size or colour, might explain the male's behaviour, he says. Or the male monkeys might seek out more physically active toys, he says.
But the study ties in with a previous experiment with green vervet monkeys showing that males favour masculine toys.
"Together the results are compelling," says Gerianne Alexander, a psychologist at Texas A&M University in College Station, who led the vervet monkey study.
She thinks that biological differences between sexes start the ball rolling toward learned preferences for play toys.
"There is likely to be a biological tendency that is amplified by society," she says.