In a study of baby vervet monkeys (a type of monkey I refuse to Google, because I misread them as "velvet monkeys" the first time and now have a wonderful image in my head that I never want to lose), scientists found that when offered trucks or dolls to play with, female monkeys tended to prefer the dolls, while the male monkeys (to a slightly lesser degree) tended to prefer the trucks. Similarly, in the wild, baby female Kanyawara chimpanzees (a monkey I refuse to Google because I don't want to know I'm pronouncing it wrong) have been observed building dolls and pretending to raise them -- and while males occasionally indulged in this play as well, it was far more rare.
Even though this study is generally used to argue that boy-girl toy divisions are natural, to me the far crazier point is how the fuck does a boy monkey know that trucks are stereotypically manly? He's never seen Mad Max: Fury Road, right? Are we showing that movie to monkeys now? If we are I'm shockingly okay with it, but I guess I just thought I'd be among the first to know.
Actually a bad example because Charlize Theron drives the truck
in that movie, but you get my point.
The answer isn't clear, but it probably has something to do with what other studies have found: In humans, baby boys prefer looking at mechanical motion (gears, wheels, rigid shapes), while baby girls prefer looking at biological motion. A boy toddler will stare at a video of moving cars, while baby girls will stare longer at videos of moving faces.