Monkeys do not show sex differences in toy preferences through their individual choices

As interest in evaluating sex differences in nonhuman animals grows, the finding that male and female monkeys have toy preferences that differ, and that parallel those documented in human children, has garnered significant attention and is leveraged as an argument in favor of a biological contribution for human sex differences. To date, however, only two studies have investigated sex differences in monkeys' toy preferences, both documenting that males prefer toys considered to be "masculine" (such as vehicles) and females prefer toys considered to be "feminine" (such as dolls). Monkeys in these studies were tested in their social groups, making it hard to determine if the sex differences reported reflect actual individual preferences or result from social dynamics present at the time of testing.

Here, we assessed the preferences of 14 rhesus macaques (N = 7 males; N = 7 females) who were singly tested in a choice test with a variety of toys characterized as masculine (hard non-zoomorphic wheeled toys), feminine (zoomorphic soft toys), neutral (hard non-zoomorphic toys) and ambiguous (zoomorphic or plush vehicles) based on criteria from previous studies.

Males and females showed similar preferences for neutral and "masculine" toys and preferred them (i.e., were more likely to interact with them) to "feminine" and sex-ambiguous toys. When they interacted with the toys, both males and females interacted more with neutral than with "masculine" toys. Females, but not males, interacted more with neutral and "masculine" toys than with "feminine" toys. The highest frequency of interaction for any single toy for the male monkeys was with the doll-standing is stark contrast to previous findings.

Our results contrast greatly with the previous study in rhesus monkeys, as well as findings in human children, suggesting that the previously documented sex differences are likely context dependent, and question the existence of a strong biological basis to sex differences in toy preferences.