We all know that little girls tend to play more with dolls and little boys with trucks and soldiers. “Blank-slaters” have long attributed this difference in behavior to socialization rather than innate genetic preferences (or a combination of the two). A new paper in Current Biology by Sonya Kahlenberg and Richard Wrangham (Sex differences in chimpanzees’ use of sticks as play objects resemble those of children), suggests, at least according to the authors, that the behavior could be partially hard-wired with some evolutionary roots. This is based on the observation that, at Kibale National Park in Uganda, female chimps play with sticks more often than do males, and in a way that suggests what the authors call “play-mothering”: the sticks are surrogate children in whom females take a greater interest.
Over 14 years of observation at the park, the authors observed juvenile chimps using sticks in several ways. They’re used as “weapons” in aggressive encounters, as solitary play objects, and during the well-known phenomenon of “probing”: chimps stick twigs into holes in search of either water or honey. But the authors were more interested in “stick-carrying” behavior, which the authors describe thusly:
Stick-carrying consisted of holding or cradling detached sticks (median length, 36 cm; median weight, 112 g; n = 6 recovered sticks). The juveniles carried pieces of bark, small logs or woody vine, with their hand or mouth, underarm or, most commonly, tucked between the abdomen and thigh.
Here’s a photo, by Sonya Kahlenberg, of a 9-year-old female carrying a big stick in the “groin pocket”:
This behavior was observed significantly more often in females than in males, peaking at about 5 years of age in males and 7 in females, and dropping off to nearly zero behaviors observed after age 11. This isn’t explained by a generally higher use of object by females, since weapon use and the use of leaves to wipe one’s body are in fact male-biased behaviors. Probing, however, is seen more often in females than in males. But the authors claim that probing isn’t correlated with stick-carrying, because females that probe more often don’t show a correlated increase in stick-carrying, nor was there an overlap between the size of probe sticks versus “carried” sticks.
I’m not fully convinced by this special pleading. First, the sample size was small (10 individuals), and unless the correlation is really high you wouldn’t observe it in such a small sample. More important, perhaps females just tend to have a penchant for carrying sticks in general, and simply differ in their preference for sticks of different sizes. If males like candy more than females, but some males prefer Skittles over Twizzlers, and vice versa, then it’s possible that overall you’d see males eating candy more than females, but individual males who ate more Skittles wouldn’t necessarily also eat more Twizzlers.
Nevertheless, the authors suggest that this is a sex-specific difference in “play mothering” for several reasons:
- Females carry sticks only before they give birth; the behavior isn’t seen after motherhood.
- Unlike other sticks, carried sticks are taken into “day nests” and fondled in a way that suggests “maternal play” to the authors.
- Young chimps reared by humans also show similar behaviors that sometimes strongly resemble the behaviors of mothers carrying infants.
The authors suggest, then, that this “maternal” stick-carrying is hard-wired in chimps, so that females are genetically predisposed to treat sticks like infants. If this is true, it could either be directly adaptive (practice for motherhood), or simply an early and nonadaptive expression of behaviors that become adaptive after motherhood.
The behavior can’t be completely hard-wired, though, because stick-carrying isn’t seen in chimps outside of the Kanyawara park. It appears to be socially learned from other juveniles. The idea, then, is that individuals learn to carry sticks in a specific “maternal” way, but that females have a greater propensity to imitate this behavior. It’s a behavior that involves gene/environment interaction.
The authors conclude that differences in male and female humans in their propensity to use toys may also have this genetic/evolutionary component:
The sex difference in stick-carrying in juvenile Kanyawara chimpanzees arises without any teaching by adults and is consistent with practice for adult roles. Our findings suggest that a similar sex difference could have occurred in the human and pre-human lineage at least since our common ancestry with chimpanzees, well before direct socialization became an important influence.
This is intriguing, and goes along with some other evidence for a genetic component in humans. For example, the authors note that girls who have been exposed to high levels of androgen in utero make more masculine toy choices (I’m not familiar enough with this work to know if there’s potential socialization bias based on appearance). But, though intriguing, the data are too scanty to make a really convincing case. This is not because of a lack of effort, for these data were collected over 14 years of observation in Uganda. It’s just tough to collect a huge number of rare behaviors in a wild, free-roaming primate.
Kahlenberg, S. M. and R. W. Wrangham. 2010. Sex differences in chimpanzees’ use of sticks as play objects resemble those of children. Current Biology 20:R1067-R1068. (doi:10.1016/j.cub.2010.11.024)