An answer to the Explainer's 2010 Question of the Year.

It turns out that young girls who were exposed in utero to high levels of androgens—a condition called congenital adrenal hyperplasia—end up showing a preference for boyish objects, even when their parents urge them in other directions. And a recent study at the Yerkes Primate Research Center found that juvenile rhesus monkeys show the same sex-related preferences as human children when offered a choice between plastic wagons and Raggedy-Ann dolls. (The boy monkeys played with the wagons; the girls went either way.) That suggests culture isn't the whole story: To some extent, at least, boys are naturally drawn to certain objects. They do indeed have a "knack" for playing with trucks.

OK, back to sticks. If sticks happen to fall in the same category as "masculine" toys like toy trucks and fake weapons, then there's good reason to believe that a boy would be drawn to them from early infancy. Which raises the question: Which aspects of a stick—or any other play object—make it more boyish or less girly?

There are plenty of theories. Some have argued that color plays a part—that girls are naturally drawn to pink objects, for example. Or that boys prefer sharp lines, while girls like rounder things. (In art class, boys tend to draw more angular shapes.) Or maybe boys like harder objects, while girls prefer softer ones. Recent research suggests that such tendencies develop relatively late: Among 1- and 2-year-olds, at least, both boys and girls seem to prefer reddish colors and rounded shapes. Less is known about which qualities might be most relevant to infants.

In any case, a look at the animal kingdom reveals that there's more than one way to play with a stick: A new study, published in December, looked at a wild group of chimpanzees in Uganda and found that the young females were using sticks as dolls. That is to say, they were holding the sticks while they slept and patting them like babies. (The chimps also used sticks to hit each other, and poke into holes looking for water or honey.)

This behavior has never before been seen in the wild and appears to be unusual among non-human primates. (The study's conclusions were also based on a rather sparse data-set collected over a 14-year period.) Girl rhesus monkeys, for example, don't bother with pretend-stick-babies: They just kidnap real babies from lower-ranked females in the group and play with those. And primates aren't the only animals that play with sticks: Birds drop them and then catch them again in midflight; sardines and needlefish leap over floating twigs for no apparent reason; and both male and female porcupines masturbate using sticks.

Got a question about today's news? Ask the Explainer.

Explainer thanks Gerianne Alexander of Texas A&M University, Gordon Burghardt of the University of Tennessee, Kim Wallen of Emory University, and Ed Yong of Not Exactly Rocket Science.

Previous questions of the year:

2009: If a Siamese twin commits murder, does his brother get punished, too?

2008: What is the most disloyal dog breed?