The battle of the sexes has just heated up—in dogs. A new study finds that when a ball appears to magically change size in front of their eyes, female dogs notice but males don't. The researchers aren't sure what's behind the disparity, but experts say the finding supports the idea that—in some situations—male dogs trust their noses, whereas females trust their eyes. The study, published online today in Biology Letters, didn't set out to find sex differences. Cognitive biologist Corsin Müller and his colleagues at the University of Vienna and its Clever Dog Lab wanted to find out how good dogs are at size constancy—the ability to recognize that an object shouldn't change size if it disappears for a moment. But they recruited 25 female and 25 male dogs for the study, just to be safe.
When a dog came to the lab for the test, first it got to play with two balls: one the size of a tennis ball and one that looked identical but was about the size of a cantaloupe. Then the dog and owner left the room while a researcher set up the experiment. When the dog came back, it sat in front of its owner, who was blindfolded so that his or her reactions wouldn't influence the pet. One of the balls sat to the left of a screen in front of the dog, and an experimenter, hiding behind another screen, slowly pulled the ball with transparent string. As the dog watched, the ball went behind the screen. Then the ball reappeared on the other side. But in some cases, it was replaced by the other ball, so the ball seemed to have magically shrunk or grown (see video). Overall, dogs looked at the ball longer when it seemed to change size. But when Müller analyzed sex differences, “I was quite surprised,” he says. Male dogs looked at the ball for about the same amount of time, whether or not it appeared to magically change size. But female dogs looked much longer at balls that changed size than at balls that remained the same—about twice as long, or 36 seconds on average. Müller warns that when animal cognition researchers put together their study groups, they may be missing this kind of effect if they aren't including equal numbers of male and female animals.