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.
Müller and his colleagues think it's unlikely there'd be an evolutionary reason for female and male dogs to have different visual skills. But psychologist and dog expert Stanley Coren, a professor emeritus at the University of British Columbia in Canada, disagrees. "Whenever you find sex differences, you can usually find an evolutionary reason as to why these things occur," he says. He speculates that females might need to rely on sight more when keeping track of a litter of puppies, which pretty much all smell the same. Or maybe there's some kind of trade-off for males. Males are more scent-oriented—people prefer them over females for tasks that require trailing and tracking—so they may pay less attention to visual differences, he says.
To test whether females are more reliant on vision because they need to track their puppies, researchers could try the experiment on female dogs that are pregnant or have new litters of puppies, to see if they're even more attentive, says psychologist Emma Collier-Baker of the University of Queensland in Australia.
Müller and his colleagues aren't pursuing the difference between the sexes. Instead, they're trying to learn whether dogs get better at understanding space if they're given educational toys when they're 2 months old. The results could show whether dogs' shortcomings in physical understanding are ingrained or have to do with the environments they grow up in.
Müller can't try this out on his own pooch, because he doesn't have one. He figures it wouldn't be fair for his dog to wait alone at home all day while he goes to work and plays with other people's puppies.
*This item has been corrected. This article originally stated that Müller and his colleagues are studying dogs' shortcomings in spatial understanding. They are actually studying physical understanding.