Researchers in Austria have discovered that females are much better than males at noticing when a tennis ball rolling behind a wall appears to change size. In fact, males don't seem to notice at all. Don't worry, guys--I'm talking about dogs.
Corsin Müller and his colleagues studied 25 male dogs and 25 female dogs. Each dog was led into a room by its owner and allowed to play with a large and a small blue tennis ball; this let the dogs get familiar with the objects in the experiment. Then the owners walked the dogs out of the room and back in again. Owners sat in a chair--blindfolded, so as not to give any inadvertent clues to their pets--and sat their dogs between their knees. Meanwhile, a hidden experimenter watched the dog with a camera. When the dog was calmly looking in the direction of a low wall, the experiment began. The experimenter tugged on hidden strings so that a blue tennis ball rolled behind the wall. A moment later, a second ball rolled out from the opposite side of the wall. For half the dogs, the second ball was the same size as the first. For the other half, it was different (either a big ball changing to a small ball or vice versa). The male and female dogs were evenly divided between the same-size and different-size groups.
As the second ball rolled out, cameras around the room recorded the dog's reaction. The experiment used a principle that's common in psychology experiments done with baby humans. If a baby sees something it doesn't expect to see, the assumption goes, the baby will stare at that object for longer that it would otherwise. Infants learn "size constancy," the rule that things should stay the same size from one moment to the next, during the first year of their lives. If they see an object appear to change size, they stare at it.
Since dogs are kind of like infants, I guess, the principle can be transferred. So the researchers studied videos of the dogs' reactions to see how long they stared at the second blue ball. The result was striking: Male dogs looked at the ball for the same amount of time, no matter what. Size constancy, schmize constancy. But female dogs stared for significantly longer when the ball appeared to change size.
(I won't tell anyone if you want to take a couple seconds and secretly cheer for the female dogs. But be aware that a girl dog is going to look dumb later on in this story.)
Why might this be? It's always tempting to invoke an evolutionary explanation. A scientist interviewed by ScienceNOW (and not involved in the study) takes the bait, suggesting that female dogs have evolved to pay more attention to visual cues so they can keep track of their puppies. The problem with such an explanation is that you can never really know whether it's true. And in the case of human sex differences--say, a difference in innate math ability that hasn't been convincingly shown to exist--some people use them as excuses to cling to outmoded generalizations such as "men like things, women like people." (Yes, I'm talking about JohnTierney.)
Müller doesn't speculate about an evolutionary basis for the difference he found between male and female dogs. Instead, he writes that the most likely explanation is a "by-product of other sex differences." In other words, male and female brains are sculpted differently by the hormones they receive during development, and affected differently by hormones throughout life--but not all of these differences have to be evolutionary adaptations.
The uninvolved dog expert in the ScienceNOW story does add, interestingly, that male dogs tend to be more scent oriented than female dogs, and are preferred for tracking. Could it be that male dogs, for whatever reason, depend more on their noses, while females depend more on their eyes?
One dog doesn't make for much of a sample size, but can still be (I think) an entertaining example: In this MythBusters video, the two hosts of the show wear highly detailed masks of each other's faces. They also swap clothes. Host Jamie Hyneman's dog, who is trained to run to her owner when asked, "Where's Jamie?" demonstrates her trick--and runs straight to Adam, who's wearing the Jamie mask. Pretty embarrassing for an animal with such a sensitive nose. To her credit, she then turns around, shakes herself and barks. "She's very confused," Adam observes. Tennis balls changing size is one thing, but humans changing faces is a challenge she's not equipped for.