There's more to a pair of rat noses than meets the eye. Like tiny, leashless dogs, rats like to sniff each other all over when they meet. Yet not all of this sniffing is aimed at gathering scents. Some of it seems to transmit messages such as "I'm in charge" or "Be cool" or "Please don't bite my face."
Rats and other animals give off odors from the "face, flanks, and anogenital region," says neuroscientist Daniel Wesson of Case Western Reserve University. So it's not surprising that these regions are where rats aim their sniffers when they cross paths. To find out whether there might be more going on, though, Wesson outfitted rats with head-mounted devices that measured the speed of their sniffs. Then, after recording videos of these rats encountering each other, he looked at how sniff frequency lined up with different stages of the rodents' interaction.
He saw that all rats sped up their sniffing when their noses were pointed at each other's flanks or rear ends. But when the rats were sniffing each other's faces, their behavior depended on whether they were socially dominant or subordinate. Higher-ranking rats sped up their sniffing as usual. Lower-ranking rats slowed down their own sniffing in response.
This seemed to be an "appeasement signal," akin to climbing into one's own locker when the school bully approaches. Wesson found that when subordinate rats didn't give this signal—when they kept up their sniffing at the usual rate—dominant rats were quicker to pick a fight.
To further test this idea, Wesson treated the insides of the rats' noses with zinc sulfate, making them temporarily lose their sense of smell. Even though they weren't gathering any odors, rats kept on sniffing. And when they were face-to-face, they acted the same as always: dominant rats sniffed faster, while subordinate ones slowed down to avoid trouble. "This sniffing behavior was interestingly resilient," Wesson says.
Sniffing seems to be a form of communication for rats—but only sniffing in the face, not other body parts. Wesson says this may be because face sniffing is an especially vulnerable position for a rat or other animal to be in. When their eyeballs and whiskers and biting parts are all in close proximity, maybe it's a good time for rats to make clear that they don't want a fight.
Alternately, face-to-face might be the only way a rat can detect another rat's sniffing; maybe the signal wouldn't get through if it were aimed at the tail end. "These are different theories we are testing now," Wesson says. There may also be ultrasonic squeaks or other signals invisible to humans that contribute to the conversation between two rats.
If rats use sniffing for communication, and not only for gathering smells, do other social sniffers do the same thing? "I would predict so," Wesson says. "Other rodents likely use this behavior, as could possibly cats and dogs." He points out that neighborhood dogs who meet on a walk will sniff each other, then either part peacefully or start fighting. Some signal in their sniffing behavior may make the difference, though this idea would have to be tested.
That's not to say dogs or rats aren't also gathering actual smells when they sniff. It would be "frankly silly" to discount the importance of smell in an animal's life, Wesson says. It seems there's much more going on, though, when an animal sticks its nose into the world.
Wesson, D. (2013). Sniffing Behavior Communicates Social Hierarchy Current Biology DOI: 10.1016/j.cub.2013.02.012
Image: Daniel Wesson.