L'Eau Pour Chien
Why do dogs rub up against things that smell bad?
I'm no perfume connoisseur, but somewhere along the way, in a cheap strip mall in Brussels, my nose found a pleasurable, enigmatic scent called Eau des Baux, by the popular French chain L'Occitane. The company describes it as a "sensual and mysterious blend of cypress and incense." How fancy. I've always wanted to be sensual and mysterious, in fact, but, sadly, nature failed to endow me with either this rich blend of natural secretions or the corresponding personality attributes. So I shelled out some 34 euros, and prior to my book launch in Ohio a few months later, I entertained this olfactory illusion with a few liberal spritzes on my upper half. I must say, while I didn't feel especially erotic that night, I did smell delicious. My dog, Gulliver, would agree, since on getting a good whiff of me, he performed a shameless act of vertical scent-rubbing by grinding his cheeks and sides vigorously against my cologne-covered body.
The curious thing about this was that, earlier that same day—and in fact I'd just gotten through bathing him to wash away the mealy intestinal bits stuck in his fur—Gulliver had done the very same thing with the week-old, rotting carcass of an albino squirrel at the park. Most men might second-guess their sillage de parfum given these facts, since heaven knows we're not all turned on by a trail of festering squirrel juice, but I still liked it, myself. In any event, all of this had me wondering why it is that dogs indulge in such scent-rubbing behavior, and in response to such diverse cues as Eau des Baux and Eau de Squirrel Innards. (Note the distinction between scent-marking, which transfers the animal's signature scent to its surroundings, and scent-rubbing, which transfers environmental scents to the animal's body.) L'Occitane wasn't in business when domesticated dog brains were evolving, of course, but perhaps the general behavior has some adaptive function.
As it happens, a team of animal behaviorists led by Jenny Ryon at Dalhousie University had explored this very question of canines' penchant for perfume in 1986. Their study wasn't with dogs—to the best of my knowledge, no controlled studies on scent-rubbing with domesticated dogs have ever been done—but with a very close relative instead: the wolf (Canis lupus). The authors note that scent-rubbing is an unconditioned response in wolves, which means that it's an instinct, more or less, and they submit that wolves have been observed to luxuriate among the pungent, nostril-pinching stews of everything from detached body parts to insect repellent to rotten fruit and cigar ashes. That's quite a wide array of stinky things, some of which did not exist through most of evolutionary history.
Ryon and her coauthors' objective, however, was to conduct a controlled test of the idea that wolves will grind up on unfamiliar, strong-smelling objects as a way to obtain chemosensory information about them. This is a relatively easy hypothesis to test, because if it's correct, then simple repeated exposure to the same stimulus should lead to a decrease in rubbing. Fortunately, the authors had a few packs of wolves at their disposal in their university labs, wolves whose normal diets consisted of "raw chicken necks and backs and whole or partial carcasses of road-killed deer." (We can only assume there was no shortage of the latter on the Madonna-blaring highways of 1980s Nova Scotia.)
Here's how the experiment worked. The researchers selected four pairs of "odiferous substances" and coated 15 cm circles of clean, demarcated ground area with these substances using a sterile wooden tongue depressor. The carnivore feces pair included the dung of black bears and cougars; herbivore feces consisted of the waste products of Aoudad sheep and the Sable Island Horse; the food class sampled albacore tuna oil and commercially produced salt pork; and, finally, the manufactured category was represented by heavy-duty motor oil and a now-discontinued Max Factor perfume called "Geminesse," which, just to be clear, was redolent of a leather-floral mix atop a musky, amber base.
The wolves were released into the pen one by one, and observed "from an automobile inside their enclosure"—there's no telling how insane Geminesse might make these beasts, after all. In contradiction to their hypothesis, explain the authors, the wolves showed a marked propensity to rub on some substances and not others, with manufactured odors eliciting the strongest response. All individuals that rubbed did so most often on the perfume. Interestingly, just as Gulliver did with me, some of the wolves waiting their turn performed the same type of "vertical scent-rubbing" on the perfumed bodies of those that had just returned from the pen. Carnivore feces had them rolling around on the malodorous dirt too, but none of the other stimuli, herbivore dung and food, did the trick.
The authors argue that it's not novelty or even pungency per se that elicits scent-rubbing; some especially smelly substances, such as the sheep shit and the tuna oil, went unmolested. Rather, "[o]ur wolves showed a negative correlation between substances rubbed and substances tasted. Manufactured and carnivore odors were most frequently rubbed but rarely tasted, whereas the reverse applied to food and herbivore odors." In other words, the wolves didn't rub against the sheep shit and tuna oil; they licked or even ate it. These findings also cast considerable doubt on another theory, not mentioned by these authors but circulating in dog-behavior crowds for some time, which is that dogs scent-roll to "camouflage" their own odors. This evolutionary theory sounds good in principle: It would have helped ancestral canines thwart detection by prey animals that bolted whenever they got wind of wolf in the air. Yet, if it were true, then we wouldn't expect the wolves in the foregoing study to behave as they did, since it's hard to imagine that smelling like a bear or cougar is much better than smelling like a wolf for this purpose, and it's certainly less effective than masking predator odor with that of a harmless ungulate.
Jesse Bering is the author of The Belief Instinct and Why Is the Penis Shaped Like That? (July 2012). He is a frequent contributor to Slate and writes the "Bering in Mind" column for scientificamerican.com. His next book will be on the curiously scandalous science of human sexuality. Follow him at www.jessebering.com, on Twitter @JesseBering, or try adding him on Facebook.
Illustration by Mark Alan Stamaty. Photograph of dog by Hemera/Thinkstock.