Back in April of this year, after being utterly revolted by our previously pristine diva-of-a-puppy rolling ecstatically in god-knows-what-filth she found on the ground, we did some research into why (…oh WHY?!) dogs find the grossest, smelliest, gooiest things on the ground apparently irresistible. We didn’t get very far. Basically, the upshot was: no one knows, but there are some theories, the dominant one of which is that it’s a holdover instinct from their wolf ancestors, who would roll in carcasses for unknown reasons. (Note that this behavior isn’t unique to wolves and dogs either, evidenced by the photo above… yes, that’s a bear, rolling in bison urine.)  As to WHY animals roll in disgusting and dead things, most think it’s due to one or more of the following reasons:

1) To learn about the substance

2) To cover up their own scent (for purposes of predation)

3) To communicate to others what they’ve found (note that this negates theory #2)

and/or

4) To claim the carcass as their own – apparently – by wearing it.

None of these theories have been convincingly supported (though all of them sound fairly plausible, if you’re thinking like a wolf).

To date, there are still no known similar studies in domestic dogs, unfortunately. However, more research has unearthed (pun completely intended!) two fairly comprehensive, and complementary, studies conducted on “scent-rubbing” behavior in wolves and hyenas (two species that are closely related to domestic dogs). Note that “scent rubbing” is different from “scent-marking,” which transfers the animal’s individual scent to the environment, most effectively accomplished by  urinating and defecating, but also by rubbing up against things.  A study of wolves and another study in hyenas showed that scent-rubbing is an “unconditioned response”, which means that it’s an instinct, simple as that.  As to what their scent preferences are? They range from the moderately yucky (cigarette ashes) to increasingly unpalatable (rotten fruit) to downright nauseating (decomposing animals).

More interesting, and new to note from these studies, are the specific preferences the animals exhibited -  for type of odor, behavioral response to the odor, and also… lack of habituation to certain scents. This means that one theory can be discounted: scent-rubbing has likely NOT evolved to serve the purpose of obtaining specific information about the thing that’s being rubbed upon. If the purpose of scent rubbing was for the animal to learn about the stinky thing (theory #1 above), then repeated exposure to it would lead to a decrease in rubbing against it (because the information about it had already been learned). That didn’t happen.

Another theory: that dogs scent-rub to “camouflage” their own odors (theory #2 above), is also probably not the reason for rolling in yuck. This theory sounds good from an evolutionary principle: camouflaging themselves with the scent of a harmless animal would not raise suspicion in the prey animals that would otherwise run once they got whiff o’ wolf.

How did this theory get discounted? Well, the researchers carefully selected four different kinds of odoriferous substances that they “coated the ground” with. In four separate areas of a field, the researchers spread four different types of nastiness: 1) carnivore feces (black bear and cougar scat), 2) herbivore feces (sheep and horse feces), 3) smelly food (tuna oil and salt pork), and 4) ‘manufactured scents’ (motor oil and a cheap but very strong perfume). The wolves were let loose and their behavior around each of these substances was carefully observed and noted.

In contradiction to what the researchers originally thought would happen, the wolves showed a consistently strong preference for rubbing themselves into some substances and not to others, with manufactured odors (the perfume and the motor oil) eliciting the strongest “rolling” response. Interesting. Can’t say I saw that one coming from an evolutionary point of view, given that these substances aren’t even natural. However, this response might in fact help substantiate theory #1 – learning about a new or novel substance introduced to the environment. Given that such human-made substances wouldn’t naturally show up in the wild, it seems very plausible that an animal would want to gather as much information as it could about the stimulus. However, just to be contradictory, this thinking is just as easily dismissed, because the wolves didn’t show a decreased preference to rub against these substances even after they had done so previously (ostensibly after they had already gathered all of the information they could).

However, carnivore feces had them rolling in ecstasy too, but, oddly, not herbivore feces. And the wolves weren’t interested in rolling in the  food products either, although they were happy to lick at or even eat the tuna oil and salted pork (the study proves one thing: even wolves love bacon).

If the “camouflaging for purposes of predation” hypothesis were true, then wouldn’t it be much better for a wolf to disguise itself in sheep’s clothing (or poo) to sneak up on a sheep, rather, than, oh, a predator equal to or worse than itself: i.e., bear or cougar?! This would suggest that the wolf would prefer the herbivore feces to carnivore feces, but the opposite happened.

The scientists behind the hyena study, on the other hand, actually showed really good evidence of theory #3 – that rolling around in rotten things serves  an important social function: the information is contained in the odor would be conveyed from one hyena to another, when they greeted each other. Similar to dogs, hyenas greet each other by mutually sniffing and inspecting each other’s face, neck, mouth, and head region, as well as the all-important “flank and behind” area.

The researchers took this information and performed a simple experiment and observed the results. They took one group of hyenas and doused them down with “eau de carrion” and introduced them to another group of normal hyenas, and compared the results with introductions between hyenas who had no scents (or a neutral non-animal based scent) forced upon them. The hyenas perfumed with carcass-odor received significantly more attention than did the neutrally-scented or non-odor-donning hyenas. In fact, the hyenas “wearing” the eau-de-carcass received more positive social interactions and receptiveness from the other hyenas.

The researchers of the wolf study did not take their observations to this level, that is, to re-introduce the odor-wearing wolves back into their non-odor-wearing brethren and observe the results in comparison to non-odoriferous wolves, though it is likely that they would see similar reactions between the group, substantiating the behavioral outcome across at least two species.

All of these results suggest that rolling around in disgusting muck serves probably no other purpose than to convey interesting information back to the pack, and perhaps, elevate the carrion-bearer’s status a bit within the group at least for a time, since the wearer probably does, in fact, own it (the carcass, that is). For those of us human members of our dogs’ “pack,” we receive the information they return to us with great disgust, outrage, and aggravation, much to their (likely) confusion.

Regardless of the reasons, the outcome is clear: Into the bath we go — it’s time for another earthbath!

Sources:

Drea CM, Vignieri SN, Cunningham SB, Glickman SE. (2002) Responses to olfactory stimuli in spotted hyenas (Crocuta crocuta): I. Investigation of environmental odors and the function of rolling. Journal of Comparative Psychology, 116(4):331-41.

Ryon, J., Fentress, J.C., Harrington,  F. H., Bragdon, S. (1986). Scent rubbing in wolves (Canis lupus): the effect of novelty. Canadian Journal of Zoology, 64:(3) 573-577.

*Photo credit: The Wild Photographer: Larry Thorngren