There are basically three ways that mammals leave scent marks. There's urinating, leaving dung around, and using glands specifically evolved to create smelly secretions. Having abandoned scent marking, we humans lack proper scent glands, and the closest we get are some modified sweat glands in the armpits and groin that create a special kind of sweat that smells, instead of the usual odourless watery stuff that the rest of our skin makes. And, really, compared with proper scent glands, that's pretty pathetic.
But if you're going to leave scent marks, by whichever of these methods works for your species, just how much information do you really need to communicate? It depends to a large extent on how sophisticated your social behaviour is. For group-living animals, like dogs, quite a lot of information may be needed, to identify members of your pack, who is dominant over whom, where territorial boundaries are, and so on. At the other extreme, if an animal spends almost all of its life alone, it may be able to cut down the amount of information needed to just two possible messages: "Go away" and "I'm currently in heat."
Of course, even then, it helps to be able to glean at least something about who left the message. If nothing else, you need to be able to tell that the "go away" messages wasn't one you put there yourself a couple of days ago. And, for the sake of genetic diversity, the second message could benefit from something to the effect of "...but I'm your sister."
But, perhaps, for solitary animals, you can get away with being quite vague about this sort of thing. If somebody else has marked out territory as theirs, does it really matter who they are, so long as you know they aren't you? If I don't want a fight, why should I care who it is that I'm not fighting with?
Yet many animals that do have more complex social lives really do need a lot of this sort of information. We know that there are a great many animals that can, to at least some extent, identify an individual purely by the scent that they leave. Dogs (and wolves) are one example, obviously, but we can also add house mice, hamsters, mongooses, pronghorns, and even ring-tailed lemurs to the list, among others. Nonetheless, we'd expect this sort of discriminatory ability to be less prominent in animals that live alone.
Such as, say, the common raccoon (Procyon lotor).
While there, in fact, two other living species of raccoon, this animal is so much better known than either of them that it is generally called simply "the raccoon". And, since it's the only one of the three found in the United States, it's almost certainly the one you're thinking of when you think "raccoon", as well. Raccoons basically live alone, with each carving out their own bit of territory in which to forage for food. A male will share parts of his territory with those of neighbouring females, and vice versa, but otherwise they tend to stay apart outside of the mating season.
Unsurprisingly, we know that raccoons do scent mark their territories, and, indeed, they have scent glands on their rear ends similar to the ones that dogs so like to sniff on one another. However, it has recently become clear that males, at least, sometimes gather together more than previously thought, forming temporary coalitions. Since we know that they aren't doing so on the basis of being close relatives (brothers that had recently left their mother, say), it seems plausible that they have more ability to tell one apart than one might think. So can they do so purely on the basis of smell, or do they only recognise one another by different means?
To test the idea, researchers captured a number of wild raccoons, and took samples of their urine, dung, and gland secretions, then presented them to different wild-caught raccoons, housed in artificial dens. The attempt to use glandular secretions was a complete failure, as none of the test raccoons took a blind bit of notice of them, perhaps indicating something about how the animals would normally deposit the scent that the researchers had missed.
The raccoons did, however, investigate the boxes in which the urine and dung had been placed, as witnessed by watching them with infrared video cameras. (Raccoons being mostly nocturnal, an infrared camera is the sort that you want). Of course, all that tells us is that they pay attention to such things - place a stranger's scent sample in their habitat, and they're going to check it out. That much we already knew, or they wouldn't bother scent marking in the first place.
But, since the scent samples were in sealed boxes with small holes in them, and the raccoons couldn't actually get at the contents, eventually they're going to get bored. That is, they will become "habituated", used to the fact that a particular other raccoon has apparently come past and had a dump in a nearby box.
It's this habituation that allows us to see how good they are at recognising one another. Place a different raccoon's scent in their den later on, and what happens? If they can't tell the difference, they'll continue ignoring it, but if they know that this is a new, third, raccoon, hopefully their interest will be piqued, and they will behave differently than they will if you just present them with a new sample from a raccoon they already "know about".
So that's what the researchers tried. When they tried it with dung, there was no clear evidence that the raccoons noticed anything had changed. They may have paid a little more attention, but the difference in behaviour was too small to be sure that it wasn't just a coincidence - by sheer chance, the raccoons will spend slightly more time examining some things than others, and it may not mean anything. So, either raccoon poo just smells like raccoon poo to them, regardless of who left it, or, alternatively, they could tell the difference, but simply didn't care.
(The researchers speculate that one reason the raccoons might not care is if poo smells different all the time anyway, depending on what the animal's last meal was. In which case, yeah, this new dung smells different, but then, it would, wouldn't it?)
The urine, however, had quite a different effect. Present them with a second sample from a raccoon whose urine they have previously smelled, and they check it out, apparently decide "oh, it's him again", and go off to do something else. But if the second sample is from a stranger, the study showed that they spend about twice as long sniffing at it and acting curious. That they behave so differently towards the two samples shows quite clearly that they can tell them apart, and know that there's something different about the new one.
So we know that raccoons can tell one another apart just by the way that their urine smells. It's a difference in smell that's quite beyond the ability of the human nose to detect, but the researchers also took samples from nine different raccoons, selected and random, and analysed them with a gas chromatograph. Each was identifiably unique, with some volatile chemicals being present in different quantities in each animal, and as many as half of the 51 chemicals the researchers identified being missing altogether in one or more of the samples. So there's different stuff in there, and the stuff that is the same may be present in different amounts.
Nine animals, nine unique signatures. We may need complex equipment to tell us that, but all a raccoon needs is his or her nose.
[Photo by D. Gordon E. Robertson, from Wikimedia Commons]
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