|Southern long-nosed bat|
Having said that, humans can't really be considered to have scent glands in the sense that some other animals do, and our sense of smell is pretty rubbish compared with most non-primate mammals. We do not, as a general rule, communicate by wafting our smelly armpits at one another. But many other mammals do something that isn't quite so far from that, and in a variety of different ways.
Bats are no exception to this general rule. Many bat species have scent glands, often located on the throat or the upper part of the chest, and they can leave scent marks behind to communicate with others of their kind - often signalling something like sexual availability. By no means all bats have fully formed scent glands, but that doesn't mean that the others don't use scent in some way.
The reality is, of course, that there are a great many bat species about which we know very little indeed. Bats are the second largest order of mammals, after the rodents, and we are still discovering new species every year. Thus, it really shouldn't be much of a surprise that we only discovered the scent-producing habits of lesser and southern long-nosed bats (Leptonycteris yerbabuenae and L. curasoae) within the last decade.
The lesser long-nosed bat lives primarily in Mexico, but migrates north to the scrublands of southern California, Arizona, and New Mexico during the summer, and south to Central America during the winter. The other species lives in northern Colombia and Venezeula, and both kinds of bat prefer dry, scrubby habitats, not least because the great bulk of their diet comes from drinking cactus nectar. In fact, they are major pollinators of cactuses, and, while they are no longer common, and are both currently considered "threatened" species, they are important parts of their local ecosystems.
In 2008, it was reported that the males of these species often had a patch of hair between their shoulder blades that was covered in a smelly, greasy substance. Examination of the skin beneath these patches revealed that it had an unusual density of sebaceous glands, and that the glands were unusually active when the patch was present - which only occurred at certain times of the year.
Sebaceous glands are found in almost all mammal species (dolphins and the like being an exception) and secrete an oily substance the primary function of which appears to be water-proofing the skin. However, the secretion can also carry out other functions, including the production of scent. This is even true in humans, where modified sebaceous glands around the nipple produce a subtle scent during lactation that attracts newborn babies to the source of milk.
So the idea that it was these glands that produced the scent in the bats was not unreasonable. However, a further investigation, by different researchers, in 2009, revealed that what's going on is not as simple as it first appeared. This study seemed to show that, while the sebaceous glands under the patch are indeed more active, they aren't the primary source of the smell. Instead, the bats are actually smearing body fluids, such as saliva and urine, onto their fur in this patch.
A follow-up in 2011 showed that, as might be expected, given that this smearing behaviour is only found in males, and only at certain times of the year, the scent so produced attracts females, and presumably indicates willingness and fitness to mate. The bats in question are highly promiscuous, with the females mating with any male that feels like it - that is, males don't form harems, or defend their territory from rivals, or anything like that - so this may be quite a useful feature. Nonetheless, it is an unusual behaviour, one that, so far as we know, is not found in any other bat species.
Or is it?
The reason that this question arises is that there, in fact, three species in the genus Leptonycteris. The third one is the greater long-nosed bat (L. nivalis), and it, too, is native to Mexico, along with extreme southern Texas. This bat is much rarer than the other two species, which probably explains why it wasn't studied together with them. Overall, its population seems to be declining, although the size of its roosting colonies varies so much from year to year that it's difficult to know for sure. For example, the only known colony in the US, at a cave near Emory Peak in the Big Bend National Park, was completely abandoned by the bats in 1992, despite populations of over two thousand in both 1991 and 1993. Nonetheless, the overall pattern is strong enough that the greater long-nosed bat was listed as an endangered species in 1996, and there is no sign that this status will be lifted any time soon.
Still, it's not as if these bats haven't been studied at all (we'd know even less about their population size were that the case). But, if they engage in the same unusual "smearing behaviour that their two close relatives evidently do, nobody seems to have noticed. However, since the behaviour is a fairly recent discovery even in the other two species, there's no particular reason that anyone would even have thought to look. So, to check, a study specifically designed to look for the behaviour was conducted at a breeding roost in Mexico between 2010 to 2015.
And... well, actually, no they don't.
Interestingly, this does give us a pretty good idea of when the bats first took up this behaviour. That's because we already have genetic evidence that the lesser and southern species are more closely related to each other than to the greater species. Moreover, using our best estimate of how fast genes change in these species, we can say that the last common ancestor of the three species lived around a million years ago, and the two closely related species separated about 540,000 years ago. This strongly implies that the smearing behaviour evolved between these two dates.
The question then becomes, why did such a unique behaviour arise in the ancestor of these two species, and yet not in their closest relative? Here, unfortunately, we have to enter the realm of speculation. One thing we do know, however, is that bats with patches of smeared body fluids between their shoulder blades tend to be less infested with blood-sucking bat-flies.
These, technically known as streblid flies, are relatives ofthe better-known tsetse flies. They are unusual in a number of ways, being nearly blind, and often (though not always) having limited powers of flight, if they even have wings at all. They also spend their entire larval life inside their mother's body, who eventually "lays" a pupa, rather than an egg, which "hatches" as a new adult. Significantly, though, they live only on bats, being completely unable to survive without them. So far as we can tell, they are quite significant parasites, and, while it's hard to disentangle cause and effect, it seems more likely that, in this case, the flies limit the production of the smears, rather than the smears driving off the flies. But either way, the presence of the smelly patches could well indicate to potential mates that the male in question is relatively fit and free from parasites.
If so, it might be that the ancestors of the greater long-nosed bats had less of a problem with bat-flies than their cousins elsewhere. That's pretty much impossible to know, and it could all be a coincidence, so long as the two types of bat were already breeding in different locations by the time that one of them developed the smearing behaviour. At some later point this ancestral "smearing" species, presumably influenced by the changing climate of the Ice Ages, spread into new ranges, and split into the two, geographically separate species that practice the behaviour today.
But the females of the greater long-nosed bat, despite living in broadly the same area (modern-day Mexico) as one of those new species, could well have been put off by the weird smell emanating from the fittest of their males. Even when the opportunity to cross-breed arose, it evidently didn't happen... and smearing themselves with pee may well have been part of the reason for that.
[Photo by the US National Park Service, in the public domain.]