Sunday 10 February 2019

Land Mines and Scent Marks

For primates such as humans, vision is our primary sense. For the majority of mammal species, however, the most important sense is probably that of smell. In addition to using this sense to scope out the natural environment around them, many mammals leave scent marks as messages to one another, marking out a territory, advertising their willingness to mate, or whatever else they wish to convey.

Much research on the nature of scent marking focuses on how and where the animal leaves these messages, or how they may differ based on sex, maturity, and so on. This is obviously important if we want to understand why they're doing it, and what it is that they're trying to 'say'. But, naturally, it's also useful to understand how other animals of the same species respond to those marks - there's no point leaving a message if everyone else is just going to ignore it.

There are clearly a number of different ways that an animal might react to finding a scent mark left by another of its kind, but one of them is countermarking - leaving another message of their own, either beside the original to complement it, or directly on top of it, typically to cover it up and assert their own dominance. With so many different mammal species in existence, and most of them using scent marking of some kind, this is potentially a large area of study. (Of course, there are exceptions; apart from humans, scent marking would be a bit pointless for, say, dolphins).

One mammal that's known to have a particularly good sense of smell is the giant pouched rat (Cricetomys spp.) recently divided into two distinct species. You might not have heard of them, but they, and their sense of smell, have proven particularly useful in recent years. That's because an organisation in Tanzania has been training them to detect landmines, something that's much cheaper than teaching dogs to do the same thing. They can also detect tuberculosis in sputum samples (I'm not sure this is really any cheaper than the standard ZN test, but it's probably more convenient for screening samples in remote rural communities) and there have been efforts to train them for other similar tasks, too.

These useful animals were long considered to be, if not 'true' rats (that is, closely related to the sort you often find down sewers), at least members of the same biological family. However, genetic studies early in the last decade showed instead that they belong to a unique grouping of rodents native to Africa, distinct from the larger and better-known lineages that led to (among other things) 'true' mice and voles. This new grouping is now generally considered a family in its own right, but, due to lack of a pre-existing common name, can only really be referred to as the "nesomyids".

There are over 60 species of nesomyid, and there's considerable variety among them, ranging from small "mice" to... well, giant pouched rats. Both the common and scientific names (the latter translates as "hamster mouse") refer to the fact that they have large cheek pouches in which to store food, while the former also implies that they are pretty damn big. In fact, they are among the world's largest "rats", being up to 45 cm (18 inches) long even before you count the tail - about 50% longer and three times heavier than the largest of sewer rats.

There has only been one previous study on scent marking in these rats (or, indeed, any nesomyids whatsoever), and that was back in 1967, as part of a broader study of the animal's behaviour. This showed that males urinate when introduced to a new location, presumably marking it as their own, and also rub their cheeks on suitable surfaces. The same study also showed that males tend to be aggressive towards one another, especially when there's a female about, so they're probably territorial and compete for mates.

Now, however, a new study has been published on scent marking behaviour in these animals, and it focuses particularly on countermarking. When brought into a new enclosure that had previously been scent marked by another male, a number of the rats responded by leaving a mark of their own close by. Often, this was by urinating on the ground or rubbing something with their cheeks, but some rats instead chose to scrape their backsides along the surface, presumably depositing scent in the process. This method had not been seen in the previous study, but it's unclear whether that is just because the 1967 study was small, or because it used what is now known to be a different species - the northern (C. gambianus) rather than southern (C. ansorgei) giant pouched rat.

There are a couple of other interesting points here, that may help to explain how these animals use their olfactory powers when they aren't sniffing out landmines. Firstly, when scraps of material soaked with another male's urine were placed in their own cages, they investigated them, but didn't bother to respond. In other words, they scent mark only when on neutral ground, not when they're at home. Perhaps their home cage is already sufficiently saturated with their own odour that they don' t think there's much point, but, for whatever reason, it seems that they only leave marks when exploring somewhere new, advertising their presence to others of their kind.

The other point is that they always left any new mark close by the existing one, but not covering it up. There is, it seems, no intention of hiding the presence of the rival male. One possible explanation for this, as seen in some voles, is that the scent degrades over time, allowing a female to determine which mark is the newer one. If the male leaving the newer mark can do so frequently, he can demonstrate dominance over his potential rival, effectively telling a female that he is a better choice for a mate, and that she should ignore the alternative if she ever meets him. There's also the possibility that, if the new mark doesn't completely obscure the older one, the female might get confused, not recognising the exact scent when she does meet its creator.

This is just one species of nesomyid (two, if we count the one in the 1967 study), and it's impossible to know how typical it is of more distantly related members of the group, such as climbing mice and African rock mice. It's also only a study on males, and, for that matter, only on males responding to the scent of other males. We'd need to include females in the study to get a more complete picture and to see how they would react.

But, for whatever reason, it seems that male pouched rats don't merely want to announce their presence, but to present others of their kind with as much information as possible about who else is in the area. We may not know what females (or other males) do with that information, but at least nobody's trying to hide the truth from them.

[Photo by Derek Keats, from Wikimedia Commons.]


  1. That sight is more important to us humans than smell is fairly obvious - I can't really imagine even a blind person disagreeing - but is there any objective measurement to tell which is the more important sense in edgier cases?

    1. I agree that it's hard to know for sure without being able to ask them, and it's subjective anyway, which is why I hedged my bets in the phrasing :) Having said that, the relative size of various brain regions can, I think, provide something of a clue. The olfactory centres are particularly large in rodents, and many other mammals - as are the areas connected with hearing in cetaceans.

    2. Thanks.

      Of course, I've only got the vaguest ideas how large the various brain centra are in humans ...