The simplest way to answer the question is probably just to follow the animal around at a suitable distance and watch what it does. In the case of large, visible animals such as lions or moose, this is likely an effective, if somewhat time consuming, method. But it works rather less well with something small or difficult to observe, especially since small mammals spend most of their time eating. (They have to, to maintain their body temperature, since small objects lose heat more rapidly, and they need, proportionally speaking, a lot of calories).
A second method would be to open the animal up and see what's in its stomach. This has obvious disadvantages, not least for the animal, and is doubly a problem if the creature is endangered. We can also infer what the animal eats from the shape of its teeth, and even the exact patterns of wear and microscopic pitting on their tooth enamel. This is usually the best you're going to manage with an extinct species, but it's hardly very specific, or the most accurate method for something that's actually still alive.
So, instead, what you generally do is examine its poo. Collect samples of dung from various different habitats in which your animal lives over a reasonable period of time, and you can tell not only what it's been eating, but how that varies depending on the local environment, or the time of year. Does it only eat one thing, for instance, or does it eat more or less whatever is in front of it?
But even this does have some limitations. Because, of course, by the time he food has come out of the other end of the animal, its no longer the same as when it went in. For instance, you might have bits of bone, hard seeds, or, if the animal is small enough, bits of insect carapace. These can give you a reasonable idea of what the animal eats, but it's not always terribly precise. So I might be able to tell that an animal has been eating beetles, but it might be a lot harder to tell exactly what kind of beetle it has been eating. And if the animals eats, say, earthworms or leaves, that don't have any significant hard parts, it might be difficult to get any evidence of that at all.
The solution, at least in modern times, is probably clear: genetically analyse the dung. Since about the early 2000s, the standard way of doing this has been "DNA barcoding". This has nothing to do with actual barcoding, and is instead a means of examining a standard stretch of DNA that varies subtly between species to narrow down what sort of organism it originally came from. In the case of animals, the stretch in question is usually the COX-1 gene, which is responsible for a vital step in the biochemical pathway that makes it possible to breath oxygen - as such, absolutely every animal has the gene in some form or another. (As do plants, although, in practice, different genes are used to "barcode" those).
Pyrenean desmans (Galemys pyrenaicus) are one of only two living species of desman. These are odd-looking, long-snouted, members of the mole family, but, unlike their relatives, they live and feed in rivers, not underground - although they do dig burrows to sleep in. They are found, as their name suggests, in the Pyrenean Mountains between France and Spain, but also across north-western Spain and into northern Portugal. In the latter country, they are known, appropriately enough, as "water moles", but France and Spain most commonly use variants of the English name.
These are not the most thoroughly studied of European mammals, but one thing we do know is that their numbers are in decline, especially in Spain. There are enough of them not to be formally listed as an endangered species, although they are considered "vulnerable", and, while they are legally protected in all three of the countries in which they live, if the present decline in their population continues, they may yet slide into that status (as the only other living desman, which lives in Russia, already has). The threats to their survival come mostly from changes to the habitat, as hydroelectric dams and water reservoirs are constructed where they live. In Spain, American mink that have escaped from fur farms add to their woes, as do increasing droughts due to climate change.
Gaining a better understanding of how desmans live their lives may help to prevent these strange and virtually unique animals from becoming truly endangered. And this is where using DNA barcodes to examine their dung can come in. The textbooks state that Pyrenean desmans feed on insect larvae that live on the bottom of streams, but a new genetic study is able to go into far more detail than that.
The study was conducted over the summer on the French side of the Pyrenees, and so, as always with these things, we don't know how applicable it is to desmans living elsewhere. Nor do we know what they eat in winter, when insects will be at different stages in their life cycles. Nonetheless, we can say that the study was able to identify 156 different kinds of prey animal in the droppings of the desmns, significantly more than the 20 kinds identified in the largest previous study, which had used the traditional "examine the poo under a microscope" method.
At first sight, the details of these prey are not hugely surprising. While the detail is more fine-tuned, the new study confirms what we already knew, which is that favourite prey of Pyrenean desmans are the larvae of caddisflies, mayflies, and stoneflies, all of which live on the bottom of streams, rivers, and clear lakes. But then, these are just about the most common things where the desmans live that they could eat, and it may be significant that the rarer components of their diet varied significantly between samples collected at different sites. This may suggest that they don't eat caddisfly larvae and the like because they're particularly good at catching them, but that they'll eat pretty well any small invertebrate that they happen to come across, and, in most places, that means caddisflies.
That they may be less reliant on specific types of insect than previously thought is encouraging, but it's also noteworthy that about 8% of their diet turned out not to live underwater at all, including such things as spiders, flies, and millipedes. It's possible that at least some of these animals happened to have fallen into the water and drowned before the desman found them, but it could also be that, perhaps during times of summer drought, desmans are capable of at least temporarily switching their diet to other kinds of prey.
Perhaps further supporting the idea of desmans as opportunistic predators of tiny invertebrates, the researchers had hoped to show that the genetically different desmans at the Atlantic and Mediterranean ends of the mountain range hunted and ate different prey... but they don't. Instead, variations in their diet are more due to the precise types of streams or rivers in which they live. Streams high up in the mountains will have different insects and tiny crustaceans living in them than low altitude floodplains, and, again, the desmans adapt to what's around them.
This is all good news for desmans. To be sure, there are still threats to their existence, whether from fishermen who mistakenly think they threaten fish stocks (they don't eat fish at all, although they do eat the very occasional tadpole), from pollution, or from other unintentional human changes to their habitat. But, if the changes are small enough, this suggests that they can adapt and that, with the right kind of conservation effort we can help protect the mountain-dwelling water moles.
[Photo by David Perez, from Wikimedia Commons.]
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