But that's not always as simple a question to answer as one might like. If the animal is an extreme specialist, then it's not such an issue - it eats whatever it eats and that's that. But most creatures have at least some variety in their diet, and, in practice, not all of them will be eating exactly the same mix of foods. In most parts of the world, for instance, diet will change with the seasons. In temperate environments, different foods are available throughout the year, with fruits and seeds, for example, being most available in the autumn, and insects being more readily available in summer than in winter. Even in the tropics, there are usually wet seasons and dry seasons, which affect both the plant life and the creatures that feed upon it, which, in turn, affects the carnivores.
Then there are animals that inhabit different habitats across their range. Here, diet can be affected simply by where you happen to live. In turn, what, and how much, food is available can affect other aspects of the animal's behaviour. For example, it may affect how they interact with others of their kind, how big an area they need to forage in if they are to keep themselves healthy, and such things as birth rates and mortality.
Primates, being tree-dwelling animals, are most often found in forests, and warm, tropical or subtropical forests at that. Here, food is abundant, even during the dry season, and, while diet certainly does vary throughout the year for many species, it's not as if they have to deal with cold winters. Of course, there are primates that live outside the forests - grassland baboons being an obvious example - but there are even less outside the tropics. The clearest exception, and the most northerly dwelling of any non-human primate, is the Japanese macaque (Macaca fuscata).
Japanese macaques are found on three of the four major islands of Japan, as well as a number of smaller ones. Like most primates, they inhabit the forests, but forests in question vary considerably across their range. Japan has, for the most part, a subtropical climate, but the more northerly parts are considered warm temperate (the northernmost major island, Hokkaido, is colder still, but that's the one without macaques).
Furthermore, Japan is noted for being mountainous, so you may not have to go far inland before things start getting quite a bit colder. Since Japanese macaques can live anywhere from the southern shores to mountain slopes at least 1500 metres (5000 feet) in elevation, we can fairly say that they inhabit both subtropical evergreen forests and sub-alpine deciduous ones. That's more rather more variety than is typical for primates. Significantly, many of them live in areas where you can expect significant snow cover in winter.
One way to evaluate how this geographical variation affects the macaques would be to conduct studies at two different ends of the island chain, and compare them. That would undeniably be a useful start, but it would take a lot of work and organisation to set up, and it's still only two areas. To get a broader picture, we need a meta-analysis.
This means that we take a lot of published studies, conducted by other people, and compare what they have to say, building a picture that's larger, and hopefully more useful, than any one study can be on its own. Just such a meta-analysis has recently been published for Japanese macaques, comparing the monkeys at 29 different sites across the archipelago.
One of the main pitfalls with this sort of thing is, since you didn't conduct any of the primary studies yourself, they aren't necessarily directly comparable with one another. They may not even have been trying to answer the exact question that you are. In this case, for instance, we want to compare how the macaques cope as the climate and terrain varies, which means that we may have to gather that part of the information ourselves, from sources such as the Japanese Meteorological Agency.
Like most primates, Japanese macaques are primarily herbivorous, with only about 4% of their diet consisting of small vertebrates, insects, and the like. However, if the findings of this review are correct, they eat pretty much the same plant matter regardless of whether they live in subtropical evergreen or temperate deciduous forest. This is surprising, not only because you'd think that the nature of the forest that they happen to be in would be the most important factor when it comes to their diet, but because we know that these sorts of changes do occur with other animals. Baboons, for instance, eat less leaves the further north they are.
The macaques, on the other hand, while they do eat more bark and buds in the deciduous forests (both of which are low quality food for them), always focus on things like leaves, fruit, and seeds, which form about half of their food intake regardless of which sort of forest they live in. It may be that this is something they really need to stay healthy, and they can't afford to cut back on this part of their diet even if the leaves are from different kinds of plants, or it takes more effort to find juicy ones.
This does not, however, mean that they ate the same food regardless of where they lived. It just isn't the local forest type that makes the difference. The favourite food of Japanese macaques is fruit, because it's far more nutritious and easily digestible than leaves. In areas where such fruit was more common, the macaques did eat more of it, and, while such areas could be found in both forest types, large edible fruits are more common in warmer climes, so there is a pattern there.
But the biggest factor turned out to be the snow cover. Many of the areas surveyed have heavy snowfalls in winter, and fields of snow up to 2 metres (6 feet) deep are hardly uncommon. And this, it turns out, really did affect what the monkeys ate, causing them to avoid the higher quality food in favour of poorer material such as bark. The precise reasons for this aren't clear, but the authors of the review suggest a couple of possibilities.
Firstly, the snow may actually have been covering the better food, which would obviously make it harder to find. But secondly, and I suspect more significantly, the monkeys may just have found it harder to move about in such deep snow drifts. In which case, they'd have been forced to eat whatever was nearby, regardless of how nice it was.
Clearly this doesn't work very well for them, as mortality rates and general malnutrition both increase during harsh winters, but, presumably, they have little alternative. It's worth noting that, so far as we can tell, Japanese macaques haven't lived in the northern parts of Japan for very long. They first reached the islands during the Pleistocene, but initially stuck to the south, only heading north when the Ice Ages ended. Finding no other primates that could compete with them, they didn't have to be enormously successful to be the best at what they do - it's easy to come first when there's nobody else in the race.
If they really have lived in the high mountains for less than 100,000 years they may simply not have had time to evolve to truly cope with deep winter snow.
[Photo by Frank Gualtieri, from Wikimedia Commons.]
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