There are many ways of determining where animals travel on a regular basis and different methods will work better for some animals than they do for others. For example, we hear a lot in modern times about the use of remote telemetry, attaching GPS tags to animals and watching to see where they go. This is a useful technique, especially for tracking long-distance migrations. But it still requires that you capture your animal to fit it with the tag and then hope that having the thing attached won't affect its behaviour... which may, in turn, depend on how large it has to be. Not to mention the risk that it might fall off.
There's also the issue that the number of different species we might want to track in this way is large. And that's even if we only want to look at threatened species, not the more common ones that might, for example, affect the habitat that the rarer ones live in or the prey that they're able to feed on. Either way, it should not be surprising that there are gaps in our knowledge and that filling them could have practical benefits.
Mandrills (Mandrillus sphinx) are a highly distinctive species of forest-dwelling monkey and one of the largest species of monkey in the world. They are closely related to the savannah-dwelling baboons, and look similar enough that most people would probably refer to them as such. Unlike baboons in the narrow, scientific, sense, however, mandrills have highly visible blue-and-red snouts, which are particularly bright in the larger males.
Mandrills are most common in Gabon in West Africa, but there are also populations in Equatorial Guinea and neighbouring regions of Cameroon and Congo. They are considered a "threatened" species because, while we don't have any accurate census of their numbers, hunting for bushmeat is sufficiently extensive that they are thought to be declining and they aren't especially common to begin with.
Assessing the movement of mandrills is not easy for a number of reasons, even aside from the fact that are many other monkey species that one might also wish to look at. Perhaps the main reason for this is that they live in dense jungle, where it's much harder to follow animals than it would be in the sort of open country where true baboons are found. They also live in large groups that can have over 600 members and (more significantly) wander over an exceptionally large area, making tracking difficult.
This is not to say that nobody has tried. The first such study was in the 1980s using the longstanding method of "watching them to see what they do". A second, using radiotelemetry, was conducted more recently, and confirmed that the way that mandrills travel changes between the seasons, presumably as their food supply changes. This is because about 50% of their diet consists of fruit, the sort of thing that would be expected to be more common at some times of the year than others, depending on the seasonal reproduction of the plants in question. The fact that their own breeding cycle is, as in many mammals, also seasonal, could well play a part, too.
A few weeks ago, a third study was published, looking at the some of the mandrills living in Moukalaba-Doudou National Park, in southern Gabon. This is close to the southern edge of the jungle, with the park including some patches of open woodland and has a distinct rainy season between October and April, making it possible to see the effects that season has on the monkey's behaviour.
One difference from the earlier studies is that the groups studied here were larger. Both of those were on groups of around 100 individuals, which may sound a lot, but is actually at the small end of the scale for mandrill hordes. (Yes, at some point, somebody decided that baboons have "troops", but mandrills have "hordes". Go figure.) Three hordes live in the new study area, with the largest of them having around 440 members, and known to have existed since at least 2008.
In this instance, the researchers chose not to use the more typical method of capturing and radio-tagging the mandrills before releasing them to see where they went. Partly, they went out and simply watched the animals, keeping at a safe distance in the hope that this wouldn't spook them and affect their behaviour. But they also confirmed their findings by using motion-activated video cameras strapped to trees at locations throughout the area. Whilst this provides only a snapshot rather than longer-term information, it does have the advantage that the mandrills probably wouldn't even have noticed anything and that they could be observed when humans weren't available.
That latter fact, for instance, confirmed that mandrills really don't move about at night at all, something that would otherwise be difficult to know. Of course, it was hardly a new discovery that the monkeys are diurnal and sleep at night. They would also have good reason to fear nocturnal predators, such as leopards, so their motivation to stay hidden is clear enough. But both chimpanzees and macaques have been observed moving around at night, despite living in smaller groups that could potentially be more vulnerable to nocturnal predators. Neither do so regularly, but mandrills seem even more reluctant to do so... or, at least, if they do, it's up in the trees where the cameras wouldn't see them, rather than on the ground.
In broader terms, the study confirmed earlier findings that mandrills are more active, and move further during times when ripe fruit are readily available, during the rainy season. It turns out, however, that they take something of a siesta during the hottest parts of the day. Which, given that we're talking daily highs of around 35°C (95°F) and humid with it, is hardly something one can blame them for; similar behaviour is known in other tropical animals, and even some in cooler climes.
It's unlikely that heat explains why they move further during the hotter rainy season than they do in the cooler dry season when one would, if anything, expect the reverse. That may instead be down to what they're eating. While they prefer to eat fruit, that's not an option during the dry season, when they are instead forced to focus on seeds buried in the forest litter. It may well be that this requires careful searching to find, slowing down their movement, rather than them all heading off to the nearest cluster of suitable-looking trees.
It's also notable that, while, in this part of the park, they had the opportunity to travel into patches of savannah, where the terrain is relatively open but there are still trees, some of them with edible fruit or seeds. Yet they never did. Mandrills regularly travel over very wide areas, with a single large horde perhaps occupying 180 km² (70 square miles) so knowing what they require and that they use it differently at different times of the year can be useful information if we want to avoid them slipping onto the endangered species lists.
[Photo by Sanjay Acharya, from Wikimedia Commons.]
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