Discovery of the World's Newest Monkey

Hamlyn's monkey (left) and lesula (right)

Even today, new species of mammal are being discovered all the time. The majority are small animals, often nocturnal, or otherwise difficult to find. Just in the last few months I've seen the announcement of four new bats and a mole, and that's without me particularly trying - there may well be more discoveries I haven't seen. In many cases - as happened with the bats, for example - it's not that the animal had never been seen before, it's just that it wasn't obviously a different species. Until you look closely, one mole may look much like another.

The discovery of larger species is a rarer event. Which is why the announcement of an entirely new species of monkey is so exciting. This new animal, the lesula (Cercopithecus lomamiensis), is the first new species of African monkey to be discovered since the critically endangered kipunji in 2004, and only the second since 1984.

Perhaps one of the first questions that one might ask is "new to whom?" In June 2007, field researchers in the Democratic Republic of the Congo came across a captive monkey at the home of a primary school teacher in the town of Opala. Realising that it was one they had never seen before, they asked what it was, and were told that it was a "lesula" - an animal well-known enough to the locals to have a name in their native language, but entirely unknown to western science. It turned out that the animals actually came from further south, an area of dense jungle between the Lomami and Tshuapa rivers, and the researchers spotted the first one in the wild in December of that year.

They continued to observe the animals over the next few years, confirming their identity as a new species; the formal announcement and description was published just this month. The newly coined scientific name means something like "tailed monkey from the Lomami River". As part of the naming process, the researchers, led by John Hart, identified a holotype specimen, which helps to define the discovery. This becomes the archetypal representative of its species, and it's important because, if in the future anyone decides that there are really two species (or subspecies) of the animal, we will know which is the "original" and which is the "new" one - and therefore in need of its own name. Obviously, the holotype has to be dead, so that it's possible to go back and check it, if necessary, but  there's fortunately no need to go and kill it yourself. It's currently housed in the Yale Peabody Museum.

So, what is this new monkey? It's a guenon, a member of a group of forest-dwelling African monkeys that already contains 24 species. It's closest relative appears to be Hamlyn's monkey (Cercopithecus hamlyni). This lives only a short way to the east, on the other side of the Congo River, known along this part of its length as the Lualaba River. You can see the two species side-by-side in the photo at the top of this post, and its immediately clear that they aren't the same.

Probably the most obvious difference to human eyes is the colour: while lesulas do have dark fur on their limbs, the head and back are much paler. On a second look, you can also tell that the lesula has noticeably larger eyes than Hamyln's monkey. There are a number of other differences, notably in the shape of the skull and the relative size of the teeth, that can clearly distinguish the two, confirming that lesulas are not just a different coloured form of the previously known species.

So how do we even know that Hamlyn's monkey is the closest relative? After all, there are many other guenons living in the same general area, such as mona monkeys and red-tailed monkeys. It turns out that, for all their clear differences, the two species also have a lot in common. For example, looking at the picture again, you can see a pale stripe down the nose in both animals. It's far more obvious in Hamlyn's monkey, and not just because it's contrasting with much darker skin, but it is present in both species. Crucially, no other guenon has this particular feature.

Infant Hamyln's monkey (left) and lesulas (right)

Something you can't tell from the picture is that the males of both species have bright blue buttocks and scrotums. While this isn't unique, the coloured patch is far larger in these two species than in other guenons, again, implying a close relationship. The females also have a mane around the head in both species, something that no other guenons have. Indeed, before the adult colour is fully developed, the young of the two species do look remarkably similar, as this second picture shows. Judging from the one individual they were able to follow for long enough, the full adult colour and size are reached by an age of about fifteen months.

However, we don't have to rely on just their physical resemblance to confirm that these two species, while different, are very closely related. By taking DNA samples, and comparing them to those from other monkeys, it's possible to draw up a family tree showing which species are most genetically similar. Comparing lesulas with eleven other species of guenon, it was possible to show both that the relationship is genuine, and that they really aren't the same thing.


In fact, armed with their DNA, we can go one step further. Because we have some idea of how fast genetic material changes in guenons, its possible to compare particular genes present in the two species, and come up with a good guess as to when they diverged. The researchers tried this in two different ways, and didn't get quite the same answer, because not all genes really mutate at the same rate. Nonetheless, there was considerable overlap, and we can say that lesulas and Hamlyn's monkeys probably last shared a common ancestor somewhere between 1.5 and 3 million years ago.

That's either the early Pleistocene or the late Pliocene, and most likely before the dramatic climatic shifts of the Ice Ages. That suggests that the two populations probably became separated by the rivers that lie between their ranges today. According to the researchers, the region between the Congo and Lomami rivers, where neither species is found, is less dense, and not so suitable for monkeys, as the jungles on either side.

Simplified family tree showing how lesulas relate
to other guenons

Even once they knew they existed, it proved quite difficult for the researchers to find lesulas in the wild. They live in dense, lowland jungle, in an area that has been little surveyed by biologists, and (perhaps sensibly) hide from humans whenever they can. There were clearly plenty of them around, because, like Hamlyn's monkeys, and unlike other guenons, they make short booming calls to greet the dawn, and also occasionally at night. Again, the sounds were similar to those of the other species, but not quite the same, being, among other things, slightly higher in pitch.

When they were seen, it was often in the company of other monkeys that were, presumably, less effective at hiding. These incldued other local guenon species, but also red colobus monkeys. They don't seem to be very sociable, though, with no more than five ever seen together at one time, and frequently less. Like their relatives, they seem to be pure herbivores, eating fruits, flowers, and soft leaves from a variety of plants.

The researchers estimate that lesulas can be considered a threatened species, although probably not in the higher category of "endangered". A formal decision on that will have to wait for evaluation by the IUCN Red List, but it's based on the encroachment of hunters looking for bushmeat. The area in which they live is so remote that logging, mining and so on are not (yet) an issue, but its worth noting that all but one of the dead specimens the researchers examined - including the holotype - were obtained from local hunters. The sole exception, incidentally, died from injuries sustained in a fight with an eagle that had presumably wanted to eat it...

Living in a relatively small area, lesulas are vulnerable to hunters in a way that more widespread animals are not. There are moves to have the area declared a national park, with some protection for its wildlife. That's largely due to the presence of bonobos there, along with three - now four - distinct kinds of monkey found nowhere else in the world. The new discovery can only make that more important.

[Pictures by Noel Rowe, Maurice Emetshu, and John Hart. Released under the Creative Commons attribution license. Cladogram adapted from Hart et al, 2012].