Saturday 22 August 2020

The Case of the Missing Sengi

The rediscovery of a species of mammal thought "lost" for almost 50 years was a significant enough story this week to appear (briefly) on the front page of the BBC News website. I'm not sure how much further I can add to the BBC story in terms of detail, but perhaps I can put it into context, as well as providing it for those who may not have seen the original. So here goes.

To start with, what exactly is a sengi?

Sengis used to be (and often still are) called "elephant shrews". The term is falling out of favour because they aren't literally shrews, in the sense of belonging to the actual shrew family, although, as descriptive terms go, it's not a bad one. Since the late '90s most scientists, when they aren't using the more technical term "macroscelidean", instead use the Swahili name for the animal, which is also the one I'll stick with.

Sengis are distinctive animals, being small, somewhat shrew-like creatures with relatively long, slender legs, large brains for their size, and a flexible proboscis, which is the source of the fancied resemblance to elephants. They are fast-running and agile, almost like miniature gazelles, and feed primarily on insects and other invertebrates. They live only in Africa and, being small and not especially numerous, are not among the most thoroughly studied of mammals.

It's long been clear that sengis are distinctive enough to be placed in their own family, but until recently, less clear where that group fits in the larger mammalian family tree. Up until the end of the 20th century, they were placed in the order Insectivora, a collection of mostly small, invertebrate-eating mammals, which also included such things as shrews and moles. These were thought to be the most primitive living placental mammals and, indeed, it's fair to say that many of them do quite closely resemble some of the earliest fossils of that group.

In 1998, molecular evidence blew that apart, demonstrating conclusively that the Insectivora, as traditionally thought of, were not a single evolutionary group. Instead, they represent at least two different lineages of animal that just happen to have a similar body form and diet, perhaps because they had no reason to change a winning formula. (And that, of course, ignores the obvious specialisations for, say, burrowing in moles or running in sengis).

Popular news coverage of this story has described sengis as "related to elephants". This is broadly true, and not just in the sense that everything is related to everything if only you go back far enough. But it's more accurate to say that elephants are the closest familiar animal to which they are related; they're not really all that close in the grand scheme of things.

Instead, while moles and shrews belong to a branch of the mammalian family tree that includes everything from antelope to bats, sengis belong to a separate, very early, lineage that originated in Africa when it was still an island continent. Collectively, the animals in this branch are called "afrotheres" and there aren't very many of them alive today. Elephants are surely the best-known animals in this group, but they diverged from the sengis very early on. In fact, there are two evolutionary branches within the afrotheres, perhaps separating around the time of the extinction of the non-avian dinosaurs; elephants are in one, and sengis in the other. 

In fact, their closest relatives are other animals formerly considered to be members of the Insectivora, but possibly even more obscure than they are: tenrecs and golden moles.

There are currently twenty recognised species of sengi, one of them only having been discovered in 2017. Most of them, if not studied to the extent that, say, elephants, have been, are at least well enough known to scientists that we can be confident that their existence isn't endangered or anything. (One species is listed as "vulnerable" because it lives in only two small areas in central Tanzania which could potentially be destroyed in a bushfire or suddenly turned into farmland, but so far this hasn't happened). 

However, there are four species about which we know virtually nothing.

One, native to South Africa, can only be readily distinguished from its neighbours by genetic analysis, which makes it hard to study as a distinct species. The others were all first described in the 19th century, and there are preserved specimens of them in museum drawers but, otherwise, they're just... missing.

In 2017, the charity Global Wildlife Conservation published a list of over a thousand such "missing" species that just hadn't been seen in recent times. Out of those hundreds of species, the Somali sengi (Elephantulus revoilii) made the Top 25 "Most Wanted" shortlist. It now becomes the fifth species, and second mammal, to make it off that shortlist.

As its name implies, the Somali sengi was first discovered in Somalia, and thought to be unique to the northern half of that country, close to the Gulf of Aden. It was first described in 1881, on the basis of a specimen preserved in the Paris Museum of Natural History and named for George Revoil, a French naturalist and explorer who had collected it during an expedition to the area. It can be distinguished from other sengis by the white ring of fur around the eyes, with a dark "spectacle-like" spot just behind them and by having a long tuft of fur at the end of the tail - no other species has all of these features. It is considered "missing" because no specimen has been collected since 1973, and the scientific literature contains exactly zero accounts of the animal ever being seen alive.

Until now.

Interestingly, this new observation, which includes both photographs of living animals and specimens of dead ones, did not take place in Somalia. Instead, the animals were found at a number of different sites across Djibouti, which lies just to the west along the Gulf of Aden... and is a rather safer place to conduct field research right at the moment.

In fact, this new discovery adds considerably to our knowledge of this obscure species. For one thing, we now know that it lives further west than we had previously thought. From more precise details of where they were captured, we can also get some idea of its preferred habitat, which seems to be semi-arid rocky areas with limited vegetation. There are plenty of these along the northern part of the Horn of Africa, and they're hardly great for agriculture, meaning that, risks from climate change aside, the Somali sengi is probably reasonably secure in terms of having enough places to live. 

Genetic analysis on three of the specimens, collected from different parts of Djibouti, also shed light on the evolutionary history of the species, and of sengis in general. The Djiboutian animals were genetically distinct from the one Somalian animal previously analysed, although not enough to justify naming a new subspecies. 

Comparison with other species shows that its closest relatives are the four-toed and North African sengis, from which its ancestors diverged around 20 million years ago, during the Early Miocene. It probably first evolved in (more or less) its present location, but its ancestors must have travelled a long distance, given that the North African sengi lives from Tunisia to Morocco, on the other side of the continent, and the four-toed sengi is found from Zaire and Kenya to Mozambique, far to the south.

Purely anatomical evidence, however, had placed it in the genus Elephantulus, along with a number of southern African species of sengi that now turn out not be especially close relatives. Indeed, the newly improved family tree means that it can no longer be considered to belong to its prior genus and therefore it needs a new name. That new name is Galegeeska revoilii, which roughly translates as "Revoil's small creature from the horn [of Africa]". 

Earlier on, though, I mentioned that there were two other species of sengi known since the 19th century, but now considered "lost" - a single individual of one of them was trapped in 2005, and the other apparently hasn't been since at least the 1960s. For those, we have no virtually no genetic information at all, and what there is doesn't make a lot of sense. If we ever find them again - and here's hoping we do - they will probably shed yet further light on the evolution of this little-known group of animals.

[Photo by Stephen Heritage. Available under CC Attribution 4.0 license.]

No comments:

Post a Comment