How the Lemming Got its Coat

The Ice Ages were, without doubt, the most dramatic natural climatic changes on Earth in the last few million years. The last one was particularly severe, with vast ice sheets covering much of northern Europe and northern North America. This, naturally enough, forced many species of animals in the Northern Hemisphere to move south. Even those well-suited to the cold, such as reindeer, musk oxen, and woolly mammoths, would have had to avoid the barren ice sheets, even if they were happy in the broad tundra belt to the south.

In Europe, in particular, there is only so far south you can go before hitting the coastline. This meant that many animals were forced into small areas, some of which may still have been marginal habitat for them, to avoid extinction. These areas are called "refugia", and their small size and isolation were a driver for evolutionary change. Sometimes populations were split apart for so long that they became separate species and, for example, we can date many species of northern birds to this time, even though, for them, the Mediterranean would not have been an issue. 

On the other hand, sometimes closely related species were forced into the same area where a combination of limited living space and limited choice of mates meant that they ended up interbreeding. Modern brown bears (Ursus arctos), for example, are now known to have inherited up to 2.4% of their DNA from the extinct cave bears (Ursus spelaeus). 

Among the species likely to have been affected are the lemmings. This is because they live in the north, where ice sheetswere likely to encroach on their homelands. The exact definition of "lemming", from a taxonomic point of view, can be debated. They are a kind of vole, albeit larger than most other species, but we can use the term variably to refer either to a single genus (Lemmus) or a set of three closely related genera. The latter includes the wood lemmings and bog lemmings in the definition, while the former does not. 

Even if we're going with the first definition, leaving us with what we might call the "true lemmings", there is debate as to how many species there are. We know that there are at least four. The Norwegian lemming (L. lemmus) lives, not just in Norway, but also in northern Sweden, Finland, and the Russian Kola Peninsula to the east. The Siberian lemming (L. sibiricus) lives along almost the whole of the northern coast of Russia, from the White Sea to the East Siberian Sea. East of that, we come to the brown lemming (L. trimucronatus), which lives both in far eastern Siberia and in Alaska, northwest Canada, and Baffin Island. The fourth species is the almost entirely unstudied Amur lemming (L. amurensis), found only around the Amur River in southeastern Siberia, just north of the Chinese border.

In the last decade or so, genetic evidence has indicated that there may really be six species. This scheme, which is commonly, but not yet universally, accepted as accurate, sees both the brown and Siberian lemmings being split into two. (Perhaps surprisingly, the split in the brown lemming is not at the Bering Strait, and instead sees the Asia/Alaska/Yukon population separated from that further east).

The Norwegian lemming is interesting for a couple of reasons. Firstly, it is the only mammal species entirely unique to the Fennoscandia region. Secondly, it is slightly different from the other lemmings, having a bright black-and yellow coat rather than the dull brown of other species, and being noticeably more aggressive. (It has been hypothesised that these two features may be linked, with the bright colours being a warning to predators that "this lemming will fight back".) 

However, we would expect that its closest relative is the Siberian lemming, given that the others are much further away and analysis of mitochondrial DNA not only supports this, but indicates that it may be a remarkably young species. Based on these studies, dating back to the late '90s, the Ice Ages, perhaps even the most recent of these, have been implicated in its origins.

Since that time, we have become a lot better at the sort of genetic analysis that enables to tell how and when species originated and diverged. This is because mitochondrial DNA represents only a tiny proportion of the total, making it much easier to trace changes, but, at the same time, not giving us the whole picture. In modern times, we have whole-genome sequencing and computers powerful enough to crunch the vast amounts of data that generates. Time then, perhaps, for another look.

A recently published study examined the genomes of nine individual lemmings across a wide area. These included five Norwegian and three Siberian lemmings, plus a brown lemming from Alaska to serve as a reference. In addition, the researchers were able to recover partial genomes from two ancient specimens. One of these was from coastal Siberia, roughly where the eastern species (or subspecies) currently meet up, and had been dated to 9,600 years ago. The other, dated to 12,500 years ago, was recovered from Somerset in England. This is well outside the range of lemmings today, but that date falls during the twelve centuries of the Younger Dryas, a time when temperatures fell by 3°C (5°F) across the whole of Europe, and essentially the last gasp of the Ice Ages.

The analysis showed that the lemmings in the samples grouped together into four distinct groups. In the technical language, these are "monophyletic clades", which is to say that each appears to descend from a common, unique ancestor and can therefore be regarded as a genuine evolutionary unit. Significantly, the Siberian lemmings from the White Sea region fell into a different group than those from the eastern end of the continent. Using fossil-calibrated estimates for the rate of evolutionary change, the researchers estimate that the two groups diverged between 124 and 149 thousand years ago, somewhere around the end of the Next-to-Last Ice Age.

More importantly, they did so before the western Siberian lemmings diverged from the Norwegian species. This confirms the placement of western and eastern Siberian lemmings as different species. Since the first proposal for this was made in 1914 (and then ruled out for lack of evidence), the name it was given back then, L. paulus, is now given to the eastern species.

Since only one brown lemming was used in the study, we can't say the same about any split between its two putative species. For what it's worth, though, this was, as might be expected, radically different from the other two, suggesting that brown lemmings (however many species they may be) diverged from the others first, and did so at least 0.25 million years ago. There's enough wiggle room in the date here that we can't be certain whether that happened during one of the Ice Ages, or during a warm gap between them.

The two ancient samples both proved to be Norwegian lemmings, despite one coming from well to the east of the current range. They do, however, predate the last common ancestor of all the Norwegian lemmings in the sample. Which means that if you, really, really, wanted to, you could put them in separate subspecies from the living sort. Not that there's any suggestion that anyone does.

The headline finding, however, was just how recently the data in this study shows the split between Norwegian and West Siberian lemmings to have been. While this was already thought to be recent by the standards of such things, the new finding gives an even later date of just 35,000 years ago. That's not long before the very harshest point of the Last Ice Age and would make the two species among the youngest mammal species known to science. 

The researchers point out that, being rodents, lemmings breed rapidly. For them, a generation is just one or two years. This means that, counting the number of generations since the origin of the species and today (rather than years), it's broadly in line with the estimated divergence between polar bears and brown bears, or between Himalayan and Chinese takins. Even in actual years, it's also about in line with the estimated date of origin of some other, smaller, vole species. So it's short, yes, but not unreasonable, all things considered.

Another advantage of genome sequencing, as opposed to mitochondrial DNA analysis is that it becomes possible not only to see how many genes have changed, but which some of the key ones are. Determining what those genes do is a lot more difficult but, in some cases, this work has already been done for other species, which can give us a clue.

In the case of the Norwegian lemming, one of the genes that had changed significantly from its form in the other species of lemming is known to affect coat colour in mice and mink. Here, it presumably has something to do with the yellow-and-black fur of the Norwegian species. Interestingly, the same gene was in its normal form in the two ancient samples, suggesting that this colouration may be an adaptation even more recent than the origin of the species, perhaps occurring in a single population then spreading until all modern Norwegian lemmings possess it.

Another altered gene is responsible for certain forms of colour blindness in humans, but has been implicated in male fertility and responses to stress in other species, so what it does in lemmings is anyone's guess. Several are known to be involved in the sense of smell, although since these tend to change a lot between mammalian species anyway, and olfaction has many uses, this is no great surprise. Other changed genes, however, are related to fat metabolism, including one that has been linked to non-alcoholic fatty liver disease - something that Norwegian lemmings are known to be prone to. 

Putting this together, it seems that Norwegian lemmings originated as a species shortly before the worst part of the Last Ice Age, presumably trapped in some small locality by hostile climate and separated from their west Siberian kin (which, by definition, became a distinct species at the same time). Although it didn't happen immediately, at some later point, some of the lemmings developed a new coat colour. There is no evidence of past interbreeding in the genomes sampled here, so perhaps this put off the regular grey-brown lemmings from breeding with them, or possibly the changes in the sense of smell meant that one or the other no longer smelled quite 'right'. 

Either way, the Ice Ages had created another new species.

[Photo by Fährtenleser, from Wikimedia Commons. Cladogram adapted from Lord et al., 2025.]