There are a couple of reasons why zokors are not as well-known as some other rodent groups. For one thing, they only live in northeast Asia, an area that it's fair to say doesn't receive much attention in the Western world. For another, even if you did live there, you wouldn't see them very often, because they are burrowing animals that don't like coming to the surface if they can help it. Rather like moles, you might see the mounds of earth they leave, but not often the animal itself.
How the zokors fit into the wider rodent family tree has undergone significant revision over the last few decades. Back in the 20th century, they were considered a subfamily in the mouse family, but then, far more rodents were in those days than is the case now, the older broad definition of "murid" having been refined. Then, at the dawn of the current century, early molecular studies showed that they weren't mice at all but a kind of hamster. Which was a bit surprising, since, while they look a bit like hamsters superficially, they are are different in almost every other respect.
In 2004, however, a flaw was discovered in the original genetic analysis. Specifically, it turned out that the "zokor" whose genetics had been analysed was, in fact, a Siberian hamster. Reanalysis of an actual zokor showed that it belonged to a different rodent family, the blind mole-rats. Some zoologists have argued since then that they're unique enough to be given their own family, but today, they are generally placed as one of three subfamilies with the blind mole-rat family, technically referred to as the spalacids.
Zokors are moderately large rodents, between about 15 and 25 cm (6 to 10 inches) in length. They burrow through the soil using the claws on the front feet, which is a different method than that employed by the mole-rats, which use their teeth. Like the blind mole-rats, they try to avoid venturing above ground, and they have very small eyes and no external ears. (Blind mole-rats, it should be noted, are not the same thing as the African mole-rats, which include the naked sort). They survive by eating roots and underground seeds, and are thought to be quite common in those areas where they live.
There are six living species of zokor, three belonging to the genus Eospalax, and living in central China, and three others living in parts of Siberia, Mongolia, and Manchuria. These last three are usually placed together in the genus Myospalax, from which the subfamily takes its technical name (Myospalacinae), although one species sometimes gets its own, Siphneus.
But we also know of fossil species and, using these, we can piece together something of the history of the group. It's sometimes the case, when there are a small number of living species, that they represent the last gasp of some group that was once more diverse and perhaps wider spread. But that's not the case with the zokors, which don't ever seem to have spread far beyond their current home, and to have lived much the same specialised lifestyle as they do now.
Blind mole-rats as a whole are thought to be a very early branch within the mouse-like rodents, splitting off before, for example, the hamsters and voles separated from the true mice and their kin. This would place their origin around 20 million years ago, but the oldest known fossils that we can specifically call zokors date back to only 9 million.
These belong to Prosiphneus, which lived during the Late Miocene across much of eastern Siberia and northern China and even reached into southwestern Tibet, close to the border with India This is a much wider range than any living species of the group, but the world was warmer then than it is now, with relatively mild conditions likely persisting across the region before they became split into more isolated pockets around 5 million years ago at the dawn of the Pliocene. This breakup of formerly hospitable terrain may have led to the geographic fragmentation of the family we see today, where each species lives in relatively restricted areas.
This change in the climate would have isolated early zokors into at least two different regions, perhaps divided by the Takla Makan desert and the surrounding Kunlun and Tien Shan mountain ranges. One group, represented by the Pliocene genus Pliosiphneus, remained in China, where it is likely that the very first zokors had appeared a few million years earlier. That died out by the end of the epoch, but one isolated group had survived, most likely after becoming trapped in Mongolia by the changing climate of the surrounding regions, and evolved into the genus Episiphneus. Common during the Ice Ages, it eventually moved back south again, giving rise to the modern Chinese zokors, which include the most well-studied species.
Over on the other side, the Pliocene sees the appearance of Siberosiphneus, initially in the western reaches of Siberia. Indeed, the oldest known species of the genus lived on the banks of what is now the Irtysh River, on the eastern side of the Ural Mountains, and thus about as far west as you can get without leaving Siberia. This new species has several features in common with its presumed ancestor Prosiphneus, indicating that it may represent one of the first members of its lineage, shortly after the split with the Chinese species.
This, or something like it, would have given rise to the modern species living further north. During the Ice Ages, this 'Siberian' lineage spread further afield, perhaps driven by the expansion and retreat of the glaciers. While Siberia itself had relatively little in the way of massive ice sheets, being too dry to accumulate enough snow to form them, the permafrost tundra would have shrunk and returned as the interglacials came and went, and permanently frozen subsoil is obviously not going to be encouraging for an animal that would have had to dig through it and still find enough fleshy plant roots to eat.
So it's at this time that the Siberian zokors would have spread east, leading to the appearance of the two modern species in Mongolia and Manchuria. To back up this pattern of sudden dispersal, we also have fossils that appear all-but indistinguishable from the modern Siberian species discovered on the western side of the Urals - and thus in Europe, not Asia - where they would have lived during the Middle Pleistocene. They probably didn't remain there long, perhaps just during an interglacial, but that they got there at all is surprising for something long assumed to be unique to Asia, especially when there was a mountain range in the way.
[Photo by "Avustfel", from Wikimedia Commons.]
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