Saturday 1 February 2020

A History of the Bamboo Rats

In terms of number of species, the mouse-like rodents are the most successful group of mammals alive today. The group is typically considered to consist of six families, with the vast majority of species crammed into just two - the mouse family itself, and the cricetids, which includes the voles and hamsters. These, however, are relatively late arrivals, and there are two families that split from the main line of mouse-like rodents long before the ancestors of the mice and voles diverged from one another.

One of these consists of a grand total of three species (maybe), living in the forests of southern Asia. The other goes by the technical name of the Spalacidae, and it consists of animals that have the rather unusual trait of spending almost their entire lives underground. There are at least 35 species of these animals, found in various places across Europe, Asia, and Africa. This is a fairly wide distribution, which raises the interesting question of how they managed to spread that far when they seem to have fairly narrow habitat preferences. Until recently, the evolutionary history of these oddities has been something of a mystery but now, it seems, we are starting to get enough information to piece together an overview.

Loosely speaking, the Spalacidae are the "blind mole rat family", since these are the animals they are named for. They are not closely related to the much more famous naked mole rats (which are not, in this sense, "mouse-like rodents"), although, aside from being furry, they do have some physical similarities. However, the family also includes two other kinds of subterranean rodent, the zokors and bamboo rats.

It's been clear for a while that the family is a "real" one, in the sense that all of its members share a single common ancestor that isn't also the ancestor of something else. How the three subgroups within it interrelate has been rather less clear, although opinion at the moment seems to be swinging round to the idea that the zokors and blind mole rats are more closely related to each other than to the bamboo rats.

This places the bamboo rats as the oldest branch within the family. The bamboo rat subfamily (Rhyizomyinae) is also the most puzzling, since it includes not only the bamboo rats proper, which live in eastern Asia, but a number of more typical-looking mole rats in East Africa. How did they manage to get so far?

The oldest known fossil bamboo rat, Prokanisamys, lived in Pakistan around 24 million years ago, shortly before the dawn of the Miocene epoch. They first appear in Africa around 5 million years later, which is still remarkably early, although not necessarily before the first temporary land bridge between Asia and Africa. More significantly, perhaps, these early bamboo rats don't appear to have been adapted to underground living.

In fact, they appear to have been forest-dwelling animals, preferring a warm, humid environment with plenty of lush vegetation. Ironically, though, their long-term survival seems to have been aided by the precise opposite: geological times when the monsoons were at their weakest. The most likely explanation for this was that such times saw a reduction in their preferred habitat, forcing the ancestral bamboo rats to adapt to new environments, creating new species in the process, and some of them moving as far as Africa.

Another surprise is that the mole-rats living in East Africa today don't seem to be the direct descendants of those early immigrants. This implies a second invasion of the continent during the Late Miocene, by which time Asia and Africa were well and truly joined. By this point, around 10 million years ago, they had, judging from their skeletons, only recently got the hang of burrowing, and spent far less time below ground than they do today, something that probably made it easier for them to travel the long distances required.

When Asia really began to cool and dry up in the Pliocene, the ancestral bamboo rats may have left the lush forests altogether - there does seem to be a wave of extinctions in the group at this time, so most of them probably failed - with those still living on the continent shifting further east and adapting to eat the roots of bamboo and other plants as they do today.

Aside from the fact that they are closer in size to a rat, zokors look rather like moles. This is because, unlike the bamboo rats and mole-rats, they burrow with their forepaws rather than gnawing through the soil with their teeth. They are the smallest subfamily today, with only six living species, and their fossil history isn't great, either. The oldest fossils known were found in northern China and date from around 12 million years ago, although the subfamily as a whole must be much older than this.

Even these oldest fossils, belonging to the genus Prosiphneus, looked to have belonged to animals that were pretty good at digging, so we don't quite know when or how they took up this lifestyle. The Tibetan Plateau was still rising at this time, which seems to have resulted in the evolution of a number of different species of zokor over the next few million years, as various mountain ranges separated the ancestral groups. However, they never seem to have spread very far and died back significantly during the Ice Ages, which is why so few species survive today.

The third group, the blind mole rats, live today in southeastern Europe and the Middle East. Their oldest fossils are about the same age as the oldest bamboo rats, at 24 million years old, and come from Bosnia. They never seem to have been very numerous, or to have spread very far from their homeland, and probably only diversified as much as they have because the creation of the Bosporus and Dardanelles forced the Asian and European populations apart.

Genetic evidence implies that the origin of the family as a whole dates back to around 32 million years ago. The history of how they spread out across the continents implies that this probably took place somewhere in southern Asia. We don't, however, have any fossils dating back this far, possibly because the very earliest species didn't look very different from whatever they had evolved from - not yet being subterranean - but also perhaps because we just don't have fossils from the right area.

However, this is some supporting evidence for this in the form of a few fossil rodents from Tibet hat have been rather difficult to place. Going by the rather cumbersome name of "tachyoryctoidines" they have been variously classified as early relatives of mice or voles, but some recent evidence suggests that earlier theories of them being spalacids may have been correct all along. They certainly seem to have been good at digging, and they seem to follow a similar pattern to the Asian bamboo rats in terms of their diversification, before their eventual extinction a little over 10 million years ago. Crucially, the earliest examples date back 28 million years, which might fill in at least part of the gap in the family's earliest history.

Creatures that live underground are likely less affected by the vagaries of climate than those that have to endure actual weather. They also probably have difficulty travelling long distances, which explains why animals such as the blind mole rats have never spread very far, even given millions of years of opportunity. That the only members of the family to have reached a significant distance into Africa did so before they developed a subterranean lifestyle is likely not a coincidence.

But, as the history of the bamboo rats shows, climate can affect you indirectly even if you shelter from the rain and the sun. You might be able to avoid bad weather or increasing aridity, but if your food supply can't, you're still going to have to change. Or die out; take your pick.

[Artwork by John Kuelemans, in the public domain.]

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