The wildlife of the Nansei Shoto is a fascinating mixture of species, and as is clear from recent research on the spiny rats that are unique to the central islands, there may be more species there than we realize.
As I mentioned last time, the Nansei Shoto are separated geographically into three groups of islands (Northern, Central and Southern) by the Kerama and Tokara straits, and the mammals too can accordingly be separated. Even among those species native to the archipelago, some, such as the Ryukyu fruit bat, are widespread and occur on many of the islands. Other species, though, are unique to just two or three islands, while some are known only from single islands.
The mammals of the northern part of the archipelago, such as the Japanese field mouse, are closely related to those of Honshu, Shikoku and Kyushu, while the mammals of the southern part, such as the Iriomote wildcat, are more closely related to those of Taiwan and the Asian continent. Those populations from the northern and southern islands have only low levels of genetic distinctiveness.
In strong contrast, the Amami black rabbit, the spiny rat and the longhaired rat are confined to the central islands of the Nansei Shoto: Okinawa, Tokunoshima and Amami Oshima. These three species are very distinctive, indicating that they diverged a very long time ago from other lineages. Their continued survival in such a small cluster of islands is remarkable, and they form a vital part of Japan’s biodiversity legacy.
Apart from the spiny rat and the longhaired rat, discussed in my last article, the most distinctive mammal of the region is the Amami black rabbit. This forest creature, though immediately recognizable as a rabbit of sorts, has, on closer inspection, so many distinctive characteristics that it becomes clear it is not closely related to the other rabbits and hares of Eurasia or North America.
At first sight, the extremely dark fur seems most striking. In fact, though, coat color is extremely variable among mammals, and I recall once seeing an almost black European rabbit on Fair Isle in Scotland.
More distinctive, on closer inspection, are the Amami rabbit’s proportions. It lacks either the rangy, long-limbed appearance of the hares or the cute proportions of the European rabbit. It has rather a large head, a short body and very stout hind legs. Its rounded ears are shorter than those of any other rabbit species, and tipping its stout legs are the most prominent claws on any rabbit I know. All these points make for a most distinctive creature and one that is instantly recognizable when glimpsed in the beam of a headlamp along a quiet forest track at night.
The phrase “breeding like rabbits” is a common one in English, yet while other rabbits may rear large litters of young several times in a year, the Amami rabbit reproduces very slowly, breeding just twice, in October and December and again during April and May, and producing only one young each time.
This stout, nocturnal creature is well adapted to its island forests. If left alone, even its slow rate of reproduction is just one more way that it remains in tune with its environment. Unfortunately, though, rapid forest destruction and the introduction of alien predators to its home islands may prove its undoing all too soon.
Because the Amami rabbit is somewhat similar in appearance to the volcano rabbit of Mexico, the two have generally been considered relatives belonging to the same group or subfamily. Recent DNA research has, however, compared many species of the large Leporid family, which includes all the rabbits and hares, and has revealed that the Amami rabbit’s lineage is ancient and independent of any of the others.
In fact Amami rabbits share no special affinities with the Mexican rabbit at all; any visual similarities are entirely coincidental and perhaps the result of parallel evolution in relation to their ecology. It appears that the divergence of the genus Pentalagus, of which the Amami rabbit is the only living species, occurred some 10 or 20 million years ago.
So in the Amami rabbit we see a truly ancient relict species. Great fuss is made over human artifacts dating back just a few thousand years, yet here is a work of nature’s art dating back millions of years that deserves every bit as much attention.
Hokkaido, at the opposite end of Japan, also has a rich mammal fauna, yet all of its species, with the exception of one small rodent, a vole known as Clethrionomys rex, also occur elsewhere in northeast Asia. The three main is-lands of Honshu, Shikoku and Kyushu have many distinctly native species, and some of them are very distinct indeed. Mostly, however, they are small mammals which go un-noticed, such as the Japanese dormouse, several species of moles and various voles. The larger mammals, such as the Japanese serow, the giant fly-ing squirrel and the Japanese macaque, though interesting, have close relatives elsewhere.
What makes the Nansei Sho-to exceptional? The survival of isolated genetic lineages for millions of years despite the very small area of the is-lands.
What we still don’t know about the evolutionary history of the animals of the Nansei Shoto is whether their ances-tors arrived in there 10-20 mil-lion years ago, and diverged after they had arrived, or whether their lineages di-verged first, somewhere out-side the region, and then came to what are now the Nansei Shoto Islands. Per-haps as research techniques become increasingly sophisti-cated we shall be able to solve even this conundrum.
Whether they arrived, then diverged, or diverged, then arrived, the animals of the Nansei Shoto are very special indeed.