Hebridean Grass Mouse (M. agrestis exsul), common on several islands of the Hebrides. Distinguished from the Field Vole by its much larger size and duller brown colour.
Eigg Grass Mouse (M. agrestis mial) restricted to the island of Eigg. Differs from M. a. exsul in its shaggy coat of abundant long hairs.
Highland Grass Mouse (M. agrestis neglectus) found on the summits of the highest Scottish mountains. It is larger than the Field Vole, with thicker fur and darker, browner upper side. It differs from M. a. exsul in the simpler character of the first molar tooth.
Muck Grass Mouse (M. agrestis luch), of which only three specimens have been taken, all on the island of Muck. About the same size as the Field Vole, it has a buff underside.
Orkney Vole (Microtus orcadensis, Millais).
So far back as 1805 the Rev. George Barry, in his "History of the Orkney Islands," mentions a rodent that was known locally as the Vole Mouse, which he believed to be the same as the agrestis of Linnæus. He says it "is very often found in marshy grounds that are covered with moss and short heath, in which it makes roads or tracks of about three inches in breadth, and sometimes miles in length, much worn by continual treading, and warped into a thousand different directions."
Towards the end of last century Mr. J. G. Millais obtained specimens, and on a critical examination found that they differed from the known forms in several details of skull structure and in the folds and angles of the teeth, sufficient in his opinion to constitute a new species, which he called Microtus orcadensis. It is larger than the Field Vole, with a longer and slightly broader head.
It was found subsequently that specimens from different islands in the Orkney group showed differences due to their segregation over a long period, and they have consequently been distinguished as five sub-species. These differences are minute, and it would be wearisome and out of place in a popular work such as the present to detail them. Generally speaking, they are much alike, and their habits are practically identical, so far as at present known.
The runs are a conspicuous feature of the islands, among the heather and the rough vegetation of the fields and hillsides, running along the surface and at intervals entering tunnels about two and a quarter inches in diameter—just sufficient to clear the spread of the Vole's whiskers. Their nesting places, like those of the Mole, are under small mounds connected with a network of runs. The nest itself is made of grass and roots in a rounded chamber, where at intervals during the spring and summer several litters, varying from three to six, are produced. Before they are three weeks old they are capable of independent existence, but for a time are still guarded by the mother.
The Orkney Vole appears to be specially fond of the roots of Heath Rush (Juncus squarrosus), but also feeds on grass and the crops in cultivated fields to which they can gain access. Mr. Millais found that in cold weather his captive Voles became inactive. It has many enemies to hold its increase in check, for every bird and beast large enough to capture it will eat it readily.