MUSTELA RIXOSA (Bangs)
Least Weasel
(Synonymy under subspecies)
Type.—Putorius rixosus Bangs, Proc. Biol. Soc. Washington, 10:21, February 25, 1896.
Range.—From Norway and Switzerland eastward through Siberia and all the way across North America, but unknown from Iceland, Greenland and the Arctic islands west of Greenland; in North America, from the Arctic Life-zone south to Central British Columbia, Montana and into parts of the Upper Austral Life-zone as in the eastern half of the continent.
The southern extension of range in the Appalachians (to North Carolina) is not duplicated in the Rocky Mountains of western North America probably because the region there suitable for rixosa south of Central British Columbia and Montana is occupied by the almost equally small Mustela erminea muricus and related subspecies which seem to fill the ecological role that rixosa plays where it occurs. The small size of females of M. erminea cicognanii in New England may similarly account for the absence of rixosa there.
Characters for ready recognition.—Differs from both Mustela erminea and Mustela frenata by tail a fourth or less of length of head and body and without a black tip (at most a few black hairs at extreme tip in rixosa), and from M. frenata and from M. erminea in regions where it and rixosa occur together, by basilar length of skull less than 32.5 in males and less than 31.0 in females.
Characters of the species.—Size small: Total length less than 250 in males and 225 in females; tail a fourth or less of length of head and body, and without a black pencil and at most with a few black hairs at extreme tip; caudal vertebrae 11 to 16, normally 15 in M. r. rixosa, and 11 in one M. r. eskimo examined; skull with long braincase and short precranial portion, thus essentially same shape as in M. erminea but the largest males of M. rixosa always with a lesser basilar length that even the smallest females of M. erminea or M. frenata of the same geographic area. In fact no specimens of M. frenata have skulls so small as the largest M. rixosa, and skulls of equal size of M. erminea and M. rixosa, for example, M. erminea muricus of Colorado and M. rixosa eskimo of Alaska, differ in that when the skulls are viewed from directly above those of rixosa have the mastoid processes more prominent, or the braincase is higher in relation to its width or both differences together prevail. Stated in another way, comparison of skulls of equal size of rixosa and erminea shows that in the latter the braincase is more nearly flat and is wider above and in front of the mastoid processes; therefore, the greatest breadth of the braincase equals or exceeds the mastoid breadth, whereas the reverse is ordinarily true of rixosa.
Geographic variation.—In the Old World four subspecies are currently recognized (see Allen, 1933:316) and the same number is here recognized in North America. Length of the tail, length of head and body and hind foot, breadth of the rostral part of the skull in relation to its length, and position on the side of the head of the line of demarcation between the dark color of the upper parts and the white underparts, are the features in which geographic variation has been detected. The general impression is that the amount of geographic variation is much less than in Mustela frenata and only slightly less than in Mustela erminea of the same geographic area.
Nomenclature.—It is exceptional for a species which occurs in both the Old- and New-World to take its specific name from New World material, especially if the name was proposed as recently as 1896; most circumboreal species take their names from descriptions of European specimens. Although the least weasel, Mustela rixosa (Bangs) 1896, seems now to be an exception, it may yet turn out that the first available name was based on European material. Zimmermann (1943) shows that the least weasel actually was named on the basis of European material long before 1896 and concludes that the name Putorius minutus Pomel, 1853, based on a specimen from France, is the first available name.
Because Putorius nowadays is relegated to subgeneric rank under the generic name Mustela, we have for consideration the name-combination Mustela minuta (Pomel). Unfortunately for Zimmermann's conclusion, Mustela minuta Pomel is not available because it is preoccupied by Mustela minuta Gervais [= Palaeogale minuta (Gervais), 1848-1852—see Simpson, 1946: 2, 12], a name applied to another species of small mustelid from the Oligocene or lower Miocene deposits of Europe.