Under Construction May 2011


Mustela rixosa


Length: 6-10mm (Toth 2002) 7mm (Furskin 2006)  7-10mm (ASM #1)

Diameter Range: 92-176 microns (Brown 1942) 87-110 microns (Toth 2002) 45-80 microns (Furskin 2006) 70 microns (ASM #1)

Medullary Index: 0.80 (Brown 1942) 0.75 ave (Toth 2002) 0.64 (ASM #1)

Medulla: Cells in the medulla not separated by pigment masses, like we see in canids.  Look at the base, edges have deep wavy s-curves? Toth describes medulla of shaft as unicellular ladder, then an irregular unicellular ladder in the transitional part, and then cloisonné in the shield with scalloped medullar margins. (Toth 2002)

Color: Light brown at the tip and white at proximal end (ASM#1) Never banded (Mathiak 1938)

Scales: Pectinate and diamond petal near the base, then mosaic and elongate petal in the transitional part, and then irregular or regular wave in the shield. (Toth 2002)


Length: 3-5mm (ASM#1)

Diameter Range: 15-30 (Brown 1942) 7-12 microns (Furskin 2006)

Medullary Index: 0.66 (Brown 1942)

Medulla: uniserial ladder present in all underfur hairs

Color: present in the cortex (Brown 1942)

Scales: long, pointy narrow spiky scales

Macro Qualities: 7.2 – 8.5” long.  White in winter with a few black hairs at tip of tail, summer has a brown back and a white underside (Forsyth 1999).  Up to 10” long.  Summer pelage of medium to dark brown above and yellowish white underparts, fully white in the winter except for a few black hairs in the tip of the tail (ADF&G 2008). 6″ long with 1″ of it tail.  Guard hair 2-3 times as long as underfur? (Bachrach 1953) Skins are 14-15.5 cm and tail is 4.5cm.  Summer they are a red brown color with a white belly (Furskin 2006).  Fibers are very fine and short overall.  Longest fibers on tail (ASM#1). UAF email suggests that their longest specimen in about ½” longer than a pencil, and the tail should be shorter to or equal to the length of the hind foot (image from UAF Museum of the North gallery).  Samet (1950) says the tail is about an inch long.  The weasel itself is 4-5” long.  Skin is dark grey-blue if caught too early, and skin is clear white when prime.


Notes: Family Mustelidae (the mustelids)

Troubleshooting: Toth 2002: says that Mustela ermine and Mustela nivalis show significant differences only in length of hair, with M. ermine being longer.  If the length of the guard hair is more than 10mm, it seems that M. ermine is more likely.  M. nivalis averages about 8mm and M. ermine averages around 14mm.  He also says that the two weasels can be distinguished from the other mustelids because their hair shaft is narrow and then widens rapidly into a shield, while the hair shaft of other mustelids thickens more gradually. (Toth 2002) It seems that the least weasel is smaller and has only a few black hairs in the tip of its tail when compared to the similar short-tailed weasel. (ADF&G 2008) 10” without tail (ASM #1).  Can we distinguish the two weasels from the idea that the short-tailed weasel has a lot more variation in the length of the hair on the tail?  Short Tailed Weasel and Least Weasel may be difficult to distinguish, although the guard hair is a significant clue.  The longest least weasel guard hair rarely exceeds 10mm, while short tailed weasel guard hair is usually over 10mm, especially on the tail where it may be 50mm.  The prominent black tip on the tail is also indicative of short tailed weasel, since the least weasel usually only has a few black hairs at the tip of the tail. If the back feet are present, the toes of the least weasel’s outstretched feet are only as long as the tail, but tail is much longer than feet on the short-tailed weasel.

Range: All of Alaska (Forsyth 1999). NOT in islands of the Bering Sea or most of the Aleutian Islands beyond Unimak Island.  Also not on Kodiak Island or most of the islands of Southeast Alaska (ADF&G 2008).  A 2009 article by McDonough and Olson discusses the first record of them on the Kenai Peninsula (Northwestern Naturalist Vol 90 p 256-258).  Not seen at all in Southeast Alaska, uncommon and sparse elsewhere, mostly seen on the Arctic slope (MacDonald and Cook 2009).

Names:  Wildlife Notebook Series lists Mustela rixosa as the least weasel found in Alaska, but Wikipedia says that “rixosa” is a subspecies of Mustela nivalis whose range is the Mackenzie River Delta, across Canada and south into the lower 48.  It lists Mustela nivalis eskimois as the least weasel found in Alaska and the Yukon Territory (AFD&G 2008).  Also called “mouse-weasel” (MacDonald and Cook 2009).

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