WEASELS

Under construction May 2011

WEASELS

ORDER CARNIVORA (the carnivores)

Family Mustelidae (the mustelids)

Fisher (Martes pennati)

Least Weasel (Mustela nivalis)

American Marten (Martes americana)

Mink (Neovison vison)

River Otter (Londra canadensis)

Sable (Martes zibellina)

Sea Otter (Enhydra lutris)

Short-tailed Weasel (Mustela erminea)

Wolverine (Gulo gulo)

LENGTH: Most of the weasel guard hair is under 60mm maximum length, except for wolverine which is usually over 70mm and samples measured up to 140mm on the flanks of the animal.  Length may help distinguish least weasel (under 10mm) from short-tailed weasel (15-30mm).  It also may aid in distinguishing mink (up to 30mm) from marten and fisher who can often reach 60mm.  Tail hairs are often longer than any of the body hairs.

WIDTH: Weasel guard hairs often broaden toward the tip into a wide shield.  This means the hair begins narrowly at the base like a stalk, stays narrow for a considerable distance, and then suddenly broadens out into a much wider area before tapering at the tip.  This gives the hair a paddle shape from base to tip.  All the mustelids in Alaska have this feature, and most of the other mammals do not, with possible exception of marmot and muskrat.  The shield width can be helpful in distinguishing between the otters.  A sea otter has a shield less than twice as wide as the stalk, but the river otter shield is typically 3X as wide as its stalk.  On the wolverine, the stalk is rather short but the shield goes on for a great distance, as the wolverine typically has very long guard hairs.  Maximum width of the shield for all the weasels is in the 150-200mm range, with only the wolverine measured wider in our samples.

MEDULLA: Generally not a lot of pigment between cells in the medulla (Brown 1942)  The medulla of the weasel shows closely packed dark cells in the stalk, often taking the shape of bubbly letterforms like H,M,V,Y,X and so on.  Sometimes this is referred to in the literature as “fingers of cortex” extending into the medulla.  In the shield section, the individual cells are dark and closely packed, some cells look dark and some light, but there are usually distinct outlines around each cell, unlike in the canids where pigment masses between the cells tend to obscure the individual cells.  Typical weasel medulla is scalloped along the edge, while canids tend to be smoother.  Both sea otter and river otter have fragmentary or interrupted medullas in the stalk of the guard hair.  Other weasels don’t tend to have an interrupted medulla? 

MEDULLARY INDEX:  The index of each weasel tends to have some variation, with the typical range between 0.45 and 0.8.  Only in the smallest weasels does the medullary index go above 0.8 (least weasel, ermine and sable).  The medulla of the sea otter is very narrow, with an index of less than 0.3 as diagnostic among weasels and smaller than the river otter whose MI is closer to 0.5.

SCALE: Weasels have a distinct scale pattern, beginning at the base with smooth-edged wide squat scales that are often shaped a little bit like puzzle pieces.  This pattern lasts only briefly before becoming a distinctive diamond petal shape with very visible individual long pointed scales, rather like snake skin.  These long pointy petal-shaped scales do not appear on rodent guard hair.  This pattern lasts the entire stalk but changes abruptly again at the shield into closely packed wide scales with ragged serrated or jagged edges that extend until the tip.  The underfur of the weasels always has long sharp petal shapes, while canids and rodents usually have more stacked crown-shaped underfur scales (exceptions: Arctic fox and raccoon).

UNDERFUR:  Almost all weasels have uniserial ladder medulla and a scale pattern with long pointed petals for most of the length but a few stacked crowns at tip and base.  Otters are unlike the other weasels in having no medulla in the underfur, and the long pointy petal shaped scales extend all the way to the tip.  The wolverine’s underfur may also have no medulla, but there are usually a few intermediate fur samples present with interrupted or fragmentary medullas.

COLOR:  (Moore et al 1974) says unbanded hairs are characteristic of Mustelidae except for badgers (no badgers native to Alaska.)  In rare cases, the banding includes a lighter section next to the skin instead of the prominent dark band seen in canids and raccoons.  Generally, mink are lighter brown in color than marten, and marten are a deeper chocolate color with a distinct orange throat patch.  Fishers are larger than mink and marten, and their coloration tends to be darker.

SUMMARY:  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.

Mink, martin and fisher can be difficult to distinguish.  Overall fur color can be helpful, and if the head is present, marten have more prominent ears than mink or fisher.  In our only sample of fisher, it seemed that the tip of the guard hair may be so darkly pigmented that the medulla can no longer be observed.  This did not happen in any of our mink or marten samples.  One mink sample had an absent or fragmentary medulla in the stalk region.  The underfur of the fisher and marten (15-25mm) seems longer than the underfur of the mink (less than 15mm) and this may be a clue.  Generally, guard hairs on the mink will not exceed 25mm, perhaps extending to 30mm on the tail.  If the length of the guard hairs is longer than 30mm, marten or fisher is more likely.  Tail hairs on the marten can be up to 90mm.  Marten and fisher are difficult to distinguish, but fisher are rare in Alaska.

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