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The Least Weasel

By Dr. Jim Pease, Emeritus
Extension Wildlife Specialist (ISU - Retired)

Pound for pound, one of the toughest predators around, the least weasel (Mustela nivalis) ranks as “the world’s smallest carnivore”. (Though the short-tailed shrew is smaller, it is classified as an insectivore, even though its diet contains rodents, too.) Found statewide in a variety of habitats, from prairies to brush to savannah woodlands, the adaptability of this little weasel probably accounts for its success. Though thorough, recent surveys have not been done, the least weasel appears to have survived the massive changes in the Iowa landscape better than its cousins, the ermine (Mustela erminea) and the long-tailed weasel (Mustela frenata).

Least Weasel (Mustela nivalis) © James F. Parnell - email: jparnell@ec.rr.comWith a body barely 5-6 inches long and a tail usually only 1-2 inches, this slender bodied fellow is ideally suited for rodent patrol. It slips easily and quickly through the burrows of mice and other small rodent and feeds readily on them, killing them quickly with a bite to the back of the skull. Like its weasel cousins, the least weasel eats about ½ its body weight each day in mouse and rat meat, making it an important predator on voles, mice and other Rodentia.

It’s short fur is milk chocolate brown in the summer with a light underbelly and white feet and black eyes. In the winter months, it molts into its camouflage white color with a few black hairs on the tip of its tail. Their round ears top a flattened skull that is rather large and indicates the intelligence of the weasel family. Found in both North America and in the boreal forests and steppes across Europe and parts of Asia, the least weasel has scent glands like all of its cousins. Though usually used to mark territory, when disturbed, they may issue a high-pitched squeak and release musk as a warning.

While many Mustelids (the family that includes weasels, mink, badgers, skunks, otters, wolverines, fisher and marten) have the interesting adaptation of delayed implantation, the least weasel apparently does not and can breed more than once per year. (Delayed implantation is where after summer or fall breeding, embryos develop to a 50-cell, “blastocyst” level after breeding. Implantation does not occur for several months into late spring when the condition of the female determines how many implant and develop.) Least weasels are pregnant for about 34 days and give birth to naked and deaf young in hollow logs and tree stumps or rodent burrows the mother has occupied. Litter sizes average about 5 and young are weaned at about 24 days, hunting on their own at about 12 weeks of age. Predators on least weasels include hawks and owls and some snake species.

Though usually traveling in rodent runs under the duff of grasses or tunneling under the snow, the diagonal lines, about 1 inch across and 6-8 inches apart in shallow snow will identify the loping gait and presence of this fellow. Quietly follow the circuitous route of this hunter to a burrow. Stand quietly and make mouse-like sounds by “kissing” the back of your hand and you may catch a glimpse of this curious weasel as it peaks from its burrow, hoping to find another mouse to eat!

Least Weasel Photo © James F. Parnell