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Yet another mustelid from the Land of the Rising Sun!

The Japanese Weasel (Mustela itasi) is often confused as a subspecies of the Siberian Weasel (more on them later in the series) because their coloring is so similar, and they are distantly related. Significant differences in body length and tail ratios, as well as genetic study, however, have confirmed that the Japanese Weasel is actually a distinct species. They are endemic to the islands of Japan, and have been intentionally introduced to the Ryukyu Islands (a small island chain that runs from southern Japan to Taiwan) and Sakhalin Island in Russia as a means of controlling the rodent populations.


Adult Japanese Weasels have the classic elongated weasel body, and their pelts vary in color depending on the season. During the summer, they are usually an orange-brown color with darker markings on their eyes, like a mask, as well as pale patches on their chins and necks. During the winter, their bodies stop producing melanin and their coats start to turn white as their thicker winter coats grow in. They can grow up to 20.5 inches (52 cm) from nose to tail, and weigh just under a pound (450 grams). Females are smaller than males, on average.

The diet of Japanese Weasels consists mainly of other small mammals like rats and mice, as well as insects, birds and eggs, fish, crustaceans, frogs and reptiles. As previously stated, their taste for rodents have made them extremely useful in controlling the populations of pests on small islands, and it’s likely that they also help in spreading seeds around by eating fruits and other vegetation and depositing the seeds in their feces. Japanese Weasels are sometimes targeted as prey by larger animals, mostly birds of prey.

The length of the mating season depends on the abundance of prey - in times of plenty, the mating season will last from late spring to late summer, instead of just May and June. Males seek out females to mate with, tracking their scent and then putting on mating displays to entice the female to mate with him. After they copulate several times, they go their separate ways and will most likely each have different partners the next season. After a gestation period of 29 days, the female will give birth to a litter of about five kits, although litter sizes have been as big as twelve! The female will care for her young until they are fully weaned, about eight weeks, after which the kits will strike off on their own.


Japanese Weasels are generally solitary, but they do communicate with each other in a number of ways. The primary method of communication is through smell, as they will mark their territories by rubbing their musk glands on trees and rocks. In weasel-to-weasel interactions, they have a wide variety of vocalizations to choose from. Female Japanese Weasels will trill to their young to soothe them, and any weasel will hiss, bark or screech when they sense danger. They are considered to be a species of least concern by the IUCN.

Source for all images used in this post.

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