Japanese weasel

Japanese Weasel alone on a rock

Mustela Itatsi


  • Ecology
  • Diet
  • Hunting
  • Distribution & Habitat
  • Vermin Control
  • In Japanese Culture
  • Folklore

The Japanese Weasel (Mustela itatsi) is a carnivorous mammal belonging to the genus Mustela in the family Mustelidae. The nearest phylogenetically Mustela species is the Siberian weasel (Mustela sibirica). The name of the taxonomic species, itatsi is a corruption of the Japanese word for weasel, Itachi (イ タ チ). It is native to Japan where it occurs on the islands of Honshu, Kyushu, and Shikoku. It was introduced to Hokkaido and the Ryukyu Islands for rodent control, and was also introduced to Sakhalin Island in Russia.

It has an orange-brown coat with darker markings on the head and varies in size depending on the sex. It has a long slender body, a long tail, relatively short legs and sharp claws. It is often confused with the Siberian weasel, which has a different tail to head ratio and body length. Unlike other weasel species, their coat does not change color in winter. The average lifespan of the Japanese weasel depends heavily on food availability and less on other factors affecting its life. In the wild, it can live from 2 to 3 years, with a maximum life span of about 5 years.The Japanese weasel has a tail ratio of 36-50%, while the Siberian weasel has more than 50%. The Japanese weasel population has declined by 25% in the last 3 generations, which has been used to justify its endangered status on the IUCN Red List. The Japanese government has banned the hunting of female Japanese weasels as a conservation measure for the species.

This is often classified as a subspecies of the Siberian weasel (M. sibirica). The two species are very similar in appearance, but differ in the ratio of tail length to head length and body length. There are also genetic differences that suggest that the two species diverged about 1.6-1.7 million years ago. Mitochondrial sequencing of the two species suggests that the two species diverged in the early Pleistocene. Their ranges now overlap in western Japan, where the Siberian weasel was introduced.

Adult male Japanese weasels can reach 35 cm (14 inches) in body length with a tail length of up to 17 cm (6.7 inches). Females are smaller. This species is usually found in mountainous or forested areas near water.Its diet includes mice, frogs, reptiles, insects, and crayfish.

japanese weasel characteristic

Japanese weasels are sexually dimorphic, with males weighing about three times as much as females   Despite this, there is no significant observable difference in tail ratio between the sexes. Both sexes are capable of producing a smelly secretion known as musk from the anal gland used to deter predators which is used by rubbing this secret against rocks, branches and other natural obstacles. They become sexually mature before they are a year old and have four to five offspring. Offspring are weaned after 8 weeks.

Japanese weasels are responsible for protecting young trees in winter because they prey on rodents that gnaw at the roots, while wintering snakes and owls cannot control the rodent population.

Diet of the Japanese Weasel

The Japanese weasel’s diet includes a variety of animals, including mice, reptiles, crayfish, and frogs. They eat non-meat foods such as berries, seeds, and fruit, but usually only when hungry   In addition, there is a difference between the diets of males and females because of their sexual dimorphism. 1 Males tend to eat more mammals and crustaceans, while females have a more varied diet of insects, fruits, and earthworms. Their stomachs can only hold 10-20 grams of food, and since small rodents weigh 15-30 grams, Japanese weasels cannot eat more than one small rodent in one sitting.

Their diet varies seasonally with the scarcity and availability of certain foods   Fish and insects form part of their diet at all times of the year. However, seasonal variation means that the most consumed foods in season are hard-winged insects in spring, fruit in summer, straight-winged insects and crustaceans in fall, and fish and fruit in winter. One of the dangers of urbanization to Japanese weasels is their intolerance to artificial food.

Japanese Weasel Temperament

japanese weasel

Japanese weasels live solitary lives and because they are prey to many species, weasels can be observed behaving cautiously. The only situations that cause them to abandon their solitary lifestyle are mating and mothers feeding their cubs. Their temperament and activity is largely controlled by a balance of several basic needs: finding food to maintain metabolism, avoiding inclement weather and predators, finding partners, and feeding their cubs.

They move cautiously into open areas and live in dens in logs and stumps. They react quickly when they think they are in danger and quickly flee to safety. Their dens are lined with grass or feathers and are known to store food in special hiding places. Communication between weasels is achieved acoustically and chemically. Territory is determined by the marking of the ground with musk. 

Aromatic markings contain information that other weasels can understand about “sex, identity, social status, and breeding conditions, but also about the likely outcome of a confrontation.” (King, K.) The cries and sounds made by the Japanese weasel are varied and include trills, squeals, hisses, and barks. Each sound is used in different scenarios and circumstances, from marking imminent danger to comforting the young. The vision of the Japanese weasel is not hampered by darkness.

Japanese weasels have a polygynous mating system, as the male often never sees the female again after copulation. The male tracks the female by analyzing her scent traces. After the female is tracked, there are hours or days of playful biting before the relatively brief act of copulation.

Like other weasel species, Japanese weasels, especially males, are ferocious fighters. In spite of this, they are able to avoid confrontation, retreating from areas where another weasel’s scent remains.

Japanese Weasel Hunting

Weasels hunt regardless of whether it is day or night, instead they hunt according to their hunger level. Japanese weasels hunt most often along rivers, although they sometimes enter suburban areas and meadows in search of prey.

Because of their high metabolism, weasels have to hunt constantly to meet their energy needs. Japanese weasels, like other weasels, use their agility to their advantage because they are confident swimmers, climbers, runners, and are able to reach prey in hard-to-reach places. They use their long body to aid in the hunt, as they hug their prey to suppress its struggles. In addition, prey can be reoriented in its bodily grip. Even after a deep bite, the Japanese weasel’s prey hangs on for a while. Prey can be found by smell, sight, or hearing, although the kill is ultimately by eye.

japanese weasel hunting

Japanese weasels explore holes and cracks to track down smaller prey. Small prey, such as mice, are hunted by piercing the brain and neck with their teeth and carrying them by the neck. Japanese weasels prey on some larger animals. Foxes, martens, birds of prey, and domestic cats prey on Japanese weasels and pose a threat to their lives. Despite this, Japanese weasels have been known to hunt larger prey, such as rabbits. Even as prey, rabbits are a threat to weasels and are capable of kicking and dragging in defense. In winter in snowy areas, Japanese weasels chase rodents through tunnels in the snow, using their slender bodies to their advantage.

Caching is used by Japanese weasels when they kill multiple victims in a single session   This occurs when weasels see multiple victims because their willingness to hunt is immediate and instinctive when a victim enters their sight regardless of hunger level. Japanese weasels maintain this supply of excess food in their hiding places, and this is especially important in winter, as there are days when the weather interferes with hunting. Where there are chick flips or other areas with extremely vulnerable prey, the stored food supply can far exceed their dietary needs  

Distribution & Habitat of Japanese Weasel

Japanese forest

Because they have thin fur and a long body, Japanese weasels choose their nest. Males and females choose their nests differently, because females need habitats with higher quality food than males. This is due to the need to feed both themselves and their young.

The Japanese weasel inhabits grasslands, shrublands, forests. And plantations. They are not greatly affected by altitude, with an upper limit of 336 meters. Of the forests they inhabit, they inhabit both natural and secondary forests. They are widespread throughout Japan, but have evacuated much of western Japan and the lowlands, where the introduced Siberian weasel has become the dominant weasel.

The coexistence of countryside and Japanese weasels poses a danger to weasel life, as many dead Japanese weasels die on the roads. In addition, they are not usually found in large cities. They are not found at all in urban Tokyo. Reduced human activity in rural areas leads to the return of Japanese weasels to these areas. This has been seen in the rural areas of Oichiya, Japan. The area that has seen the greatest decline in Japanese weasels is western Japan.

Japanese Weasels used as Vermin Control

The Japanese weasel was used by the Russian and Japanese governments as a means of controlling vermin.

To solve the rat problem on Hokkaido, the Japanese weasel was introduced to the island in the 1880s. It was also introduced to southern Sakhalin in Russia in 1932, but there has been no reliable data on the current status of the Japanese weasel population there since 1980.

Introduction of the species in some cases, especially Miyake Island has unintentionally affected the population of Japanese songbirds as they prey on the bird’s eggs and chicks. Before the introduction of the Japanese weasel, predation on songbirds was rare. After the introduction of the Japanese weasel, the survival rates of the warbler at the chick and incubation stages were about 0.49% and 0.84%.

Because they prey on domesticated birds such as chickens, they are thought to have a negative economic impact on humans.

In Japanese Culture

The Japanese weasel has entered everyday Japanese language. In Japanese, “itachi gokko” or “weasel play” refers to the vicious circle of repeating the same message. The scent of the musk they emit gives rise to the saying “itachi no saigo-pei,” which translates to “the last fart of the weasel,” which is used to describe the last words of an unpopular speaker. A person who displays bravado only when he has no formidable opponents can be called “itachi” or “weasel.” Despite the everyday use of these phrases, they do not reflect the true temperament of the Japanese weasel.

A village Japanese legend states that the Japanese weasel turns into the Japanese marten (Martes melampus) when it reaches 100 years of age.

Although not originally depicted as a weasel, the yōkai To explain the phenomenon of cutting winds was eventually represented by the Japanese weasel. Named Kamaitachi (鎌 鼬) the yōkai is depicted with long, sharp nails shaped like sickles. It is a very recognizable yōkai that is found throughout Japan, but is most commonly found in the snow-covered north of Honshu. The characteristics of the yokai vary slightly throughout Japan. For example, it is a trio of three weasels, each with its own distinctive action. One to push the victim, another who inflicts injury with a blade, and a third who administers a healing ointment

Japanese weasel - shrine

The existence of kamaitati was recognized as early as 1911, when the British Medical Journal reported on the so-called “Kamaitati disease,” which was called a spontaneous wound.56 The wound is crescent-shaped and arose spontaneously during a thunderstorm as a result of a temporary vacuum. (Y. Tanaka), writing for the British Medical Journal, explains the phenomenon that “. during a thunderstorm, a temporary vacuum may form in some places as a result of random air currents, and if a part of the body enters such a space, the tears may be the result of internal pressure not altered by the action of external pressure.