1. Do Weasels Really Live in House Attics?
At night, if you hear pattering footsteps in the attic, the culprit might be a weasel. It is explained that weasels make nests in burrows, but they can also enter houses, live under floors or in attics, and breed there [5]. So, where do they live before coming to a house? The clue lies in their food. According to an Okinawa prefecture control plan, weasels are mainly nocturnal and active all year. They eat a very wide range of foods, including frogs, rodents, birds, insects, crustaceans, and fish [6]. Because their diet is so broad, any place with food becomes a place they can live in. A human house is just one of those places. The next question is that the animal called "weasel" is actually not just one species.
2. What Are the Two Types of Weasel, and How Do Tails and Faces Tell Them Apart?
The animal native to Japan is the Japanese weasel. It naturally lives in Honshu, Shikoku, Kyushu, and surrounding islands [1]. The other type is the Siberian weasel, an animal whose name changed in 2021; it was previously called the Korean weasel. It naturally lives only in Tsushima Island in Japan. Those brought in around 1930 have spread, and it is now mainly found in western Japan, with Ishikawa, Gifu, and Aichi prefectures as its eastern limit [3]. A rough guide for telling them apart is the tail. If the tail is more than half the length of the head and body, it is considered a Siberian weasel; if shorter, a Japanese weasel [5]. However, the tail ratio alone is not enough to decide, as some individuals have overlapping values, so facial patterns are also checked. Comparing females, the Japanese weasel has a head-and-body length of 16–25 cm (about 6–10 inches) and a tail length of 7–9 cm (about 3–4 inches), while the Siberian weasel has a head-and-body length of 25–31 cm (about 10–12 inches) and a tail length of 13–16 cm (about 5–6 inches), making the Siberian weasel larger [5]. There are also differences in the face. The Japanese weasel has a dark brown patch from the center of the forehead to the nose, while the Siberian weasel may have white patches around the nose and lower jaw [5]. Note that "half" is just a rough guide, and it is organized that adult Japanese weasels, both male and female, do not exceed a tail ratio of 50% [1].
3. In What Kinds of Places Does the Japanese Weasel Live, from Plains to Mountains?
The Japanese weasel lives in a fairly wide range. The Oita Prefecture Red Data Book explains that it lives widely from plains to sub-alpine zones and can adapt to farmland, grasslands, rivers, and forests [2]. Materials from the National Institute for Environmental Studies also state that its main habitat is the plains, but in western Japan, it is mainly found in mountainous areas [1]. Then, the Siberian weasel entered these areas. According to Oita Prefecture’s description, since the invasion of the Siberian weasel after the 1960s, the Japanese weasel is rarely seen in the plains, and there is concern that it may disappear in some areas due to competition. It is confirmed to be present in mountainous areas and along streams within the prefecture [2]. This is the description of the Japanese weasel in Oita Prefecture. It is important to note that this data is specific to Oita. In Oita’s materials, it is not known how the two species live in the same place [2]. Therefore, we cannot say that Japanese weasels have disappeared from plains nationwide. In Gifu Prefecture, a 1989 museum report states that out of 11 weasels examined in the prefecture, only 1 individual had a tail ratio of 55% and was identified as a Siberian weasel. The other 10 individuals had ratios of 50% or less, with an average of 42% excluding females [4]. This is a record of a small number of specimens in the prefecture, and the distribution across the entire prefecture is not known.
4. Why Were Weasels Brought to Islands in Japan?
Weasels were brought to islands within Japan to reduce rodents and, in the past, for their fur. According to the National Institute for Environmental Studies, the first examples were introductions to Hokkaido in the 1880s and to the Tokara Islands in 1925–27 [1]. The Okinawa Prefecture control plan is more specific. Between 1957 and 1971, about 12,000 weasels were released on 21 inhabited islands, including the Okinawa Islands, Daito Islands, Miyako Islands, and Yaeyama Islands, as a measure against agricultural damage caused by rodents. Populations remain on at least 12 islands today [6]. If you divide about 12,000 weasels by 21 islands, the average is about 570 per island. However, this is a simple average, and the number on each island likely differed. The background to weasels spreading on islands was the human decision to try to reduce crop damage from rodents. Today, the impact on native reptiles and amphibians on the islands is a problem [6].
5. How Can the Same Weasel Be Native in One Place and Invasive in Another?
A Kagoshima Prefecture invasive species card summarizes how the status of the weasel changes by location. On the mainland of the prefecture, it is a native species. However, it was brought to the Tokara Islands, Kikai Island, Okinoerabu Island, and Yoron Island, where it is considered an invasive species. On these islands, it is designated as an emergency control species [7]. The animal itself did not change; whether it is native or invasive depends on whether humans brought it there. On the mainland, it is an animal that has been there from the beginning. On the islands, it becomes a newcomer because humans brought it there.
6. Where Does the Stoat, Which Turns White in Winter, Live?
Among weasel relatives, there is also the stoat, which turns white in winter. It is a different species from the Japanese weasel and is a much smaller animal. According to Gifu Prefecture’s explanation, the stoat lives in limited areas from the mountain zone to the alpine zone and moves to lower elevations in winter. In summer, its back is brown and its belly is white; in winter, its whole body becomes white. It eats small mammals like rodents and moles, as well as birds [8]. The Japanese weasel lives from plains to mountains and islands, while the stoat lives in limited areas from mountain to alpine zones. Even within the same family, the range of places they can live differs.
7. How Can You Safely Observe Weasels Near You?
If you see a weasel, look at its tail length and face color. However, since it is a wild animal, do not approach it, touch it, or give it food. Just watch it quietly from a distance. Look at a Japanese map you have at home and find your prefecture. Next, find where Oita, Kagoshima, and Okinawa are, and trace the distance and sea separation from your prefecture with your finger. Think about how the weasel’s way of life and its relationship with people might differ depending on the environment.