Japanese Weasel

MammalJapan

Sep 26

Mustela itatsi

The Japanese weasel's agility and lithe body allow it to pursue its prey anywhere; down tunnels, up trees, or into water. During winter, it spends its time chasing rodents through snowy tunnels and, after catching and eating its target, it lays down to enjoy the warmth of its prey's burrow.

The Japanese weasel is a creature both sly and shy, bold and bashful. A nimble-footed hunter and a solitary recluse. It is pretty but also pungent. An inspirer of Yōkai myths and an exterminator of vermin. It is a many-faceted creature, naturally found nowhere else but the Japanese archipelago.

A Lithe Assassin

The body of a Japanese weasel is slender and long, carried on stubby little legs and ending in a thick, bushy tail. It measures some 48 centimetres (19 in) from head to tail-tip, with males larger than females — some sources claim a male can weigh three times as much. The weasel's coat is one of golden-orange fur, lighter along the belly and dark across its face, like a thief's mask. Meanwhile, below its nose and down its chin, its fur is stark white. And more often than not, the white fur of its mouth is painted red with the blood of its prey.

The weasel's lanky body retains little heat and energy. To compensate, the weasel is on a near-perpetual hunt. Its advantage is that of speed and lithe agility. Anywhere that its victims can flee, the weasel can follow. Despite its short legs, the weasel can spring and sprint in a furious pursuit, like a guided arrow on the heels of a rodent or rabbit. Its slender form can slip into the narrowest of tunnels, of soil or snow, to follow sheltering prey. Or bound up bark to hunt in the arboreal realm, where it snatches birds — like the Japanese bush warbler — and their eggs. Not even aquatic denizens are safe, for the weasel has semi-webbed feet and strong swimming skills, able to seize slippery amphibians and armoured crustaceans. The Japanese weasel is a little assassin; a ninja that can scale any wall, swim any waterway, and sneak into any shelter.

So ingrained is its killing instinct that the Japanese weasel will make chase even when full and satisfied. If prey enters its line of sight, its lifespan will likely be short. All of the weasel's extra kills go into its hidden cache; a winters-day stash, for when prey is less abundant or weather prevents the weasel from hunting. Weasels that live near places plentiful in vulnerable prey, such as chicken coops, may have caches overflowing with the bodies of their victims — more meat than they could ever hope to eat. Other weasels, living in more remote areas, may have to resort to foraging berries and other fruit during leaner times.

Japan's Weasel ¹

In the past, this weasel was found only on three of Japan's main islands; Honshu, Kyushu, and Shikoku. It is as flexible in habitat preference as it is in hunting strategy; residing in a variety of environments, but most abundant in mountainous and forested areas, usually near rivers.

The Japanese weasel is rarer in large cities, apparently absent in urban Tokyo. But it is present in central Osaka — to this I can personally attest. My only encounter with this weasel was with one that I saw slink out of some parking lot bushes before scurrying away into an alley, just a few streets from the Umeda Sky Building. ² So, at least some individuals seem to have adapted to urban living. And, not only has it infiltrated our cities, but the Japanese weasel has long since spread beyond its natural range.

The Island Exterminator

In the late 19th century, this weasel's hunting prowess was noticed by people — people who had pest problems, specifically with rodent populations overrunning islands. The Japanese weasel was captured, transported, and released onto these various islands; from the scattered, subtropical Ryukyu Islands in southwestern Japan to the large and cold island of Hokkaido in the north. The hope was that the weasel would act as an exterminator. It was good at its job. Too good, in some cases.

A Japanese bush warbler — prey to the Japanese weasel.

Miyake-jima is a volcanic island some 180 kilometres (110 miles) southeast of Tokyo. From its dense thickets and forest edges, the “uuuuuu-guisu" warbles of Japanese bush warblers would reverberate across the island. With few to no predators preying on their nests, these little songbirds lived tranquil lives; the chicks blessed with safe childhoods. That all changed In the early 1980s, when the Japanese weasel was purposefully introduced to Miyake-jima. A study from 2009 found that the survival rate of nestling chicks had dropped to around 85%, while that of incubating eggs was only 50%. No predators were seen attacking the nests save one; the introduced weasel. As such, the main factor determining a nest's safety was its proximity to a weasel's range — if it was within a weasel's hunting territory, the chances of the eggs hatching, or the chicks growing up, were significantly lower.

The takeaway of this study was that the "Japanese Weasels must be regarded as a major threat to breeding birds on Miyake Island". This is just one case, on one island, studying one bird species. But the lesson is applicable to anywhere that such a proficient predator is introduced. It must be asked whether the goal — rodent control, in this case — is worth the cost — the decline or loss of native bird species. ³

Melees, Musk, and Mating

For much of its life, the Japanese weasel is alone. And it seems to like it that way. Any breach of territory is taken as a personal affront, resulting in aggressive barking, screeching, and hissing, especially between dominant males. This weasel's main form of communication is olfactory. It possesses anal glands that secrete a pungent substance known as musk. Although not as offensive as a skunk's spray, this musk is noxious enough that it dissuades most mammalian predators from making the weasel into a meal.

If its musk isn't enough, the weasel puts up a vicious fight, even against much larger foes. However, less aromatically-picky, aerial predators, such as birds of prey, are more willing to hunt the aggressively stinky noodle.

The weasel's musk is also applied liberally throughout its territory — turning branches and stones into smelly signposts — and can act as a warning to trespassers or as a kind of dating profile; communicating things like the weasel's gender, age, social status, health, and breeding condition.

The Japanese weasel dating scene is very casual, comprised of many one-night stands and little commitment. Sometime between May and late June, a male weasel will begin to try his luck. He tracks down a female by the musky marks she leaves behind. Upon finding her, the two embark on a date that can last between several hours and several days, and once the female is ready to mate, she lets the male know. The male, however, is not much of a gentleman. He bites the female by the neck and holds her down. The act is brief, and when it's done, the two go their separate ways and never meet again.

Maternal Care

The raising of a family is left wholly in the female's paws. After about a 30-day pregnancy, the new mother gives birth to anywhere between 2 and 12 little kits (litters typically number 5 or 6 kits). She shelters them in a nest — one tucked in some form of cavity, whether an abandoned burrow or a log hollow — which she lines with soft furs and grasses. They remain safe and snug, suckling their mother's milk, until their razor-sharp canines begin to grow in.

Soon the kits learn some control over their noodly bodies and begin their journey to becoming master assassins. Their first practice comes in the form of play with one another. As they grow older, they accompany their mother on her hunting trips, tagging along to witness how it's really done. Only once the kits learn to hunt for themselves, once they can make their own kills, are they considered independent. They leave their mother then, and find territories of their own. The whole process, from birth to independence, only takes some 8 weeks.

The Japanese weasel lives fast, but not long. At one year, a weasel is considered sexually mature. At two to three, it is a seasoned adult. At five it has lived an exceptionally lengthy life. In captivity, some weasels have made it to the elderly age of eight, but the wilds are less kind — a lifespan of two years is the average.

Kamaitachi: The "Sickle Weasel"

It is the extraordinary — and probably non-existent — weasel that makes it to 100 years old. Japanese countryside folklore declares that upon hitting such an age, a weasel will mysteriously transform into not one, but ten martens (another weasel-like mustelid, present throughout Japan). Why an aged weasel should spontaneously divide into ten slightly different mustelids is anyone's guess.

A more widespread legend pertains to a particular yōkai — a Japanese supernatural spirit or monster. Travelling the trails of the Japanese Alps, you may find that you lose your footing amidst a strong gale. This could be the work of a mischievous yōkai trio. The kamaitachi, or “sickle weasels", ride the mountain whirlwinds in threes. A gust of wind carries them speeding past you; the first slices at your legs and topples you, the second uses its sickle claws to inflict a thousand little cuts, and the third applies a healing balm. So fast and precise are the kamaitachi that you don't even notice their attack; you just chide yourself for your clumsiness and grumble at all the cuts you've taken in your fall.

Clockwise from top left; a Japanese badger (*Meles anakuma*), Japanese marten (*Martes melampus*), sea otters (*Enhydra lutris*), and a Siberian weasel (*Mustela sibirica*).

¹ The Japanese weasel is far from Japan's only mustelid. There is a Japanese badger and a Japanese marten. The sable and Eurasian stoat are native to Japan. Meanwhile, the American mink is an invasive species. There are even sea otters in the waters along Hokkaido's eastern coast.

There is also another weasel, the Siberian weasel, which was introduced to Japan between 1920 and 1930 (in return, in 1932, the Japanese weasel was introduced to the southern part of Sakhalin Island in Russia). The two species look very similar — so similar that some taxonomists consider the Japanese a subspecies of the Siberian. The only obvious physical difference is their tail-to-body ratios; the Siberian has a longer tail relative to its body (over 50%) than the Japanese (36–50%). However, genetic differences suggest that the two species split some 1.6 to 1.7 million years ago, and although they now overlap in parts of Japan, where one species dominates the other is typically absent — in the western lowlands, for example, where the Siberian weasel is more common.

² While in Japan, I travelled through most of Honshu southwest of Tokyo — exploring the cities, but also more rural areas and hiking spots like the mountainous forests around Karuizawa or those of Mt. Daisen. However, this unexpected glimpse, in the very centre of Osaka, is the only sighting I've had of the elusive Japanese weasel (if it was indeed a Japanese and not a Siberian weasel — I didn't get the opportunity to measure its tail).

Introduced Mustelids & Endangered Island Birds

From top left to bottom right; a kākāpō (*Strigops habroptilus*), South Island takahē (*Porphyrio hochstetteri*), rifleman (*Acanthisitta chloris*), kea (*Nestor notabilis*), wrybill (*Anarhynchus frontalis*), and a morepork (*Ninox novaeseelandiae*).

³ The Japanese bush warbler happens to be a pretty common species, found throughout most of Japan and a few surrounding countries. As such, the introduction of the Japanese weasel to Miyake-jima is a small threat to the species as a whole (the Japanese bush warbler is considered to be of 'least concern'). However, there was a marked decline in the warbler's breeding success after the weasel's arrival. Now, imagine that the warbler isn't common — that the entire species is found only on one island, where its nests are now being raided by an invading apex killer. That is exactly the situation many endangered island birds find themselves in.

To witness this desperate situation writ large, we must travel far to the south, to Oceania. The islands of New Zealand — the main North and South Islands and the many smaller ones — are an ornithologist's paradise. These islands host over one hundred unique bird species found nowhere else, species that happen to be some of the most fascinating birds on the planet. From the iconic wingless kiwis, to tiny rotund riflemen, inquisitive mountain-dwelling keas, bent-beaked wrybills, and adorably big-eyed moreporks.

Of the 245 species of birds breeding in New Zealand before the arrival of humans, 71% were considered endemic. These endemic birds evolved in the absence of any mammalian ground predators and, as a result, had few to no defences against them. The most obvious example is the loss of flight in the likes of the kiwis, kākāpō, and takahē. These now-flightless species forfeited a means of escaping predators and, unlike flightless birds of the mainland, they didn't evolve compensatory defences — such as the giant size of cassowaries or ostriches. Nor could they secret their nests in the safety of tall trees anymore, leaving their eggs and young particularly vulnerable.

In the 1880s, people brought a trio of mustelid murderers — ferrets, stoats, and weasels — to control the population of rabbits in New Zealand (which themselves were introduced in the 1830s for sport and food). Upon their arrival on these distant islands, the mustelids found that the native and naive birds, and their very accessible nests, made for much easier meals than the spry rabbits. To quote the New Zealand Department of Conservation; "These natural enemies of rabbits rarely controlled rabbit numbers effectively but they have been disastrous for our native species."

Clockwise from top left; a yellowhead (*Mohoua ochrocephala*), kākās (*Nestor meridionalis*), black stilt (*Himantopus novaezelandiae*), and a little penguin (*Eudyptula minor*).

The New Zealand birds already suffered at the claws and jaws of feral cats, who gradually spread across the country after the European's arrival in 1769. The introduction of weasels, ferrets, and stoats — small but proficient killers — made the situation far worse. Today, stoats are the number-one killers of many of New Zealand’s endangered native species. Like the Japanese weasel, the stoat doesn't stop hunting when it's full, but builds up a larder of its surplus kills. As its prey, it takes New Zealand dotterels, black-fronted terns, and baby kiwis, as well as cavity-nesting yellowheads, yellow-crowned parakeets, and kākās. Even big-bodied birds, like adult kākāpōs and takahēs, fall victim to the stoat. Ferrets, meanwhile, terrorise wetland birds, killing black stilts, dotterels, and pied oystercatchers, as well as sea bird chicks, like those of the royal albatross, yellow-eyed penguin and little penguin. Weasels, while less abundant, are just as voracious. Today, over 80% of New Zealand's native birds are at risk of becoming threatened or are already facing extinction.

The point of this footnote is not the villanize these mustelids. The weasels only continue to do as they've always done, hunting to survive, whether in their native range or wherever they've been introduced. The ferrets have no malevolent intentions. The stoats don't wish to drive native birds to extinction.

What I do want to do is bring attention to the harm these predators can cause when introduced, by humans, into foreign ecosystems. With our increasing knowledge of ecosystems, and the infinitely complex interactions that compose them, comes an increasing charge to be responsible for how we affect them. Progress won't come from blaming the immediate cause — e.g. weasels hunting birds — but looking at the ultimate causes — e.g. the initial introduction of weasels, or the rabbits before them, or maybe another underlying issue, like habitat loss or a changing climate. We can learn from our own past and our own follies, and then proceed into the future, wiser, carrying knowledge of how to better safeguard the vulnerable species with which we share the planet.

**Where Does It Live?**

⛰️ Mountainous and forested habitats near rivers.
📍 Native to most of Japan; introduced to Hokkaido and southern Sakhalin in Russia.

‘Near Threatened’ as of 03 March, 2015.

IUCN

Size // Medium
Length // 45 - 52 cm (18 - 20 in) body and tail
Weight // 400 grams (14.1 oz)
Activity: Diurnal and Nocturnal ☀️/🌙
Lifestyle: Solitary 👤
Lifespan: 2 years average (wild), up to 8 years (captivity)
Diet: Omnivore (mostly carnivorous)
Favourite Food: Wide range of small animals 🐤
Class: Mammalia
Order: Carnivora
Family: Mustelidae
Genus: Mustela
Species: M. itatsi

Because of its skinny body, the Japanese weasel retains little heat or energy, and so must hunt often.

Even when this weasel is full, it will attack any prey within its line of sight. It's extra kills go into its hidden cache.

It can be active during both day and night.

The Japanese weasel is a mostly solitary creature. Any breach of territory, especially among dominant males, is taken as an invitation for a fight — involving lots of aggressive barking, screeching, and hissing.

The Japanese weasel can produce a pungent musk from its anal gland.

This musk can be used as a defence, making the weasel unpalatable to keen-nosed mammalian predators. Birds of prey, however, are less picky about how their meals smell, and will readily hunt this weasel.

It rubs its musk across its territory; communicating information like the weasel's gender, age, social status, health, and breeding condition.

A male will track down a female by her musky "signposts". The two then go on a "date" that can last anywhere from a few hours to a few days.

The female lets the male know when she's willing to mate. Once the act is done, the male promptly dips out — chivalry never existed among weasels.

The female raises her kits alone. She keeps them safe in a soft hollow, feeds them milk until their teeth grow in, and takes them on practice hunts.

The entire process, from birth to independence — a weasel's childhood — lasts only around 8 weeks.

The average lifespan for a wild weasel is only 2 years. Some make it to 5 but rarely more. In captivity, a Japanese weasel can live up to 8 years.

The Japanese weasel is native to most of Japan — on the three islands of Honshu, Kyushu, and Shikoku — but because of its high hunting success rate it's been purposefully introduced to many other islands as a form of rodent pest control.

Its introduced range includes many of the subtropical Ryukyu Islands in the southwest, Hokkaido in Japan's cold north, and even beyond, to the southern tip of Sakhalin Island in Russia.

In the early 1980s, it was introduced to Miyake-jima, an island some 180 kilometres (110 miles) southeast of Tokyo, after which the breeding success of the island's bush warblers decreased markedly.

The Japanese weasel can easily be mistaken for the Siberian weasel — which was introduced to Japan between 1920 and 1930. The only noticeable physical difference is tail length; the Siberian weasel has a longer tail, relative to body length, than the Japanese.

There is Japanese countryside folklore that tells of how this weasel, if it makes it to the age of 100 years old, will spontaneously transform into ten martens.

This weasel is also associated with the yōkai called kamaitachi, or “sickle weasels". These yōkai travel in trios on mountain winds; knocking down victims, slicing them up with sickle-like front claws, and then disappearing.