Mustela russelliana

The Sichuan weasel is 14-21 cm long, with a tail length of 3-7 cm and a weight of 50-130 grams. It is very small. It has a slender body. It has very small ears. It has short and dense fur. It has short limbs and walks on its feet. The soles of its feet are covered with short fur, and the toes and palm pads are hidden in the fur. Both the front and rear limbs have five toes, and the claws are slightly curved, slender, and very sharp. There are several long white hairs on the wrists of the forelimbs that point outward. The tip of the penis bone of the male is curved and hooked. There are two pairs of nipples under the armpits of the female, and three pairs of nipples in the shrew. The tail is very short, about 1/5 of the body length.
The Sichuan weasel sheds its fur twice a year, in spring from April to August, and in autumn from late October to early November. It starts from the head, then gradually the center of the back and the sides of the body, and finally the hair on the limbs.

Distributed in Sichuan, southwest China. It only lives in a very small area in China. There are only three collection records of Sichuan weasels, with a total of four specimens collected. Tiancunlu (now Lucheng, 30°03'N, 102°01'E, 4,480 meters above sea level) in Sichuan Province, (Thomas 1911a, b); Taibai Mountain, Zhouzhi County, Qinhuangdao, Hebei (34°N, 108°E, 1950-2750 meters above sea level); and an unnamed point from Beijing at 34°N, 109°E (2400 meters above sea level), and Zhashui County (Groves, 2007).
It lives in mountain coniferous and broad-leaved mixed forests, coniferous forests, forest edge shrubs and other areas. It often invades the nests of small rodents as its nests, and also uses fallen trees, caves, grass and earth holes as hiding places.

Sichuan Weasel (scientific name: Mustela russelliana) is also known as Sichuan Weasel and Lackedteeth Pygmy Weasel. It was once a subspecies of Pygmy Weasel. It was classified as an independent species in 2003 without subspecies.

Sichuan Weasel is fast and agile. Vision, hearing and smell are very good. Usually solitary. Often forage during the day, hunting areas are generally fixed, and will not leave their living area unless food is extremely scarce.

Weasels use abandoned nests of prey species or other animals as roosting areas, and only maintain an area around temporary nests when the rodent population in the area is large. Boundaries are marked by the release of scent secretions from the anal glands, but there is a significant overlap between territories. Males are dominant and will enter the territory of females at all times. Generally, each weasel has its own defined territory, but there is sometimes overlap because the male's territory is larger than the female's. Overlap can also occur when the population density of weasels is high or when the territory of a single male will overlap the territory of other individuals due to dominance. This is thought to be because the dominant male is able to move without conflict, unlike subordinate weasels. Non-dominant weasels rarely have overlapping territories due to strong territoriality and fighting between lower ranking individuals. It is common for males and females to have overlapping territories. The size of the territory also varies with prey density, with higher prey densities resulting in shorter prey hunting times and therefore smaller distances to hunt.

Weasels rely heavily on olfaction for intra- and interspecific communication and are known to "use their noses" when searching for prey in underground environments. While olfaction is considered of primary significance, weasels also use vision and hearing when hunting. Weasels have long whiskers, which can help detect the environment and spatial orientation using the vibrations of the whiskers. Many vocalizations have been recorded from the whiskers: bark rubbing, hissing, chirping, squeaking, screaming, etc. These sounds vary with external stimuli. Hissing is interpreted as a response to smaller threats, while chirping is a response to more pressing threats. A squeak is emitted when the tentacles are forced away, and the quiet trill is thought to be a greeting between mother and pup, and also between kin. Pups are vocal, using squeaks and chirps to communicate with their mother and siblings.

Like other species in the genus, weasels have a habit of killing prey much larger than themselves, then storing and burying the carcasses for later use. Weasels are highly specialized carnivores that rely heavily on rodents for food. They are also opportunistic foragers and will not overlook easy foods like carrion. Voles, wood mice (Apodemus sylvaticus), and European mice (Clethrionomys glareolus) make up the diet of many weasels in more southern populations. If food is abundant, it is almost 100% composed of rodents. When rodents are scarce, weasels will also eat bird eggs, lizards, amphibians, small fish, and invertebrates.

The mortality rate of the weasel is highest early in life, when it is born blind and helpless, and relies entirely on its mother's breastfeeding. Juvenile mortality is common from predation by snakes and foxes, while adults are usually preyed on by owls, hawks, and other birds of prey. The weasel may also be preyed on by other larger weasels, such as minks and long-tailed weasels. The weasel can counteract predators by using its camouflaged cortex and aggressive behaviors (such as vocalization and biting) and hiding in shelters. When the weasel is startled or frightened, it also releases a foul smell from its anal glands to drive away enemies. The secretions from the scent glands contain strong-smelling sulfur compounds such as thiacyclobutane and dithiacyclobutane, which are thought to deter predators.

The weasel has a highly promiscuous mating system, but males do not compete for females, meaning that males and females mate with multiple partners at many times. The breeding season is mostly limited to the spring and summer (because early births greatly increase the survival rate of the young), but breeding occurs intermittently throughout the year. Females estrus for an average of 4 days, during which they will mate with several males. Male weasels defend territories, typically against other males. But during the breeding season, they abandon their territories to seek out females in estrus. Because of the sexual differences and rigid dominance between males and females, males are able to invade females' territories at any time of the year. Females largely defend their territories only in relation to other females. But during late pregnancy and lactation, they will fiercely defend the family's territorial range regardless of the sex of the intruder.

Female weasels need to be induced to ovulate, and courtship is a rough process that includes fighting, biting, and rolling until the male is able to get a hold of the female by the neck. Once the position is established, the male will hang on to the female and copulate for more than an hour. Prolonged copulation is required to stimulate the female to ovulate. Prey density has a significant effect on the number of litters per year and the number of pups per litter. In years when prey is abundant, up to 15 offspring can be born, and up to 3 litters may be produced each year. On average, there are only one or two litters per year. Gestation lasts about 1 month, and there are an average of 4-5 litters. The pups are born hairless and helpless, and weigh between 1.0-1.7 grams.

There are probably only some meaningful guesses about the threats that weasels may face. They live in an area where hunting can be carried out by various methods. This could theoretically lead to a decline in weasel numbers. But the most obvious relative, the common weasel, remains severely persecuted in some parts of its range. This cannot be assumed to apply to the Sichuan weasel. Habitat degradation, fragmentation and conversion are extensive throughout its range. But there is no evidence that it depends on forests or any other natural habitat, and the impacts of this habitat change are unknown. The most urgent and priority for the conservation of the Sichuan weasel is to find an existing species and then understand the basic threats faced. Otherwise, it is difficult to speculate on its conservation needs. The wide coverage of protected areas in Sichuan, especially at high altitudes, suggests that the species may inhabit one or more protected areas and may not be immediately threatened with extinction unless it has highly specific habitat use.

Listed on the 2016 IUCN Red List of Threatened Species ver 3.1 - Data Deficient (DD).

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