Least weasels are the smallest carnivorous predator in the world and have a typical weasel body shape, with a long, tubular body, short limbs and a short tail that is less than a quarter of their body length. Their skull is long and flat, with short, round ears, long whiskers and large, dark eyes. Their white paws are five-toed with non-retractile claws at the end of each of the five toes. Least weasels have a total of 34 teeth. Their coat color changes seasonally in northern populations, but not in southern populations. In the winter, their coat is pure white, but always lacks the black tail tip seen in two similar species: ermines, or stoats (Mustela erminea), and long-tailed weasels (Mustela frenata). In summer, their coat turns rusty-chocolate brown on their back and stays white on their belly. Their body size changes depending on where they live. The largest least weasels are found in warm climates, such as individuals in North Africa, which can weigh over 70 grams and can reach total lengths (including the tail) over 217 mm. The smallest weasels are in North American and weigh about 45 grams, and average about 190 mm in length (including the tail). Males are larger than females by approximately 20 to 30 cm and 30 to 50 grams. There are about 10 subspecies of least weasels that live in different areas but look the same physically. (Casey and Casey, 1979; Feige, et al., 2012; Innes and Hay, 1991; Iversen, 1972; Naughton, 2012; Sheffeld and King, 1994; Zub, et al., 2012)
Least weasels are widespread throughout the Northern Hemisphere. They are native to North America, from Alaska, south throughout Canada and the northern United States, and Europe (excluding Ireland, Greenland, and Iceland). They have also been introduced onto islands such as New Zealand, the Azores, Crete, and Malta. (Naughton, 2012; Sheffeld and King, 1994)
Least weasels are able to thrive in many habitats. They are found in prairie grasslands, forests, open tundra, bushy areas, and rainforests that have a dry season in the summer months. Least weasels are comfortable above and below ground, moving easily through leaf litter, underground, and under snow. Weasels have dens made of different materials in different habitats but do not burrow or dig dens; they use the abandoned dens from other species. Weasels only live in dens temporarily and many dens can be found in a single least weasel’s territory. Weasels often choose dens at the base of trees in habitats with tree stands, such as mixed forests. When trees are not available, weasels will reside in brush or log piles and tall grass patches, such as those found in prairie habitats or on agricultural lands. The plants in their habitat are not as important as the amount of cover for hunting, as cover is very important for a successful hunt. (Innes and Hay, 1991; King, 1989; Murphy and Dowding, 1994; Naughton, 2012; Sheffeld and King, 1994; Ylönen, et al., 2003)
Least weasels are polygynandrous, which means males and females mate many times with multiple partners. Although least weasels mate with many partners, males do not compete for females. They mostly breed in the spring and summer months as birthing earlier in the year increases the survival rate of the young, but breeding may also occur at different times throughout the year. (Sheffeld and King, 1994; Sundell, 2003)
Male least weasels defend territories, typically against other males, but during the breeding season, they abandon their territories in search of females in estrous. Because males are larger than females, males are able to invade a female’s territory at any point in the year. Females defend a territory largely against other females, but will defend a home range, regardless of the intruders’ gender, during late-stage pregnancy and lactation. Courtship is a rough process, which includes fighting, biting, and tumbling about until the male is able to grasp the female at the nape of her neck. The amount of prey available impacts the number of litters per year and the number of young per litter. During years of high prey and in areas such as the Arctic, where lemming populations can reach high numbers, up to 15 offspring can be born, and up to three litters per year can occur. On average, there are only one or two breeding events per year. Gestation lasts about one month, and an average of four or five young, called “kits”, are born. Kits are born hairless and helpless, and weigh between 1.0 to 1.7 grams at birth. (East and Lockie, 1965; Erlinge, et al., 1982; Innes and Hay, 1991; King, 1989; Sheffeld and King, 1994; Sundell, 2003)
Soon after mating, males leave and females care their offspring, with no help from the male. Young are born hairless, blind, deaf, and helpless and rely completely on their mother for their survival. Kits rely on their mother’s milk for about 32 days, after which weaning begins, although the mother may bring meat to the kits when they are about two weeks old. At about 47 days, the kits are able to kill prey for themselves and at about nine to ten weeks, the kits leave and become independent. Kits raised in captivity kill prey with no previous experience or exposure to prey; however, kits that stay with their mother and gain hunting experience have higher hunting success rates. (Naughton, 2012; Sheffeld and King, 1994; Sundell, 2003)
Because least weasels are so small, have limited fat stores, and have high metabolic demands, they likely do not live long in the wild. The majority of young do not make it to the age of weaning, especially in the second and third litters of the year, likely due to the increased predation risk on kits in nests. The average lifespan for wild individuals is short; only 1 or 2 years, whereas the longest recorded captive lifespan is 10 years. (Naughton, 2012; Sheffeld and King, 1994; Sundell, 2003)
Least weasels, similar to other weasel species, are usually alone, except during the breeding season, where males, which normally stay in one place, travel to find females. Least weasels form hierarchies, where older males are dominant over females and juvenile males. Males are thought to be dominant over females because they are larger in mass and length. Dominant males make physical and vocal threats, and will attack submissive males, which retreat, sometimes squealing as they leave. These attacks rarely happen to females, as females fight back unlike submissive males, although females will submit to the dominant male. Least weasels must eat very regularly to avoid starving and often foraging at all hours, day or night. Weasels can consume over 50% of their body weight every day and even more so in the winter months. Least weasels can only eat a few grams of meat per meal, although they can kill prey larger than themselves, in such cases they store the food for later, a practice known as caching. Caching is especially important for females with kits. Caches are concealed around the den entrance. An individual will scent-mark around a den site using secretions from their anal glands. When cornered or startled, anal gland secretions can be discharged, which release a foul-smelling fluid. Weasels also occasionally perform the “weasel war dance”, which is a series of leaps and twists that often include bark-like vocalizations, stiff limbs, and an arched back, with their fur standing up. The purpose of this dance is not known. One theory suggests they perform this dance to confuse their prey, however, they sometimes dance when there is no prey present. Weasels of all ages perform the dance, but it is more common in younger animals, especially kits playing with their siblings. (Innes and Hay, 1991; King and Powell, 2007; King, et al., 2001; Naughton, 2012; Sheffeld and King, 1994)
Least weasels use abandoned dens left by other species and maintain a territory around the den site only if the rodent population within the territory is large. The boundaries are marked by releasing scent secretions from anal glands, but there can be overlap between territories. Males, being dominant, will enter a female’s territory at any given time. In general, each individual has its own defined territory, but there is sometimes overlap as males have a larger territory size than females. Overlap also occurs when the weasel population is high or due to dominance, where a single male’s territory will cover other individual’s territory. Non-dominant weasels rarely overlap territories because of fighting between lower ranked individuals. The dominant male commonly overlaps others, both males and females. Territory size also varies with prey populations, where higher prey populations lead to a smaller home range because they need less time to find prey. (King, 1975; Naughton, 2012; Sheffeld and King, 1994)
Least weasels use their sense of smell for communication and have been known to hunt “with nose” while searching for prey in underground environments. Although their sense of smell is thought to be their most important sense, weasels also use vision and hearing while hunting. Least weasels have long whiskers that may help them sense vibrations. Weasels make many sounds including barks, hisses, chirps, squeaking, squealing, and trilling. Hissing is used in response to a lesser threat, while chirping is in response to a more urgent threat. Squeals are used when the weasel is cornered, and quieter trills are thought to be greetings between mothers and kits, and also greetings between kin. Kits are vocal, using squeaks and chirps to communicate with their mother and siblings. (Burn, 2008; King and Powell, 2007; Naughton, 2012; Ylönen, et al., 2003)
Least weasels, like many other weasel species, are able to kill prey much larger than themselves, then store the remains. Least weasels mostly hunt rodents, if they are available, almost 100 percent of their diet will be made up of rodents, however, they will not overlook an easy meal. Field voles, wood mice, and bank voles make up much of their diet in southern populations. When rodents are scarce, least weasels will also feed on birds’ eggs, lizards, amphibians, small fish, and invertebrates. Rodents, especially collared lemmings in the northern arctic area, are very important for breeding success. The population sizes of least weasels found in northern areas cycle due to the population sizes of lemmings. (Andersson and Erlinge, 1977; Day, 1968; King, 1975; King, et al., 2001; Naughton, 2012; Sheffeld and King, 1994; Sundell, 2003)
Least weasels are most vulnerable to predators during the early days of life while they are still helpless in their nests. Juveniles are often preyed upon by snakes and foxes, while adults are typically preyed upon by owls, such as tawny owls, and other birds of prey, such as falcons, eagles, and hawks. Least weasels may also be preyed upon by other larger weasel species, such as ermines and long-tailed weasels. Least weasels use their camouflaged fur to blend in with their environment and aggressive behavior, such as vocalizations and biting, and by hiding in shelters to escape predators. Least weasels can also release their anal glands when startled or fearful. These secretions contain the strong-smelling compounds that may deter attacks from predators, especially from those that rely heavily on their sense of smell. (Innes and Hay, 1991; Naughton, 2012; Powell, 1973; Sheffeld and King, 1994)
As rodent predators, least weasels help maintain rodent populations. This is especially important in the tundra ecosystem, where least weasels help keep lemming populations in check. Bird species in New Zealand, where least weasels were introduced, are negatively affected by weasel predation, especially ground-dwelling brown kiwis. (Feige, et al., 2012; Henttonen, 1987; King, et al., 2001)
Although least weasels have been blamed for attacks on domesticated chickens and other livestock, there is little proof to suggest that least weasels prey upon any domestic livestock. (Naughton, 2012)
Least weasels are effective rodent predators. By preying on rodents, which can transmit disease, eat economically valuable crops, and cause extensive property damage, humans directly benefit both economically and health-wise from least weasels. Trappers are also able to benefit from least weasels caught in traps set for larger animals. Least weasel pelts are not very valuable in Canada, but some weasel pelts are used as lining and trim on coats and mittens. (Naughton, 2012; Sheffeld and King, 1994; Naughton, 2012; Sheffeld and King, 1994)
While they are not considered rare in North America, least weasels are more common in Europe and Asia, and are not globally threatened. As a whole, populations of least weasels are considered stable. (Tikhonov, et al., 2013)
Gina Campbell (author), University of Manitoba, Jane Waterman (editor), University of Manitoba, Leila Siciliano Martina (editor), Animal Diversity Web Staff.
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