Family Mustelidae - Mustelids

PENNSYLVANIA WILDLIFE (VOL. XIX, NO. 1) – Weasels attack prey by biting hard on the base of it’s preys skull. Often, it will only eat the brain, internal organs and major muscle masses. The long-tailed weasel, at 17" (head to tail tip) is the largest and most common of the three common North American weasels (along with the short-tailed – or ermine (at 11"), and least weasel (at 7 to 8", the smallest carnivore on the continent). In October, weasels molt into an almost pure white pelage that creates a perfect camouflage when the snow flies. In the southern part of their range, including the southern two-thirds of PA, the long-tailed weasels molt into a pale brown winter coat. It’s body is designed to allow it to enter underground burrows in search of mice, squirrels, and other subterranean nesters. Males are sized larger than females to eliminate competition for food prey. For example, females (25% smaller than males) may feed on deer mice, while males feed on larger chipmunks. Least weasels can breed throughout the year and produce two to three litters per year. Long-tail and short-tail weasels, however, bear a single litter of five to eight each year. They mate in summer, when food is abundant and physical conditions are at peak, but the tiny embryos do not implant on the female’s uterine wall until ten months later, sometime in the spring. This delayed implantation allows for birthing to occur at the beginning of the summer season and mating to not have to take place under stressful conditions of winter. The father stays with the mother for only a few weeks to help bring in food. The young are on their own by fall. For most of the year, weasels are solitary animals.

BACKPACKER (FEB 1997) – When the snows reach 6 to 10”, the weasel begins to burrow under the snow, in subnivean tunnels.  Until snowmelt, this dark environment becomes the world of the non-hibernating predaceous weasels, along with their prey species, the shrew, mouse and vole, and their prey, insects and spiders.  With such a high surface area to volume ration of these small mammals, they must compensate through hyperactivity and naturally high metabolic rates that burn a lot of calories and produce heat.  A weasel’s heart beats 360 times a minute, while the heart rate of a mouse being hunted can reach 600 beats per minute.  Such high metabolism requires a large dietary intake, in some shrew species reaching as much as their own weight each day. 

While the blanket of snow provides considerable insulation (up to 50° warmer than the ambient air temperature), mid-winter conditions can be quite stress-free, what with the mice and voles munching on roots and insects, and the weasels munching on the mice and voles.  The only concern at these times is the buildup of carbon monoxide,  that must be vented by the digging of tunnels to the surface.  A good snow cover in winter often means a large summer rodent population, thus, a good predator site. 

 The worst situations exist when no snow cover exists, or when the snow is melting, saturating the snow or filling the tunnels with water.  Such conditions often exist at the end of the winter season when their natural body fat reserves have been utilized and the food resources hare lowest.  Flooded tunnels cause drownings and hypothermia, since water or wet snow conducts hear ten times faster than air.

NATURAL HISTORY (11/91) - Three weasel exist in the Northern Hemisphere; the long-tailed, the short-tailed (also called the ermine, or stoat) and the least, or common weasel. Weasel are extremely aggressive. Stories tell of birds of prey being beaten by their intended prey (a buzzard killed by a weasel it attacked, another one in flight with a weasel attached to it’s belly by teeth and feet, and an eagle that, although surviving an encounter with a weasel, was left with only the bleached head and teeth of a long gone weasel permanently affixed to it’s neck.) The weasel lives in a world of trade-offs between the costs and benefits of being a small predator. The small size of weasels make them the unintentional victim of attacks by bobcat, fox and raptors. But it also enables weasel to follow rodents into their burrows. But even voles, rabbits and the like can cause serious injury to weasels, so the weasel usually will only attack when it has the advantage, or when hungry enough to warrant the risk of injury. Apparently, the small size increases a weasel's opportunity to find a meal faster than it increases it's risks of becoming one. They prefer voles and lemmings, but in years of shortage, larger prey are sought out. What they lack in size, they compensate with length to overcome their prey. Also, in decreasing size, weight decreases faster than strength. Proportionately, weasel are more powerful than mammals much larger.

Weasels evolved near the end of the Tertiary, when the newly evolved grasses were replacing forests over vast areas of the North Temperate Zone (cooler/dryer). The voles/lemmings of this habitat were the prey of weasel, who could track down the prey in the tall grass, while being concealed itself from predators. Weasel don't make their own dens, rather, inspect the dens of rodents, inviting them to stay for dinner(!). When glacial climates began to grip the northern regions with frigid winds and paralyzing frosts, the weasels were already equipped to make the most of these new conditions. They could continue to hunt rodents and to raise litters of their own in fur-lined grass nurseries, snug under the insulating blanket of snow. During interglacial periods, such as the present one, the weasel's habitat thawed out and became repopulated with warmer-weather prey, such as rabbits an birds. Along with them came the larger predators, but the weasel carries on.

Weasel can release "stink bombs" like it's cousin, the skunk. However, this defense is not effective against owls and other raptors with poor olfactory senses, thus, weasel stay away from clearings (note how martens won't cross roads in earlier article.) Roger Powell (noted in Smokies bear studies), showed how the black tip on the tail of the winter coat of the long-tailed weasel attracted the attack of raptors, thus enabling the weasel to escape (sometimes). Annual mortality is 75 - 85 %. Pops are closely related to vole pops.

Weasel heart rate is 400-500 beats per minute. Everything is done at the cost of high energy use. The small size (and long body length, thus large surface to body ratio) also accounts for high energy loss/ need, especially in colder conditions. Thus, they must eat constantly. Every three or four years, pop booms in lemmings/voles make life easy for weasels.

Males are larger than females. Large variation within races and individuals. Size variations indicate the delicate balance between risks and rewards of size within the weasel family. Largest weasel ( long-tailed) diet is rabbits, squirrels and game birds while smallest (least) feeds on small rodents, songbirds, and insects. Even within the same species, the larger male diet has a greater concentration of rabbits than the female. It appears that regional size variations are due to prey size differences.