Whether the prey is found by vision, hearing, or scent, the final kill is certainly done by eye and stimulated by movement (Heidt 1972). Voles that respond to the presence of a weasel by running away can be overtaken in a couple of bounds and dispatched with hardly time for a squeak. Some rodents, however, especially deer mice and wood mice, may respond by climbing upward or "freezing,"
and both these tactics are often successful (Cushing 1985). A weasel does not seem immediately to recognize as prey a mouse that, though in plain view, is sitting motionless, and in the wild this must sometimes give the mouse a chance to escape. Wood mice react less to the pungent musk of weasels or stoats than do field voles, so are less likely to give themselves away (Stoddart 1976; Gorman 1984).
Even larger, more visible mammals can use this trick so long as they sit still. Murie (1935), perched up a tree, watched a long-tailed weasel trying to catch a snowshoe hare in winter. Both were in their white winter coats, and there was some 15 cm of snow on the ground. The hare came loping along, then crisscrossed its tracks in a small area just in front of Murie's tree and settled down. The weasel followed, tracking the hare's prints through the maze exactly, at times passing quite close to the hare. When it had followed every turn of the trail to within a meter of its end, the hare skipped off. The same game was played twice more before the hare ran off in top gear, and the weasel gave up.
Jgdrzejwska and Jgdrzejwski (1998) closely watched wild, radio-collared common weasels killing small rodents in Poland. Small rodents in the open were seldom chased farther than a bound or few. When a least weasel entered a rodent hole, rodents erupted from all the emergency exits. Only six documented attacks involved a chase of 5 to 20 m, and all of these were on yellow-necked mice. Of the kills observed, a third were down rodent holes or tunnels, especially bank voles' holes. Another 45% were on the ground along fallen logs or in ground vegetation, especially those on voles and yellow-necked mice. Finally, 22% of kills, all of mice, were in tree cavities. In 1990, when rodent populations were high, less than half of all of attacks were successful. Roughly two thirds of the attacks were on single rodents, and one third on rodents in huddles of two to six. Of 19 attacks on huddles of rodents, the weasel killed two only once, providing rodents in groups a 20% lower chance of being killed by a weasel than singles. In summer when rodents are reproducing, 70% of kills may be nestling voles.
Laboratory studies have examined the preferences of weasels for different prey. The only generality that we can deduce from the information reported so far is that, not surprisingly, weasels appear to attack first those prey that are easiest to kill. Derting (1989) found that captive American least weasels caught woodland voles most often, meadow voles least often, and deer mice at an intermediate level. Meadow voles were the largest of the three species presented, and they put up the most aggressive fight. Woodland voles tended to be slow, timid, and easily cornered. Wild woodland voles, however, usually live in extended family groups, in which they can offer each other the double protection of the safety of numbers and of warning behavior (Powell & Fried 1992).
In general, voles are more easily caught than mice (Erlinge 1975). Finnish least weasels, given the choice, preferred bank to field voles (Pekkarinen & Heikkila 1997; Sundell et al. 2003). Males killed bank voles more quickly, and females ate bank voles first, whenever they had the option. Both sexes also ate juvenile bank voles before eating adults. The stoats observed by Raymond et al. (1990) and Vaudry et al. (1990) in a large, complex enclosure were more successful at hunting meadow voles and deer mice than house mice and short-tailed shrews. Male stoats took longer to find meadow voles than did females, especially when the stoats were hungry. Both sexes of stoats got the least energy gain from house mice, and both found the shrews either hard to find or hard to kill.
Such lists of preferences established in artificial conditions are interesting but, in real life, they may not influence a wild weasel's diet except during the short periods when rodents are abundant. When food is scarce, the racing metabolism of a hungry weasel requires it to take whatever it can get.
Where behavior or physiology makes some kinds of prey more vulnerable, weasels might be expected to take advantage. Cushing (1985) suggested that least weasels can perceive the pheromones of deer mice, and prefer to hunt female mice in estrus. Estrous mice emerged from their holes sooner than did diestrous (sexually inactive) mice, and were more likely to flee and less likely to freeze, making them significantly easier to catch. When given the choice in a Y-maze between odors of estrous and diestrous mice, the weasels sought the odors of estrous mice first. Unfortunately, when further Y-maze experiments were done with a larger group of least weasels, each tested only once, and with four age/ sex groups of bank voles, Ylonen et al. (2003) confirmed that least weasels used olfactory cues in hunting, but not to distinguish between categories of voles. It is not clear yet whether this difference between mice and voles is real or an artefact of doing such experiments in captivity.
A weasel orientates its killing bite to the back of the head or neck of a rodent, stimulated by movement and guided by visual cues, particularly the position of the eyes and ears of the prey (Heidt 1972). The final strike is made with deadly accuracy. The long upper canines pierce the back of a rodent's skull or the vertebral column, and meet the lower canines entering below the ear or the throat. This neck bite is very characteristic, and death is just about instantaneous. Skinned mice killed by weasels often show no injury besides the two pairs of needle-like punctures in the neck.
If necessary, in the heat of the moment, the weasel may first grab the mouse almost anywhere, often wrapping its long slender body around its victim and using its feet to manipulate it and to gain leverage to transfer its hold to the neck (Figure 6.7). The whole process takes only from a few seconds to less than a minute, though the weasel often keeps its grip until the mouse stops kicking, perhaps shaking it a bit in the meantime. Weasels may attack larger prey by jumping on their backs, to get at the neck from above, or by darting in to reach unprotected parts from below. An animal that backs into a corner or makes cries of threat or fear only increases the weasel's excitement.
Once its prey is dead, a weasel may lick any blood coming from the prey's mouth or wounds before starting to eat, but there is no truth in the old belief that weasels suck the blood of their prey. Weasels do not—in fact, physically cannot—suck blood.
A weasel presented with any of the common small rodent species already dead will eat them equally readily. Nearly always, a weasel begins its meal by eating the brain, often leaving completely the point of the nose and teeth, and then proceeding backward to the rest of the body. The feet, tail, and intestines, especially the stomach, are left until last. If many prey are supplied, a weasel may simply eat the brains and abandon the rest, or eat the brains of all and the body of only one, or even alternate among several carcasses. Strict carnivores, however, often have trouble obtaining enough carbohydrates from their animal diet. Their digestive tracts cannot digest plant material, which are the most ready source of carbohydrates. Consequently, weasels usually eat the small intestines of their prey, and thereby obtain partially digested plant matter with readily available carbohydrates.
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