When several live prey are in sight, a weasel will kill one after another, and even search around for more, until it is exhausted. This is not because it enjoys killing, but because the entire sequence of killing behavior is instinctive and is set off by the sight of moving prey, whether or not the weasel is hungry. A weasel in a chicken coop is psychologically unable to ignore the fluttering of the live ones and settle down to eat one that it has killed. For a weasel, this behavior is completely logical, and has evolved because it is in an individual weasel's best interests to behave that way. In the wild, weasels never find prey as abundant as they are in a chicken coop. Most weasels do not know where the next meal will come from, and must search hard to find it. Consequently, when presented with more than one meal, a weasel will catch as many as it can, while it can, and store the extras for later. People do exactly the same thing when they buy more food at the grocery store than they need for one meal and put the rest in the pantry.
Weasels have a strong tendency to store surplus food. Where prey are very abundant, or unusually vulnerable, carcasses can be accumulated in astonishing numbers. For example, in the days when haystacks were common and always infested with huge numbers of rats and mice, weasel caches were often found inside when the stacks were dismantled for threshing. A typical cache would contain 40 to 50 freshly killed mice with the telltale puncture marks of the weasel's teeth in the neck. Fifteen caches made by common weasels in Russia, found by Parovshchikov (1963), contained an average of 30 carcasses each. Besides various kinds of voles and mice, the weasels had stored water voles, common shrews, moles, frogs, lizards, garden dormice, and goldcrests. One stoat cache found under a rock in Greenland by Sittler (1995) contained the remains of nearly 150 lemmings.
Weasels do, of course, prefer fresh meat, but if none can be had and the need is pressing, they can remember the locations of their caches and return to them. Svendsen (2003) describes a female longtail hunting in an alpine meadow in Colorado. She found two nests of golden-mantled ground squirrels, killed all nine young, and carried them to an old pocket gopher burrow. She visited the cache three times in the next few hours, and added new items to the same store during the following 2 weeks. She was lactating, and had young in another burrow about 175 m away.
In warm climates, caching food may produce only a short-term benefit, or even a liability. Surely, one might argue, it is wasteful if the stored food spoils, and killing multiple prey at once could increase the chances that later hunting expeditions will not be so lucky. But weasels need to eat frequently, although not much at a time, so almost all food is cached for a while. For example, the weasels studied by Jgdrzejwska and Jgdrzejwski (1998) needed to eat one to two small rodents per day. In summer, when weasels hunted on 90% of days, they captured a mean of two rodents per hunting day. In winter, when they hunted on only 60% of the days, they captured a mean of 3.1 rodents per day. During a frost, when the temperature fell below -5°C, they ventured outside much less often, and captured a mean of only 0.57 rodents per day and made up the difference from their stores. Cached food allowed the weasels to survive the coldest days when they dared not go outside at all.
Even in summer, food does not rot immediately when stored in cool holes in the ground, and a cached prey item, even if somewhat putrid, could make the difference between surviving and starving for a weasel. But in cold climates, where energy demands are high and alternative prey few, the caching habit has real survival value. Oksanen et al. (1985) and Jgdrzejwska and Jgdrzejwski (1989) suggested that caching should be regarded not as an unusual event but as a positive strategy, at least in winter and in the far north. This is a much more likely explanation: Really damaging behavior should be weeded out in the course of evolution, and after all, weasels evolved in cold climates, and are still completely at home in them.
The killing and caching of prey as a regular and predictable behavior of weasels differs from "surplus killing," as described by Kruuk (1972). Kruuk observed that large numbers of game animals were killed by spotted hyenas during a foggy night. The prey were disoriented and unable to escape from the hyenas, and the hyenas, stimulated by the movement of the prey, continued to kill for as long as opportunity offered. Under normal circumstances, the hyenas might have been able to kill only one or two prey before the rest scattered. Such "surplus killing" may be maladaptive for hyenas hunting large mammals in a hot climate, but for them, such events are very rare and have no evolutionary significance. By contrast, weasels predictably kill and cache as an adaptive strategy.
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