Large Mammals

In theory, the larger a predator is, the larger the prey it can kill—but, also, the larger its food requirements. We would expect small predators to avoid the risk of injury by avoiding large prey. Small prey are often more numerous than large ones, and they take refuge in places more easily searched by small predators than large ones. Weasels are small predators, and small prey certainly are their bread and butter, so to speak. Weasels, however, are also bold and confident out of proportion to their size, and they seem to play by a different set of rules. Their extraordinary courage, strength, and tenacity put them among the few solitary predators able to attack prey larger than themselves (Figure 6.3).

For example, Harestad (1990) witnessed a longtail attempting to attack a Columbian ground squirrel, a stocky rodent weighing 350 to 800 g, considerably more than a longtail (see Table 4.1). The longtail's plan was foiled when two other squirrels mobbed it, and it fled. Such incidents may not happen every day, but eyewitness accounts of weasels attacking large prey are too common to be ignored, and are filled with vivid details describing, for example, the lightning thrusts with the teeth, the legs propped on widely straddled paws, and the tail furiously bristling with excitement.

Figure 6.3 Weasels are among the few predators that regularly kill prey larger than themselves without having the advantage of hunting in a pack. The risks from attacking large prey are considerable because large prey can injure or kill a weasel that makes a wrong move. This stoat is showing the typical "bottle-brush tail" reaction to excitement and danger as it makes a grab for the rabbit's neck.

Figure 6.3 Weasels are among the few predators that regularly kill prey larger than themselves without having the advantage of hunting in a pack. The risks from attacking large prey are considerable because large prey can injure or kill a weasel that makes a wrong move. This stoat is showing the typical "bottle-brush tail" reaction to excitement and danger as it makes a grab for the rabbit's neck.

We define "large prey" as those as large or larger than the weasel attacking them. Thus, rabbits, many ground squirrels, and most rats are large prey for all weasels, but even chipmunks and large voles may count as large prey for the smallest least weasels.

Carbone et al. (1999) explored the diets of carnivorous mammals in general, examining especially the amounts of invertebrate prey eaten and the proportion of vertebrate prey as large as or larger than the predator in relation to energy requirements and intake rate. For their size, weasels take an exceptionally large proportion of vertebrate prey, and they eat large prey (relative to their own size) much more often than expected.

Carbone et al.'s model predicted that most predators larger than 20 to 25 kg should eat predominantly large vertebrates, but because invertebrates are easier to catch and can be harvested fast enough to sustain a body smaller than 20 kg, invertebrates should often constitute a large proportion of the diet of predators below that size The weasels stand out starkly among the small predators for having diets like big predators, because they do often kill prey larger than themselves. True, weasels are specialists on small rodents, which are mostly smaller than themselves, but in contrast to other small predators like shrews and hedgehogs, they do not seem able to live on invertebrates.

Why are weasels willing to run the risk of injury that comes with attacking prey larger than themselves? We suggest that this question has two answers. First, large prey provide a lot of food for a successful attacker, and weasels are perpetually hungry. Consequently, large prey may at times be critical for their survival (Powell & Zielinski 1983). The analysis presented by Carbone et al. (1999) suggests that when the density of small rodents falls below some critical level and alternatives are few, weasels must tackle large prey to meet their food requirements. The risk is balanced by the fact that, once a large prey item has been killed and cached, the weasel may be able to eat without hunting for days and thereby avoid exposure to the dangerous world outside its den.

Second, weasels always approach large prey with care. Just as a wolf takes caution when attempting to kill a large deer, which could kill the wolf with one well-placed strike from a hoof, a weasel takes caution when attacking large prey. Once, R.A. Powell (unpubl.) watched a 40-g female least weasel hunt and kill a 60-g female meadow vole. The least weasel approached the vole cautiously under cover. She watched the vole, which appeared unaware of her, from several vantage points before choosing one with perfect access, hiding the weasel until she was very close to the vole yet allowing an escape if necessary. Although the vole was large enough to kill the weasel, when the weasel's attack came it was swift and safe. The vole struggled violently, but only for the briefest time while the weasel held it firmly with her teeth at the back of the head and with all four paws. The weasel made a safe kill of a potentially dangerous prey.

Unfortunately, no one has made extensive, well-documented observations of weasels killing large prey. The abundant anecdotes are difficult to interpret, at best. For example, many well-known stories (some repeated in the first edition of this book) describe stoats killing rabbits and other large birds by "dancing" to distract attention from imminent attack, or stoats mesmerizing rabbits by their behavior or odor. The dancing of weasels and stoats is so well known in Britain that there has been for years a restaurant near Manchester called The Waltzing Weasel. "Stoated" rabbits (supposedly mesmerized by stoats), rescued without a mark on them, may recover from their paralysis and totter away, only to sink down and die later. Hewson and Healing (1971) examined carefully several rabbits killed by stoats. They concluded that, as the teeth and jaws of a stoat are somewhat small compared with the well-muscled neck of a rabbit, and the injuries inflicted did not seem to be severe, the rabbits must have "died of fright."

Intriguing though these stories are, there is another, simpler explanation of at least some of these reports, related to diseases. Lagomorphs in North America are periodically ravaged by epizootics of tularemia. During these epizootics, sick, live-trapped animals often die from stress-related causes in the hands of researchers despite every effort to give them humane care. Some rabbits suffering from tularemia or other diseases may be easy prey for weasels, and may die from stress when attacked, or perhaps even when approached.

Opinions are divided on whether "dancing" weasels are merely playing, or deliberately using the "dance" as a hunting technique. In favor of the first interpretation is the fact that these "dances" are not confined to situations offering a potential hunting opportunity. During months of radio tracking stoats in Scotland, Pounds (1981) watched 13 "dances," some of which were performed without any audience at all. In favor of the second interpretation is the fact that weasels in general are intelligent and opportunistic hunters, and if they find themselves surrounded by curious rabbits or birds, for whatever reason, they will certainly take the chance to catch one if they can. If they realize the connection between their behavior and the subsequent kill, they might well learn to "dance" on purpose. A third, completely different explanation is that the "dances" are an involuntary response to the intense irritation caused by parasitic worms lodged inside the skull (Chapter 11), and are quite unrelated to hunting behavior.

Whatever the interpretation of "dancing" or "mesmerizing," one consistent factor is that, when associated with an attack on large prey, these behavior patterns appear to reduce or minimize the risk of injury to the weasel. Such a benefit could eventually reinforce the behavior, whether it was deliberate or not.

Another way to reduce the risk inherent in attacking large prey is for two or more predators to cooperate. Although all weasels are normally solitary, Bullock and Pickering (1982) described in detail an incident involving two common weasels that persisted in attacking an adult brown hare. They suggested that the two weasels were cooperating. More frequent are anecdotal accounts of families of weasels hunting together before the young disperse. But we doubt if these stories are evidence of true cooperation, either. Weasel families do not stay together long, so the time available for learning of cooperative behavior is short. Our own anecdotal observations suggest that real cooperation is limited—rather, the mothers in family groups do the hunting and the youngsters tag along (p. 223).

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