How are we to understand the great variability in the spacing behavior of weasels? It helps to remember the basic economics: A weasel will spend the time and effort required to defend its home range only if it can thereby guarantee priority access to a resource in short supply, such as a population of voles, for itself or to protect its investment in a litter (Carpenter & MacMillen 1976; Powell 2000). If the voles become more abundant, a weasel can economize on energy by reducing the size of its home range, but only to a certain extent, so the size of the area over which a resident weasel hunts depends on the density of the local small rodents, down to a minimum of a few hectares.
If the habitat is patchy, with good spots and bad spots for voles (which is always the case), at some level of abundance the good patches together provide more voles than the resident weasel needs. To delete one or some of the patches from its home range, however, could leave the owner of such a territory short in the future, so it is likely to keep all patches and allow home range overlap with neighbors. This often happens in autumn, when voles are at their seasonal peak and transient weasels can settle into rich areas overlapping the home ranges of residents. At the other extreme, if the voles become so scarce that no home range can support enough of them to feed even one weasel, then the whole system breaks down and the resident weasels move on to seek better hunting elsewhere.
The home range sizes of weasels do in fact show variation of just the sort predicted. For example, the male common weasels studied by Lockie (1966) when field voles were very abundant (110 to 540 per ha), did well on home ranges of only 1 to 5 ha. Those at Wytham, where wood mice and bank voles were much scarcer (together only 21 to 39 per ha), had to hunt over much larger areas and were still perpetually hungry (King 1975c).
If prey density is the most important consideration deciding home range size, the limitations of the weasel's own body is probably the next one. Large weasels can cover more ground than small ones, but the smallest weasels are better able to exploit small rodents, so can make a living on a small area where a large weasel, excluded from tunnels, would starve. The home range estimates compiled in Table 8.1 are influenced by a host of different variables, but there is still a broad correlation between the size of the weasel and the area it occupies. Every study shows that the ranges of females are smaller than those of the males of the same species, measured at the same time and place. The ranges of common and least weasels are clearly smaller than those of stoats and longtails; the smallest local races of stoats (e.g., in Ontario) have the smallest ranges of that group.
When the home ranges of a male and female weasel overlap, the hunting activities of each reduce the prey available to the other. Each catches voles and, probably more important, alerts prey while hunting, making the prey wary and harder to catch (Chapter 7). Female weasels simply tolerate the loss of prey to males, except perhaps in a small, core area worth defending, but have smaller appetites and greater searching efficiency instead. Males, however, benefit from overlapping ranges that provide information on where females live, which is especially useful in the breeding season (Powell 1994). Field data confirm that home ranges often overlap, particularly between sexes (Murphy & Dowding 1994, 1995; Alterio 1998), but many individuals have separate core ranges that do not overlap.
Why do weasels bother to establish a home range and try, to varying degrees, to keep others out of it? The most critical reason is that local knowledge is valuable, for several reasons. First, the weasel that knows its home range knows where rodents are to be found and where they are not, and knows when it can return to each good hunting area. Weasels are good dispersers and are fully capable, when the local rodent population crashes, of wandering across a landscape in search of new habitats dense with voles and unclaimed by other weasels. That is one of the characters of small, short-lived "weed" species like rodents and their specialist predators. But since rodents are not evenly spread throughout all parts of any habitat, and the dangers of wandering on unfamiliar ground are high, a weasel will obviously stick to a good area when it finds one.
Second, weasels are always vulnerable to larger predators, and the best defense against attack is to have an intimate knowledge of one's own ground, the position of every refuge, and the safest, quickest way to get to it. A weasel who knows where the good escape holes are has a better chance of surviving if spied by a hawk (Chapter 11).
Third, energy conservation is all-important to weasels, which means that there is a high premium on minimizing the energy spent in hunting (Chapter 2). A weasel that knows exactly where to go to find a meal is more likely to be able to meet its needs and to get back into its warm nest in the least possible time. Once a weasel has established a home range, keeping others out allows it to keep track of the best times to return to favorite vole-rich patches.
Obeying rules of trespass and scent marking to warn others that it is willing to challenge intruders allow a weasel to maintain its ground with minimal cost. One might say that, through evolution, weasels have become experts in economics, game theory, and information theory by daily experience in the School of Hard Knocks. All these are important to weasels, because their day-to-day survival depends on finding ways to meet the huge costs of living in a small, thin body.
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