Hunters small and thin enough to enter rodent burrows must not be too small to execute a kill, so weasels make up extra size in length rather than in girth. A weasel can wrap its long body around a catch, holding it with all four feet while it struggles (see Figure 6.7). Even so, weasels have not sacrificed muscular strength for size. On the contrary, weasels are relatively stronger than large carnivores.
The difference is a result of scaling. The apparently disproportionate strength of a weasel is one of the mechanical advantages of being small. The force that can be exerted by an individual muscle is the same, per unit of cross-sectional area, in mammals of any size (Schmidt-Neilsen 1984). As animals get larger, the strength of their muscles increase in proportion to the square of body length, but the total weight of the body to be moved increases in proportion to its volume, as the cube of body length. Consequently, the force that can be exerted by the muscles, relative to the mass of the body, is greatest in small animals.
The economics of hunting have some peculiar costs and benefits for a small predator (King 1989a). For example, the energy cost of running is relatively high in short-legged animals, because they have to take many small steps or bounds to move one unit of body mass over one unit of distance, each step requiring work in proportion to mass. Weasels never merely walk anywhere. They either glide along with a straight-backed scuttle or bound hump-backed at speed— and either method takes a lot of energy. Bounding is especially energetic, because the supple back is used as an extension of the legs, so the whole body is involved in every step. On the other hand, when a weasel climbs a tree or a steep slope, the additional energy required to work against gravity is negligible. Running is always an expensive activity to a weasel, but, as in all small animals, it makes hardly any difference whether it is going along the ground, straight up, or straight down (Schmidt-Neilsen 1984).
Small predators also have the problem of carrying prey that may be as big as themselves or bigger. The energy cost of carrying a load increases in direct proportion to its weight. If the load is 100% of the weasel's own mass, its oxygen consumption (a measure of energy expended) increases by 100% (Schmidt-Neilsen 1984). Weasels routinely carry prey that heavy. Even the smallest of them have the strength, but the cost is high. A stoat can kill a rabbit of twice its own weight, and then carry it away, looking like a terrier bounding off with a sheep. On the other hand, no lion can run at speed carrying a carcass of even half its own weight. For its size, the weasel is a fiercer, stronger, and tougher predator than any lion.
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