The Skeleton

Everything about a weasel is attuned to the profession of hunting for small prey in dark, confined spaces. In motion, weasels appear almost boneless. We have seen weasels leap into a hole and then look out again in a single, fluid action so fast that the tail was not in before the nose came out again. A weasel can do this because the articulations between its vertebrae are so flexible that it can turn over and walk back over its own hindquarters. Living and working in tunnels are normal and natural activities for weasels; the short legs (less than half the length of the body) (Holmes 1980) swing through their normal arc in a space only twice the depth of the animal's head (Figure 2.2). In an "average-shaped"

Figure 2.2 Weasels are perfectly adapted for hunting in tunnels. Their short legs allow a normal gait, and their long necks allow them to carry prey without tripping.

mammal, the legs and the body are about the same length, and in a confined space the legs are cramped, folded, and hampered in movement.

A weasel's neck is so long that prey can be carried in its mouth without tripping its front feet. No part of the skeleton of a weasel is wider than the skull, so that anywhere the head can go, the rest of the body can follow. The limb girdles at the shoulders and hips do not bear any great weight, so they are small; they fit with the sinuous lines of the vertebral column and allow great freedom of movement to the limbs. It is said that a common weasel can pass through a man's wedding ring. This tale may be an exaggeration for a living weasel, but it is not far off the mark for a skull.

The vertebrae of the neck are very strong and provide anchorage for the large muscles of the neck (Figure 2.3). The joint between the skull and the atlas (the first vertebra in the neck) is wide with smoothly curved surfaces. Each side of the atlas has a large flat wing, which receives muscles from the skull that move the head up and down. The axis (the second vertebra) is a completely different shape, compressed sideways rather than horizontally, because it receives a different set of muscles from the skull that allow a weasel to tilt its head.

The rest of the vertebrae are much smaller than these two, and number five more in the neck (total seven cervical vertebrae), 14 to 15 in the chest (the thoracic), 5 to 6 in the abdomen (the lumbar), 2 to 4 in the hips (sacral), and 11 to

Figure 2.3 The skeleton of a stoat. Weasels have elongate heads, necks, and vertebral columns and short legs. Otherwise, their skeletons are generalized and typical of mammals.

33 in the tail (caudal). The tails of least and common weasels are short (11 to 16 vertebrae, <25% of body length; see Table 1.1), for several reasons. One is that an important function of a tail, maintaining balance while jumping and turning (as in tree-dwelling mammals), is not needed in burrows; another is that a tail may be a source of serious heat loss, especially in very cold climates. On the other hand, a long thin tail carrying a deflection mark can decrease the risk of a weasel being caught by other predators (see Figure 11.6). The long-tailed weasel, living up to its name, has 19 to 33 caudal vertebrae, stretching its tail to 40% to 70% of its body length. The tails of stoats are intermediate in length (30% to 45% of body length) and in numbers of vertebrae.

Other than their very long necks and spines, weasels have a generalized type of mammalian body, with no particular specializations (Ondrias 1960, 1962; Holmes 1980). They have five toes on each foot, and walk with their heels on the ground in a plantigrade posture, much as the earliest mammals did. Their paws seem large for their body size—good for grabbing a fleeing vole—and, like those of their close relatives the martens, their claws are semiretractable but not sheathed (Powell 1993). Weasels are good climbers, another ancient characteristic of mammals in general and mustelids in particular (Holmes 1980), and they frequently climb trees and shrubs in their search for food (Chapter 6). Like tree squirrels, they can rotate their ankles and hang or climb head-downward as naturally as they can climb straight up (see Figure 6.5).

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