The skulls of weasels are where their specialized life style is best demonstrated. Weasel skulls are unusually long and narrow, have exceptionally large areas for jaw and neck muscles, and enclose large brains (Radinsky 1981). The bones of an adult weasel's skull are fully fused, forming a single, box-like unit of massive strength. Sturdy, boney crests along the top (the sagittal crest) and back (the nuchal crest) of a full-grown skull provide extra space to anchor the temporalis muscles, which are responsible for a weasel's powerful bite.
This feature is unusual among small mammals: In general, only large mammals have crests on their skulls (Hildebrand 1974). For example, the large cats must have huge jaw muscles and need large crests to anchor them. Most small mammals need no crests. Even a wild cat the size of a house cat, which may weigh 10 times more than a weasel, can function well with no crests on its skull at all. Similarly, the skulls of adult red foxes show no large sagittal crests for jaw muscles. By contrast, the exceptionally well-developed temporalis muscles and tremendously powerful jaws of the large weasels need to be anchored to crests on the skull, just as in the large carnivores.
The zygomatic arches (cheekbones) on a weasel's skull are widely separated, to leave room for the temporalis muscles to pass through from the sagittal crest to the large coronoid process on the back of the jawbone (Figure 2.4). The zygomatic arches anchor the masseter muscles, which in herbivorous animals, such as voles or horses, provide a grinding motion to the molars as well as closing the jaws. Because the jaws of weasels can move only up and down, and because the masseter also prevents the jaws from opening very wide, the masseter muscles of a weasel are small. The zygomatic arches are therefore slim and weak, which shows that there are no strong pressures on them (Ewer 1973).
A short thick jaw is a better lever than a long slender one, and it can exert a stronger force (Biknevicius & van Valkenburgh 1996). The jaws of weasels are short, and their teeth are specialized for a diet of flesh, to a degree matched only in the cat family. The four carnassials, the last upper premolar and the first lower molar on each side of the jaw, are critically important. They are strategically
placed at the back of the jaw so as to take the utmost advantage of the leverage of the jawbone and the huge strength of the temporalis muscles. This arrangement explains why weasels, like dogs, chew bones at the corners of their mouths (Figure 2.5)
The upper and lower carnassials on each side are parallel and slightly offset, and they shear closely past each other, to slice flesh and bones from a carcass into swallowable lumps. Weasels do not grind up their food finely; indeed, they physically cannot and do not need to, because meat is digestible in chunks without grinding. In the upper jaw, the first molar is set a little behind and crosswise, with a deep recess in its center (Figure 2.4). The rearmost blade of the lower carnassial slots exactly into it to form a slicing guillotine. Not only is this arrangement devastatingly effective, it is also self-sharpening and self-tightening (Mellett 1981). The joint between the lower jaw and the skull is cylindrical, and flanges from the skull wrap around the mandibular condyle on the lower jaw to prevent any sideways motion or dislocation of the jaw. Sideways movement would make the carnassials ineffective, like scissors held together with a loose screw. When the temporalis muscles contract, they pull
on the coronoid process, causing the jaw to rotate around the jaw joint and creating torque (Biknevicius & van Valkenburgh 1996).
The next most highly specialized teeth are the long, slim canines, whose function is not to slice food but to catch it. The canines act together as a trap and humane killer, grabbing hold of a fleeing mouse and dispatching it with a piercing bite through the bones of the neck and skull. The crunch can be audible several meters away.
The rest of the teeth of a weasel have little to do, and most of them are small or have been permanently lost. The incisors are jammed into such a tiny space at the narrow front of the mouth that, in the lower jaw, two of the six have been squeezed out into a second row behind. Between the incisors and canines in the upper jaw is a large gap on each side, which, when the mouth is closed, is filled by the canines of the lower jaw. The first premolars and the third molars are absent in both jaws, and so are the second molars in the upper jaw.
This reduction in the number and size of the nonessential teeth has permitted the front part of the face to become shorter, forming the characteristic wedge-shaped head. The dental formula is I3/3, C1/1, P3/3, M1/2 = 34. This means that weasels have three incisors, one canine, three premolars and one molar on each side of the upper jaw, and the same in the lower jaw except for an extra molar on each side. Most of the work is done by only 10 of the 34 teeth.
With this equipment, a weasel can kill a mouse or bird in seconds or less. The big cats of Africa are usually regarded as the ultimate in predatory power but, in relation to their size, the weasels are equally formidable predators.
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