2.2 Cracking nuts and other ways of eating

We can tell a great deal about a mammal's diet, and therefore about its way of life, by studying its dentition. Before you study the armoury that enables the rodents to chisel into nuts, I'll remind you of the standard mammalian dental pattern and describe the differences found in rodent dentition.

Mammals have teeth of different types, differentiated from front to rear: incisor, canine, premolar and molar. (Where teeth are different in shape and size, the dentition is said to be heterodont.) For the hedgehog (see course S182_2) the dental formula is I3/2, C1/1, P3/2, M3/3. The pygmy shrew dental formula is different with respect to the lower incisors and premolars; I3/1, C1/1, P3/1, M3/3.

All rodents are like the mole-rat and have only four incisors, two in the upper jaw and two in the lower jaw. As DA points out 'The wear on the incisors as they cut through the soil is particularly heavy, so it is essential that they grow fast and continuously. Consequently, the mole-rat's incisors have extremely long open roots that extend far back along the jaw, even beyond the roots of their cheek teeth' [p. 76]. These incisors are described as being rootless, as distinct from the rooted teeth described in S182_2. They retain the open pulp cavity with a good blood supply and so the teeth continue to grow throughout the animal's life. The action of opposing teeth as they gnaw and grind against one another gives the incisors chisel-sharp edges. As DA explains [p. 61], the incisor enamel is found only on the anterior (front) surfaces and enamel is harder than the underlying dentine, which consequently is more rapidly worn away on that side [p. 61]. The TV programme shows diagrammatically (at 02.52) how such sharpening is achieved.

Rodents have other distinctive dental features, also mentioned by DA. Canine teeth are entirely absent from the group. The gap between the gnawing front teeth and the chewing teeth behind is termed the diastema (Figure 1); when the lips can be drawn into this space, the mouth is effectively sealed off, so reducing the chances of swallowing any indigestible material gnawed off by the incisors. In some species the diastema provides deep pockets within which food can be temporarily stored during the to-and-fro shuttling associated with building up food hoards, an activity that is wonderfully illustrated in the TV programme. Many rodents have no premolars, though the molar teeth are often very substantial and sculptured in complex ways that promote the grinding up of plant material - features reminiscent of the dentition of the plant-eating mammals, which are highlighted in the next course in this series.

Figure 1 shows some features of rodent jaws and skulls and they indicate just how intricate the processes of gnawing and chewing in rodents are. The hinge of the jaw is complex. When the lower jaw is protruded forward, the incisor teeth engage, and can chisel or nip. In this position, the surfaces of the upper and lower teeth do not meet. But when the jaw slides backwards, the molars can grind the food, while the incisors hang over the lower ones, giving the animal the characteristic 'buck teeth' appearance.

Figure 1: adapted from Young, J. Z. (1962) The Life of Vertebrates, Oxford University Press ©

Figure 1: adapted from Young, J. Z. (1962) The Life of Vertebrates, Oxford University Press

Figure 1 Some rodent skulls and jaws - the identity of each needn't concern you. Two sets of muscles, represented schematically in each case, are used for gnawing and for closing the jaw. (At one time, rodents were classified into three suborders on the basis of their jaw musculature.)

The muscles of the jaw are massive and are able to produce this backward and forward movement of the jaw. The major muscle is called the masseter, consisting of two main components, which are attached (i.e. inserted) at particular points in the upper and lower jaw. Figure 1a shows the arrangement typical of some types of squirrel, with points of insertion of these muscles drawn as arrow heads. That part of the masseter represented in red can close the jaw; while the masseter muscle shown in blue can pull the jaw forward. Figure 1b-c show the very different arrangement in other rodents and although the details needn't concern us, note that in rats and mice (Figure 1b), the insertion of the masseter is more forward onto the face, with one part of the muscle passing through the eye socket onto the muzzle. Note too that the lower jaw of many rodents has a large flange, where the masseter muscle is inserted. What these side views can't show is how the two halves (i.e. left and right) of the rodent lower jaw (the mandible) sit side by side. Rather than being fused, as with many mammals, these bones are linked by a ligament, a band of flexible tissue like a strap. This independent mobility allows the lower incisors to be brought together or pushed apart, facilitating manipulations such as prising open a nut. The teeth are firmly held in their sockets and not easily dislodged.

It seems then that a nut cannot generally be cracked unless it can be manoeuvred into the right position. You saw the squirrel using its forepaws to manipulate the white acorn before it started to gnaw; this was no random movement. As you know, the second programme extract in Activity 1 (05.35-08.56) was of a squirrel biting out the embryo before storing the remainder of the nut. Other research has shown that squirrels learn to gnaw along the weak 'seams' of a nut. Their proficiency improves with age - a result of learning, not just maturation. A squirrel reared on mashed food and not given the opportunity to handle nuts does not improve its handling technique with age.