2 How insect eaters obtain their food
2.1 Mammalian dentition
Insects are generally very small animals. Many kinds are hard work to collect and not very nutritious because a high proportion of their mass is a protective and indigestible outer layer, called cuticle. Insectivorous mammals need to eat large numbers of insects to fulfil their energy requirements.
Insect eaters have diverse ways of catching and dealing with their prey; teeth play a crucial role. Indeed, teeth are of such enormous significance to mammalian diets in general (and are so readily preserved by fossilisation) that they feature extensively in this series of units as a whole. A general introduction to teeth is useful at this point.
Mammals have several kinds of teeth that operate together as a set, of which there are only two - the 'milk' set that emerges just before weaning, and the permanent set that replaces it usually before sexual maturity. Because the young are fed on milk for some time after birth, the emergence of the teeth is delayed until the jaws have grown large enough to accommodate them. Consequently, teeth in the upper and lower jaws grow to be opposite each other. This matching, together with the tough hard materials and complex structure of each tooth, are crucial to their role as precision tools in feeding. In fact, the possession of differentiated and exactly opposing teeth is a defining character of mammals. The downside is that, although reptiles such as snakes and crocodiles can replace damaged or diseased teeth throughout life, mammals have a fixed number of teeth and, as many of us know to our cost, defective teeth are not replaced.
At the front of the jaw there are incisors, used for grasping and biting food. Canine teeth behind the incisors are pointed, and often sharp, but may be small and poorly developed in animals that are not flesh eaters and do not use their teeth for defence and in threat displays. Premolars and molars behind the canines have several roots and are usually broad and ridged. These teeth are the main grinding surface for chewing food, a uniquely mammalian ability that entails complex movements of the tongue, cheeks and jaws. Sheep, cattle, deer and their relatives chew by side-to-side movements of the jaw, while rats, squirrels, guinea-pigs, etc. grind their food by sliding the lower jaw forwards and backwards. Other mammals including ourselves use combinations of these movements. The numbers of each type of tooth in a mammal can be shown in shorthand by the dental formula, of which you see examples later. Such a formula describes the numbers of incisors (I), canines (C), premolars (P) and molars (M) on one side of the upper and lower jaws. For example, if a mammal were to have three premolars on one side of its upper jaw and two premolars on one side of its lower jaw, this would be abbreviated to P3/2. The milk set, which develops before the jaw is fully grown, includes premolars but not molars. The milk set may be partially or entirely eliminated in some kinds of mammals, and some, notably the great whales, are completely toothless.
A typical mammalian tooth is made up of three regions: the crown, the neck and the root (Figure 1). The visible part of the tooth that emerges from the gum is the crown; the neck of the tooth is surrounded by the gum. The tooth is anchored in the gum by the root, and a layer of bone-like tissue, called cement, fixes the root to the jaw bone. In most mammals, the permanent teeth eventually stop growing and their blood supply is reduced, at which point the teeth are described as 'rooted'.
In vertical cross-section, the tooth can be seen to consist of three layers. The outermost layer of the crown is hard enamel, made of mainly calcium phosphate with a small proportion of calcium fluoride. The enamel resists damage from hard foods and protects the inner layers. Most of the tooth is made up of dentine, which is similar to bone in structure, made up of a fibrous framework, (rather like the mish-mash of coconut fibres in a doormat, but on a microscopic scale) in which are embedded crystals of calcium phosphate. The central pulp cavity contains blood vessels and nerves and becomes almost closed off at maturity.
Members of the Insectivora have numerous (up to 44) small teeth. Look at Figure 2a, which shows the skull and lower jaw of the hedgehog.
How many incisors, canines, premolars and molars are there in the upper jaw of the hedgehog? Express these values using the shorthand given above.
There are three incisors (I3), one canine (C1), three premolars (P3) and three molars (M3).
Counting the corresponding numbers in the lower jaw of the hedgehog gives: I2, C1, P2, M3. Bear in mind that the dental formula gives, for each type of tooth, the ratio of the number in one side of the upper jaw to the number in one side of the lower jaw. The dental formula of the hedgehog is thus I3/2, C1/1, P3/2, M3/3, and the hedgehog has a total of 36 teeth.
The pygmy shrew has three incisors, one canine, three premolars and three molars in one side of the upper jaw, and one incisor, one canine, one premolar and three molars in one side of the lower jaw. What is its dental formula and how many teeth does it have in total?
The dental formula of the pygmy shrew is I3/1, C1/1, P3/1, M3/3, giving 16 teeth on each side and so a total of 32 teeth. (Figure 2b shows just the pygmy shrew's teeth nearer the front of each jaw.)