3 Herbivore teeth
Tables are a useful way of recording key information. The headings for Tables 1 and 2 have been prepared for you, and you can copy and complete the tables in your notebook. If you need to find any of this information again later, then it is very useful to have it summarised in a table.
If you are working through all the units in this series, you will recall the basic design and arrangement of mammalian teeth from course S182_2 Studying mammals: the insect hunters.
Look back at Figure 2 in course S182_2. Which sorts of teeth would be particularly useful for biting off and chewing plant food?
Incisors could be used to cut off pieces of food (grass or leaves) and molars (and premolars) for chewing the food.
The molars of herbivores become worn down by constant grinding of plant material between them. The teeth are composed of softer and harder materials, notably dentine and enamel. The overlying enamel is a hard, resistant layer, especially important in the incisor teeth of rodents. When they first erupt, the molar teeth of herbivores often have an extra coating layer of cement (similar to the layer surrounding the root). So in herbivores, cement, enamel and dentine are worn down unevenly, such that differential wear usually produces ridges, especially in the molars and premolars, which make chewing more efficient. These cutting ridges of hard enamel are evident in a vertical section through the single functional molar tooth from the lower jaw of an elephant (Figure 2a, below).
In all mammals, including ourselves, the molar teeth do not all erupt through the surface of the gum at the same time, but in mature adults all can be functional simultaneously.
Reread LoM p. 29 and p. 122 to identify two very different herbivores that exploit this delayed eruption. Write a few sentences to explain how they do so, making clear the similarities and the main difference between them.
The two animals are the kangaroo, which is a marsupial, and the elephant, a placental mammal. In both of them, new molar teeth form at the back of the jaw and move forward to replace the old worn teeth, which then drop out. There are only limited numbers of teeth, so in each case, when all the teeth have been worn out, the elderly adults are unable to feed properly and starve to death. The main difference (apart from the fact that elephant teeth are much bigger) is that kangaroos have a total of only four pairs of molars on each jaw, whereas elephants have six pairs. Figure 2b shows a single elephant molar from above.
The ridges on the teeth of elephants and horses lie across the jaw, and the jaws move mostly backwards and forwards while chewing. Those on sheep and deer run from front to back, and the jaw movement is mostly side to side. You might be able to see this movement for yourself if you have the opportunity to watch some of these herbivores eating.
In LoM information is given about the diet and teeth of some other herbivores. Scan through the chapter then copy and complete Table 1 in your notebook using the headings provided below.
Here is a completed version of Table 1.
|tapir||leaves mostly, but also fruits and nuts [p. 91]||two kinds of teeth: chisel-shaped incisors to snip off leaves; molars behind toothless gap, flat and ridged for grinding [p. 90]|
|chevrotain||fallen fruit and leaves [p. 94]||two teeth, one on either side of upper jaw, enlarged into short tusks [p. 92], molars [p. 94]|
|rabbit||mainly grass (and other herbaceous plants) [p. 95]||large pair of chisel-shaped teeth, with smaller pair at side [p. 95]|
|hippopotamus||grass (and other herbaceous plants) [p. 99]||Do not use teeth like conventional grazers (though their grinding molars are huge). Nip the grass with huge leathery lips, ripping up the leaves with sways of the head [p. 99]. The front teeth (i.e. the canines and incisors) have become tusks, used for display and fighting. Front incisors may be used for fighting, and thus become chipped [p. 102]|
Now use the information you've entered in Table 1 to answer the following questions.
Which teeth are missing in tapirs? Which other group of animals have you already met with a 'toothless gap' (scientifically it is called a diastema) and what did it enable them to do? (Check back to LoM p. 62 if you don't recall.) Do you think that the tapir's 'toothless gap' has a similar function?
Tapirs only have incisors and molars, so the canine teeth and premolars are missing. Rodents have a similar gap. This can be used to store the shavings of the coatings of the seeds on which many of them feed. Tapirs feed mostly on leaves, so probably their diastema performs a different function. It would allow food to be pushed between the upper and lower jaw through the gap, to be stored briefly in the cheek. Thus it is possible for the animal to eat more food without stopping to chew and swallow each mouthful.
Teeth can be used for activities other than chewing. Which teeth are modified in the chevrotain and hippopotamus?
You probably correctly surmised that it is the canine teeth, which are enlarged into short tusks protruding from the upper jaw in the chevrotain, though it is not possible to see them in the LoM photograph [p. 95]. Canine teeth are used by carnivores for tearing off pieces of meat, but in herbivores these teeth are often used for other purposes. In the hippopotamus, both the incisor teeth at the front and the canine teeth, enlarged into tusks, are used for display and fighting.
Biting off and chewing up the plant material is the start of the process of breaking it down. These actions break the food into smaller pieces and squash the pieces to give a larger surface area for the chemical part of the breakdown, called digestion, to begin. The real process of extracting the nutritious material from inside the cells happens after the food is swallowed.