The chewed food particle size and shearing capacity of the lower molars of two primate species, the fat-tailed dwarf lemur,Cheirogaleus medius and the bushbabyGalago senegalensis, and an insectivoran, the tree shrew,Tupaia glis, were compared. Differences in the shearing design of the lower molars correlate strongly with the chewed food particle size in these species: the greater the shearing capacity, the smaller the chewed food particles.
These three species are of comparable size but differ greatly in diet in the wild.C. medius primarily eats fruit and nectar, whileG. senegalensis andT. glis are largely insect-eaters. The lower molars ofG. senegalensis andT. glis show a much greater shearing capacity than do those ofC. medius. The average length of chewed food particles ofC. medius is significantly larger than that ofG. senegalensis, while that ofT. glis is intermediate between the two primates but is closer to that ofG. senegalensis.
Our findings that insect-eating species grind their food more finely than do fruit- and resin-eating species can be correlated with digestibility of foods: finely chewing foods such as fruits which are low in relatively undigestible cell wall components would not greatly improve their digestibility, so a highly efficient food processing apparatus would be less important to the animal's survival. Insect-eaters much more finely chew their foods, implying that there is some constituent of insect bodies difficult to digest, and that grinding increases its digestibility. We suggest that this constituent is chitin.