Asymmetry in form and complex occlusal interlocking between upper and lower mammalian cheek teeth. (a,b) The upper (a) and lower (b) fourth premolar and first molar of the bat Pipistrellus in occlusal view showing the difference in morphology between the maxillary and mandibular dentitions. Despite their differences, these teeth interlock precisely along a complex, complementary series of surfaces, as shown by (c) the teeth of the bat Barbastellus in their functional orientation. The arrows show corresponding points that slide into contact as the animal chews. In mammals with this type of occlusion, there is only one possible angle through which the lower teeth can move into occlusion with the uppers. Coordination among the forms of the individual teeth, the sizes of the upper and lower jaw, the position and structure of the jaw joint, and the vectors of movements of the several muscles of mastication are required for these mammals to be able to eat. These factors have different developmental genetic controls, and the constraining force of integrative stabilizing selection must be strong, yet this functional complex evolves quickly enough that even a non-specialist can see morphological differences in the teeth of these two con-familial, moth-eating specialists.