Form, Function, and Evolution in Skulls and Teeth of Bats

Bats provide a model system for tracking change from the primitive mammalian tooth pattern to patterns indicating the more-derived food habits of carnivory, nectarivory, frugivory, and sanguinivory. Whereas microchiropteran bats show all these transitions, megachiropterans illustrate an alternative pattern concerned only with frugivory and nectarivory. In microchiropterans, it is likely that carnivory nectarivory, frugivory, and sanguinivory are all derived from a dilambdodont insectivorous tooth pattern. Megachiropterans are troublesome because they appear as nectarivores or frugivores without a clear relationship to ancestral taxa. The nature of the food item and how teeth respond to that item evolutionarily is an issue I have addressed previously diet by diet (Freeman 1979, 1981a, 1981b, 1984, 1988, 1995). With the insectivorous family Molossidae, and among insectivorous microchiropteran bats in general, consumers of hard-bodied prey can be dstinguished from consumers of soft-bodied prey by their more robust mandibles and crania, larger but fewer teeth, longer canines, and abbreviated third upper molars (M3; Freeman 1979, 1981a, 1981b; Strait 1993a, 1993b). Carnivorous microchiropterans have distinctive large upper molars with lengthened metastylar shelves and elongated skulls with larger brain volumes and external ears than their insectivorous relatives. As in terrestrial mammals, however, there is no clear distinction between insectivorous and carnivorous species (Savage 1977; Freeman 1984). Microchiropteran nectarivores are also on a continuum with insectivores but are characteristically long-snouted with large canines and diminutive postcanine teeth (Freeman 1995). Finally among microchiropterans, frugivores differ from insectivore/carnivores and insectivore/nectarivores by having a substantially different cusp pattern on the molars. The paracone and metacone are pushed labially or buccally to become a simple, raised but sharpened ridge at the perimeter of the dental arcade (Freeman 1988, 1995). First I examine function of dfferently shaped skulls and palates of bats in different dietary groups. Among Megachiroptera, frugivores are on a continuum with nectarivores, but there are characteristics of robustness that appear to be good indicators of diet that distinguish the two (Freeman 199.5). Megachiropterans have several convergent characteristics in common with microchiropteran nectarivores. I believe this convergence is not only the key to explaining cranial and palatal shape and jaw function in bats but also is critical to understanding the evolution of nectarivory and frugivory in chiropterans. Associated with the shape of the palate is the way that allocation and emphasis of tooth material on the toothrow shift between suborders. The relative area that each kind of tooth occupies on the toothrow is quantified and serves as the basis for my interpretations. A second goal is to examine function in bat teeth. Here I synthesize my past work on tooth function, particularly with regard to canines and molars, and introduce a novel way to examine function in canines. Function in more complex teeth involves a review of the principal cusps on the upper and lower molars and how cusp patterns have evolved relative to different diets. Specifically, I contrast carnivory in terrestrial mammals and bats, insectivory in insectivorous and nectarivorous species, and frugivory in mega- and microchiropterans. Finally, I suggest that the evolution of dilambdonty can be correlated with packaging and digestibility of the food item