TESTING ADAPTIVE HYPOTHESES FOR HYPSODONTY IN SMALL MAMMALS USING PHYLOGENETIC AND PALEOECOLOGICAL DATA

The evolution of hypsodonty is a classic problem in the study of adaptation. High-crowned (hypsodont) teeth have evolved in a wide array of mammalian lineages, especially in herbivores both large and small. Hypsodonty was first hypothesized to be an adaptation to eating abrasive food, such as grasses. Recent studies of isotopic composition and tooth wear have cast some doubt on this hypothesis. A variety of other correlates of hypsodonty have been proposed as the ultimate cause of this adaptation, including climatic aridity and increasing body mass. Most studies of hypsodonty have focused primarily on large herbivores, such as horses. However, hypsodonty is more widespread among small herbivores, particularly fossorial forms, suggesting a fossorial life habit may necessitate hypsodonty in some of these cases. While the correlates of hypsodonty have been widely used in a variety of studies of environmental change, relatively few studies have examined the evolution of hypsodonty in a rigorous test of adaptation. This study tests four hypotheses for the cause of hypsodonty evolution using a well-resolved phylogeny of all described species of the rodent clade Aplodontoidea. The four hypotheses tested here are 1) adaptation to a grassy diet, 2) adaptation to an arid climate, 3) adaptation to a fossorial (ground-dwelling) life habit, and 4) hypsodonty necessitated by an increase in body size. Data used to test these hypotheses are drawn from a variety of lines of evidence, including tooth wear and isotopic composition, as well as skeletal morphology and global climate data. Preliminary results are somewhat ambiguous, as many of the hypothesized causes coincide in time; however, it appears that diet is not the most important causal factor in driving evolution of hypsodonty in this clade. Testing adaptive hypotheses within a phylogenetic context gives greater power for teasing apart the effects of closely synchronous hypothetical selective forces. Comparison with other clades in which hypsodonty evolves suggests that hypsodonty is a common response across different lineages to different causal factors.