Ecological correlates of morphological variation in the tympanic bulla of desert rodents: phylogeny, adaptation, and drift
Phylogenetic history, adaptation, and drift are the primary determinants of phenotype, although each can differ in magnitude and direction (i.e. complementary or antagonistic). Desert rodents share many distinctive features. Among the most distinctive features is the hypertrophy of the tympanic bulla. A common functional explanation for this trait is that it is an adaptation for increased sensitivity to sound that is necessary for both prey capture and predator avoidance in open habitats where sound tends to dissipate quickly. Using a new composite phylogeny of >1,000 myodont and heteromyid rodents that documents multiple transitions into deserts, we conducted phylogenetic independent contrasts to determine whether variation in tympanic bullae morphology of rodents is associated with aridity. Morphology of the bulla was quantified from specimens photographed at natural history museums using geometric morphometric approaches. We tested whether size (centroid size and relative size of the bulla to the rest of the skull) and shape variables (using semi-landmarks) are correlated with environmental variables (temperature, precipitation, and net primary productivity). Co-variation of skull shape variables with environmental variables was compared in three primarily desert dwelling rodent taxa (dipodoids, heteromyids, and gerbils) and their mesic relatives.
There was significant phylogenetic signal in the size and shape of the tympanic bulla. These were strongly correlated with habitat aridity. When desert and mesic species were pooled into two bins after discretizing environmental variables, a clear and significant difference was detected. This effect was detected in all taxa even after the effect of phylogeny was removed (i.e. phylogenetic ANOVA analysis). The pattern was less clear when environmental variables were treated as continuous variables (i.e. phylogenetic independent contrasts analysis). Other regions of the skull showed no correlation with aridity. This evidence supports the theory that adaptation plays a similar role in shaping the evolution of the tympanic bulla in various taxa of desert rodents. The general pattern that rodents in arid environments have significantly larger bulla than rodents in mesic environments holds true in most taxa even after phylogenetic correction. Evidence for the correlation between bulla shape variables with environmental variables is more tenuous.