Testing a long-standing hypothesis on the relation between the auditory bulla size and environmental conditions: A case study in two jird species (Muridae: Meriones libycus and M. crassus)

Variation in mammalian auditory bulla size has been linked to environmental conditions, and has even been claimed to provide a habitat-specific survival value. Enlarged bullae are typically shared among species adapted to living in arid habitats. Previous studies suggest that jirds also exhibit this adaptive enlargement of the bulla. However, such claims are based on the observation on a limited number of specimens, and thus they provide no quantitative support for this hypothesis. Therefore, we tested this hypothesis using a combination of geometric morphometrics and multivariate statistical techniques on a large sample of two jird species that exhibit a wide and partially overlapping geographical (and hence climatic) range, i.e., Meriones crassus (Sundevall, 1842) and M. libycus (Lichtenstein, 1823). A total of 485 intact skulls of populations originating from Africa to the eastern Iranian Plateau were analysed. The covariation between auditory bulla shape and several geoclimatic variables was explored using the two-block partial least-squares method. To some degree, the results seem to support the hypothesis that morphological variation in the auditory bulla is, indeed, correlated with geoclimatic variation. However, this finding cannot be generalised at several levels, hence refuting the hypothesis of convergent correlations between geoclimatic conditions and bulla morphology. Species-specific differences were found at different levels, suggesting that if local responses are present, they may reflect interactions with phylogenetic constraints.