Keeping an ear out: size relationship of the tympanic bullae and pinnae in bandicoots and bilbies (Marsupialia: Peramelemorphia)

Morphological variations of auditory bullae in otomyine rodents (Rodentia: Otomyini) in southern African biomes

Mammalian middle ear cavities differ from those of other taxa as they comprise three ossicles and in rodents, can be encapsulated by an auditory bulla. In small mammals, the middle ear cavity (bulla) was found to be enlarged in the desert-dwelling species; however, differences in bullar size could have been due to ancestry. In this study, we sampled seven species from three genera (Myotomys, Otomys, and Parotomys) of the African murid tribe Otomyini (laminated-toothed rats), and compared the bullar volumes and shapes between the otomyine species and within the species Myotomys unisulcatus. Photographs of museum skull specimens were taken from ventral and lateral views, and the volumes of the bullae were estimated digitally from the photographs. No sexual dimorphism in bullar volumes was found in any of the species. Corrected bullar volumes were significantly different between species and larger bullae were seen in individuals inhabiting regions with lower annual rainfall. Bullar shape (estimated using geometric morphometrics) was significantly different between the genera and the species. Parotomys have tympanic meatuses that face more anteriorly compared to both, Otomys and Myotomys. When comparing bullae within M. unisulcatus, those inhabiting regions with lower annual rainfall had significantly larger bullar volumes, but no significant difference was found in bullar shape between the regions. This study shows that otomyine rodents in more xeric habitats have different auditory structures to those inhabiting wetter habitats.

Morphological variability or inter-observer bias? A methodological toolkit to improve data quality of multi-researcher datasets for the analysis of morphological variation

OBJECTIVES

The investigation of morphological variation in animals is widely used in taxonomy, ecology, and evolution. Using large datasets for meta-analyses has dramatically increased, raising concerns about dataset compatibilities and biases introduced by contributions of multiple researchers.

MATERIALS AND METHODS

We compiled morphological data on 13 variables for 3073 individual mouse lemurs (Cheirogaleidae, Microcebus spp.) from 25 taxa and 153 different sampling locations, measured by 48 different researchers. We introduced and applied a filtering pipeline and quantified improvements in data quality (Shapiro-Francia statistic, skewness, and excess kurtosis). The filtered dataset was then used to test for genus-wide sexual size dimorphism and the applicability of Rensch's, Allen's, and Bergmann's rules.

RESULTS

Our pipeline reduced inter-observer bias (i.e., increased normality of data distributions). Inter-observer reliability of measurements was notably variable, highlighting the need to reduce data collection biases. Although subtle, we found a consistent pattern of sexual size dimorphism across Microcebus, with females being the larger (but not heavier) sex. Sexual size dimorphism was isometric, providing no support for Rensch's rule. Variations in tail length but not in ear size were consistent with the predictions of Allen's rule. Body mass and length followed a pattern contrary to predictions of Bergmann's rule.

DISCUSSION

We highlighted the usefulness of large multi-researcher datasets for testing ecological hypotheses after correcting for inter-observer biases. Using genus-wide tests, we outlined generalizable patterns of morphological variability across all mouse lemurs. This new methodological toolkit aims to facilitate future large-scale morphological comparisons for a wide range of taxa and applications.

Cranial differences in three-toed jerboas (Dipodinae, Dipodidae, Rodentia) according to recent taxonomic revisions

Recent phylogenetic studies amended the taxonomy of three-toed jerboas (subfamily Dipodinae), including raising subspecies to full species. Here, we use geometric morphometrics to compare scaled-shape differences in dipodine crania while considering their revised taxonomy. We sampled Dipus deasyi, D. sagitta halli, D. s. sowerbyi, Jaculus blanfordi blanfordi, J. hirtipes, J. jaculus, J. loftusi, J. orientalis gerboa, J. o. mauritanicus, and Stylodipus andrewsi. Crania were not sexually dimorphic. Common allometry explained some of the shape variation, for example, reduced braincases in larger specimens. Most operational taxonomic unit pairs differed in both size and shape. Dipus and Stylodipus clustered together based on their cranial shape. Jaculus differed from the aforementioned genera by its larger tympanic bulla, broader braincase, larger infraorbital foramen, along with reduced molars and rostra. Jaculus orientalis differed from other Jaculus by its broader face versus reduced cranial vault. Jaculus blanfordi (subgenus Haltomys) resembles members of the subgenus Jaculus more than its consubgener (J. orientalis). Jaculus loftusi, previously considered a synonym of J. jaculus, clearly differed from the latter by its shorter rostrum, smaller infraorbital foramen, and more caudolaterally expanded tympanic bulla. Jaculus hirtipes, another recent synonym of J. jaculus, resembled J. blanfordi more in scaled cranial shape than it did J. jaculus. Dipus sagitta halli and D. s. sowerbyi were indistinguishable, but they clearly differed from D. deasyi (recently raised to full species) with the latter having a larger molar row, more inflated tympanic bulla, and shorter, slenderer rostrum. Ecological explanations for detected cranial shape differences are considered, including diet and habitat (particularly substrate).