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Abstract This special issue of The Anatomical Record is inspired by and dedicated to Professor Kunwar P. Bhatnagar, whose lifelong interests in biology, and long career studying bats, inspired many and advanced our knowledge of the world's only flying mammals. The 15 articles included here represent a broad range of investigators, treading topics familiar to Prof. Bhatnagar, who was interested in seemingly every aspect of bat biology. Key topics include broad themes of bat development, sensory systems, and specializations related to flight and diet. These articles paint a complex picture of the fascinating adaptations of bats, such as rapid fore limb development, ear morphologies relating to echolocation, and other enhanced senses that allow bats to exploit niches in virtually every part of the world. In this introduction, we integrate and contextualize these articles within the broader story of bat ecomorphology, providing an overview of each of the key themes noted above. This special issue will serve as a springboard for future studies both in bat biology and in the broader world of mammalian comparative anatomy and ecomorphology. 

Nasal turbinals, scrolled thin bones of the nasal cavity, increase surface area for conditioning inspired air or for olfaction in mammals. To assess function in Eptesicus fuscus (Big Brown Bat), we quantify surface area of respiratory and olfactory turbinals from birth to adult size, using data from microCT scans before and after iodine staining. Surface area of each turbinal is significantly correlated with postnatal age and cranial length. The surface area of the maxilloturbinal and first ethmoturbinal (ET I) grows faster, relative to skull size, than surface area of caudal ethmoturbinals or the frontoturbinal. Histological examination of selected specimens reveals ET I grows disproportionately more presumptive respiratory mucosa than olfactory mucosa, supporting the hypothesis that ET I has a dual function. Lastly, we find that distribution of olfactory mucosa in the caudal nasal cavity diminishes with age. Our findings suggest a reduction in olfactory function in E. fuscus, perhaps due to a diminished role in food acquisition by this aerial insectivore. 

Synopsis The lower reproductive tract of female mammals has several competing functions including mating, tract health maintenance, and parturition. Diverse vaginal anatomy suggests interactions between natural and sexual selection, yet despite its importance, female copulatory morphology remains under-studied. We undertook a comparative study across the species-rich mammalian order Chiroptera (bats) with a focus on the suborder Yangochiroptera (Vespertilioniformes) to examine how female vaginal features may have coevolved with male penis morphology to minimize mechanical damage to their tissues during copulation. The penis morphology is diverse, presenting great potential for post-copulatory sexual selection and coevolution with the female morphology, but vaginas have not been carefully examined. Here we test the hypotheses that vaginal thickness and collagen density have coevolved with features of the male penis, including the presence of spines and a baculum. We present histological data from females of 24 species from 7 families of bats, and corresponding data on male penis anatomy. We also examine the role of phylogenetic history in the morphological patterns we observe. We found evidence that female vaginal thickness has coevolved with the presence of penile spines, but not with baculum presence or width. Collagen density did not appear to covary with male penile features. Our findings highlight the importance of considering interactions between the sexes in influencing functional reproductive structures and examine how these structures have been under selection in bats. 

In olfactory systems, convergence of sensory neurons onto glomeruli generates a map of odorant receptor identity. How glomerular maps relate to sensory space remains unclear. We sought to better characterize this relationship in the mouse olfactory system by defining glomeruli in terms of the odorants to which they are most sensitive. Using high-throughput odorant delivery and ultrasensitive imaging of sensory inputs, we imaged responses to 185 odorants presented at concentrations determined to activate only one or a few glomeruli across the dorsal olfactory bulb. The resulting datasets defined the tuning properties of glomeruli - and, by inference, their cognate odorant receptors - in a low-concentration regime, and yielded consensus maps of glomerular sensitivity across a wide range of chemical space. Glomeruli were extremely narrowly tuned, with ~25% responding to only one odorant, and extremely sensitive, responding to their effective odorants at sub-picomolar to nanomolar concentrations. Such narrow tuning in this concentration regime allowed for reliable functional identification of many glomeruli based on a single diagnostic odorant. At the same time, the response spectra of glomeruli responding to multiple odorants was best predicted by straightforward odorant structural features, and glomeruli sensitive to distinct odorants with common structural features were spatially clustered. These results define an underlying structure to the primary representation of sensory space by the mouse olfactory system.