Functional microanatomy of the vomeronasal complex of bats.

Recently, Yohe and Krell (The Anatomical Record, vol. 306:2765-2780) lamented the incongruence between genetics and morphology in the vomeronasal system of bats. Here, we studied 105 bat species from 19 families using histology, iodine-enhanced computed tomography (CT), and/or micro-CT. We focused on structural elements that support a functional peripheral vomeronasal receptor organ (vomeronasal organ [VNO]), together comprising the "vomeronasal complex." Our results support prior studies that describe a functional VNO in most phyllostomid bats, miniopterids, and some mormoopids (most known Pteronotus spp.). All of these species (or congeners, at least) have vomeronasal nerves connecting the VNO with the brain and some intact genes related to a functional VNO. However, some bats have VNOs that lack a neuroepithelium and yet still possess elements that aid VNO function, such as a "capsular" morphology of the vomeronasal cartilages (VNCs), and even large venous sinuses, which together facilitate a vasomotor pump mechanism that can draw fluid into the VNO. We also show that ostensibly functionless VNOs of some bats are developmentally associated with ganglionic masses, perhaps involved in endocrine pathways. Finally, we demonstrate that the capsular VNC articulates with the premaxilla or maxilla, and that these bones bear visible grooves denoting the location of the VNC. Since these paraseptal grooves are absent in bats that have simpler (bar-shaped or curved) VNCs, this trait could be useful in fossil studies. Variable retention of some but not all "functional" elements of the vomeronasal complex suggests diverse mechanisms of VNO loss among some bat lineages.

Decoupled evolution of the cranium and mandible in carnivoran mammals.

The relationship between skull morphology and diet is a prime example of adaptive evolution. In mammals, the skull consists of the cranium and the mandible. Although the mandible is expected to evolve more directly in response to dietary changes, dietary regimes may have less influence on the cranium because additional sensory and brain-protection functions may impose constraints on its morphological evolution. Here, we tested this hypothesis by comparing the evolutionary patterns of cranium and mandible shape and size across 100+ species of carnivoran mammals with distinct feeding ecologies. Our results show decoupled modes of evolution in cranial and mandibular shape; cranial shape follows clade-based evolutionary shifts, whereas mandibular shape evolution is linked to broad dietary regimes. These results are consistent with previous hypotheses regarding hierarchical morphological evolution in carnivorans and greater evolutionary lability of the mandible with respect to diet. Furthermore, in hypercarnivores, the evolution of both cranial and mandibular size is associated with relative prey size. This demonstrates that dietary diversity can be loosely structured by craniomandibular size within some guilds. Our results suggest that mammal skull morphological evolution is shaped by mechanisms beyond dietary adaptation alone.

Mind the gap: natural cleft palates reduce biting performance in bats.

Novel morphological traits pose interesting evolutionary paradoxes when they become widespread in a lineage while being deleterious in others. Cleft palate is a rare congenital condition in mammals in which the incisor-bearing premaxilla bones of the upper jaw develop abnormally. However, ∼50% of bat species have natural, non-pathological cleft palates. We used the family Vespertilionidae as a model and linear and geometric morphometrics within a phylogenetic framework to (1) explore evolutionary patterns in cleft morphology, and (2) test whether cleft morphological variation is correlated with skull shape in bats. We also used finite element (FE) analyses to experimentally test how presence of a cleft palate impacts skull performance during biting in a species with extreme cleft morphology (hoary bat, Lasiurus cinereus). We constructed and compared the performance of two FE models: one based on the hoary bat's natural skull morphology, and another with a digitally filled cleft simulating a complete premaxilla. Results showed that cleft length and width are correlated with skull shape in Vespertilionidae, with narrower, shallower clefts seen in more gracile skulls and broader, deeper clefts in more robust skulls. FE analysis showed that the model with a natural cleft produced lower bite forces, and had higher stress and strain than the model with a filled cleft. In the rostrum, safety factors were 1.59-2.20 times higher in the model with a filled cleft than in the natural model. Our results demonstrate that cleft palates in bats reduce biting performance, and evolution of skull robusticity may compensate for this reduction in performance.

Maxilloturbinal Aids in Nasophonation in Horseshoe Bats (Chiroptera: Rhinolophidae).

Horseshoe bats (Family Rhinolophidae) show an impressive array of morphological traits associated with use of high duty cycle echolocation calls that they emit via their nostrils (nasophonation). Delicate maxilloturbinal bones inside the nasal fossa of horseshoe bats have a unique elongated strand-like shape unknown in other mammals. Maxilloturbinal strands also vary considerably in length and cross-sectional shape. In other mammals, maxilloturbinals help direct respired air and prevent respiratory heat and water loss. We investigated whether strand-shaped maxilloturbinals in horseshoe bats perform a similar function to those of other mammals, or whether they were shaped for a role in nasophonation. Using histology, we studied the mucosa of the nasal fossa in Rhinolophus lepidus, which we compared with Hipposideros lankadiva (Hipposideridae) and Megaderma lyra (Megadermatidae). Using micro-CT scans of 30 horseshoe bat species, we quantified maxilloturbinal surface area and skull shape within a phylogenetic context. Histological results showed horseshoe bat maxilloturbinals are covered in a thin, poorly vascularized, sparsely ciliated mucosa poorly suited for preventing respiratory heat and water loss. Maxilloturbinal surface area was correlated with basicranial width, but exceptionally long and dorsoventrally flat maxilloturbinals did not show enhanced surface area for heat and moisture exchange. Skull shape variation appears to be driven by structures linked to nasophonation, including maxilloturbinals. Resting echolocation call frequency better predicted skull shape than did skull size, and was specifically correlated with dimensions of the rostral inflations, palate, and maxilloturbinals. These traits appear to form a morphological complex, indicating a nasophonatory role for the strand-shaped rhinolophid maxilloturbinals. Anat Rec, 2018. © 2018 American Association for Anatomy.

3D Digitization in Functional Morphology: Where is the Point of Diminishing Returns?

Modern computational and imaging methods are revolutionizing the fields of comparative morphology, biomechanics, and ecomorphology. In particular, imaging tools such as X-ray micro computed tomography (µCT) and diffusible iodine-based contrast enhanced CT allow observing and measuring small and/or otherwise inaccessible anatomical structures, and creating highly accurate three-dimensional (3D) renditions that can be used in biomechanical modeling and tests of functional or evolutionary hypotheses. But, do the larger datasets generated through 3D digitization always confer greater power to uncover functional or evolutionary patterns, when compared with more traditional methodologies? And, if so, why? Here, we contrast the advantages and challenges of using data generated via (3D) CT methods versus more traditional (2D) approaches in the study of skull macroevolution and feeding functional morphology in bats. First, we test for the effect of dimensionality and landmark number on inferences of adaptive shifts during cranial evolution by contrasting results from 3D versus 2D geometric morphometric datasets of bat crania. We find sharp differences between results generated from the 3D versus some of the 2D datasets (xy, yz, ventral, and frontal), which appear to be primarily driven by the loss of critical dimensions of morphological variation rather than number of landmarks. Second, we examine differences in accuracy and precision among 2D and 3D predictive models of bite force by comparing three skull lever models that differ in the sources of skull and muscle anatomical data. We find that a 3D model that relies on skull µCT scans and muscle data partly derived from diceCT is slightly more accurate than models based on skull photographs or skull µCT and muscle data fully derived from dissections. However, the benefit of using the diceCT-informed model is modest given the effort it currently takes to virtually dissect muscles from CT scans. By contrasting traditional and modern tools, we illustrate when and why 3D datasets may be preferable over 2D data, and vice versa, and how different methodologies can complement each other in comparative analyses of morphological function and evolution.

Signatures of echolocation and dietary ecology in the adaptive evolution of skull shape in bats.

Morphological diversity may arise rapidly as a result of adaptation to novel ecological opportunities, but early bursts of trait evolution are rarely observed. Rather, models of discrete shifts between adaptive zones may better explain macroevolutionary dynamics across radiations. To investigate which of these processes underlie exceptional levels of morphological diversity during ecological diversification, we use modern phylogenetic tools and 3D geometric morphometric datasets to examine adaptive zone shifts in bat skull shape. Here we report that, while disparity was established early, bat skull evolution is best described by multiple adaptive zone shifts. Shifts are partially decoupled between the cranium and mandible, with cranial evolution more strongly driven by echolocation than diet. Phyllostomidae, a trophic adaptive radiation, exhibits more adaptive zone shifts than all other families combined. This pattern was potentially driven by ecological opportunity and facilitated by a shift to intermediate cranial shapes compared to oral-emitters and other nasal emitters.

Jaw-Dropping: Functional Variation in the Digastric Muscle in Bats.

Diet and feeding behavior in mammals is strongly linked to the morphology of their feeding apparatus. Cranio-muscular morphology determines how wide, forcefully, and quickly the jaw can be opened or closed, which limits the size and material properties of the foods that a mammal can eat. Most studies of feeding performance in mammals have focused on skull form and jaw muscles involved in generating bite force, but few explore how jaw abduction is related to feeding performance. In this study, we explored how the morphology of the digastric muscle, the primary jaw abducting muscle in mammals, and its jaw lever mechanics are related to diet in morphologically diverse noctilionoid bats. Results showed that insectivorous bats have strong digastric muscles associated with proportionally long jaws, which suggests these species can open their jaws quickly and powerfully during prey capture and chewing. Short snouted frugivorous bats exhibit traits that would enable them to open their jaws proportionally wider to accommodate the large fruits that they commonly feed on. Our results support the hypothesis that digastric muscle and jaw morphology are correlated with diet in bats, and that our results may also apply to other groups of mammals. Anat Rec, 301:279-290, 2018. © 2018 Wiley Periodicals, Inc.

Unique Turbinal Morphology in Horseshoe Bats (Chiroptera: Rhinolophidae).

The mammalian nasal fossa contains a set of delicate and often structurally complex bones called turbinals. Turbinals and associated mucosae function in regulating respiratory heat and water loss, increasing surface area for olfactory tissue, and directing airflow within the nasal fossa. We used high-resolution micro-CT scanning to investigate a unique maxilloturbinal morphology in 37 species from the bat family Rhinolophidae, which we compared with those of families Hipposideridae, Megadermatidae, and Pteropodidae. Rhinolophids exhibit numerous structural modifications along the nasopharyngeal tract associated with emission of high duty cycle echolocation calls via the nostrils. In rhinolophids, we found that the maxilloturbinals and a portion of ethmoturbinal I form a pair of strand-like bony structures on each side of the nasal chamber. These structures project anteriorly from the transverse lamina and complete a hairpin turn to project posteriorly down the nasopharyngeal duct, and vary in length among species. The strand-like maxilloturbinals in Rhinolophidae were not observed in our outgroups and represent a synapomorphy for this family, and are unique in form among mammals. Within Rhinolophidae, maxilloturbinal size and cross-sectional shape were correlated with phylogeny. We hypothesize that strand-shaped maxilloturbinals may function to reduce respiratory heat and water loss without greatly impacting echolocation call transmission since they provide increased mucosal surface area for heat and moisture exchange but occupy minimal space. Alternatively, they may play a role in transmission of echolocation calls since they are located directly along the path sound travels between the larynx and nostrils during call emission. Anat Rec, 300:309-325, 2017. © 2016 Wiley Periodicals, Inc.

Repeated loss of frontal sinuses in arctoid carnivorans.

Many mammal skulls contain air spaces inside the bones surrounding the nasal chamber including the frontal, maxilla, ethmoid, and sphenoid, all of which are called paranasal sinuses. Within the Carnivora, frontal sinuses are usually present, but vary widely in size and shape. The causes of this variation are unclear, although there are some functional associations, such as a correlation between expanded frontal sinuses and a durophagous diet in some species (e.g., hyenas) or between absent sinuses and semiaquatic lifestyle (e.g., pinnipeds). To better understand disparity in frontal sinus morphology within Carnivora, we quantified frontal sinus size in relationship to skull size and shape in 23 species within Arctoidea, a clade that is ecologically diverse including three independent invasions of aquatic habitats, by bears, otters, and pinnipeds, respectively. Our sampled species range in behavior from terrestrial (rarely or never forage in water), to semiterrestrial (forage in water and on land), to semiaquatic (forage only in water). Results show that sinuses are either lost or reduced in both semiterrestrial and semiaquatic species, and that sinus size is related to skull size and shape. Among terrestrial species, frontal sinus size was positively allometric overall, but several terrestrial species completely lacked sinuses, including two fossorial badgers, the kinkajou (a nocturnal, arboreal frugivore), and several species with small body size, indicating that factors other than aquatic habits, such as space limitations due to constraints on skull size and shape, can limit sinus size and presence.

Beyond the sniffer: frontal sinuses in Carnivora.

Paranasal sinuses are some of the most poorly understood features of mammalian cranial anatomy. They are highly variable in presence and form among species, but their function is not well understood. The best-supported explanations for the function of sinuses is that they opportunistically fill mechanically unnecessary space, but that in some cases, sinuses in combination with the configuration of the frontal bone may improve skull performance by increasing skull strength and dissipating stresses more evenly. We used CT technology to investigate patterns in frontal sinus size and shape disparity among three families of carnivores: Canidae, Felidae, and Hyaenidae. We provide some of the first quantitative data on sinus morphology for these three families, and employ a novel method to quantify the relationship between three-dimensional sinus shape and skull shape. As expected, frontal sinus size and shape were more strongly correlated with frontal bone size and shape than with the morphology of the skull as a whole. However, sinus morphology was also related to allometric differences among families that are linked to biomechanical function. Our results support the hypothesis that frontal sinuses most often opportunistically fill space that is mechanically unnecessary, and they can facilitate cranial shape changes that reduce stress during feeding. Moreover, we suggest that the ability to form frontal sinuses allows species to modify skull function without compromising the performance of more functionally constrained regions such as the nasal chamber (heat/water conservation, olfaction), and braincase (housing the brain and sensory structures).