DIETARY HARDNESS, LOADING BEHAVIOR, AND THE EVOLUTION OF SKULL FORM IN BATS

Optimal Gearing of Musculoskeletal Systems

Movement is integral to animal life, and most animal movement is actuated by the same engine: striated muscle. Muscle input is typically mediated by skeletal elements, resulting in musculoskeletal systems that are geared: at any instant, the muscle force and velocity are related to the output force and velocity only via a proportionality constant G, the "mechanical advantage". The functional analysis of such "simple machines" has traditionally centered around this instantaneous interpretation, such that a small vs large G is thought to reflect a fast vs forceful system, respectively. But evidence is mounting that a comprehensive analysis ought to also consider the mechanical energy output of a complete contraction. Here, we approach this task systematically, and deploy the theory of physiological similarity to study how gearing affects the flow of mechanical energy in a minimalist model of a musculoskeletal system. Gearing influences the flow of mechanical energy in two key ways: it can curtail muscle work output, because it determines the ratio between the characteristic muscle kinetic energy and work capacity; and it defines how each unit of muscle work is partitioned into different system energies, that is, into kinetic vs "parasitic" energy such as heat. As a consequence of both effects, delivering maximum work in minimum time and with maximum output speed generally requires a mechanical advantage of intermediate magnitude. This optimality condition can be expressed in terms of two dimensionless numbers that reflect the key geometric, physiological, and physical properties of the interrogated musculoskeletal system, and the environment in which the contraction takes place. Illustrative application to exemplar musculoskeletal systems predicts plausible mechanical advantages in disparate biomechanical scenarios, yields a speculative explanation for why gearing is typically used to attenuate the instantaneous force output ($G_{\text{opt}} \lt 1)$, and predicts how G needs to vary systematically with animal size to optimize the delivery of mechanical energy, in superficial agreement with empirical observations. A many-to-one mapping from musculoskeletal geometry to mechanical performance is identified, such that differences in G alone do not provide a reliable indicator for specialization for force vs speed-neither instantaneously, nor in terms of mechanical energy output. The energy framework presented here can be used to estimate an optimal mechanical advantage across variable muscle physiology, anatomy, mechanical environment, and animal size, and so facilitates investigation of the extent to which selection has made efficient use of gearing as a degree of freedom in musculoskeletal "design."

Maxillary morphology of chimpanzees: Captive versus wild environments

Morphological studies typically avoid using osteological samples that derive from captive animals because it is assumed that their morphology is not representative of wild populations. Rearing environments indeed differ between wild and captive individuals. For example, mechanical properties of the diets provided to captive animals can be drastically different from the food present in their natural habitats, which could impact cranial morphology and dental health. Here, we examine morphological differences in the maxillae of wild versus captive chimpanzees (Pan troglodytes) given the prominence of this species in comparative samples used in human evolution research and the key role of the maxilla in such studies. Size and shape were analysed using three-dimensional geometric morphometric methods based on computed tomography scans of 94 wild and 30 captive specimens. Captive individuals have on average larger and more asymmetrical maxillae than wild chimpanzees, and significant differences are present in their maxillary shapes. A large proportion of these shape differences are attributable to static allometry, but wild and captive specimens still differ significantly from each other after allometric size adjustment of the shape data. Levels of shape variation are higher in the captive group, while the degree of size variation is likely similar in our two samples. Results are discussed in the context of ontogenetic growth trajectories, changes in dietary texture, an altered social environment, and generational differences. Additionally, sample simulations show that size and shape differences between chimpanzees and bonobos (Pan paniscus) are exaggerated when part of the wild sample is replaced with captive chimpanzees. Overall, this study confirms that maxillae of captive chimpanzees should not be included in morphological or taxonomic analyses when the objective is to characterise the species.

Facing the facts: adaptive trade-offs along body size ranges determine mammalian craniofacial scaling

The mammalian cranium (skull without lower jaw) is representative of mammalian diversity and is thus of particular interest to mammalian biologists across disciplines. One widely retrieved pattern accompanying mammalian cranial diversification is referred to as 'craniofacial evolutionary allometry' (CREA). This posits that adults of larger species, in a group of closely related mammals, tend to have relatively longer faces and smaller braincases. However, no process has been officially suggested to explain this pattern, there are many apparent exceptions, and its predictions potentially conflict with well-established biomechanical principles. Understanding the mechanisms behind CREA and causes for deviations from the pattern therefore has tremendous potential to explain allometry and diversification of the mammalian cranium. Here, we propose an amended framework to characterise the CREA pattern more clearly, in that 'longer faces' can arise through several kinds of evolutionary change, including elongation of the rostrum, retraction of the jaw muscles, or a more narrow or shallow skull, which all result in a generalised gracilisation of the facial skeleton with increased size. We define a standardised workflow to test for the presence of the pattern, using allometric shape predictions derived from geometric morphometrics analysis, and apply this to 22 mammalian families including marsupials, rabbits, rodents, bats, carnivores, antelopes, and whales. Our results show that increasing facial gracility with size is common, but not necessarily as ubiquitous as previously suggested. To address the mechanistic basis for this variation, we then review cranial adaptations for harder biting. These dictate that a more gracile cranium in larger species must represent a structural sacrifice in the ability to produce or withstand harder bites, relative to size. This leads us to propose that facial gracilisation in larger species is often a product of bite force allometry and phylogenetic niche conservatism, where more closely related species tend to exhibit more similar feeding ecology and biting behaviours and, therefore, absolute (size-independent) bite force requirements. Since larger species can produce the same absolute bite forces as smaller species with less effort, we propose that relaxed bite force demands can permit facial gracility in response to bone optimisation and alternative selection pressures. Thus, mammalian facial scaling represents an adaptive by-product of the shifting importance of selective pressures occurring with increased size. A reverse pattern of facial 'shortening' can accordingly also be found, and is retrieved in several cases here, where larger species incorporate novel feeding behaviours involving greater bite forces. We discuss multiple exceptions to a bite force-mediated influence on facial proportions across mammals which lead us to argue that ecomorphological specialisation of the cranium is likely to be the primary driver of facial scaling patterns, with some developmental constraints as possible secondary factors. A potential for larger species to have a wider range of cranial functions when less constrained by bite force demands might also explain why selection for larger sizes seems to be prevalent in some mammalian clades. The interplay between adaptation and constraint across size ranges thus presents an interesting consideration for a mechanistically grounded investigation of mammalian cranial allometry.

The hierarchical radiation of phyllostomid bats as revealed by adaptive molar morphology

Adaptive radiations are bursts in biodiversity that generate new evolutionary lineages and phenotypes. However, because they typically occur over millions of years, it is unclear how their macroevolutionary dynamics vary through time and among groups of organisms. Phyllostomid bats radiated extensively for diverse diets-from insects to vertebrates, fruit, nectar, and blood-and we use their molars as a model system to examine the dynamics of adaptive radiations. Three-dimensional shape analyses of lower molars of Noctilionoidea (Phyllostomidae and close relatives) indicate that different diet groups exhibit distinct morphotypes. Comparative analyses further reveal that phyllostomids are a striking example of a hierarchical radiation; phyllostomids' initial, higher-level diversification involved an "early burst" in molar morphological disparity as lineages invaded new diet-affiliated adaptive zones, followed by subsequent lower-level diversifications within adaptive zones involving less dramatic morphological changes. We posit that strong selective pressures related to initial shifts to derived diets may have freed molars from morpho-functional constraints associated with the ancestral molar morphotype. Then, lineages with derived diets (frugivores and nectarivores) diversified within broad adaptive zones, likely reflecting finer-scale niche partitioning. Importantly, the observed early burst pattern is only evident when examining molar traits that are strongly linked to diet, highlighting the value of ecomorphological traits in comparative studies. Our results support the hypothesis that adaptive radiations are commonly hierarchical and involve different tempos and modes at different phylogenetic levels, with early bursts being more common at higher levels.

Ecomorphology and sensory biology of bats

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.

Morphological specialization to nectarivory in Phyllostomus discolor (Wagner, 1843) (Chiroptera: Phyllostomidae)

Dedicated nectarivory is a derived feeding habit that requires specialized cranial and soft-tissue morphologies to extract nectar from flowers. Nectarivory has evolved many times in terrestrial vertebrates, and in four bat families (Pteropodidae, Phyllostomidae, Vespertilionidae, and Mystacinidae). Within phyllostomids, specializations to nectarivory have been well documented in two subfamilies, Glossophaginae and Lonchophyllinae. However, nectarivory has also evolved independently in the genus Phyllostomus (subfamily Phyllostominae). Since Phyllostomus species have an omnivorous diet with a high consumption of nectar, they can be used to explore the basic morphological modifications linked to evolving a nectarivorous habit. Here, we focused on describing and comparing the morphological features potentially associated with nectarivory in Phyllostomus discolor. We present the first detailed tongue and palate morphological descriptions for P. discolor and perform skull morphometric analysis including 10 species. We found hair-like papillae on the tongue of P. discolor, a convergent feature with Glossophaginae and nectarivorous Pteropodids; these papillae likely confer an advantage when feeding on nectar. P. discolor does not show skull morphological features characteristic of nectarivorous bats, such as a long and narrow snout. We pose that the consumption of a variety of food, such as hard insects and fruits, and the large size of P. discolor relative to specialized nectarivores may create trade-offs against morphological specialization of the skull towards nectarivory. In contrast, a long and mobile tongue with hair-like papillae may be an evolutionary solution for nectar extraction that does not have a major impact on this species' ability to feed on other resources.

Disentangling Mechanical and Sensory Modules in the Radiation of Noctilionoid Bats

AbstractWith diverse mechanical and sensory functions, the vertebrate cranium is a complex anatomical structure whose shifts between modularity and integration, especially in mechanical function, have been implicated in adaptive diversification. Yet how mechanical and sensory systems and their functions coevolve, as well as how their interrelationship contributes to phenotypic disparity, remain largely unexplored. To examine the modularity, integration, and evolutionary rates of sensory and mechanical structures within the head, we analyzed hard and soft tissue scans from ecologically diverse bats in the superfamily Noctilionoidea, a clade that ranges from insectivores and carnivores to frugivores and nectarivores. We identified eight regions that evolved in a coordinated fashion, thus recognizable as evolutionary modules: five associated with bite force and three linked to olfactory, visual, and auditory systems. Interrelationships among these modules differ between Neotropical leaf-nosed bats (family Phyllostomidae) and other noctilionoids. Consistent with the hypothesis that dietary transitions begin with changes in the capacity to detect novel food items followed by adaptations to process them, peak rates of sensory module evolution predate those of some mechanical modules. We propose that the coevolution of structures influencing bite force, olfaction, vision, and hearing constituted a structural opportunity that allowed the phyllostomid ancestor to take advantage of existing ecological opportunities and contributed to the clade's remarkable radiation.

Diversification of the ruminant skull along an evolutionary line of least resistance

Clarifying how microevolutionary processes scale to macroevolutionary patterns is a fundamental goal in evolutionary biology, but these analyses, requiring comparative datasets of population-level variation, are limited. By analyzing a previously published dataset of 2859 ruminant crania, we find that variation within and between ruminant species is biased by a highly conserved mammalian-wide allometric pattern, CREA (craniofacial evolutionary allometry), where larger species have proportionally longer faces. Species with higher morphological integration and species more biased toward CREA have diverged farther from their ancestors, and Ruminantia as a clade diversified farther than expected in the direction of CREA. Our analyses indicate that CREA acts as an evolutionary "line of least resistance" and facilitates morphological diversification due to its alignment with the browser-grazer continuum. Together, our results demonstrate that constraints at the population level can produce highly directional patterns of phenotypic evolution at the macroevolutionary scale. Further research is needed to explore how CREA has been exploited in other mammalian clades.

The interplay between spatiotemporal overlap and morphology as determinants of microstructure suggests no 'perfect fit' in a bat-flower network

Plant-pollinator interactions in diverse tropical communities are often predicted by a combination of ecological variables, yet the interaction drivers between flower-visiting bats and plants at the community level are poorly understood. We assembled a network between Neotropical bats and flowering plants to describe its macrostructure and to test the role of neutral and niche variables in predicting microstructure. We found a moderately generalized network with internally nested modules comprising functionally similar plant and bat species. Modules grouped bats and plants with matching degrees of specialization but had considerable overlap in species morphologies and several inter-module interactions. The spatiotemporal overlap between species, closely followed by morphology, and not abundance, were the best predictors of microstructure, with functional groups of bats also interacting more frequently with plants in certain vegetation types (e.g., frugivores within forests) and seasons (e.g., long-snouted nectarivores in the dry season). Therefore, flower-visiting bats appear to have species-specific niche spaces delimited not only by their ability to exploit certain flower types but also by preferred foraging habitats and the timing of resource availability. The prominent role of resource dissimilarity across vegetation types and seasons likely reflects the heterogeneity of Neotropical savannas, and further research in biomes beyond the Cerrado is needed to better understand the complexity of this system.

Mechanical compensation in the evolution of the early hominin feeding apparatus

Australopiths, a group of hominins from the Plio-Pleistocene of Africa, are characterized by derived traits in their crania hypothesized to strengthen the facial skeleton against feeding loads and increase the efficiency of bite force production. The crania of robust australopiths are further thought to be stronger and more efficient than those of gracile australopiths. Results of prior mechanical analyses have been broadly consistent with this hypothesis, but here we show that the predictions of the hypothesis with respect to mechanical strength are not met: some gracile australopith crania are as strong as that of a robust australopith, and the strength of gracile australopith crania overlaps substantially with that of chimpanzee crania. We hypothesize that the evolution of cranial traits that increased the efficiency of bite force production in australopiths may have simultaneously weakened the face, leading to the compensatory evolution of additional traits that reinforced the facial skeleton. The evolution of facial form in early hominins can therefore be thought of as an interplay between the need to increase the efficiency of bite force production and the need to maintain the structural integrity of the face.

Bat Dentitions: A Model System for Studies at The Interface of Development, Biomechanics, and Evolution

The evolution of complex dentitions was a major innovation in mammals that facilitated the expansion into new dietary niches that imposed selection for tight form-function relationships. Teeth allow mammals to ingest and process food items by applying forces produced by a third-class lever system composed by the jaw adductors, the cranium, and the mandible. Physical laws determine changes in jaw adductor (biting) forces at different bite point locations along the mandible (outlever), thus individual teeth are expected to experience different mechanical regimes during feeding. If the mammal dentition exhibits functional adaptations to mandible feeding biomechanics, then teeth are expected to have evolved to develop mechanically-advantageous sizes, shapes, and positions. Here, we present bats as a model system to test this hypothesis and, more generally, for integrative studies of mammal dental diversity. We combine a field-collected dataset of bite forces along the tooth row with data on dental and mandible morphology across 30 bat species. We (1) describe, for the first time, bite force trends along the tooth row of bats, (2) use phylogenetic comparative methods to investigate relationships among bite force patterns, tooth and mandible morphology, and (3) hypothesize how these biting mechanics patterns may relate to the developmental processes controlling tooth formation. We find that bite force variation along the tooth row is consistent with predictions from lever mechanics models, with most species having the greatest bite force at the first lower molar. The cross-sectional shape of the mandible body is strongly associated with the position of maximum bite force along the tooth row, likely reflecting mandibular adaptations to varying stress patterns among species. Further, dental dietary adaptations seem to be related to bite force variation along molariform teeth, with insectivorous species exhibiting greater bite force more anteriorly, narrower teeth and mandibles, and frugivores/omnivores showing greater bite force more posteriorly, wider teeth and mandibles. As these craniodental traits are linked through development, dietary specialization appears to have shaped intrinsic mechanisms controlling traits relevant to feeding performance.

Impact of Diet Consistency on the Mandibular Morphology: A Systematic Review of Studies on Rat Models

Apart from genetics, environmental factors, such as food consistency, may affect craniofacial morphology and development. The present systematic review aims to systematically investigate and appraise the available evidence regarding the effect of diet consistency on the anatomical structures of the basal bone of the rat mandible. The search was performed without restrictions in five databases (PubMed, Scopus, Web of Science, ProQuest Dissertations and Theses Global, including grey literature) and hand searching through January 2022. A total of 14,904 references were initially identified, and 16 articles were finally included in the systematic review. Rats that consumed hard diets were found to exhibit an increase inbigonial width, corpus height, condylar depth, condylar base inclination, condylar process inclination, mandibular plane inclination, height and length of angular process, mandibular body height, depth of antegonial notch, growth rate in the gonial angle, angular process convexity and height of condylar process. It was also noted that mandibular depth, mandibular height, ramus angle and angle between the angular process and mandibular plane were decreased in rats that were fed with a hard diet. On the other hand, there were conflicting results about the growth of mandibular length and width, corpus length, mandibular body length, ramus height, condylar length and width, gonial angle and height of coronoid process. From the abovementioned results, it can be concluded that food consistency may affect the morphology of anatomical structures and the overall growth and development of rat mandibles in various ways.

Geographic variation in the skulls of the horseshoe bats, Rhinolophus simulator and R. cf. simulator: Determining the relative contributions of adaptation and drift using geometric morphometrics

The relative contributions of adaptation and genetic drift to morphological diversification of the skulls of echolocating mammals were investigated using two horseshoe bat species, Rhinolophus simulator and R. cf. simulator, as test cases. We used 3D geometric morphometrics to compare the shapes of skulls of the two lineages collected at various localities in southern Africa. Size and shape variation was predominantly attributed to selective forces; the between-population variance (B) was not proportional to the within-population variance (W). Modularity was evident in the crania of R. simulator but absent in the crania of R. cf. simulator and the mandibles of both species. The skulls of the two lineages thus appeared to be under different selection pressures, despite the overlap in their distributions. Difference in the crania of R. cf. simulator was centered largely on the nasal dome region of R. cf. simulator but on the cranium and mandibles of R. simulator. It is likely that the size and shape of the nasal dome, which acts as a frequency-dependent acoustic horn, is more crucial in R. cf. simulator than in R. simulator because of the higher echolocation frequencies used by R. cf. simulator. A larger nasal dome in R. cf. simulator would allow the emission of higher intensity pulses, resulting in comparable detection distances to that of R. simulator. In contrast, selection pressure is probably more pronounced on the mandibles and cranium of R. simulator to compensate for the loss in bite force because of its elongated rostrum. The predominance of selection probably reflects the stringent association between environment and the optimal functioning of phenotypic characters associated with echolocation and feeding in bats.

Skull Morphology, Bite Force, and Diet in Insectivorous Bats from Tropical Dry Forests in Colombia

In Neotropical bats, studies on bite force have focused mainly on differences in trophic ecology, and little is known about whether factors other than body size generate interspecific differences in bite force amongst insectivorous bats and, consequently, in their diets. We tested if bite force is related to skull morphology and also to diet in an assemblage of Neotropical insectivorous bats from tropical dry forests in the inter-Andean central valley in Colombia. It is predicted that the preference of prey types among insectivorous species is based on bite force and cranial characteristics. We also evaluated whether skull morphology varies depending on the species and sex. Cranial measurements and correlations between morphological variation and bite force were examined for 10 insectivorous bat species. We calculated the size-independent mechanical advantage for the mandibular (jaw) lever system. In all species, bite force increased with length of the skull and the jaw more than other cranial measurements. Obligate insectivorous species were morphologically different from the omnivorous Noctilio albiventris, which feeds primarily on insects, but also consumes fish and fruits. Our results show that bite force and skull morphology are closely linked to diets in Neotropical insectivorous bats and, consequently, these traits are key to the interactions within the assemblage and with their prey.

The influence of divergent reproductive strategies in shaping modularity and morphological evolution in mammalian jaws

Marsupial neonates are born at an earlier developmental stage than placental mammals, but the rapid development of their forelimbs and cranial skeleton allows them to climb to the pouch, begin suckling and complete their development ex utero. The mechanical environment in which marsupial neonates develop is vastly different from that of placental neonates, which exhibit a more protracted development of oral muscles and bones. This difference in reproductive strategy has been theorized to constrain morphological evolution in the oral region of marsupials. Here, we use 3D morphometrics to characterize one of these oral bones, the lower jaw (dentary), and assess modularity (pattern of covariation among traits), morphological disparity and rates of morphological evolution in two clades of carnivorous mammals: the marsupial Dasyuromorphia and placental fissiped Carnivora. We find that dasyuromorph dentaries have fewer modules than carnivorans and exhibit tight covariation between the angular and coronoid processes, the primary attachment sites for jaw-closing muscles. This pattern of modularity may result from the uniform action of muscles on the developing mandible during suckling. Carnivorans are free from this constraint and exhibit a pattern of modularity that more strongly reflects genetic and developmental signals of trait covariation. Alongside differences in modularity, carnivorans exhibit greater disparity and faster rates of morphological evolution compared with dasyuromorphs. Taken together, this suggests dasyuromorphs have retained a signal of trait covariation that reflects the outsized influence of muscular force during early development, a feature that may have impacted the ability of marsupial carnivores to explore specialized regions of morphospace.

Foraging strategies, craniodental traits, and interaction in the bite force of Neotropical frugivorous bats (Phyllostomidae: Stenodermatinae)

Bats in the family Phyllostomidae exhibit great diversity in skull size and morphology that reflects the degree of resource division and ecological overlap in the group. In particular, the subfamily Stenodermatinae has high morphological diversification associated with cranial and mandibular traits that are associated with the ability to consume the full range of available fruits (soft and hard).We analyzed craniodental traits and their relationship to the bite force in 343 specimens distributed in seven species of stenodermatine bats with two foraging strategies: nomadic and sedentary frugivory. We evaluated 19 traits related to feeding and bite force in live animals by correcting bite force with body size.We used a generalized linear model (GLM) and post hoc tests to determine possible relationships and differences between cranial traits, species, and sex. We also used Blomberg's K to measure the phylogenetic signal and phylogenetic generalized least-squares (PGLS) to ensure the phylogenetic independence of the traits.We found that smaller nomadic species, A. anderseni and A. phaeotis , have a similar bite force to the large species A. planirostris and A. lituratus; furthermore, P. helleri registered a bite force similar to that of the sedentary bat, S. giannae. Our study determined that all the features of the mandible and most of the traits of the skull have a low phylogenetic signal. Through the PGLS, we found that the diet and several cranial features (mandibular toothrow length, dentary length, braincase breadth, mastoid breadth, greatest length of skull, condylo-incisive length, and condylo-canine length) determined bite force performance among Stenodermatiane.Our results reinforce that skull size is a determining factor in the bite force, but also emphasize the importance of its relationships with morphology, ecology, and phylogeny of the species, which gives us a better understanding of the evolutionary adaptions of this highly diverse Neotropical bat group.

Functional and ecological correlates of the primate jaw abductors

While the adductor musculature of the primate jaw has been extensively analyzed within the context of dietary and social ecology, little is known about the corresponding muscles of jaw abduction. Nonetheless, these muscles significantly contribute to a species' maximum gape potential, and thus might constrain dietary niche diversity and impact social display behaviors. In this study, we quantify the architectural properties of the digastric (a jaw abductor) and lateral pterygoid (a jaw abductor and anterior translator) across a broad sample of male and female anthropoid primates. We test the hypothesis that the abductor musculature reflects specialization to dietary and behavioral ecology. Our sample comprises 14 catarrhine and 13 platyrrhine species spanning a wide range of dietary and social categories. All specimens were sharp dissected and muscles subsequently chemically digested using a standardized protocol. Our findings demonstrate that relative fascicle lengths within the lateral pterygoid (but not the digastric) are significantly greater within species that habitually consume larger food items. Meanwhile, canine length is more strongly associated with fascicle lengths in the digastric than in the lateral pterygoid, particularly within males. Neither dietary mechanical resistance nor the intensity of social competition relates to the size or architectural properties of the jaw abductors. These findings suggest that dietary-and to a lesser extent, socioecological-aspects of a primate's life history may be reflected in the architecture of these muscles, albeit to varying degrees. This underlines the importance of considering the complete masticatory apparatus when interpreting the evolution of the primate jaw.

Dietary Diversification and Specialization in Neotropical Bats Facilitated by Early Molecular Evolution

Dietary adaptation is a major feature of phenotypic and ecological diversification, yet the genetic basis of dietary shifts is poorly understood. Among mammals, Neotropical leaf-nosed bats (family Phyllostomidae) show unmatched diversity in diet; from a putative insectivorous ancestor, phyllostomids have radiated to specialize on diverse food sources including blood, nectar, and fruit. To assess whether dietary diversification in this group was accompanied by molecular adaptations for changing metabolic demands, we sequenced 89 transcriptomes across 58 species and combined these with published data to compare ∼13,000 protein coding genes across 66 species. We tested for positive selection on focal lineages, including those inferred to have undergone dietary shifts. Unexpectedly, we found a broad signature of positive selection in the ancestral phyllostomid branch, spanning genes implicated in the metabolism of all major macronutrients, yet few positively selected genes at the inferred switch to plantivory. Branches corresponding to blood- and nectar-based diets showed selection in loci underpinning nitrogenous waste excretion and glycolysis, respectively. Intriguingly, patterns of selection in metabolism genes were mirrored by those in loci implicated in craniofacial remodeling, a trait previously linked to phyllostomid dietary specialization. Finally, we show that the null model of the widely-used branch-site test is likely to be misspecified, with the implication that the test is too conservative and probably under-reports true cases of positive selection. Our findings point to a complex picture of adaptive radiation, in which the evolution of new dietary specializations has been facilitated by early adaptations combined with the generation of new genetic variation.

Non-model systems in mammalian forelimb evo-devo

Mammal forelimbs are highly diverse, ranging from the elongated wing of a bat to the stout limb of the mole. The mammal forelimb has been a long-standing system for the study of early developmental patterning, proportional variation, shape change, and the reduction of elements. However, most of this work has been performed in mice, which neglects the wide variation present across mammal forelimbs. This review emphasizes the critical role of non-model systems in limb evo-devo and highlights new emerging models and their potential. We discuss the role of gene networks in limb evolution, and touch on functional analyses that lay the groundwork for further developmental studies. Mammal limb evo-devo is a rich field, and here we aim to synthesize the findings of key recent works and the questions to which they lead.

Mechanical properties of food and masticatory behavior in llamas, Llama glama

Mammals typically process food items more extensively in their oral cavities than do other vertebrates. Dental morphology, jaw-muscle activity patterns, mandibular movements, and tongue manipulation work to facilitate oral fragmentation of dietary items. While processing mechanically challenging foods, mammals modulate mandibular movements and bite forces via recruitment of greater jaw-adductor muscle forces and protracted biting or chewing. Because jaw-loading patterns are influenced by magnitude; frequency; and duration of muscular, bite, and reaction forces during routine feeding behaviors, relatively larger jaws are thought to be more characteristic of mammals that experience higher masticatory loads due to the processing of mechanically challenging foods. The ease of food fracture during post-canine biting and chewing is mainly determined by food stiffness and toughness. Such foods have been associated with increased loading magnitude and/or greater amounts of cyclical loading (i.e., chewing duration). Dietary properties are thought to modulate cyclical loading through changes in chewing frequency and chewing investment. On the other hand, chewing frequency has been found to be independent of dietary properties in rabbits and primates; however, little evidence exists regarding the influence of dietary properties on these parameters in a broader range of mammals. Here, we assessed chewing behavior in seven adult llamas (Llama glama) processing foods with a wide range of mechanical properties (grain, hay, carrots, and dried corn). Each subject was filmed at 60 frames/s, with video slowed for frame-by-frame computer analysis to obtain length of feeding bout and number of chewing cycles for each food type. These parameters were used to calculate chewing frequency (chews/s), chewing investment (chews/g), and chewing duration (s/g). Chewing frequency was not significantly related to mechanical properties of food, but chewing investment and chewing duration were significantly related to dietary stiffness and toughness. Therefore, cyclical loading is positively influenced by stiff and tough foods. This suggests that variation in jaw morphology in extinct and extant mammals is positively related to dietary stiffness and toughness, which requires greater chewing investment and increased chewing duration.

Ecological Morphology of Neotropical Bat Wing Structures

Morphology has a direct influence on animal fitness. Studies addressing the identification of patterns and variations across several guilds are fundamental in ecomorphological research. Wings are the core of ecological morphology in bats; nevertheless, individual bones and structures that support the wing, including metacarpals, phalanges and the length of digits, have rarely been the subject of comprehensive research when studying wing morphology. Here, I analyzed morphological variations of wing structures across 11 bat guilds and how individual bone structures are correlated to diet, foraging mode and habitat use. I obtained wing measurements from 1512 voucher specimens of 97 species. All the specimens analyzed came from the Mammalian Collection at the Museo Javeriano de Historia Natural of Pontificia Universidad Javeriana (MPUJ-MAMM) (Bogotá, Colombia). Positive correlations between size and the length of the third and fifth digit were detected. Bat guilds that capture their preys using aerial strategy in uncluttered habitats had longer third digits but short fifth digits compared to guilds that rely on gleaning strategy and forage in highly cluttered space. Although terminal phalanges were shown to be important structures for guild classification, metacarpals were strongly related to aerial foragers from uncluttered habitats because of their potential role in flight performance and ecological adaptations. Results show that habitat use, as well as foraging mode, are reflected in wing structures. Different wing traits to those evaluated in this study should be considered to better understand the ecological interactions, foraging strategy, wing adaptations, and flight performance in Neotropical bats.

Functional correlates of skull shape in Chiroptera: feeding and echolocation adaptations

Morphological, functional, and behavioral adaptations of bats are among the most diverse within mammals. A strong association between bat skull morphology and feeding behavior has been suggested previously. However, morphological variation related to other drivers of adaptation, in particular echolocation, remains understudied. We assessed variation in skull morphology with respect to ecology (diet and emission type) and function (bite force, masticatory muscles and echolocation characteristics) using geometric morphometrics and comparative methods. Our study suggests that variation in skull shape of 10 bat families is the result of adaptations to broad dietary categories and sound emission types (oral or nasal). Skull shape correlates with echolocation parameters only in a subsample of insectivorous species, possibly because they (almost) entirely rely on this sensory system for locating and capturing prey. Insectivores emitting low frequencies are characterized by a ventrally tilted rostrum, a trait not associated with feeding parameters. This result questions the validity of a trade-off between feeding and echolocation function. Our study advances understanding of the relationship between skull morphology and specific features of echolocation and suggests that evolutionary constraints due to echolocation may differ between different groups within the Chiroptera.

Find the food first: An omnivorous sensory morphotype predates biomechanical specialization for plant based diets in phyllostomid bats

The role of mechanical morphologies in the exploitation of novel niche space is well characterized; however, the role of sensory structures in unlocking new niches is less clear. Here, we investigate the relationship between the evolution of sensory structures and diet during the radiation of noctilionoid bats. With a broad range of foraging ecologies and a well-supported phylogeny, noctilionoids constitute an ideal group for studying this relationship. We used diffusible iodine-based contrast enhanced computed tomography scans of 44 noctilionoid species to analyze relationships between the relative volumes of three sensory structures (olfactory bulbs, orbits, and cochleae) and diet. We found a positive relationship between frugivory and both olfactory and orbit size. However, we also found a negative relationship between nectarivory and cochlea size. Ancestral state estimates suggest that larger orbits and olfactory bulbs were present in the common ancestor of family Phyllostomidae, but not in other noctilionoid. This constellation of traits indicates a shift toward omnivory at the base of Phyllostomidae, predating their radiation into an exceptionally broad range of dietary niches. This is consistent with a scenario in which changes in sensory systems associated with foraging and feeding set the stage for subsequent morphological modification and diversification.

A test for rate-coupling of trophic and cranial evolutionary dynamics in New World bats

Morphological evolution is often assumed to be causally related to underlying patterns of ecological trait evolution. However, few studies have directly tested whether evolutionary dynamics of-and major shifts in-ecological resource use are coupled with morphological shifts that may facilitate trophic innovation. Using diet and multivariate cranial (microCT) data, we tested whether rates of trophic and cranial evolution are coupled in the radiation of New World bats. We developed a generalizable information-theoretic method for describing evolutionary rate heterogeneity across large candidate sets of multirate evolutionary models, without relying on a single best-fitting model. We found considerable variation in trophic evolutionary dynamics, in sharp contrast to a largely homogeneous cranial evolutionary process. This dichotomy is surprising given established functional associations between overall skull morphology and trophic ecology. We suggest that assigning discrete trophic states may underestimate trophic generalism and opportunism, and that this radiation could be characterized by labile crania and a homogeneous dynamic of generally high morphological rates. Overall, we discuss how trophic classifications could substantively impact our interpretation of how these dynamics covary in adaptive radiations.

Phylogeny and foraging behaviour shape modular morphological variation in bat humeri

Bats show a remarkable ecological diversity that is reflected both in dietary and foraging guilds (FGs). Cranial ecomorphological adaptations linked to diet have been widely studied in bats, using a variety of anatomical, computational and mathematical approaches. However, foraging-related ecomorphological adaptations and the concordance between cranial and postcranial morphological adaptations remain unexamined in bats and limited to the interpretation of traditional aerodynamic properties of the wing (e.g. wing loading [WL] and aspect ratio [AR]). For this reason, the postcranial ecomorphological diversity in bats and its drivers remain understudied. Using 3D virtual modelling and geometric morphometrics (GMM), we explored the phylogenetic, ecological and biological drivers of humeral morphology in bats, evaluating the presence and magnitude of modularity and integration. To explore decoupled patterns of variation across the bone, we analysed whole-bone shape, diaphyseal and epiphyseal shape. We also tested whether traditional aerodynamic wing traits correlate with humeral shape. By studying 37 species from 20 families (covering all FGs and 85% of dietary guilds), we found similar patterns of variation in whole-bone and diaphyseal shape and unique variation patterns in epiphyseal shape. Phylogeny, diet and FG significantly correlated with shape variation at all levels, whereas size only had a significant effect on epiphyseal morphology. We found a significant phylogenetic signal in all levels of humeral shape. Epiphyseal shape significantly correlated with wing AR. Statistical support for a diaphyseal-epiphyseal modular partition of the humerus suggests a functional partition of shape variability. Our study is the first to show within-structure modular morphological variation in the appendicular skeleton of any living tetrapod. Our results suggest that diaphyseal shape correlates more with phylogeny, whereas epiphyseal shape correlates with diet and FG.

Correlation of skull morphology and bite force in a bird-eating bat (Ia io; Vespertilionidae)

Background

Genetic and ecological factors influence morphology, and morphology is compatible with function. The morphology and bite performance of skulls of bats show a number of characteristic feeding adaptations. The great evening bat, Ia io (Thomas, 1902), eats both insects and birds (Thabah et al. J Mammal 88: 728-735, 2007), and as such, it is considered to represent a case of dietary niche expansion from insects to birds. How the skull morphology or bite force in I. io are related to the expanded diet (that is, birds) remains unknown. We used three-dimensional (3D) geometry of the skulls and measurements of bite force and diets from I. io and 13 other species of sympatric or closely related bat species to investigate the characteristics and the correlation of skull morphology and bite force to diets.

Results

Significant differences in skull morphology and bite force among species and diets were observed in this study. Similar to the carnivorous bats, bird-eaters (I. io) differed significantly from insectivorous bats; I. io had a larger skull size, taller crania, wider zygomatic arches, shorter but robust mandibles, and larger bite force than the insectivores. The skull morphology of bats was significantly associated with bite force whether controlling for phylogeny or not, but no significant correlations were found between diets and the skulls, or between diets and residual bite force, after controlling for phylogeny.

Conclusions

These results indicated that skull morphology was independent of diet, and phylogeny had a greater impact on skull morphology than diet in these species. The changes in skull size and morphology have led to variation in bite force, and finally different bat species feeding on different foods. In conclusion, I. io has a larger skull size, robust mandibles, shortened dentitions, longer coronoid processes, expanded angular processes, low condyles, and taller cranial sagittal crests, and wider zygomatic arches that provide this species with mechanical advantages; their greater bite force may help them use larger and hard-bodied birds as a dietary component.

The Evolutionary Dynamics of Mechanically Complex Systems

Animals use a diverse array of motion to feed, escape predators, and reproduce. Linking morphology, performance, and fitness is a foundational paradigm in organismal biology and evolution. Yet, the influence of mechanical relationships on evolutionary diversity remains unresolved. Here, I focus on the many-to-one mapping of form to function, a widespread, emergent property of many mechanical systems in nature, and discuss how mechanical redundancy influences the tempo and mode of phenotypic evolution. By supplying many possible morphological pathways for functional adaptation, many-to-one mapping can release morphology from selection on performance. Consequently, many-to-one mapping decouples morphological and functional diversification. In fish, for example, parallel morphological evolution is weaker for traits that contribute to mechanically redundant motions, like suction feeding performance, than for systems with one-to-one form-function relationships, like lower jaw lever ratios. As mechanical complexity increases, historical factors play a stronger role in shaping evolutionary trajectories. Many-to-one mapping, however, does not always result in equal freedom of morphological evolution. The kinematics of complex systems can often be reduced to variation in a few traits of high mechanical effect. In various different four-bar linkage systems, for example, mechanical output (kinematic transmission) is highly sensitive to size variation in one or two links, and insensitive to variation in the others. In four-bar linkage systems, faster rates of evolution are biased to traits of high mechanical effect. Mechanical sensitivity also results in stronger parallel evolution-evolutionary transitions in mechanical output are coupled with transition in linkages of high mechanical effect. In other words, the evolutionary dynamics of complex systems can actually approximate that of simpler, one-to-one systems when mechanical sensitivity is strong. When examined in a macroevolutionary framework, the same mechanical system may experience distinct selective pressures in different groups of organisms. For example, performance tradeoffs are stronger for organisms that use the same mechanical structure for more functions. In general, stronger performance tradeoffs result in less phenotypic diversity in the system and, sometimes, a slower rate of evolution. These macroevolutionary trends can contribute to unevenness in functional and lineage diversity across the tree of life. Finally, I discuss how the evolution of mechanical systems informs our understanding of the relative roles of determinism and contingency in evolution.

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.

Field key to the bats of Costa Rica and Nicaragua

With more than 1,400 species of bats described worldwide, the order Chiroptera is second only to rodents in ecological and taxonomic diversity. Bats play critically important roles in natural systems as seed and pollen dispersers, predators of invertebrates and vertebrates, and sanguinivores. The Central American countries of Costa Rica and Nicaragua have at least 123 species of bats (in nine families and 66 genera), or nearly 10% of the world's known species. Because of the importance of proper species identification for ecological and systematic studies and conservation efforts, we present a dichotomous key to the bats of this region. Our goal is the positive, in-hand identification of living bats that may be released unharmed after identification. Identifying Neotropical bats and understanding the taxonomic changes that affect the names used for the various species over time can be a challenge. This key includes the 123 species known to occur in Costa Rica and Nicaragua as well as three that are expected to occur in these countries but which have not yet been recorded. We provide illustrations of key characters useful for differentiating bats to species and updated taxonomic notes to assist the reader in assessing the literature.

Con más de 1,400 especies de murciélagos descritos en todo el mundo, el orden Chiroptera es el segundo más diverso después de los roedores respecto a taxonomía. Los murciélagos juegan papeles de importancia crítica en los sistemas naturales como dispersores de semillas, polinizadores, depredadores de vertebrados e invertebrados, así como hematófagos. Costa Rica y Nicaragua presentan al menos 123 especies de murciélagos (en 9 familias y 66 géneros), casi el 10% de las especies conocidas en el mundo. Debido a la importancia de la identificación precisa de las especies para los estudios ecológicos, sistemáticos y de conservación, presentamos una clave dicotómica para los murciélagos de esta región. Nuestro objetivo es la identificación correcta con los murciélagos in vivo y que se pueden liberar sin daño después de la identificación. La caracterización de los murciélagos neotropicales y el seguimiento del estatus taxonómico de cada especie puede ser un reto en el proceso de identificación. Esta clave incluye las 123 especies conocidas en Costa Rica y Nicaragua, así como 3 no registradas pero con distribución potencial. Proporcionamos ilustraciones de características claves útiles para diferenciar murciélagos al nivel de la especie y notas taxonómicas actualizadas para ayudar al lector a la identificación.

Body size and diet–related morphological variation of bats over the past 65 years in China

We examined both historical (1960s) and recent (2017) specimens of an insectivorous bat species (Hipposideros armiger) and a phytophagous bat (Rousettus leschenaultii) from the same latitudinal range to explore phenotypic responses to environmental change in China over the past 65 years. Hipposideros armiger exhibited significant increases in forearm length and three diet-related cranial traits, as well as carbon and nitrogen stable isotope composition, suggesting that modern H. armiger must travel farther for food and may now use different food resources. In contrast, R. leschenaultii showed no change in forearm length but displayed significant increases in diet-related cranial traits. This study provides evidence for differential responses to recent environmental changes in bat species with different diets. The changes in diet-related traits of the two species and the forearm length change on the insectivorous bats suggest that recent phenotypic changes may be adaptions to land-use changes rather than to climate change.

The roles of joint tissues and jaw muscles in palatal biomechanics of the savannah monitor (Varanus exanthematicus) and their significance for cranial kinesis

Numerous vertebrates exhibit cranial kinesis, or movement between bones of the skull and mandible other than at the jaw joint. Many kinetic species possess a particular suite of features to accomplish this movement, including flexible cranial joints and protractor musculature. Whereas the musculoskeletal anatomy of these kinetic systems is well understood, how these joints are biomechanically loaded, how different soft tissues affect joint loading and kinetic capacity, and how the protractor musculature loads the skull remain poorly understood. Here, we present a finite element model of the savannah monitor, Varanus exanthematicus, a modestly kinetic lizard, to better elucidate the roles of soft tissue in mobile joints and protractor musculature in cranial loading. We describe the 3D resultants of jaw muscles and the histology of palatobasal, otic and jaw joints. We tested the effects of joint tissue type, bite point and muscle load to evaluate the biomechanical role of muscles on the palate and braincase. We found that the jaw muscles have significant mediolateral components that can impart stability across palatocranial joints. Articular tissues affect the magnitude of strains experienced around the palatobasal and otic joints. Without protractor muscle loading, the palate, quadrate and braincase experience higher strains, suggesting this muscle helps insulate the braincase and palatoquadrate from high loads. We found that the cross-sectional properties of the bones of V. exanthematicus are well suited for performing under torsional loads. These findings suggest that torsional loading regimes may have played a more important role in the evolution of cranial kinesis in lepidosaurs than previously appreciated.

Peramorphosis, an evolutionary developmental mechanism in neotropical bat skull diversity

BACKGROUND

The neotropical leaf-nosed bats (Chiroptera, Phyllostomidae) are an ecologically diverse group of mammals with distinctive morphological adaptations associated with specialized modes of feeding. The dramatic skull shape changes between related species result from changes in the craniofacial development process, which brings into focus the nature of the underlying evolutionary developmental processes.

RESULTS

In this study, we use three-dimensional geometric morphometrics to describe, quantify, and compare morphological modifications unfolding during evolution and development of phyllostomid bats. We examine how changes in development of the cranium may contribute to the evolution of the bat craniofacial skeleton. Comparisons of ontogenetic trajectories to evolutionary trajectories reveal two separate evolutionary developmental growth processes contributing to modifications in skull morphogenesis: acceleration and hypermorphosis.

CONCLUSION

These findings are consistent with a role for peramorphosis, a form of heterochrony, in the evolution of bat dietary specialists.

Scientific statement on the coverage of bats by the current pesticide risk assessment for birds and mammals

Bats are an important group of mammals, frequently foraging in farmland and potentially exposed to pesticides. This statement considers whether the current risk assessment performed for birds and ground dwelling mammals exposed to pesticides is also protective of bats. Three main issues were addressed. Firstly, whether bats are toxicologically more or less sensitive than the most sensitive birds and mammals. Secondly, whether oral exposure of bats to pesticides is greater or lower than in ground dwelling mammals and birds. Thirdly, whether there are other important exposure routes relevant to bats. A large variation in toxicological sensitivity and no relationship between sensitivity of bats and bird or mammal test-species to pesticides could be found. In addition, bats have unique traits, such as echolocation and torpor which can be adversely affected by exposure to pesticides and which are not covered by the endpoints currently selected for wild mammal risk assessment. The current exposure assessment methodology was used for oral exposure and adapted to bats using bat-specific parameters. For oral exposure, it was concluded that for most standard risk assessment scenarios the current approach did not cover exposure of bats to pesticide residues in food. Calculations of potential dermal exposure for bats foraging during spraying operations suggest that this may be a very important exposure route. Dermal routes of exposure should be combined with inhalation and oral exposure. Based on the evidence compiled, the Panel concludes that bats are not adequately covered by the current risk assessment approach, and that there is a need to develop a bat-specific risk assessment scheme. In general, there was scarcity of data to assess the risks for bat exposed to pesticides. Recommendations for research are made, including identification of alternatives to laboratory testing of bats to assess toxicological effects.

Normal embryonic development of the greater horseshoe bat Rhinolophus ferrumequinum, with special reference to nose leaf formation

The order Chiroptera (bats) is the second largest group of mammals, composed of more than 1,300 species. Although powered flight and echolocation in bats have attracted many biologists, diversity in bat facial morphology has been almost neglected. Some bat species have a "nose leaf," a leaf-like epithelial appendage around their nostrils. The nose leaf appears to have been acquired at least three times independently in bat evolution, and its morphology is highly diverse among bats species. Internal tissue morphology of nose-leaves has been investigated through histological analyses of late-stage fetuses of some bat species possessing the nose leaf. However, the proximate factors that bring about chiropteran nose-leaves have not been identified. As an initial step to address the question above, we describe the normal embryonic development of the greater horseshoe bat Rhinolophus ferrumequinum, and examine development of the tissues associated with their nose leaf during embryogenesis through histological analyses. We found that the nose leaf of R. ferrumequinum is formed through two phases. First, the primordium of the nose leaf appears as two tissue bulges aligned top and bottom on the face at embryonic stages 15-16. Second, the sub-regions of the nose leaf are differentiated through ingrowth as well as outgrowth of the epithelium at stage 17. In embryogenesis of Carollia perspicillata, a phyllostomid species with a nose leaf, the nose leaf primordium is formed as a small tissue bulge on the nostril at stage 17. This tissue bulge grows into a dorsally projected thin epithelial structure. Such differences in the nose leaf developmental process between chiropteran lineages may suggest that distinct developmental mechanisms have been employed in each lineage's nose leaf 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.

What drives changes in morphological diversity? Here, Arbour et al. analyse skull 3D shape evolution across the bat radiation using µCT scan data, finding two phases of skull shape diversification, early adaptive shifts related to echolocation, and more recent shifts related to diet transitions.

Postcranial heterochrony, modularity, integration and disparity in the prenatal ossification in bats (Chiroptera)

BACKGROUND

Self-powered flight is one of the most energy-intensive types of locomotion found in vertebrates. It is also associated with a range of extreme morpho-physiological adaptations that evolved independently in three different vertebrate groups. Considering that development acts as a bridge between the genotype and phenotype on which selection acts, studying the ossification of the postcranium can potentially illuminate our understanding of bat flight evolution. However, the ontogenetic basis of vertebrate flight remains largely understudied. Advances in quantitative analysis of sequence heterochrony and morphogenetic growth have created novel approaches to study the developmental basis of diversification and the evolvability of skeletal morphogenesis. Assessing the presence of ontogenetic disparity, integration and modularity from an evolutionary approach allows assessing whether flight may have resulted in evolutionary differences in the magnitude and mode of development in bats.

RESULTS

We quantitatively compared the prenatal ossification of the postcranium (24 bones) between bats (14 species), non-volant mammals (11 species) and birds (14 species), combining for the first time prenatal sequence heterochrony and developmental growth data. Sequence heterochrony was found across groups, showing that bat postcranial development shares patterns found in other flying vertebrates but also those in non-volant mammals. In bats, modularity was found as an axial-appendicular partition, resembling a mammalian pattern of developmental modularity and suggesting flight did not repattern prenatal postcranial covariance in bats.

CONCLUSIONS

Combining prenatal data from 14 bat species, this study represents the most comprehensive quantitative analysis of chiropteran ossification to date. Heterochrony between the wing and leg in bats could reflect functional needs of the newborn, rather than ecological aspects of the adult. Bats share similarities with birds in the development of structures involved in flight (i.e. handwing and sternum), suggesting that flight altriciality and early ossification of pedal phalanges and sternum are common across flying vertebrates. These results indicate that the developmental modularity found in bats facilitates intramodular phenotypic diversification of the skeleton. Integration and disparity increased across developmental time in bats. We also found a delay in the ossification of highly adaptable and evolvable regions (e.g. handwing and sternum) that are directly associated with flight performance.

Digitizing extant bat diversity: An open-access repository of 3D μCT-scanned skulls for research and education

Biological specimens are primary records of organismal ecology and history. As such, museum collections are invaluable repositories for testing ecological and evolutionary hypotheses across the tree of life. Digitizing and broadly sharing the phenotypic data from these collections serves to expand the traditional reach of museums, enabling widespread data sharing, collaboration, and education at an unprecedented scale. In recent years, μCT-scanning has been adopted as one way for efficiently digitizing museum specimens. Here, we describe a large repository of 3D, μCT-scanned images and surfaces of skulls from 359 extant species of bats, a highly diverse clade of modern vertebrates. This digital repository spans much of the taxonomic, biogeographic, and morphological diversity present across bats. All data have been published to the MorphoSource platform, an online database explicitly designed for the archiving of 3D morphological data. We demonstrate one potential use of this repository by testing for convergence in skull shape among one particularly diverse group of bats, the superfamily Noctilionoidea. Beyond its intrinsic utility to bat biologists, our digital specimens represent a resource for educators and for any researchers seeking to broadly test theories of trait evolution, functional ecology, and community assembly.