Small skeletons show size-specific scaling: an exploration of allometry in the mammalian lumbar spine

Studies of vertebrate bone biomechanics often focus on skeletal adaptations at upper extremes of body mass, disregarding the importance of skeletal adaptations at lower extremes. Yet mammals are ancestrally small and most modern species have masses under 5 kg, so the evolution of morphology and function at small size should be prioritized for understanding how mammals subsist. We examined allometric scaling of lumbar vertebrae in the small-bodied Philippine endemic rodents known as cloud rats, which vary in mass across two orders of magnitude (15.5 g-2700 g). External vertebral dimensions scale with isometry or positive allometry, likely relating to body size and nuances in quadrupedal posture. In contrast to most mammalian trabecular bone studies, bone volume fraction and trabecular thickness scale with positive allometry and isometry, respectively. It is physiologically impossible for these trends to continue to the upper extremes of mammalian body size, and we demonstrate a fundamental difference in trabecular bone allometry between large- and small-bodied mammals. These findings have important implications for the biomechanical capabilities of mammalian bone at small body size; for the selective pressures that govern skeletal evolution in small mammals; and for the way we define 'small' and 'large' in the context of vertebrate skeletons.

Anatomical and Morphological Structure of the Skull of a Juvenile Specimen of Myotis myotis (Chiroptera: Vespertilionidae)

Few studies analyze the morphology and anatomy of the bat skull, and most of them are incomplete. Some of the difficulties stem from the fact that, in the representatives of the order Chiroptera, the interosseous sutures disappear by fusing together before active flight begins, which takes place over only a few months. This study presents a detailed morphological and anatomical description of the skull of a juvenile specimen of Myotis myotis (Borkhausen, 1797). Juvenile skulls are difficult to preserve and often incomplete. Previously inconsistent terminology related to bones, sutures, and other cranial structures was unified, which will provide insight on the distribution of each structure in both juvenile and adult specimens to be investigated. The description fill in the gaps in knowledge about the cranial structures of Myotis myotis and the representatives of the family Vespertilionidae. This will allow for precise descriptions of the skulls of bats.

Living with voracious roommates: Factors that explain isotopic niche variation in a mixed colony of insectivorous bats

Theory predicts that in resource-limited environments, coexisting species may overlap their niche dimensions but must differ in at least one to avoid competitive exclusion. Specifically, it has been suggested that the coexistence of competing species within a guild, could be sustained with mechanisms of resource partitioning, such as segregation along a trophic dimension. Among the most gregarious mammals are bats, which present diversification in their diet based on habitat choice and body size. Despite differences that could explain specialization in prey selection, there are insufficient studies that explore food overlap in mixed bat colonies and the factors that determine the selection of prey, both at intra- and inter-specific levels. To fill this gap, we analyzed the isotope signal (δ13C and δ15N) in feces collected in a mixed colony of Tadarida brasiliensis and Myotis chiloensis. To understand how several factors could influence these isotopic signals, intrinsic explanatory variables were analyzed, including body mass, body length, age, and sex. Also, extrinsic variables were analyzed, including monthly temporality and moonlight intensity. Our findings support age-dependent specialization in M. chiloensis, with a significant role of moonlight intensity and sex on δ15N. In T. brasiliensis, we identified a significant effect of size, sex, and ear length on δ15N. Our analysis indicates that both species of bats experience diverse degrees of overlap through austral summer months, affected by several factors that explain the variability in their fecal isotopic signals.

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.

How to make a vampire

Herein, we compared the developmental maturity of the cranium, limbs, and feeding apparatus in a perinatal common vampire bat relative to its mother. In addition, we introduce a method for combining two computed tomographic imaging techniques to three-dimensionally reconstruct endocasts in poorly ossified crania. The Desmodus specimens were scanned using microcomputed tomography (microCT) and diffusible iodine-based contrast-enhanced CT to image bone and soft tissues. Muscles of the jaw and limbs, and the endocranial cavity were segmented using imaging software. Endocranial volume (ECV) of the perinatal Desmodus is 74% of adult ECV. The facial skeletal is less developed (e.g., palatal length 60% of adult length), but volumes for alveolar crypts/sockets of permanent teeth are nearly identical. The forelimb skeleton is uniformly less ossified than the distal hind limb, with no secondary centers ossified and an entirely cartilaginous carpus. All epiphyseal growth zones are active in the brachium and antebrachium, with the distal radius exhibiting the greatest number of proliferating chondrocytes arranged in columns. The hind limb skeleton is precociously ossified from the knee distally. The musculature of the fore limb, temporalis, and masseter muscles appear weakly developed (6-11% of the adult volume). In contrast, the leg and foot musculature is better developed (23-25% of adult volume), possibly enhancing the newborn's capability to grip the mother's fur. Desmodus is born relatively large, and our results suggest they are born neurally and dentally precocious, with generally underdeveloped limbs, especially the fore limb.

Ontogeny of cranial musculoskeletal anatomy and its relationship to allometric increase in bite force in an insectivorous bat (Eptesicus fuscus)

Bite force is a performance metric commonly used to link cranial morphology with dietary ecology, as the strength of forces produced by the feeding apparatus largely constrains the foods an individual can consume. At a macroevolutionary scale, there is evidence that evolutionary changes in the anatomical elements involved in producing bite force have contributed to dietary diversification in mammals. Much less is known about how these elements change over postnatal ontogeny. Mammalian diets drastically shift over ontogeny-from drinking mother's milk to feeding on adult foods-presumably with equally drastic changes in the morphology of the feeding apparatus and bite performance. Here, we investigate ontogenetic morphological changes in the insectivorous big brown bat (Eptesicus fuscus), which has an extreme, positive allometric increase in bite force during development. Using contrast-enhanced micro-computed tomography scans of a developmental series from birth to adult morphology, we quantified skull shape and measured skeletal and muscular parameters directly related to bite force production. We found pronounced changes in the skull over ontogeny, including a large increase in the volume of the temporalis and masseter muscles, and an expansion of the skull dome and sagittal crest that would serve to increase the temporalis attachment area. These changes indicate that development of the jaw adductors play an important role in the development of biting performance of these bats. Notably, static bite force increases with positive allometry with respect to all anatomical measures examined, suggesting that modifications in biting dynamics and/or improved motor coordination also contribute to increases in biting performance.

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.

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.

Sex-specific ontogenetic patterns of cranial morphology, theoretical bite force, and underlying jaw musculature in fishers and American martens

The carnivoran cranium undergoes tremendous growth in size and development of shape to process prey as adults and, importantly, these ontogenetic processes can also differ between the sexes. How these ontogenetic changes in morphology actually relate to the underlying jaw musculature and overall bite performance has rarely been investigated. In this study, I examined sex-specific ontogenetic changes in cranial morphology, jaw adductor muscles, and theoretical bite force between subadults and adults in the fisher (Pekania pennanti) and American marten (Martes americana). I found evidence that cranial size alone does not completely explain ontogenetic increases in bite forces as found in other mammalian species. Instead, cranial shape development also drives ontogenetic increases in relative bite force by broadening the zygomatic arches and enlargement of the sagittal crest, both of which enable relatively larger jaw adductor muscles to attach. In contrast, examination of sexual dimorphism within each age-class revealed that cranial shape dimorphism did not translate to dimorphism in either size-corrected bite forces or size-corrected physiological cross-sectional area of the jaw adductor muscles. These results reveal that morphological size and shape variation can have different influences on bite performance depending on the level of intraspecific variation that is examined (i.e. ontogenetic versus sexual dimorphism).

Decoupled ontogeny of in vivo bite force and mandible morphology reveals effects of weaning and sexual maturation in mice

The link between performance, morphology and their sources of variation is a major target of evolutionary functional biology. In vertebrates, many studies have linked in vivo bite force to skull morphology, mostly at the interspecific level. Within species, however, the ontogeny of bite force, in relation to the development of the mandible, remains poorly known, despite its relevance for life history and for the co-evolution of form and function. Here, ontogenetic trajectories of bite force, correlated with mandible size and shape, are reported for the first time in a wild-derived colony of laboratory mice. Bite forces were measured in vivo and mandible morphology was assessed using geometric morphometrics. Most coordinated changes in morphology and in vivo bite force occur during the first stages of growth, prior to weaning. Mandible shape stabilizes after day 23. The increases in mandible size and body mass slow down around day 40, but still increase during adulthood. Despite slowing down after weaning, bite force increases through a second phase during sexual maturation (days 30–40). This may be linked to the progressive tempering of weaning stress, continued growth and synchronization of the muscular and osteological systems, together with hormonal changes, as we observed a concomitant appearance of sexual dimorphism.

Dimorphism throughout the European eel's life cycle: are ontogenetic changes in head shape related to dietary differences?

A well-known link exists between an organism's ecology and morphology. In the European eel, a dimorphic head has been linked to differences in feeding ecology, with broad-headed eels consuming harder prey items than narrow-headed ones. Consequently, we hypothesized that broad-heads should exhibit a cranial musculoskeletal system that increases bite force and facilitates the consumption of harder prey. Using 3D-reconstructions and a bite model, we tested this hypothesis in two life stages: the sub-adult yellow eel stage and its predecessor, the elver eel stage. This allowed us to test whether broad- and narrow-headed phenotypes show similar trait differences in both life stages and whether the dimorphism becomes more pronounced during ontogeny. We show that broad-headed eels in both stages have larger jaw muscles and a taller coronoid, which are associated with higher bite forces. This increased bite force together with the elongated upper and lower jaws in broad-headed eels can also improve grip during spinning behavior, which is used to manipulate hard prey. Head shape variation in European eel is therefore associated with musculoskeletal variation that can be linked to feeding ecology. However, although differences in muscle volume become more pronounced during ontogeny, this was not the case for skeletal features.

A Bigger Picture: Organismal Function at the Nexus of Development, Ecology, and Evolution: An Introduction to the Symposium

Over the past 40 years of research, two perspectives have dominated the study of ecomorphology at ontogenetic and evolutionary timescales. For key anatomical complexes (e.g., feeding apparatus, locomotor systems, sensory structures), morphological changes during ontogeny are often interpreted in functional terms and linked to their putative importance for fitness. Across larger timescales, morphological transformations in these complexes are examined through character stability or mutability during cladogenesis. Because the fittest organisms must pass through ontogenetic changes in size and shape, addressing transformations in morphology at different time scales, from life histories to macroevolution, has the potential to illuminate major factors contributing to phenotypic diversity. To date, most studies have relied on the assumption that organismal form is tightly constrained by the adult niche. Although this could be accurate for organisms that rapidly reach and spend a substantial portion of their life history at the adult phenotype (e.g., birds, mammals), it may not always hold true for species that experience substantial growth after one or more major fitness filters during their ontogeny (e.g., some fishes, reptiles). In such circumstances, examining the adult phenotype as the primary result of selective processes may be erroneous as it likely obscures the developmental configuration of morphology that was most critical to early survival. Given this discrepancy-and its potential to mislead interpretations of how selection may shape a taxon's phenotype-this symposium addresses the question: how do we identify such ontogenetic "inertia," and how do we integrate developmental information into our phylogenetic, ecological, and functional interpretations of complex phenotypes?