Morphogenesis in bat wings: linking development, evolution and ecology

Grossnickle D, Santana SE, Law CJ. Gliding toward an understanding of the origin of flight in bats

Bats are the only mammals capable of powered flight and have correspondingly specialized body plans, particularly in their limb morphology. The origin of bat flight is still not fully understood due to an uninformative fossil record but, from the perspective of a functional transition, it is widely hypothesized that bats evolved from gliding ancestors. Here, we test predictions of the gliding-to-flying hypothesis of the origin of bat flight by using phylogenetic comparative methods to model the evolution of forelimb and hindlimb traits on a dataset spanning four extinct bats and 231 extant mammals with diverse locomotor modes. Our results reveal that gliders exhibit adaptive trait optima (1) toward relatively elongate forelimbs that are intermediate between those of bats and non-gliding arborealists, and (2) toward relatively narrower but not longer hindlimbs that are intermediate between those of non-gliders and bats. We propose an adaptive landscape based on limb length and width optimal trends derived from our modeling analyses. Our results support a hypothetical evolutionary pathway wherein glider-like postcranial morphology precedes a bat-like morphology adapted to powered-flight, setting a foundation for future developmental, biomechanical, and evolutionary research to test this idea.

Insights into the formation and diversification of a novel chiropteran wing membrane from embryonic development

BACKGROUND

Through the evolution of novel wing structures, bats (Order Chiroptera) became the only mammalian group to achieve powered flight. This achievement preceded the massive adaptive radiation of bats into diverse ecological niches. We investigate some of the developmental processes that underlie the origin and subsequent diversification of one of the novel membranes of the bat wing: the plagiopatagium, which connects the fore- and hind limb in all bat species.

RESULTS

Our results suggest that the plagiopatagium initially arises through novel outgrowths from the body flank that subsequently merge with the limbs to generate the wing airfoil. Our findings further suggest that this merging process, which is highly conserved across bats, occurs through modulation of the programs controlling the development of the periderm of the epidermal epithelium. Finally, our results suggest that the shape of the plagiopatagium begins to diversify in bats only after this merging has occurred.

CONCLUSIONS

This study demonstrates how focusing on the evolution of cellular processes can inform an understanding of the developmental factors shaping the evolution of novel, highly adaptive structures.

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.

Movement syndromes of a Neotropical frugivorous bat inhabiting heterogeneous landscapes in Brazil

BACKGROUND

There is growing evidence that individuals within populations can vary in both habitat use and movement behavior, but it is still not clear how these two relate to each other. The aim of this study was to test if and how individual bats in a Stunira lilium population differ in their movement activity and preferences for landscape features in a correlated manner.

METHODS

We collected data on movements of 27 individuals using radio telemetry. We fitted a heterogeneous-space diffusion model to the movement data in order to evaluate signals of movement variation among individuals.

RESULTS

S. lilium individuals generally preferred open habitat with Solanum fruits, regularly switched between forest and open areas, and showed high site fidelity. Movement variation among individuals could be summarized in four movement syndromes: (1) average individuals, (2) forest specialists, (3) explorers which prefer Piper, and (4) open area specialists which prefer Solanum and Cecropia.

CONCLUSIONS

Individual preferences for landscape features plus food resource and movement activity were correlated, resulting in different movement syndromes. Individual variation in preferences for landscape elements and food resources highlight the importance of incorporating explicitly the interaction between landscape structure and individual heterogeneity in descriptions of animal movement.

Prenatal Developmental Trajectories of Fluctuating Asymmetry in Bat Humeri

Fluctuating asymmetry (random fluctuations between the left and right sides of the body) has been interpreted as an index to quantify both the developmental instabilities and homeostatic capabilities of organisms, linking the phenotypic and genotypic aspects of morphogenesis. However, studying the ontogenesis of fluctuating asymmetry has been limited to mostly model organisms in postnatal stages, missing prenatal trajectories of asymmetry that could better elucidate decoupled developmental pathways controlling symmetric bone elongation and thickening. In this study, we quantified the presence and magnitude of asymmetry during the prenatal development of bats, focusing on the humerus, a highly specialized bone adapted in bats to perform under multiple functional demands. We deconstructed levels of asymmetry by measuring the longitudinal and cross-sectional asymmetry of the humerus using a combination of linear measurements and geometric morphometrics. We tested the presence of different types of asymmetry and calculated the magnitude of size-controlled fluctuating asymmetry to assess developmental instability. Statistical support for the presence of fluctuating asymmetry was found for both longitudinal and cross-sectional asymmetry, explaining on average 16% of asymmetric variation. Significant directional asymmetry accounted for less than 6.6% of asymmetric variation. Both measures of fluctuating asymmetry remained relatively stable throughout ontogeny, but cross-sectional asymmetry was significantly different across developmental stages. Finally, we did not find a correspondence between developmental patterns of longitudinal and cross-sectional asymmetry, indicating that processes promoting symmetrical bone elongation and thickening work independently. We suggest various functional pressures linked to newborn bats’ ecology associated with longitudinal (altricial flight capabilities) and cross-sectional (precocial clinging ability) developmental asymmetry differentially. We hypothesize that stable magnitudes of fluctuating asymmetry across development could indicate the presence of developmental mechanisms buffering developmental instability.

On the sequence heterochrony of cranial ossification of bats in light of Haeckel's recapitulation theory

Haeckel's recapitulation theory has been a controversial topic in evolutionary biology. However, we have seen some recent cases applying Haeckel's view to interpret the interspecific variation of prenatal ontogeny. To revisit the validity of Haeckel's recapitulation theory, we take bats that have undergone drastic morphological changes and possess a characteristic ecology as a case study. All members of Rhinolophoidea and Yangochiroptera can generate an ultrasonic pulse from the larynx to interpret surrounding objects (laryngeal echolocation) whereas Pteropodidae lacks such ability. It is known that the petrosal bone is particularly derived in shape and expanded in laryngeal echolocators. If Haeckel's recapitulation theory holds, the formation of this derived trait should occur later than those of other bones. Therefore, we compared the prenatal ossification timing of the petrosal in 15 bat species and five outgroup species. We found that the ossification of the petrosal is accelerated in laryngeal echolocators while it is the last bone to ossify in non-laryngeal echolocating bats and non-volant mammals, which runs counter to the prediction generated by Haeckel's recapitulation theory. We point out the evolutionarily labile nature of trait developmental timing and emphasize that Haeckel's recapitulation theory does not hold in many cases. We caution that generating predictions on ancestral conditions and evolutionary history leading from Haeckel's recapitulation theory is not well supported.

Making a bat: The developmental basis of bat evolution

Bats are incredibly diverse, both morphologically and taxonomically. Bats are the only mammalian group to have achieved powered flight, an adaptation that is hypothesized to have allowed them to colonize various and diverse ecological niches. However, the lack of fossils capturing the transition from terrestrial mammal to volant chiropteran has obscured much of our understanding of bat evolution. Over the last 20 years, the emergence of evo-devo in non-model species has started to fill this gap by uncovering some developmental mechanisms at the origin of bat diversification. In this review, we highlight key aspects of studies that have used bats as a model for morphological adaptations, diversification during adaptive radiations, and morphological novelty. To do so, we review current and ongoing studies on bat evolution. We first investigate morphological specialization by reviewing current knowledge about wing and face evolution. Then, we explore the mechanisms behind adaptive diversification in various ecological contexts using vision and dentition. Finally, we highlight the emerging work into morphological novelties using bat wing membranes.

Behavioural Patterns and Postnatal Development in Pups of the Asian Parti-Coloured Bat, Vespertilio sinensis

Simple Summary

During the development of animals from juvenile to adult, their behaviour, morphology, and physiology can be altered to accommodate specific developmental periods. Early development in young animals directly affects the acquisition and maturation of behavioural skills and neurological functions in adults. Studying the behavioural development of young animals can help reveal the formation and evolution of animal behaviour. As the only mammalian group that flies, studying the behavioural development of young bats can help improve our understanding of bat flight behaviour. From our observations in the laboratory, we found that wing flapping and wing spreading behaviours in young bats promote earlier flight attempts.

Abstract

Behavioural development is an important aspect of research on animal behaviour. In bats, many studies have been conducted on the development of flight behaviour, but the postnatal behavioural development of bats remains largely unexplored. We studied the behaviours and postnatal development of infant bats by conducting controlled video recorded experiments. Our results showed that before weaning, Asian parti-coloured bats (Vespertilio sinensis) were able to exhibit four types of behaviours, namely, crawling, head moving, wing flapping, and wing spreading, and these behaviours are different from those observed in experiments with adult bats. The number of occurrences of these behaviours was correlated with age and scaled mass index. Furthermore, the number of occurrences of these behaviours in young bats could also reflect their physical developmental status. In young bats, wing flapping and spreading might be a type of play behaviour. These behaviours were negatively correlated with the time of the first flight, indicating that they might help to promote individual physical development. Our results provide fundamental data for revealing the ontogenetic and neurophysiological mechanisms of behavioural development in bats.

Developmental and Evolutionary Allometry of the Mammalian Limb Skeleton

The variety of limb skeletal proportions enables a remarkable diversity of behaviors that include powered flight in bats and flipper-propelled swimming in whales using extremes of a range of homologous limb architectures. Even within human limbs, bone lengths span more than an order of magnitude from the short finger and toe bones to the long arm and leg bones. Yet all of this diversity arises from embryonic skeletal elements that are each a very similar size at formation. In this review article, I survey what is and is not yet known of the development and evolution of skeletal proportion at multiple hierarchical levels of biological organization. These include the cellular parameters of skeletal elongation in the cartilage growth plate, genes associated with differential growth, and putative gene regulatory mechanisms that would allow both covariant and independent evolution of the forelimbs and hindlimbs and of individual limb segments. Although the genetic mechanisms that shape skeletal proportion are still largely unknown, and most of what is known is limited to mammals, it is becoming increasingly apparent that the diversity of bone lengths is an emergent property of a complex system that controls elongation of individual skeletal elements using a genetic toolkit shared by all.

Ultrasound Imaging Reveals Accelerated In-utero Development of a Sensory Apparatus in Echolocating Bats

Organ development, both in-utero and after birth, follows a different path for every organ depending upon how early the newborn will use it. Perception of the environment using echolocation occurs very early in the life of neonatal bats. In nostril-emitting echolocating bats of the families Hipposideridae and Rhinolophidae, the shape and area of the nasal-horseshoe is crucial for echolocation emission. We therefore hypothesized that most of this organ’s ontogeny will be completed in-utero while skull and wings will develop slower and continue their growth after birth. We used intrauterine ultrasonography of pregnant females, and measured newborn Asellia tridens (Hipposideridae) to test our hypothesis at different stages of ontogeny. We found that horseshoe development is completed in-utero and neonates begin emitting precursor echolocation calls already two days after birth. In contrast, skull and forearm only develop to 70% and 40% of adult size (respectively), and continue development after birth.

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.

Prenatal postcranial development in two species of sympatric Japanese wood mice (Apodemus argenteus and A. speciosus): a comparison of arboreal versus terrestrial congeners

Habitats of two closely related Japanese field mice, Apodemus argenteus and A. speciosus, broadly overlap in many Japanese forests. A. argenteus being more arboreal and A. speciosus being more terrestrial, it is thought that such ecological segregation allows their sympatric distribution. Comparing these two congeners, whether ecological difference is reflected in postcranial development was examined. Although overall ossification sequences were virtually identical, development of the caudal vertebrae was remarkably earlier in A. argenteus. One of the clearest morphological differences between the two species is the relative length of the tail, which is arguably related to the degree of arboreality. I suggest that accelerated ossification of the caudal vertebrae found in A. argenteus is related to its elongation of the tail.

Patterns of postcranial ossification and sequence heterochrony in bats: life histories and developmental trade-offs

The recently increased interest in studies on sequence heterochrony has uncovered developmental variation between species. However, how changes in developmental program are related to shifts in life-history parameters remains largely unsolved. Here we provide the most comprehensive data to date on postcranial ossification sequence of bats and compare them to various boreoeutherian mammals with different locomotive modes. Given that bats are equipped with an elongated manus, we expected to detect characteristic heterochronies particularly related to wing development. Although heterochronies related to wing development were confirmed as predicted, unexpected heterochronies regarding the pedal digits were also found. The timing of ossification onset of pedal phalanges is earlier than other mammals. Particularly, bats deviate from others in that pedal phalanges initiate ossification earlier than manual phalanges. It is known that the foot size of new born bats is close to that of adults, and that it takes several weeks to month until the wing is developed for flight. Given that the foot is required to be firm and stable enough at the time of birth to allow continued attachment to the mother and/or cave walls, we suggest that the accelerated development of the hind foot is linked to their unique life history. Since the forelimb is not mature enough for flight at birth and requires extended postnatal time to be large enough to be fully functional, we postulate that bats invest in earlier development of the hindlimb. We conclud that energy allocation trade-offs can play a significant role in shaping the evolution of development.

Unique expression patterns of multiple key genes associated with the evolution of mammalian flight

Bats are the only mammals capable of true flight. Critical adaptations for flight include a pair of dramatically elongated hands with broad wing membranes. To study the molecular mechanisms of bat wing evolution, we perform genomewide mRNA sequencing and in situ hybridization for embryonic bat limbs. We identify seven key genes that display unique expression patterns in embryonic bat wings and feet, compared with mouse fore- and hindlimbs. The expression of all 5'HoxD genes (Hoxd9-13) and Tbx3, six known crucial transcription factors for limb and digit development, is extremely high and prolonged in the elongating wing area. The expression of Fam5c, a tumour suppressor, in bat limbs is bat-specific and significantly high in all short digit regions (the thumb and foot digits). These results suggest multiple genetic changes occurred independently during the evolution of bat wings to elongate the hand digits, promote membrane growth and keep other digits short. Our findings also indicate that the evolution of limb morphology depends on the complex integration of multiple gene regulatory networks and biological processes that control digit formation and identity, chondrogenesis, and interdigital regression or retention.

Wing morphology of Neotropical bats: a quantitative and qualitative analysis with implications for habitat use

Wing morphology has a direct influence on the flight manoeuvrability, agility, and speed of bats. Studies addressing the relationship between bat wing morphology and ecology are biased towards Old World species and few of them have addressed the ecologically rich Amazonian bat fauna. We quantitatively and qualitatively characterized the wing shape of 51 bat species found in the Brazilian Amazonia by measuring their aspect ratio (AR) and relative wing load (RWL). We found a high variability in wing shape: AR varied from 5.0862 (pygmy round-eared bat, Lophostoma brasiliense (Peters, 1866)) to 8.2774 (brown dog-faced bat, Molossus (Cynomops) paranus (Thomas, 1901)), while RWL varied from 20.0459 (spectral bat, Vampyrum spectrum (L., 1758)) to 55.3931 (Pallas’s mastiff, Molossus molossus (Pallas, 1766)). Insectivores had the largest variability, whereas frugivores and nectarivores had intermediate values with lower variability, indicating a higher flexibility in the use of space and resources. Our predictions on flight patterns are supported by capture and behavioural data from the literature, both of which point to the use of wing shape as a good proxy for habitat use and food partitioning among species. Our data are useful for integrative studies in ecology, physiology, behaviour, and evolution, and can contribute to a better understanding of the ecological interactions of Neotropical bat species.

Flapping tail membrane in bats produces potentially important thrust during horizontal takeoffs and very slow flight

Historically, studies concerning bat flight have focused primarily on the wings. By analyzing high-speed video taken on 48 individuals of five species of vespertilionid bats, we show that the capacity to flap the tail-membrane (uropatagium) in order to generate thrust and lift during takeoffs and minimal-speed flight (<1 m (s-1)) was largely underestimated. Indeed, bats flapped the tail-membrane by extensive dorso-ventral fanning motions covering as much as 135 degrees of arc consistent with thrust generation by air displacement. The degree of dorsal extension of the tail-membrane, and thus the potential amount of thrust generated during platform launches, was significantly correlated with body mass (P = 0.02). Adduction of the hind limbs during upstrokes collapsed the tail-membrane thereby reducing its surface area and minimizing negative lift forces. Abduction of the hind limbs during the downstroke fully expanded the tail-membrane as it was swept ventrally. The flapping kinematics of the tail-membrane is thus consistent with expectations for an airfoil. Timing offsets between the wings and tail-membrane during downstrokes was as much as 50%, suggesting that the tail-membrane was providing thrust and perhaps lift when the wings were retracting through the upstoke phase of the wing-beat cycle. The extent to which the tail-membrane was used during takeoffs differed significantly among four vespertilionid species (P = 0.01) and aligned with predictions derived from bat ecomorphology. The extensive fanning motion of the tail membrane by vespertilionid bats has not been reported for other flying vertebrates.

Postnatal development in Andersen's leaf-nosed bat Hipposideros pomona: flight, wing shape, and wing bone lengths

Postnatal changes in flight development, wing shape and wing bone lengths of 56 marked neonate Hipposideros pomona were investigated under natural conditions in southwest China. Flight experiments showed that pups began to flutter with a short horizontal displacement at 10 days and first took flight at 19 days, with most achieving sustained flight at 1 month old. Analysis of covariance on wingspan, wing area, and the other seven wing characteristics between 'pre-flight' and 'post-volancy' periods supports the hypothesis that growth had one 'pre-flight' trajectory and a different 'post-volancy' trajectory in bats. Wingspan, handwing length and area, armwing length and area, and total wing area increased linearly until the age of first flight, after which the growth rates decreased (all P < 0.001). Wing loading declined linearly until day 19 before ultimately decreasing to adult levels (P < 0.001). Additionally, the relationship of different pairwise combinations of bony components composing span-wise length and chord-wise length was evaluated to test the hypothesis that compensatory growth of wing bones in H. pomona occurred in both 'pre-flight' and 'post-volancy' periods. The frequency of short-long and long-short pairs was significantly greater than that of short-short, long-long pairs in most pairs of bone elements in adults. The results indicate that a bone 'shorter than expected' would be compensated by a bone or bones 'longer than expected', suggesting compensatory growth in H. pomona. The pairwise comparisons conducted in adults were also performed in young bats during 'pre-flight' and 'post-volancy' periods, demonstrating that compensatory growth occurred throughout postnatal ontogeny.

Forelimb versus hindlimb skeletal development in the big brown bat, Eptesicus fuscus: functional divergence is reflected in chondrocytic performance in Autopodial growth plates

The morphology of the chiropteran forelimb demonstrates musculoskeletal specializations for powered flight essentially unique among mammals, including extreme elongation of the distal skeletal elements. Recent studies have focused primarily on the relative timing and levels of gene expression during early stages of endochondral ossification in the chiropteran embryo for clues to the molecular basis of the evolutionary origins of flight in these species. The goal of the current study was to examine how elongation of skeletal elements of the forelimb autopod is achieved through a combination of cellular proliferation, cellular enlargement and matrix synthesis during a period of rapid postnatal growth in Eptesicus fuscus. Quantitative analyses were done of multiple performance parameters of growth plate chondrocytes during all phases of the differentiation cascade. Fourteen autopodial growth plates from the forelimb and hindlimb of one individual, as well as the proximal tibial growth plate, were collected and analyzed. Significant differences were seen in all performance parameters examined. Particularly striking were the differences between growth plates of the manus and pes in the size of the pool of chondrocytes in all cellular zones and rates of turnover of terminal cells. The magnitude of hypertrophy of chondrocytes in growth plates of the manus in E. fuscus far exceeded what has been reported previously in any species, even in rapidly elongating rodent long bones. Volume changes approaching x70 and height changes of 50-60 mum/cell (paralleling the direction of growth) occurred after proliferation in the most rapidly growing growth plates. The data demonstrate that final differences in lengths of homologous skeletal elements in the autopod of the forelimb and hindlimb of this species result not just from an initiating factor early in development, but from continued quantitative differences in chondrocytic performance during postnatal bone elongation as measured by multiple kinetic-based parameters.