Geometric morphometrics shows differences and similarities in skull shape between the red and giant pandas

Morphological examination of the visual system and orbital region in the red panda (Ailurus fulgens fulgens)

OBJECTIVES

The red panda is currently the only surviving member of the Ailuridae family in the Caniformia suborder. In this study, we provide data on anatomical, morphometric, histological and histochemical examination of the orbital region, eyelids, orbital gland, and eye tunics in two adult males Ailurus fulgens fulgens from the Wroclaw Zoological Garden, Poland.

METHODS

The study involved morphometric analysis of the eyeball and selected accessory organs of the eye, along with analysis of the bony orbit, including its morphometry, macroscopic, and microscopic evaluation. Microscopic evaluation encompassed histological and histochemical staining, with the former involving hematoxylin & eosin (H&E), Movat pentachrome, picro-Mallory trichrome, Fontana-Masson, and the latter including PAS, AB pH 1.0, AB pH 2.5; AB pH 2.5/PAS, and HDI.

RESULTS

The upper (UE) and lower (LE) eyelids presented well-developed tarsal glands, sebaceous glands, and a characteristic simple alveolar gland (producing a mucous secretion). The palpebral part of the lacrimal gland was present. A single lymphoid follicle was observed only in the upper eyelids. The superficial gland of the third eyelid (SGTE) was a multilobar acinar complex that produces mucous secretion and is contained within the interlobular ducts of numerous aggregates of lymph nodes. The third eyelid (TE) was T-shaped and composed of hyaline tissue, containing CALT. The lacrimal gland (LG) also revealed a multilobar acinar complex that produced mucous secretion, with a single lymphoid follicle. The cornea consisted of 4 layers, as Bowman's membrane was absent. The Vogt palisades composed of 7-10 layers of epithelial cells were demonstrated. The pupil was horizontally ovoid at rest (post-mortem). The sphincter pupil and the dilator pupil were well developed. Macroscopically, the tapetum lucidum appeared as a milky, non-opalescent crescent. Histologically, the choroidal tapetum lucidum cellulosum consisted of 5 to 9 layers of loosely packed oval cells. The retina showed a composition similar to that of terrestrial nocturnal carnivores.

CONCLUSIONS

The results of our research indicate that the anatomical features of the eye and orbital region in the red panda share similarities with those described in the Musteloidea clade, as well as the Canidae and Ursidae families.

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.

Comparative study of the digestion and metabolism related genes' expression changes during the postnatal food change in different dietary mammals

The changes in the expression of genes related to digestion and metabolism may be various in different dietary mammals from juvenile to adult, especially, the giant panda (Ailuropoda melanoleuca) and red panda (Ailurus fulgens), which were once carnivores but have shifted to being specialized bamboo eaters, are unique features of their changes are more unclear. To elucidate the changing patterns of gene expression related to digestion and metabolism from juvenile to adult in different dietary mammals, we performed transcriptome analysis of the liver or pancreas in giant and red pandas, herbivorous rabbits (Oryctolagus cuniculus) and macaques (Macaca mulatta), carnivorous ferrets (Mustela putorius furo), and omnivorous mice (Mus musculus) from juvenile to adult. During the transition from juvenile to adulthood, giant and red pandas, as well as rabbits and macaques, show significant upregulation of key genes for carbohydrate metabolism, such as starch hydrolysis and sucrose metabolism, and unsaturated fatty acid metabolism, such as linoleic acid, while there is no significant difference in the expression of key genes for fatty acid β-oxidation. A large number of amino acid metabolism related genes were upregulated in adult rabbits and macaques compared to juveniles. While adult giant and red pandas mainly showed upregulation of key genes for arginine synthesis and downregulation of key genes for arginine and lysine degradation. In adult stages, mouse had significantly higher expression patterns in key genes for starch hydrolysis and sucrose metabolism, as well as lipid and protein metabolism. In contrast to general expectations, genes related to lipid, amino acid and protein metabolism were significantly higher expressed in adult group of ferrets, which may be related to their high metabolic levels. Our study elucidates the pattern of changes in the expression of genes related to digestion and metabolism from juvenile to adult in different dietary mammals, with giant and red pandas showing adaptations associated with specific nutritional limitations of bamboo.

The brain of the North American cheetah-like cat Miracinonyx trumani

The cheetah Acinonyx jubatus, the fastest living land mammal, is an atypical member of the family Felidae. The extinct feline Miracinonyx trumani, known as the North American cheetah, is thought to have convergently evolved with Acinonyx to pursue fast and open-country prey across prairies and steppe environments of the North American Pleistocene. The brain of Acinonyx is unique among the living felids, but it is unknown whether the brain of the extinct M. trumani is convergent to that of Acinonyx. Here, we investigate the brain of M. trumani from a cranium endocast, using a comparative sample of other big cats. We demonstrate that the brain of M. trumani was different from that of the living A. jubatus. Indeed, its brain shows a unique combination of traits among living cats. This suggests that the case of extreme convergence between Miracinonyx and its living Old World vicar should be reconsidered.

Reaching over the gap: A review of trends in and status of red panda research over 193 years (1827-2020)

The red panda is a unique species taxonomically known for its peculiar biological and ecological characteristics, and extreme attractiveness. Despite being highly significant from conservation, scientific and economic perspectives, this species has experienced a declining population in the wild. Thus, to direct further research priorities and conservation actions and assess gaps in the current research trend of this species, a systematic literature review was conducted covering 175 journal articles published in English over 193 years (1827-2020). This review revealed that (1) the biological aspect was highly studied compared to other thematic areas of red panda (2) captive-based studies are relatively higher than the studies based in wild populations (3) China is leading the red panda studies amongst all red panda range (4) The universities were found contributing more to red panda studies than other institutions. Surprisingly, we found that the researchers from the non-range country were leading red panda study than those from range countries. Our review highlighted the need of prioritising studies in underrepresented locations and understudied thematic areas focusing on the assessment of climate change impact, bamboo distribution status, ecosystem services of red panda habitat, behavior and movement ecology, population estimation, and metapopulation dynamics. We urge landscape-level studies and long-term population monitoring. Besides, we also suggest the documentation and evaluation of the effectiveness of ongoing red panda-focused conservation programs. We also stress the need for strengthening the capacity of institutions and people from range countries.

Serial disparity in the carnivoran backbone unveils a complex adaptive role in metameric evolution

Organisms comprise multiple interacting parts, but few quantitative studies have analysed multi-element systems, limiting understanding of phenotypic evolution. We investigate how disparity of vertebral morphology varies along the axial column of mammalian carnivores — a chain of 27 subunits — and the extent to which morphological variation have been structured by evolutionary constraints and locomotory adaptation. We find that lumbars and posterior thoracics exhibit high individual disparity but low serial differentiation. They are pervasively recruited into locomotory functions and exhibit relaxed evolutionary constraint. More anterior vertebrae also show signals of locomotory adaptation, but nevertheless have low individual disparity and constrained patterns of evolution, characterised by low-dimensional shape changes. Our findings demonstrate the importance of the thoracolumbar region as an innovation enabling evolutionary versatility of mammalian locomotion. Moreover, they underscore the complexity of phenotypic macroevolution of multi-element systems and that the strength of ecomorphological signal does not have a predictable influence on macroevolutionary outcomes.

Figueirido et al. use a 3D geometric morphometric approach to study functional among-species disparity in the vertebral column of Carnivora, as well as assessing the effect of different sampling methods on homology. Disparity is generally higher in more caudal regions, compared to more cranial regions, but recruitment for locomotor function is pervasive throughout the whole studied column.

Taking a look into the orbit of mammalian carnivorans

In this study, we explore the relationship between orbit anatomy and different ecological factors in carnivorous mammals from a phylogenetic perspective. We calculated the frontation (α), convergence (β), and orbitotemporal (Ω) angles of the orbit from 3D coordinates of anatomical landmarks in a wide sample of carnivores with different kinds of visual strategy (i.e. photopic, scotopic, and mesopic), habitat (i.e. open, mixed, and closed), and substrate use (i.e. arboreal, terrestrial, and aquatic). We used Bloomberg's K and Pagel's λ to assess phylogenetic signal in frontation, convergence, and orbitotemporal angles. The association of orbit orientation with skull length and ecology was explored using phylogenetic generalized least squares and phylogenetic manova, respectively. Moreover, we also computed phylomorphospaces from orbit orientation. Our results indicate that there is not a clear association between orbit orientation and the ecology of living carnivorans. We hypothesize that the evolution of the orbit in mammalian carnivores represents a new case of an ecological bottleneck specific to carnivorans. New directions for future research are discussed in light of this new evidence.

Evolution of morphological integration in the skull of Carnivora (Mammalia): Changes in Canidae lead to increased evolutionary potential of facial traits

Morphological integration refers to the fact that different phenotypic traits of organisms are not fully independent from each other, and tend to covary to different degrees. The covariation among traits is thought to reflect properties of the species' genetic architecture and thus can have an impact on evolutionary responses. Furthermore, if morphological integration changes along the history of a group, inferences of past selection regimes might be problematic. Here, we evaluated the stability and evolution of the morphological integration of skull traits in Carnivora by using evolutionary simulations and phylogenetic comparative methods. Our results show that carnivoran species are able to respond to natural selection in a very similar way. Our comparative analyses show that the phylogenetic signal for pattern of integration is lower than that observed for morphology (trait averages), and that integration was stable throughout the evolution of the group. That notwithstanding, Canidae differed from other families by having higher integration, evolvability, flexibility, and allometric coefficients on the facial region. These changes might have allowed canids to rapidly adapt to different food sources, helping to explain not only the phenotypic diversification of the family, but also why humans were able to generate such a great diversity of dog breeds through artificial selection.

Age-associated microbiome shows the giant panda lives on hemicelluloses, not on cellulose

The giant panda feeds almost exclusively on bamboo, a diet highly enriched in lignin and cellulose, but is characterized by a digestive tract similar to carnivores. It is still large unknown if and how the giant panda gut microbiota contributes to lignin and cellulose degradation. Here we show the giant pandas’ gut microbiota does not significantly contribute to cellulose and lignin degradation. We found that no operational taxonomic unit had a nearest neighbor identified as a cellulolytic species or strain with a significant higher abundance in juvenile than cubs, a very low abundance of putative lignin and cellulose genes existed in part of analyzing samples but a significant higher abundance of genes involved in starch and hemicellulose degradation in juveniles than cubs. Moreover, a significant lower abundance of putative cellulolytic genes and a significant higher abundance of putative α-amylase and hemicellulase gene families were present in giant pandas than in omnivores or herbivores.

Genetic and Morphological Differentiation of the Semiterrestrial Crab Armases angustipes (Brachyura: Sesarmidae) along the Brazilian Coast

The genetic and morphometric population structures of the semiterrestrial crab Armases angustipes from along the Brazilian coast were examined. The influence of the Central South Equatorial Current on larval dispersal of A. angustipes also was evaluated. Six populations were sampled from estuarine areas in São Luis do Maranhão, Maranhão; Natal, Rio Grande do Norte; Maceió, Alagoas; Ilhéus, Bahia; Aracruz, Espírito Santo; and Guaratuba, Paraná. Patterns of genetic differentiation were assessed using DNA sequence data corresponding to parts of the mitochondrial cytochrome c oxidase subunit 1. Geometric morphometric techniques were used to evaluate morphological variation in shape and size of the carapace and right cheliped propodus. Our results revealed low genetic variability and lack of phylogeographic structure; geometric morphometrics showed statistically significant morphological differentiation and geographic structuring. Our data indicate the absence of possible barriers to gene flow for this mobile species, and no clear correlation of morphological or genetic variation with ocean currents and/or geographic distance. Our results also suggest that historical geological and climatological events and/or possible bottleneck effects influenced the current low genetic variability among the populations of A. angustipes.

Three-dimensional computer simulations of feeding behaviour in red and giant pandas relate skull biomechanics with dietary niche partitioning

The red (Ailurus fulgens) and giant (Ailuropoda melanoleuca) pandas are mammalian carnivores convergently adapted to a bamboo feeding diet. However, whereas Ailurus forages almost entirely on younger leaves, fruits and tender trunks, Ailuropoda relies more on trunks and stems. Such difference in foraging mode is considered a strategy for resource partitioning where they are sympatric. Here, we use finite-element analysis to test for mechanical differences and similarities in skull performance between Ailurus and Ailuropoda related to diet. Feeding simulations suggest that the two panda species have similar ranges of mechanical efficiency and strain energy profiles across the dentition, reflecting their durophagous diet. However, the stress distributions and peaks in the skulls of Ailurus and Ailuropoda are remarkably different for biting at all tooth locations. Although the skull of Ailuropoda is capable of resisting higher stresses than the skull of Ailurus, the latter is able to distribute stresses more evenly throughout the skull. These differences in skull biomechanics reflect their distinct bamboo feeding preferences. Ailurus uses repetitive chewing in an extended mastication to feed on soft leaves, and Ailuropoda exhibits shorter and more discrete periods of chomp-and-swallow feeding to break down hard bamboo trunks.

Kretzoiarctos gen. nov., the oldest member of the giant panda clade

The phylogenetic position of the giant panda, Ailuropoda melanoleuca (Carnivora: Ursidae: Ailuropodinae), has been one of the most hotly debated topics by mammalian biologists and paleontologists during the last century. Based on molecular data, it is currently recognized as a true ursid, sister-taxon of the remaining extant bears, from which it would have diverged by the Early Miocene. However, from a paleobiogeographic and chronological perspective, the origin of the giant panda lineage has remained elusive due to the scarcity of the available Miocene fossil record. Until recently, the genus Ailurarctos from the Late Miocene of China (ca. 8–7 mya) was recognized as the oldest undoubted member of the Ailuropodinae, suggesting that the panda lineage might have originated from an Ursavus ancestor. The role of the purported ailuropodine Agriarctos, from the Miocene of Europe, in the origins of this clade has been generally dismissed due to the paucity of the available material. Here, we describe a new ailuropodine genus, Kretzoiarctos gen. nov., based on remains from two Middle Miocene (ca. 12–11 Ma) Spanish localities. A cladistic analysis of fossil and extant members of the Ursoidea confirms the inclusion of the new genus into the Ailuropodinae. Moreover, Kretzoiarctos precedes in time the previously-known, Late Miocene members of the giant panda clade from Eurasia (Agriarctos and Ailurarctos). The former can be therefore considered the oldest recorded member of the giant panda lineage, which has significant implications for understanding the origins of this clade from a paleobiogeographic viewpoint.