Convergent evolution of olfactory and thermoregulatory capacities in small amphibious mammals

Turbinal bones are still one of the last frontiers of the tetrapod skull: hypotheses, challenges and perspectives

Turbinals are bony or cartilaginous structures that are present in the nasal cavity of most tetrapods. They are involved in key functions such as olfaction, heat, and moisture conservation, as well as protection of the respiratory tract. Despite recent studies that challenged long-standing hypotheses about their physiological and genomic correlation, turbinals remain largely unexplored, particularly for non-mammalian species. Herein, we review and synthesise the current knowledge of turbinals using an integrative approach that includes comparative anatomy, physiology, histology and genomics. In addition, we provide synonyms and correspondences of tetrapod turbinals from about 80 publications. This work represents a first step towards drawing hypotheses of homology for the whole clade, and provides a strong basis to develop new research avenues.

Sniffing out morphological convergence in the turbinal complex of myrmecophagous placentals

The length of the snout in mammals has important evolutionary consequences for the functional systems housed within the rostrum. However, whether increased snout lengths lead to expanded olfactory performance has rarely been examined. Here, we investigate inner rostral function among 10 species of myrmecophagous (ant- and/or termite-eating) placental mammals and 10 closely related species. We use nondestructive computed tomography scanning methods to characterize inner rostral function based on the underlying morphology of the turbinal bones in the nasal cavity. Three approaches were chosen to address this question, including the quantification of functional turbinal surface area, the quantification of functional turbinal three-dimensional complexity, and geometric morphometrics. By including non-model species from several different mammalian orders, we were able to extend the discussion surrounding turbinal homologies to comparisons across mammals. Our results show no increased olfactory function in all myrmecophagous species relative to their sister taxa, which suggests that there is no trade-off for increased olfactory capabilities in myrmecophagous species with elongated snouts. We found no evidence of convergence in turbinal morphology among all five myrmecophagous lineages. However, we found evidence of morphological convergence in the turbinals between the giant armadillo and the aardvark, suggesting a more complex interplay between the evolution of turbinal morphology and ecological correlates. While myrmecophagy alone may not be a strong enough ecological signal to overcome phylogenetic and developmental constraints, we suggest that this might be the case at the intersection of this dietary specialization with a primarily underground lifestyle where odorants may be difficult to detect.

Comparative morphology of the rhinarium and upper lip in sigmodontine rodents: Refined nomenclature, intertribal variation in a phylogenetic framework, and functional inferences

Rodents have received substantial attention in the study of olfaction. However, the rhinarium, the naked part of the nose, which plays an important role in chemical, tactile, and thermal perception, has been relatively overlooked. This study presents a comprehensive analysis of the rhinarium morphology and spatially associated structures (i.e., upper lip, and philtrum) in sigmodontines, a diverse group within the Cricetidae rodents. The research covers 483 specimens representing 145 species, accounting for 74% of genera in the clade, including all 13 recognized tribes, three incertae sedis genera, and the murid representatives Mus musculus and Rattus norvegicus. The inconsistent use of terminology in describing rhinarium traits across the literature poses a challenge for comparative analyzes. To address this issue, a standardized terminology was proposed to characterize the rhinarium. A paired complex protuberance typically with epidermal ridges (i.e., rhinoglyphics), termed here the tubercle of Hill, was identified as a distinctive feature in muroid rhinaria. Comparative assessments among tribes revealed unique sets of features defining each major clade, encompassing variations in hairiness, dorsum nasi complexity, size and positioning of the tubercle of Hill, and other key attributes. Two primary rhinarium configurations were discerned: one shared by Oryzomyalia and Sigmodontini and another specific to Ichthyomyini. The former groups display a ventrally positioned rhinarium prominently featuring the tubercle of Hill and sculptured areola circularis. In contrast, Ichthyomyini exhibit a frontally directed rhinarium characterized by an enlarged dorsum nasi fused to the tubercle of Hill, resulting in a distinctive "cherry" appearance. Convergent rhinarium structures observed in fossorial species, characterized by well-developed plica alaris and hair fringes, are presumed to mitigate potential damage during digging. Conversely, semiaquatic carnivorous sigmodontines showcase an integrated apical structure in their rhinarium, facilitating enhanced somatosensory capabilities crucial for predation activities during diving expeditions.

Disparity of turbinal bones in placental mammals

Turbinals are key bony elements of the mammalian nasal cavity, involved in heat and moisture conservation as well as olfaction. While turbinals are well known in some groups, their diversity is poorly understood at the scale of placental mammals, which span 21 orders. Here, we investigated the turbinal bones and associated lamellae for one representative of each extant order of placental mammals. We segmented and isolated each independent turbinal and lamella and found an important diversity of variation in the number of turbinals, as well as their size, and shape. We found that the turbinal count varies widely, from zero in the La Plata dolphin, (Pontoporia blainvillei) to about 110 in the African bush elephant (Loxodonta africana). Multiple turbinal losses and additional gains took place along the phylogeny of placental mammals. Some changes are clearly attributed to ecological adaptation, while others are probably related to phylogenetic inertia. In addition, this work highlights the problem of turbinal nomenclature in some placental orders with numerous and highly complex turbinals, for which homologies are extremely difficult to resolve. Therefore, this work underscores the importance of developmental studies to better clarify turbinal homology and nomenclature and provides a standardized comparative framework for further research.

Challenges and advances in measuring phenotypic convergence

Tests of phenotypic convergence can provide evidence of adaptive evolution, and the popularity of such studies has grown in recent years due to the development of novel, quantitative methods for identifying and measuring convergence. These methods include the commonly applied C1-C4 measures of Stayton (2015a), which measure morphological distances between lineages, and Ornstein-Uhlenbeck (OU) model-fitting analyses, which test whether lineages converged on shared adaptive peaks. We test the performance of C-measures and other convergence measures under various evolutionary scenarios and reveal a critical issue with C-measures: they often misidentify divergent lineages as convergent. We address this issue by developing novel convergence measures-Ct1-Ct4-measures-that calculate distances between lineages at specific points in time, minimizing the possibility of misidentifying divergent taxa as convergent. Ct-measures are most appropriate when focal lineages are of the same or similar geologic ages (e.g., extant taxa), meaning that the lineages' evolutionary histories include considerable overlap in time. Beyond C-measures, we find that all convergence measures are influenced by the position of focal taxa in phenotypic space, with morphological outliers often statistically more likely to be measured as strongly convergent. Further, we mimic scenarios in which researchers assess convergence using OU models with a priori regime assignments (e.g., classifying taxa by ecological traits) and find that multiple-regime OU models with phenotypically divergent lineages assigned to a shared selective regime often outperform simpler models. This highlights that model support for these multiple-regime OU models should not be assumed to always reflect convergence among focal lineages of a shared regime. Our new Ct1-Ct4-measures provide researchers with an improved comparative tool, but we emphasize that all available convergence measures are imperfect, and researchers should recognize the limitations of these methods and use multiple lines of evidence to test convergence hypotheses.

Localized Expression of Olfactory Receptor Genes in the Olfactory Organ of Common Minke Whales

Baleen whales (Mysticeti) possess the necessary anatomical structures and genetic elements for olfaction. Nevertheless, the olfactory receptor gene (OR) repertoire has undergone substantial degeneration in the cetacean lineage following the divergence of the Artiodactyla and Cetacea. The functionality of highly degenerated mysticete ORs within their olfactory epithelium remains unknown. In this study, we extracted total RNA from the nasal mucosae of common minke whales (Balaenoptera acutorostrata) to investigate ORs' localized expression. All three sections of the mucosae examined in the nasal chamber displayed comparable histological structure. However, the posterior portion of the frontoturbinal region exhibited notably high OR expression. Neither the olfactory bulb nor the external skin exhibited the expression of these genes. Although this species possesses four intact non-class-2 ORs, all the ORs expressed in the nasal mucosae belong to class-2, implying the loss of aversion to specific odorants. These anatomical and genomic analyses suggest that ORs are still responsible for olfaction within the nasal region of baleen whales, enabling them to detect desirable scents such as prey and potential mating partners.

phytools 2.0: an updated R ecosystem for phylogenetic comparative methods (and other things)

Phylogenetic comparative methods comprise the general endeavor of using an estimated phylogenetic tree (or set of trees) to make secondary inferences: about trait evolution, diversification dynamics, biogeography, community ecology, and a wide range of other phenomena or processes. Over the past ten years or so, the phytools R package has grown to become an important research tool for phylogenetic comparative analysis. phytools is a diverse contributed R library now consisting of hundreds of different functions covering a variety of methods and purposes in phylogenetic biology. As of the time of writing, phytools included functionality for fitting models of trait evolution, for reconstructing ancestral states, for studying diversification on trees, and for visualizing phylogenies, comparative data, and fitted models, as well numerous other tasks related to phylogenetic biology. Here, I describe some significant features of and recent updates to phytools, while also illustrating several popular workflows of the phytools computational software.

Phylogeny, Ecology, and Gene Families Covariation Shaped the Olfactory Subgenome of Rodents

Olfactory receptor (OR) genes represent the largest multigenic family in mammalian genomes and encode proteins that bind environmental odorant molecules. The OR repertoire is extremely variable among species and is subject to many gene duplications and losses, which have been linked to ecological adaptations in mammals. Although they have been studied on a broad taxonomic scale (i.e., placental), finer sampling has rarely been explored in order to better capture the mechanisms that drove the evolution of the OR repertoire. Among placental mammals, rodents are well-suited for this task, as they exhibit diverse life history traits, and genomic data are available for most major families and a diverse array of lifestyles. In this study, 53 rodent published genomes were mined for their OR subgenomes. We retrieved more than 85,000 functional and pseudogene OR sequences that were subsequently classified into phylogenetic clusters. Copy number variation among rodents is similar to that of other mammals. Using our OR counts along with comparative phylogenetic approaches, we demonstrated that ecological niches such as diet, period of activity, and a fossorial lifestyle strongly impacted the proportion of OR pseudogenes. Within the OR subgenome, phylogenetic inertia was the main factor explaining the relative variations of the 13 OR gene families. However, a striking exception was a convergent 10-fold expansion of the OR family 14 among the phylogenetically divergent subterranean mole-rat lineages belonging to Bathyergidae and Spalacidae families. This study illustrates how the diversity of the OR repertoire has evolved among rodents, both shaped by selective forces stemming from species life history traits and neutral evolution along the rodent phylogeny.

An updated synthesis of and outstanding questions in the olfactory and vomeronasal systems in bats: Genetics asks questions only anatomy can answer

The extensive diversity observed in bat nasal chemosensory systems has been well-documented at the histological level. Understanding how this diversity evolved and developing hypotheses as to why particular patterns exist require a phylogenetic perspective, which was first outlined in the work of anatomist Kunwar Bhatnagar. With the onset of genetics and genomics, it might be assumed that the puzzling patterns observed in the morphological data have been clarified. However, there is still a widespread mismatch of genetic and morphological correlations among bat chemosensory systems. Novel genomic evidence has set up new avenues to explore that demand more evidence from anatomical structures. Here, we outline the progress that has been made in both morphological and molecular studies on the olfactory and vomeronasal systems in bats since the work of Bhatnagar. Genomic data of olfactory and vomeronasal receptors demonstrate the strong need for further morphological sampling, with a particular focus on receiving brain regions, glands, and ducts.

Neural crest cells as a source of microevolutionary variation

Vertebrates have some of the most complex and diverse features in animals, from varied craniofacial morphologies to colorful pigmentation patterns and elaborate social behaviors. All of these traits have their developmental origins in a multipotent embryonic lineage of neural crest cells. This "fourth germ layer" is a vertebrate innovation and the source of a wide range of adult cell types. While others have discussed the role of neural crest cells in human disease and animal domestication, less is known about their role in contributing to adaptive changes in wild populations. Here, we review how variation in the development of neural crest cells and their derivatives generates considerable phenotypic diversity in nature. We focus on the broad span of traits under natural and sexual selection whose variation may originate in the neural crest, with emphasis on behavioral factors such as intraspecies communication that are often overlooked. In all, we encourage the integration of evolutionary ecology with developmental biology and molecular genetics to gain a more complete understanding of the role of this single cell type in trait covariation, evolutionary trajectories, and vertebrate diversity.

Mammalian maxilloturbinal evolution does not reflect thermal biology

The evolution of endothermy in vertebrates is a major research topic in recent decades that has been tackled by a myriad of research disciplines including paleontology, anatomy, physiology, evolutionary and developmental biology. The ability of most mammals to maintain a relatively constant and high body temperature is considered a key adaptation, enabling them to successfully colonize new habitats and harsh environments. It has been proposed that in mammals the anterior nasal cavity, which houses the maxilloturbinal, plays a pivotal role in body temperature maintenance, via a bony system supporting an epithelium involved in heat and moisture conservation. The presence and the relative size of the maxilloturbinal has been proposed to reflect the endothermic conditions and basal metabolic rate in extinct vertebrates. We show that there is no evidence to relate the origin of endothermy and the development of some turbinal bones by using a comprehensive dataset of µCT-derived maxilloturbinals spanning most mammalian orders. Indeed, we demonstrate that neither corrected basal metabolic rate nor body temperature significantly correlate with the relative surface area of the maxilloturbinal. Instead, we identify important variations in the relative surface area, morpho-anatomy, and complexity of the maxilloturbinal across the mammalian phylogeny and species ecology.

Developmental origin underlies evolutionary rate variation across the placental skull

The placental skull has evolved into myriad forms, from longirostrine whales to globular primates, and with a diverse array of appendages from antlers to tusks. This disparity has recently been studied from the perspective of the whole skull, but the skull is composed of numerous elements that have distinct developmental origins and varied functions. Here, we assess the evolution of the skull's major skeletal elements, decomposed into 17 individual regions. Using a high-dimensional morphometric approach for a dataset of 322 living and extinct eutherians (placental mammals and their stem relatives), we quantify patterns of variation and estimate phylogenetic, allometric and ecological signal across the skull. We further compare rates of evolution across ecological categories and ordinal-level clades and reconstruct rates of evolution along lineages and through time to assess whether developmental origin or function discriminate the evolutionary trajectories of individual cranial elements. Our results demonstrate distinct macroevolutionary patterns across cranial elements that reflect the ecological adaptations of major clades. Elements derived from neural crest show the fastest rates of evolution, but ecological signal is equally pronounced in bones derived from neural crest and paraxial mesoderm, suggesting that developmental origin may influence evolutionary tempo, but not capacity for specialisation. This article is part of the theme issue 'The mammalian skull: development, structure and function'.

Environmental and climatic drivers of phenotypic evolution and distribution changes in a widely distributed subfamily of subterranean mammals

How environmental factors shape species morphology and distributions is a key issue in ecology, especially in similar environments. Species of Myospalacinae exhibit widespread distribution spanning the eastern Eurasian steppe and the extreme adaptation to the subterranean environment, providing an excellent opportunity for investigating species responses to environmental changes. At the national scale, we here use geometric morphometric and distributional data to assess the environmental and climatic drivers of morphological evolution and distribution of Myospalacinae species in China. Based on phylogenetic relationships of Myospalacinae species constructed using genomic data in China, we integrate geometric morphometrics and ecological niche models to reveal the interspecific variation of skull morphology, trace the ancestral state, and assess factors influencing interspecific variation. Our approach further allows us to project future distributions of Myospalacinae species throughout China. We found that the interspecific morphology variations were mainly concentrated in the temporal ridge, premaxillary-frontal suture, premaxillary-maxillary suture, and molars, and the skull morphology of the two current species in Myospalacinae followed the ancestral state; temperature and precipitation were important environmental variables influencing skull morphology. Elevation, temperature annual range, and precipitation of warmest quarter were identified as dominant factors affecting the distribution of Myospalacinae species in China, and their suitable habitat area will decrease in the future. Collectively, environmental and climate changes have an effect on skull phenotypes of subterranean mammals, highlighting the contribution of phenotypic differentiation in similar environments in the formation of species phenotypes. Climate change will further shrink their habitats under future climate assumptions in the short-term. Our findings provide new insights into effects of environmental and climate change on the morphological evolution and distribution of species as well as a reference for biodiversity conservation and species management.

Strategies of vascularization of the ethmoid labyrinth in selected even-toed ungulates (Artiodactyla) and carnivores (Carnivora)

The anatomy of the nasal cavity and its structures, as well as other elements building a scaffold for olfactory organs, differs significantly among various groups of mammals. Understanding anatomical conditions of quality of olfaction are being studied worldwide and is a complex problem. Among many studies regarding bone and epithelial structures of turbinates and connected anatomical structures, few studies describe the vascularization of turbinates. Ethmoid turbinates are above all covered in olfactory epithelium containing branched axons that receive olfactory stimuli and as olfactory nerves penetrate the cribriform lamina of the ethmoid bone conveying information from smell receptors to the brain. Differences in vascularization of the cribriform plate and turbinates may add crucial information complementing studies regarding the olfactory organ's bone and soft tissue structures. In the study, we describe the vascularization of the cribriform plate of the ethmoid bone of 54 Artiodactyla and Carnivora.

Evolutionary constraint and innovation across hundreds of placental mammals

Zoonomia is the largest comparative genomics resource for mammals produced to date. By aligning genomes for 240 species, we identify bases that, when mutated, are likely to affect fitness and alter disease risk. At least 332 million bases (~10.7%) in the human genome are unusually conserved across species (evolutionarily constrained) relative to neutrally evolving repeats, and 4552 ultraconserved elements are nearly perfectly conserved. Of 101 million significantly constrained single bases, 80% are outside protein-coding exons and half have no functional annotations in the Encyclopedia of DNA Elements (ENCODE) resource. Changes in genes and regulatory elements are associated with exceptional mammalian traits, such as hibernation, that could inform therapeutic development. Earth's vast and imperiled biodiversity offers distinctive power for identifying genetic variants that affect genome function and organismal phenotypes.

When morphology does not fit the genomes: the case of rodent olfaction

Linking genes to phenotypes has been a major question in evolutionary biology for the last decades. In the genomic era, few studies attempted to link olfactory-related genes to different anatomical proxies. However, they found very inconsistent results. This study is the first to investigate a potential relation between olfactory turbinals and olfactory receptor (OR) genes. We demonstrated that despite the use of similar methodology in the acquisition of data, OR genes do not correlate with the relative and the absolute surface area of olfactory turbinals. These results challenged the interpretations of several studies based on different proxies related to olfaction and their potential relation to olfactory capabilities.

TE, Solari S, Colmenares-Pinzon J, Nivelo C, Rodríguez Herrera B, Merino W, Medina CE, Murillo-García O, Pardiñas UF. Systematics and diversification of the Ichthyomyini (Cricetidae, Sigmodontinae) revisited: evidence from molecular, morphological, and combined approaches

Ichthyomyini, a morphologically distinctive group of Neotropical cricetid rodents, lacks an integrative study of its systematics and biogeography. Since this tribe is a crucial element of the Sigmodontinae, the most speciose subfamily of the Cricetidae, we conducted a study that includes most of its recognized diversity (five genera and 19 species distributed from southern Mexico to northern Bolivia). For this report we analyzed a combined matrix composed of four molecular markers (RBP3, GHR, RAG1, Cytb) and 56 morphological traits, the latter including 15 external, 14 cranial, 19 dental, five soft-anatomical and three postcranial features. A variety of results were obtained, some of which are inconsistent with the currently accepted classification and understanding of the tribe. Ichthyomyini is retrieved as monophyletic, and it is divided into two main clades that are here recognized as subtribes: one to contain the genus Anotomys and the other composed by the remaining genera. Neusticomys (as currently recognized) was found to consist of two well supported clades, one of which corresponds to the original concept of Daptomys. Accordingly, we propose the resurrection of the latter as a valid genus to include several species from low to middle elevations and restrict Neusticomys to several highland forms. Numerous other revisions are necessary to reconcile the alpha taxonomy of ichthyomyines with our phylogenetic results, including placement of the Cajas Plateau water rat (formerly Chibchanomys orcesi) in the genus Neusticomys (sensu stricto), and the recognition of at least two new species (one in Neusticomys, one in Daptomys). Additional work is necessary to confirm other unanticipated results, such as the non-monophyletic nature of Rheomys and the presence of a possible new genus and species from Peru. Our results also suggest that ichthyomyines are one of the main Andean radiations of sigmodontine cricetids, with an evolutionary history dating to the Late Miocene and subsequent cladogenesis during the Pleistocene.

Thermal adaptation of pelage in desert rodents balances cooling and insulation

Phenotypic convergence across distantly related taxa can be driven by similar selective pressures from the environment or intrinsic constraints. The roles of these processes on physiological strategies, such as homeothermy, are poorly understood. We studied the evolution of thermal properties of mammalian pelage in a diverse community of rodents inhabiting the Mojave Desert, USA. We used a heat flux device to measure the thermal insulation of museum specimens and determined whether thermal properties were associated with habitat preferences while assessing phylogenetic dependence. Species that prefer arid habitats exhibited lower conductivity and thinner pelage relative to species with other habitat preferences. Despite being thinner, the pelage of arid species exhibited comparable insulation to the pelage of the other species due to its lower conductivity. Thus, arid species have insulative pelage while simultaneously benefitting from thin pelage that promotes convective cooling. We found no evidence of intrinsic constraints or phylogenetic dependence, indicating pelage readily evolves to environmental pressures. Thermoregulatory simulations demonstrated that arid specialists reduced energetic costs required for homeothermy by 14.5% by evolving lower conductivity, providing support for adaptive evolution of pelage. Our study indicates that selection for lower energetic requirements of homeothermy has shaped evolution of pelage thermal properties.

The orbitotemporal region and the mandibular joint in the skull of shrews (Soricidae, Mammalia)

Modern phylogenetics place the Soricidae (shrews) into the order Lipotyphla, which belongs to the relatively new superorder clade Laurasiatheria. Their most derived skull feature is the unusual position and shape of the jaw articulation: Whereas in all other mammals the glenoid region of the squamosum is more or less tightly attached to the otic capsule or petrosal, respectively, in the soricids it is attached to the nasal capsule. This new position of the jaw articulation becomes possible by the posterior extension of the nasal capsule and the rostral shift of the glenoid fossa. By the study of dated postnatal ontogenetic stages of Crocidura russula and Sorex araneus, we show that the glenoid part of the squamosal becomes fixed to the nasal capsule by the ossified alae orbitalis and temporalis. The ala orbitalis is displaced laterally by the expanded cupula nasi posterior; this posterior expansion is well documented by the lamina terminalis, which incorporates parts of the palatinum and alisphenoid. Both alae consist largely of ‘Zuwachsknochen’ (‘appositional bone’) and are then named orbitosphenoid and alisphenoid. By the forward move of the pars glenoidea and of the alisphenoid, the foramen lacerum medium (‘fenestra piriformis’) also expands rostrally. Functionally, the forward shift of the jaw joint helps to keep the incisal biting force high. Biomechanically the jaws can be considered as a tweezer, and the rostral position of the jaw joints makes the interorbital pillar and the shell-like walls of the facial skull a lever for the highly specialized incisal dentition.

Unique bone microanatomy reveals ancestry of subterranean specializations in mammals

Acquiring a subterranean lifestyle entails a substantial shift for many aspects of terrestrial vertebrates' biology. Although this lifestyle is associated with multiple instances of convergent evolution, the relative success of some subterranean lineages largely remains unexplained. Here, we focus on the mammalian transitions to life underground, quantifying bone microanatomy through high-resolution X-ray tomography. The true moles stand out in this dataset. Examination of this family's bone histology reveals that the highly fossorial moles acquired a unique phenotype involving large amounts of compacted coarse cancellous bone. This phenotype exceeds the adaptive optimum seemingly shared by several other subterranean mammals and can be traced back to some of the first known members of the family. This remarkable microanatomy was acquired early in the history of the group and evolved faster than the gross morphology innovations of true moles' forelimb. This echoes the pattern described for other lifestyle transitions, such as the acquisition of bone mass specializations in secondarily aquatic tetrapods. Highly plastic traits-such as those pertaining to bone structure-are hence involved in the early stages of different types of lifestyle transitions.

Down a Rabbit Hole: Burrowing Behaviour and Larger Home Ranges are Related to Larger Brains in Leporids

Studies on the evolution of brain size variation usually focus on large clades encompassing broad phylogenetic groups. This risks introducing ‘noise’ in the results, often obscuring effects that might be detected in less inclusive clades. Here, we focus on a sample of endocranial volumes (endocasts) of 18 species of rabbits and hares (Lagomorpha: Leporidae), which are a discrete radiation of mammals with a suitably large range of body sizes. Using 60 individuals, we test five popular hypotheses on brain size and olfactory bulb evolution in mammals. We also address the pervasive issue of missing data, using multiple phylogenetic imputations as to conserve the full sample size for all analyses. Our analyses show that home range and burrowing behaviour are the only predictors of leporid brain size variation. Litter size, which is one of the most widely reported constraints on brain size, was unexpectedly not associated with brain size. However, a constraining effect may be masked by a strong association of litter size with temperature seasonality, warranting further study. Lastly, we show that unreasonable estimations of phylogenetic signal (Pagel’s lamba) warrant additional caution when using small sample sizes, such as ours, in comparative studies.

The Arrangement of the Peripheral Olfactory System of Pleuragramma antarcticum: A Well-Exploited Small Sensor, an Aided Water Flow, and a Prominent Effort in Primary Signal Elaboration

Simple Summary

How animals perceive their surrounding environment is crucial to their reactions and behavior. Olfaction, among others, is one of the more important senses for wide-range communication and in low-light environments. This study aims to give a morphological description of the peripheral olfactory system of the Antarctic silverfish, which is a key species in the coastal Antarctic ecosystem. The head of the Antarctic silverfish is specialized to assure that the olfactory organ keeps in contact with a large volume of water, even when the fish is not actively swimming. The sensory surface area and the number of neurons in the primary olfactory brain region show that this fish invests energy in the detection and elaboration of olfactory signals. In the cold waters of the Southern Ocean, the Antarctic silverfish is therefore likely to rely considerably on olfaction.

Abstract

The olfactory system is constituted in a consistent way across vertebrates. Nasal structures allow water/air to enter an olfactory cavity, conveying the odorants to a sensory surface. There, the olfactory neurons form, with their axons, a sensory nerve projecting to the telencephalic zone—named the olfactory bulb. This organization comes with many different arrangements, whose meaning is still a matter of debate. A morphological description of the olfactory system of many teleost species is present in the literature; nevertheless, morphological investigations rarely provide a quantitative approach that would help to provide a deeper understanding of the structures where sensory and elaborating events happen. In this study, the peripheral olfactory system of the Antarctic silverfish, which is a keystone species in coastal Antarctica ecosystems, has also been described, employing some quantitative methods. The olfactory chamber of this species is connected to accessory nasal sacs, which probably aid water movements in the chamber; thus, the head of the Antarctic silverfish is specialized to assure that the olfactory organ keeps in contact with a large volume of water—even when the fish is not actively swimming. Each olfactory organ, shaped like an asymmetric rosette, has, in adult fish, a sensory surface area of about 25 mm², while each olfactory bulb contains about 100,000 neurons. The sensory surface area and the number of neurons in the primary olfactory brain region show that this fish invests energy in the detection and elaboration of olfactory signals and allow comparisons among different species. The mouse, for example—which is considered a macrosmatic vertebrate—has a sensory surface area of the same order of magnitude as that of the Antarctic silverfish, but ten times more neurons in the olfactory bulb. Catsharks, on the other hand, have a sensory surface area that is two orders of magnitude higher than that of the Antarctic silverfish, while the number of neurons has the same order of magnitude. The Antarctic silverfish is therefore likely to rely considerably on olfaction.

Venous networks in the upper airways of bats: A histological and diceCT study

Our knowledge of nasal cavity anatomy has grown considerably with the advent of micro-computed tomography (CT). More recently, a technique called diffusible iodine-based contrast-enhanced CT (diceCT) has rendered it possible to study nasal soft tissues. Using diceCT and histology, we aim to (a) explore the utility of these techniques for inferring the presence of venous sinuses that typify respiratory mucosa and (b) inquire whether distribution of vascular mucosa may relate to specialization for derived functions of the nasal cavity (i.e., nasal-emission of echolocation sounds) in bats. Matching histology and diceCT data indicate that diceCT can detect venous sinuses as either darkened, "empty" spaces, or radio-opaque islands when blood cells are present. Thus, we show that diceCT provides reliable information on vascular distribution in the mucosa of the nasal airways. Among the bats studied, a nonecholocating pteropodid (Cynopterus sphinx) and an oral-emitter of echolocation sounds (Eptesicus fuscus) possess venous sinus networks that drain into the sphenopalatine vein rostral to the nasopharynx. In contrast, nasopharyngeal passageways of nasal-emitting hipposiderids are notably packed with venous sinuses. The mucosae of the nasopharyngeal passageways are far less vascular in nasal-emitting phyllostomids, in which vascular mucosae are more widely distributed in the nasal cavity, and in some nectar-feeding species, a particularly large venous sinus is adjacent to the vomeronasal organ. Therefore, we do not find a common pattern of venous sinus distribution associated with nasal emission of sounds in phyllostomids and hipposiderids. Instead, vascular mucosa is more likely critical for air-conditioning and sometimes vomeronasal function in all bats.

Digest: New insight into sensory trade-off in phyllostomid bats. Evolution 2021;75:2946-2947. [PMID: 34498264 DOI: 10.1111/evo.14343] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Do the relative size of the olfactory bulb, cochlea, and orbit correlate with diet in phyllostomid bats? Hall et al. (2021) found that the degree of frugivory is positively correlated with the relative size of the olfactory bulb and the orbit. The degree of animalivory is negatively correlated with the relative size of the olfactory bulb and the orbit. Finally, the degree of nectarivory is negatively correlated with the relative size of the cochlea.

Anatomy of the Nasal and Auditory Regions of the Fossil Lagomorph Palaeolagus haydeni: Systematic and Evolutionary Implications

Palaeolagus, a late Eocene to early Miocene North American lagomorph genus, represented by numerous and well-preserved specimens, has been long considered a basal leporid, although it is currently understood as a stem lagomorph. Based on micro-computed tomography (μCT) data and 3D reconstructions, here we present the first description of intracranial structures of the nasal and auditory regions of a complete skull of Palaeolagus haydeni from the early Oligocene of Nebraska. Although Palaeolagus haydeni shows a puzzling mixture of extant leporid and ochotonid characters, it helps to polarize and re-evaluate already known lagomorph intracranial characters based on outgroup comparison with Rodentia and Scandentia. Common derived features of Palaeolagus haydeni and extant Lagomorpha are the dendritic maxilloturbinal and the excavated nasoturbinal that contacts the lamina semicircularis. Generally, Palaeolagus haydeni and Leporidae have several characters in common, some of which are certainly plesiomorphic (e.g., thin wall of bulla tympani and flat conic cochlea). Palaeolagus haydeni resembles Leporidae in having an interturbinal between the two frontoturbinals, and three ethmoturbinals plus one interturbinal between ethmoturbinal I and II. Now, this should also be regarded as a plesiomorphic grundplan pattern for Leporidae whereas ochotonids are derived from the lagomorph grundplan as concerns the number of frontoturbinals. Concerning the middle ear, Palaeolagus haydeni significantly contributes to the polarization of the anterior anchoring of the malleus in extant lagomorphs. Palaeolagus haydeni resembles the pattern observed in early ontogenetic stages of Ochotonidae, i.e., the attachment of the malleus to the ectotympanic via a short processus anterior. The patterns in adult ochotonids and leporids now can be regarded as two different and apomorphic character states. Autapomorphic characters of Palaeolagus haydeni are the reduced frontoturbinal 2 and the additional anterolaterally oriented process of the lamina semicircularis. Interestingly, among the investigated intracranial structures the loss of the secondary crus commune is the only apomorphic grundplan character of crown Lagomorpha.

On the Embryonic Development of the Nasal Turbinals and Their Homology in Bats

Multiple corrugated cartilaginous structures are formed within the mammalian nasal capsule, eventually developing into turbinals. Due to its complex and derived morphology, the homologies of the bat nasal turbinals have been highly disputed and uncertain. Tracing prenatal development has been proven to provide a means to resolve homological problems. To elucidate bat turbinate homology, we conducted the most comprehensive study to date on prenatal development of the nasal capsule. Using diffusible iodine-based contrast-enhanced computed tomography (diceCT), we studied in detail the 3D prenatal development of various bat species and non-bat laurasiatherians. We found that the structure previously identified as “maxilloturbinal” is not the true maxilloturbinal and is only part of the ethmoturbinal I pars anterior. Our results also allowed us to trace the evolutionary history of the nasal turbinals in bats. The turbinate structures are overall comparable between laurasiatherians and pteropodids, suggesting that pteropodids retain the ancestral laurasiatherian condition. The absence of the ethmoturbinal I pars posterior in yangochiropterans and rhinolophoids has possibly occurred independently by convergent evolution.

A unique cricetid experiment in the northern high-Andean Páramos deserves tribal recognition

While hypsodonty mostly is associated with medium to large body sizes in sigmodontine rodents, high-crowned molars combined with small bodies rarely are recorded. This latter condition is present in Neomicroxus (Sigmodontinae, incertae sedis), a genus of high-Andean cricetids also characterized by a noticeable set of cranial traits, including enlarged turbinals and rostrum, slanting zygomatic plate, and a marked backward displacement of the vertical ramus of the dentary, linked with an enlargement of the basicranial region. These morphological features, combined with the isolated position of this lineage in molecular-based phylogenies, indicate that Neomicroxus should be situated in a new tribe. We name and describe this Páramo novelty monotypic clade here. As a working hypothesis, the hypsodonty displayed by this group is considered an evolutionary response to continued volcanic ash falls that characterized the region during the Neogene. A reappraisal of tribe recognition within the two cricetid largest subfamilies, arvicolines and sigmodontines, is made, coupled with a discussion about the role of morphological convergence in “long-nose” cricetids.

Locomotory mode transitions alter phenotypic evolution and lineage diversification in an ecologically rich clade of mammals

The relationship between organismal function and form is a cornerstone of biology because functional diversity is key to generating and maintaining ecological diversity. Morphological changes often occur in unison with behavioral or ecological transitions, and this process may foster diversification, but alternately could trap a species on an adaptive peak. We estimated the most comprehensive phylogenetic hypothesis of Murinae, a young (∼15 million years) and diverse (∼700 species) clade of mammals. We then tested for correlated evolution among four morphological traits with potential links to locomotor modes (Arboreal, General, Terrestrial, and Amphibious), then investigated the effects of locomotion on morphological and lineage diversification. We found unique combinations of trait values for each locomotor mode, including strong covariance between the tail and hindfoot lengths of specialized Arboreal and ecologically flexible General species. Low diversification rates and long branch lengths suggest that specialized lineages represent stable evolutionary "cul-de-sacs." General species, characterized by the classic "rat-like" body plan and broad locomotor abilities, have narrow optimal trait values and slow phenotypic evolution, but high lineage diversification rates. Our findings suggest that versatile, generalist forms act as seeds of species diversity and morphological specialization, which together build ecologically diverse radiations.