INTRODUCTION:Many, but not all, bird taxa possess the ability to see ultraviolet (UV) light, but it is currently unclear the extent this ability exists among seabirds. Measuring retinal photoreceptor sensitivity presents many challenges, but the penetration of UV light through the ocular media (ocular transmittance) is a good proxy for UV vision. METHODS:Here, we document the ocular transmission of light wavelengths using spectrometry through the eyes of eight Procellariiform and Suliform seabird species found in Aotearoa New Zealand. RESULTS:We report that the eyes of most Procellariiformes, but not the Suliformes, can transmit UV wavelengths. Thus, there is the potential for those Procellariiformes to perceive UV. DISCUSSION:Both phylogeny and ecology could play a role in UV vision in seabirds and understanding the wavelengths that seabirds perceive can be crucial for conservation against visual threats.
INTRODUCTION:Star-nosed moles are renowned as the fastest foragers among mammals, able to identify and eat small prey in less than a quarter of a second. This ability stems in part from the mole's extraordinary mechanosensory star which has been the focus of many investigations. However, fast eating also requires a specialized motor system and associated structures. Here, the mole's unusual incisors are explored as a key adaptation for efficient foraging. METHODS:High-speed videos of foraging moles, including microscopic views at 1,000 frames per second, were used to measure prey handling time and tooth movements. Scanning electron microscopy was used to assess tooth structure. Specimens from Cornell Museum of Vertebrates were examined with light microscopy. Data from previous investigations were compared to the present results. RESULTS:A mole with worn front teeth was discovered, and this specimen often failed to secure small prey efficiently, thus doubling the mole's handling time compared to normal specimens. The manner in which the worn teeth failed suggested the mole's normal incisors are analogous to a specific type of man-made surgical forceps - so-called Yaşargil tumor forceps. CONCLUSION:The results reveal an example of serendipitous biomimicry by human surgeons in designing soft tissue forceps, highlight the importance of motor specializations in the star-nosed mole's fast foraging ability, and suggest some of the specific anatomical specialization that are the result of selection on the key variables (space clearance rate and handling time) in Holling's pioneering foraging theory equation.
Introduction: Ontogenetic brain growth in cetaceans is essential for understanding their development and evolution. This study investigates brain size changes relative to body growth in bowhead (Balaena mysticetus) and beluga (Delphinapterus leucas) whales in the framework of age estimates of pre- and postnatal specimens. Methods: We collected specimens in the field, determined brain size and endocranial volumes, as well as size of endocranial adnexa, either by direct measurement or by CT. We estimated age using baleen length (bowhead), growth layers in teeth (belugas), or fetal stages. We fitted Gompertz growth models to our data. Results: Our findings show that both bowhead and beluga whales reach nearly their full brain size by the end of weaning, unlike dolphins and humans, whose brains continue growing after weaning. Bowhead brains grow faster than those of belugas, and much faster than those of humans, and their rete mirabile occupies a much larger portion of the cranial cavity than in belugas. Encephalization quotients decline with age due to continued body growth after brain maturation. Conclusion: Brain growth in these cetacean species plateaus early, challenging the assumption that cetacean brains grow throughout life. In bowhead, the brain is significantly smaller than the cranial cavity, and this is not the case in beluga. If this observation can be generalized to all mysticetes and odontocetes, it implies that no single equation can capture the proportional volumes of the brain and cranial cavity across the entire cetacean clade.
INTRODUCTION:We reanalyzed data from Sobrero et al. [Brain Behav Evol. 2016;87:51-64] to evaluate dorsal dentate gyrus (dDG) folding or gyrification as a morphological proxy for neurogenesis and macrostructural plasticity in hippocampal or extracortical regions of wild rodents. METHODS:Using Zilles' (ZGI) and Vogeley's (VGI) indices, we quantified dDG gyrification across hemispheres in Octodon degus and Octodon lunatus, comparing populations that differ in habitat complexity and social behavior. RESULTS:Left dDG gyrification (ZGIL) showed a preliminary association with social group size, whereas right dDG gyrification (VGIR) was predicted by population differences. O. lunatus from shrub-dense environments exhibited greater dDG folding than O. degus from open habitats. Although not statistically significant, hemispheric asymmetries were suggested, consistent with previously reported right-lateralized DG cell numbers in O. lunatus and habitat-sociality effects on DG cell counts in O. degus from El Salitre. CONCLUSION:These results support dDG gyrification as an informative marker of neural plasticity shaped by habitat conditions and emphasize the value of wild models in brain-ecology research.
INTRODUCTION:Over the last century, sea surface temperatures have increased by more than 0.5°C, with predictions suggesting an increase of 1-4°C by 2100. Oceanic warming poses significant challenges to marine species, particularly those with physiological and developmental processes that are tightly linked to environmental conditions. In cartilaginous fishes, including sharks, the brain grows continually throughout life, supported by the capacity for lifelong neurogenesis. This feature suggests that the nervous system - both peripheral (sensory) and central (brain) - of sharks may be highly plastic and able to adapt dynamically to a changing environment. METHODS:We investigated the effects of elevated rearing temperature on brain development in the epaulette shark (Hemiscyllium ocellatum), a species known for its tolerance of environmental fluctuations in intertidal habitats. Eggs (n = 12) were sourced from a breeding stock at the New England Aquarium and reared in either ambient (27°C) or elevated (31°C, 4°C above ambient) seawater temperatures until 2 months post-hatch. Using histological analyses, we compared the relative volume of the nose (olfactory rosette), total brain, and major brain regions between treatment groups. RESULTS:Despite this species' natural exposure to temperature variability, generalized linear models revealed that elevated temperature significantly altered the volume of the olfactory sensory epithelium, olfactory bulbs, and medulla oblongata after accounting for overall brain size. Analyses of proportional brain region volumes also showed that elevated temperature was associated with reduced olfactory bulb size and increased subpallial volume relative to total brain size. These differences may suggest potential changes in cognitive capacity related to olfactory processing as well as sensory and/or motor functions at elevated temperatures. CONCLUSIONS:While short-term studies, such as this one, cannot capture long-term adaptive potential, understanding the impacts of elevated temperature on brain phenotypes provides critical insights into how elasmobranchs may cope with changing ocean conditions. Such knowledge will be vital for predicting the resilience of these ecologically important species to future environmental stressors.
Background: The mammalian auditory cortex can be parcellated into multiple functional subfields, with each subfield making a distinct contribution to sound processing. For example, primary fields consisting of primary auditory cortex and anterior auditory field are the first to receive information from the thalamus, and the neurons in each of these fields have different properties in terms of latency and response duration. Non-primary auditory fields consist of secondary auditory cortex, which is involved in object recognition and emotional conditioning, while dorsal auditory fields are more responsive during locomotion and spatial tasks. What is currently unknown is how the structure of each auditory cortical subfield relates to function. Thus, it is imperative to understand how anatomical substrates that make up each field contribute to function. Summary: In this review, we suggest that myelin may serve as a structural anchor for the organization of auditory cortical subfields. Myelination patterns among mammals that have been studied (primates, carnivores, rodents, and bats) show that primary auditory cortical fields are more heavily myelinated than non-primary auditory cortical fields, and upper cortical layers are less myelinated than middle and deep layers. Myelin also demonstrates experience-dependent plasticity and can be measured with a variety of invasive and noninvasive methods, and demyelination has been linked to cognitive decline. Conversely, the archicortex is lightly myelinated, and we speculate that because myelin inhibits axonal and synaptic plasticity, it would not be advantageous for the archicortex to be myelin-dense, as it has greater requirements for flexibility for learning and memory. Key Messages: This is the first review, to our knowledge, that uses a comprehensive comparative approach across mammals to determine the distribution of myelin across auditory cortical subfields. We argue that a detailed map of the myeloarchitecture of the auditory cortex must be directly aligned to the functional maps of the auditory cortex to account for individual variability and identify subfields accurately. Furthermore, myelin maps need to be compared with other anatomical markers as well to improve our understanding of the role of myelin. Finally, a detailed histological myelin map can serve as a ground truth for comparisons to noninvasive measures of myelin.
INTRODUCTION:Wet dog shake (WDS) is a motion in mammals and birds, consisting in vigorous and rapid rotations of the head and trunk around the spinal axis, which allows them to dry themselves. WDS requires fine balance control. To date, motor control in WDS has not been studied. METHODS:Here, for the first time, we investigated the trunk and limbs muscle EMG activity and correlated it with the kinematics of body movement and ground reactions force during WDS in rats. RESULTS:Strict reciprocity was revealed between the forelimb muscle on the right and left sides despite bipedal hindlimb position. Reciprocal activity was observed between the lumbar and the thoracic segments. The hindlimb muscle activity exhibited two distinct muscle synergies with strict reciprocity and atypical co-activity of flexors and extensors, which were previously observed in paw shaking behavior. These two synergies correlate with the two muscle groups of the pelvic fins of fish. The absence of typical postural responses of the hindlimb was revealed. CONCLUSIONS:(1) It is likely that WDS and paw shaking share a common nervous control. (2) The absence of typical postural responses may indicate that body balance in WDS is maintained by perfectly matched frequency and strength of the trunk muscle contractions. (3) In the hypothesis about the origin of WDS, based on the revealed characteristics, we compare it with the S-start response behavior in fish.
INTRODUCTION:Zebrin II (ZII) is a glycolytic enzyme that is expressed in cerebellar Purkinje cells. In both mammals and birds, ZII is expressed heterogeneously, such that there are sagittal stripes of Purkinje cells with a high ZII expression (ZII+) alternating with stripes of Purkinje cells with little or no expression (ZII-). To date, ZII expression studies examined at least one species from most of the major branches of the avian phylogeny including Paleognatha (tinamous, kiwi), Galloanseres (chicken), Columbaves (pigeon), and Elementaves (hummingbird). In this regard, the most glaring omission is that a species from Telluraves, a clade that contains 75% of all avian species, has not been studied. METHODS:In this paper, we examined ZII expression in the zebra finch Taeniopygia castanotis (order Passeriformes). Given that Telluraves have evolved sophisticated hindlimb movements associated with the jump to arboreality, we hypothesized that ZII expression would differ in those areas of the cerebellum that have a strong representation of the hindlimbs, namely folia II-V and IX. RESULTS:Contrary to our prediction, we found that the pattern of ZII expression in the cerebellum is highly similar to that observed in other bird species. In folium I, all Purkinje cells are ZII+. In the rest of the anterior lobe (folia II-V) there are 4 pairs of ZII+/- stripes. In the posterior lobe, folia VI-VII all Purkinje cells are ZII+, in folia VIII-IXcd there are 5-7 pairs of ZII+/- stripes, and in folium X all Purkinje cells are ZII+. Moreover, the expression of ZII+ in Purkinje cell terminals in the cerebellar and vestibular nuclei was similar to that observed in other species. CONCLUSION:These data indicate that the pattern of heterogeneous expression of ZII in cerebellar Purkinje is likely conserved across the entirety of the avian phylogenetic tree.
Introduction: Cervical and lumbar enlargements involving several spinal segments are present in the spinal cord of tetrapods, reflecting the heavy motor and sensory innervation of limbs. Such spinal enlargements are not apparent in teleost fishes. However, teleosts possess paired pectoral and pelvic fins that are homologous to forelimbs and hindlimbs, respectively, and modest spinal enlargements might be present in teleosts as well. METHODS:In the present study, therefore, we have investigated the innervation of different fins by spinal nerves in zebrafish. We then investigated the changes in transverse sectional areas of the spinal cord and gray matter, referring to the levels of spinal cord innervating different fins. RESULTS:These analyses revealed that enlargements of the spinal cord and gray matter are indeed present for pectoral and pelvic fins that are paired appendages like limbs in tetrapods. In addition, enlargements are also present for the dorsal, anal, and caudal fins. CONCLUSION:The present study thus suggests that spinal enlargements are present also in teleosts, although they are modest and can only be detected by analyses at the histological level. The present study also indicates that enlargements can be formed not only for paired fins that are homologous to limbs of tetrapods but also for unpaired fins. That is, spinal enlargements are present for all appendages or fins in teleosts. .
INTRODUCTION:The detection of novelty is a cognitive ability that is fundamental to survival. Following detection, a decision must be made to either approach (neophilia) or avoid (neophobia) the novel stimulus. The tendency to choose one strategy over the other is referred to as an animal's neotic preference. To date, the bulk of research reports that mammals are neophilic, while birds tend to be neophobic. These data, however, are differentiated not only by the class of animal (i.e., Mammalia vs. Aves), but also by the testing methods used, namely the context in which testing occurs. METHOD:To disentangle these factors, we assessed the reaction to novelty in two commonly used domesticated species, rats and pigeons, within two different contexts, a novel testing arena (common for mammals) and within the home cage (common for birds). RESULTS:Here, we show that both rats and pigeons show neophobia in the home cage and neophilia in a testing arena, demonstrating that some degree of the differences previously reported are likely due to testing protocols. Moreover, individual scores in one testing protocol did not predict testing scores in the other. CONCLUSION:These results limit the ability to: (a) compare findings across these paradigms and (b) conceive of neotic preference as a single stable trait across multiple (especially novel) contexts.
INTRODUCTION:The inner ear is a complex three-dimensional structure responsible for the detection of sound, balance, and acceleration. Detailed knowledge about the development of the inner ear of gnathostomes (jawed vertebrates) comes from studies in bony fishes and tetrapods, but comparable information about this process in chondrichthyans, the oldest gnathostome radiation, is lacking. This study describes for the first time the embryonic development of the inner ear and its innervation in the catshark Scyliorhinus canicula. METHODS:By using molecular markers of proliferating cells, migrating neuroblasts, and early differentiating neurons and genes expressed in placode-derived sensory neurons (NeuroD) and inner ear sensory patches (Sox2), we have established the spatiotemporal developmental pattern of the catshark inner ear also observed with micro-CT, and we have characterized developing sensory patches and described the establishment of the inner ear innervation. RESULTS:The development of the catshark inner ear takes place by invagination of the otic placode, as revealed by the expression of NeuroD at very early stages. From the very simple initial epithelial structure, the otic epithelium gradually grows and subdivides to form a complex three-dimensional labyrinth already recognizable at early stage 32. At this stage, the anterior semicircular canal and the horizontal semicircular canal of the catshark meet and fuse over the utricular concurrently with the beginning of the maturation of the inner ear sensory organs. We also show that the endolymphatic duct is formed as consequence of the invagination process; that the primary neurons of the statoacoustic ganglion originate by delamination from the otic epithelium, as in other vertebrates; that inner ear innervation starts when fibers immunoreactive to DCX link the otic cup to the brain at stage 20; and that the innervation pattern is completed at stage 32. CONCLUSION:Present results provide cytological data on developmental changes that may be helpful for comparison with the development of this sensory system in other vertebrates and thus to gain knowledge on the evolution of the development of the inner ear.
INTRODUCTION:Chondrichthyans represent some of the earliest diverging lineages of jawed vertebrates, making them key models for studying the evolution of vertebrate brains. Despite their evolutionary significance, Mediterranean species remain understudied. This research focuses on the speckled skate (Raja polystigma), an endemic Mediterranean benthic species with distinct life history traits, such as bathymetric segregation and postnatal shifts in diet. These traits provide a unique opportunity to explore how ecological factors influence postnatal brain development and neuroecological adaptation in cartilaginous fishes. METHODS:We examined the allometric relationship between brain mass and body mass in postnatal individuals of R. polystigma and assessed the relative growth of major brain regions, including the olfactory bulbs, telencephalon, diencephalon, optic tectum, cerebellum, and medulla oblongata. Data were analyzed using log-transformed linear regressions to determine differential growth rates and patterns of regional specialization during development. RESULTS:Our analysis revealed that brain growth scales with negative allometry relative to body mass, indicating a slowdown in brain growth as individuals mature. Region-specific trends showed that the olfactory bulbs, cerebellum, and medulla oblongata grow at a faster rate than the rest of the brain, suggesting enhanced development of sensory and motor capacities. Conversely, the optic tectum exhibited slower growth, implying a reduced visual reliance in adults. The telencephalon and diencephalon scaled isometrically with brain mass, suggesting stable roles in cognitive and integrative functions throughout postnatal development. CONCLUSION:These findings highlight how ecological and behavioral shifts during development shape brain organization in R. polystigma. Enhanced growth of non-visual sensory regions and motor centers may reflect adaptations to a benthic lifestyle and bathymetric niche. This study contributes to our understanding of neuroecological evolution in Mediterranean chondrichthyans and underscores the value of R. polystigma as a model for investigating brain development in relation to ecological specialization.
INTRODUCTION:This study analyzes the expression of the transcription factor orthopedia (Otp) in the alar hypothalamus and its evolutionary relationship with the amygdaloid complex. METHODS:Immunofluorescence analysis was used in several representative vertebrates, including sarcopterygians (mice, chickens, turtles, anuran amphibians, and lungfish) and actinopterygian fish (teleosts and polypteriforms). RESULTS:We reveal highly conserved Otp expression in all species used, supporting its critical role in hypothalamic regional specification and in the development of neuroendocrine cells and the amygdaloid complex. Our results show that hypothalamic radial migration of Otp contributes to amygdaloid populations, particularly in those with subpallial origin, in a highly conserved manner from basal actinopterygians. CONCLUSION:Differences between sarcopterygians and actinopterygians in the Otp expression patterns in cells migrated to the pallial amygdala highlight an evolutionary divergence, particularly in the complexity and cellular composition of this region, tracing its evolutionary emergence by using the studied species as reference. Moreover, present results emphasize the evolutionary and functional importance of hypothalamic-amygdaloid interactions.
INTRODUCTION:Domestication and subsequent breed selection has significantly changed the phenotype of most domesticated animal species. Not only has their external appearance changed, in many species, the brain and individual brain regions often differ in size in domesticated strains compared with their wild ancestors. Although the majority of studies on mammals focus on cortical regions, the cerebellum often differs in relative and absolute size between domestic and wild strains, but more specific data on cell sizes and numbers are often lacking. METHODS:We quantified cerebellar anatomy in two domesticated strains (Long-Evans and Sprague-Dawley) and one wild type of brown rat (Rattus norvegicus). Using unbiased stereology, we measured the total cerebellum and its layers' volumes, as well as the number and size of Purkinje cells. RESULTS:Long-Evans rats had a larger total cerebellum volume, in both absolute and relative terms, than Sprague-Dawley and wild rats, but no other significant differences were detected. Significant differences in the absolute and relative sizes of the molecular, granule cell, and white matter layers were also found, but the differences were inconsistent among strains such that the largest values alternated between the two laboratory strains. The absolute number of Purkinje cells did not differ among strains, but one population of Sprague-Dawley rats and the wild rats had more Purkinje cells relative to cerebellar volume. Last, Long-Evans rats had significantly smaller Purkinje cells than the other strains in both absolute and relative terms. CONCLUSION:Only one of the two domesticated strains differed from wild rats in cerebellar anatomy. Our results therefore demonstrate that changes in the brains of domesticated animals do not necessarily follow a universal rule; they can vary between different strains. This highlights the importance of including more than one strain in wild-domesticate comparisons in brain anatomy and avoiding the oversimplification of the effects of domestication on the brain.
Introduction: Songbirds, especially corvids, and parrots are remarkably intelligent. Their cognitive skills are on par with primates and their brains contain primate-like numbers of neurons concentrated in high densities in the telencephalon. Much less is known about cognition and neuron counts in more basal bird lineages. Here, we focus on brain cellular composition of galliform birds, which have small brains relative to body size and a proportionally small telencephalon and are often perceived as cognitively inferior to most other birds. METHODS:We use the isotropic fractionator to assess quantitatively the numbers and distributions of neurons and nonneuronal cells in 15 species of galliform birds and compare their cellular scaling rules with those of songbirds, parrots, marsupials, insectivores, rodents, and primates. RESULTS:On average, the brains of galliforms contain about half the number of neurons found in parrot and songbird brains of the same mass. Moreover, in contrast to these birds, galliforms resemble mammals in having small telencephalic and dominant cerebellar neuronal fractions. Consequently, galliforms have much smaller absolute numbers of neurons in their forebrains than equivalently sized songbirds and parrots, which may limit their cognitive abilities. However, galliforms have similar neuronal densities and neuron counts in the brain and forebrain as equally sized non-primate mammals. Therefore, it is not surprising that cognitive abilities of galliforms are on par with non-primate mammals in many domains. CONCLUSION:Comparisons performed in this study demonstrate that birds representing distantly related clades markedly differ in neuronal densities, neuron numbers, and the allocation of brain neurons to major brain divisions. In analogy with the concept of volumetric composition of the brain, known as the cerebrotype, we conclude that distantly related birds have distinct neuronal cerebrotypes. .
INTRODUCTION:A central question about the evolution of social behavior is how extensive diversity can arise when behaviors depend on shared neural, molecular, and hormonal mechanisms. Comparing close relatives can offer insights into which components of shared mechanisms are most evolvable. METHODS:We discriminate between two nonexclusive hypotheses by which conserved neural mechanisms might evolve to generate differences in social behavior: changes in the number or activity of neurons. We test these hypotheses in two recently diverged ecotypes of threespine stickleback (Gasterosteus aculeatus); the common ecotype provides parental care, while the white ecotype does not. We used double-label fluorescent immunohistochemistry with pS6, a marker of transcriptionally active neurons, to quantify the number and activity of two preoptic neuropeptidergic cell types that affect parental care across vertebrates: galanin (Gal) and oxytocin (OXT). RESULTS:Ecotypes did not differ in the overall activity of the preoptic area or the number of Gal and OXT neurons but did differ in the activity of Gal and OXT neurons. The activity of these neurons changed across reproductive stages in the common but not the white ecotype. Activity peaked after mating in commons when males began to care for their offspring, suggesting that changes in the activity of these specific preoptic neurons are required to transition from courtship to parenting. CONCLUSION:Overall, our study suggests that rapid behavioral evolution occurred via changes in the activity but not the number of specific preoptic neuropeptidergic neurons.
INTRODUCTION:The factors shaping vertebrate brain evolution and cognition are broadly categorized as being either social or environmental. Yet, their relative importance is debated, partly due to the limitations associated with standard interspecific evolutionary comparisons. Here, we adopt a complementary strategy leveraging within-population variation in fish brain size to ask how variation in social and environmental factors correlates with individual brain size. METHODS:We investigated how overall brain size and brain part sizes varied between demes of the same population in the coral reef-associated batu coris Coris batuensis. This species is ideal for our approach because its local population densities are dissociated from both interspecific densities and habitat complexity. RESULTS:We found that individuals from demes with higher population densities possess larger overall brain volumes than those from lower population density environments, caused by an enlargement of all five main brain regions. Brain anatomical measures show no correlation with interspecific density or habitat complexity. CONCLUSION:Our results suggest that variation in intraspecific social challenges is selected on individual batu coris brain size, either through phenotypic plasticity, differential survival, or habitat choice. These results conform with a broader version of the social brain hypothesis, emphasizing the importance of the entire brain over specific regions like the neocortex in mammals or the telencephalon in fishes.
Introduction: The plainfin midshipman fish (Porichthys notatus) relies on the production and reception of social acoustic signals for reproductive success. During spawning, male midshipman fish produce long duration advertisement calls to attract females, which use their auditory sense to locate and access calling males. While seasonal changes based on reproductive state in inner-ear auditory sensitivity and frequency encoding in midshipman are well documented, little is known about reproductive-state-dependent changes in central auditory sensitivity and auditory neural responsiveness to conspecific advertisement calls. Previous research indicates that forebrain dopaminergic neurons are preferentially active in response to conspecific advertisement calls and during female auditory-driven behavior in the breeding season. These dopamine neurons project to both the inner ear and central auditory nuclei and contribute to regulation of inner-ear auditory sensitivity based on reproductive state. The present study tested the hypothesis that exposure to the male advertisement call would elicit differential activation in auditory brain nuclei and in the forebrain auditory-projecting dopaminergic nucleus in reproductive versus non-reproductive male midshipman. Methods: Fish were collected during the spring reproductive and winter non-reproductive months and were exposed to a playback of the advertisement call or ambient noise (control). Immunohistochemistry identified activated neurons (pS6-ir; proxy for neural activation) in midbrain and forebrain auditory and dopaminergic nuclei. Results and Conclusions: Our results revealed that in key auditory and dopaminergic areas, the greatest activation (most pS6-ir cells) occurred in reproductive males exposed to the advertisement call.
Prairie voles (Microtus ochrogaster) are one of the few mammalian species that are monogamous and engage in the biparental rearing of their offspring. Biparental care impacts the quantity and quality of care the offspring receives. The increased attention by the father may translate to heightened tactile contact the offspring receives through licking and grooming. In the current study, we used electrophysiological multiunit recording techniques to define the organization of the perioral representation in the primary somatosensory area (S1) of prairie voles. Functional representations were related to myeloarchitectonic boundaries. Our results show that most of S1 is occupied by the representation of the contralateral mystacial whiskers and the lower and upper lips. The mystacial vibrissae representation encompassed a large portion of the caudolateral S1, while the representation of the lower and upper lips occupied a large portion of the rostrolateral aspect of S1. We found that neuronal populations representing the perioral structures tended to have small receptive fields relative to other body part representations on the head. The representation of the mystacial whiskers and perioral structures was coextensive with cytoarchitectonically defined barrel fields that extend from the caudolateral to a rostrolateral aspect of S1. We discuss our findings in the context of the magnification of behaviorally relevant sensory surfaces in other rodents, the ubiquity of the barrel systems in rodents, and behaviors associated with specialized sensory surfaces.
INTRODUCTION:Raoellidae are small artiodactyls retrieved from the middle Eocene of Asia (ca. -47 Ma) and closely related to stem Cetacea. Morphological observations of their endocranial structures allow for outlining some of the early steps of the evolutionary history of the cetacean brain. The external features of the brain and associated sinuses of Raoellidae are so far only documented by the virtual reconstruction of the endocast based on specimens of the species Indohyus indirae. These specimens are however too deformed to fully access the external morphology, surface area, and volume measurements of the brain. METHODS:We bring here new elements to the picture of the raoellid brain by an investigation of the internal structures of an exceptionally well-preserved cranium collected from the Kalakot area (Jammu and Kashmir, India) referred to the species Khirtharia inflata. Micro-CT scan investigation and virtual reconstruction of the endocast and associated sinuses of this specimen provide crucial additional data about the morphological diversity within Raoellidae as well as reliable linear, surfaces, and volumes measurements, allowing for quantitative studies. RESULTS:We show that, like I. indirae, the brain of K. inflata exhibits a mosaic of features observed in earliest artiodactyls: a small neocortex with simple folding pattern, widely exposed midbrain, and relatively long cerebellum. But, like Indohyus, the brain of Khirtharia shows unique derived characters also observed in stem cetaceans: narrow elongated olfactory bulbs and peduncles, posterior location of the braincase in the cranium, and complex network of blood vessels around the cerebellum. The volume of the brain relative to body mass of K. inflata is markedly small when compared to other early artiodactyls. CONCLUSION:We show here that cetaceans that nowadays have the second biggest brain after humans derive from a group of animals that had a lower-than-average expected brain size. This is probably a side effect of the adaptation to aquatic life. Conversely, this very small brain size relative to body mass might be another line of evidence supporting the aquatic habits in raoellids.
