Somatosensory Areas of Manatee Cerebral Cortex: Histochemical Characterization and Functional Implications

Open questions in marine mammal sensory research

Although much research has focused on marine mammal sensory systems over the last several decades, we still lack basic knowledge for many of the species within this diverse group of animals. Our conference workshop allowed all participants to present recent developments in the field and culminated in discussions on current knowledge gaps. This report summarizes open questions regarding marine mammal sensory ecology and will hopefully serve as a platform for future research.

Manatee cognition in the wild: an exploration of the manatee mind and behavior through neuroanatomy, psychophysics, and field observations

Cognition refers to the mechanisms for acquiring, processing, storing, and acting on information, all of which are critical to understanding the behavior of animals. These mechanisms are poorly known in manatees, especially how they are expressed in the wild. To expand our understanding of manatee cognition, we gathered information from behavioral experimentation in the laboratory, neuroanatomical research, controlled field studies, integrated laboratory and field measurement, and natural history observations (published reports, written surveys, and interviews with knowledgeable observers). Laboratory research, both neuroanatomical and behavioral, provided the most empirical data, primarily on sensory/perceptual capacities. Inferences from these data and narratives from surveys and interviews illuminated possibilities for higher order cognition. Evidence from field measurements was sparse, although substantial amounts of information have been collected from tracking data and to a lesser extent vessel impact studies, which can be used to infer cognitive attributes. Manatees are tactile-auditory specialists with complementary visual and chemosensory abilities. They demonstrate learning characteristics typical of vertebrates. Movement tracking data plus direct observations suggest that they have good spatial cognition, indicated by their ability to traverse complicated water networks and memory for foraging and warm water sites. They engage in a wide range of play-like, object manipulation, and mimetic behaviors, which suggests cognitive capacities beyond basic associative learning. Understanding manatee cognition beyond the laboratory will be necessary for conservation of manatees as they face challenges such as habitat degradation and threats from water-borne vessel traffic. There is a clear need for more direct research in natural settings.

Orientation selectivity in the visual cortex of the nine-banded armadillo

Orientation selectivity in primary visual cortex (V1) has been proposed to reflect a canonical computation performed by the neocortical circuitry. Although orientation selectivity has been reported in all mammals examined to date, the degree of selectivity and the functional organization of selectivity vary across mammalian clades. The differences in degree of orientation selectivity are large, from reports in marsupials that only a small subset of neurons are selective to studies in carnivores, in which it is rare to find a neuron lacking selectivity. Furthermore, the functional organization in cortex varies in that the primate and carnivore V1 is characterized by an organization in which nearby neurons share orientation preference while other mammals such as rodents and lagomorphs either lack or have only extremely weak clustering. To gain insight into the evolutionary emergence of orientation selectivity, we examined the nine-banded armadillo, a species within the early placental clade Xenarthra. Here we use a combination of neuroimaging, histological, and electrophysiological methods to identify the retinofugal pathways, locate V1, and for the first time examine the functional properties of V1 neurons in the armadillo (Dasypus novemcinctus) V1. Individual neurons were strongly sensitive to the orientation and often the direction of drifting gratings. We uncovered a wide range of orientation preferences but found a bias for horizontal gratings. The presence of strong orientation selectivity in armadillos suggests that the circuitry responsible for this computation is common to all placental mammals.NEW & NOTEWORTHY The current study shows that armadillo primary visual cortex (V1) neurons share the signature properties of V1 neurons of primates, carnivorans, and rodents. Furthermore, these neurons exhibit a degree of selectivity for stimulus orientation and motion direction similar to that found in primate V1. Our findings in armadillo visual cortex suggest that the functional properties of V1 neurons emerged early in the mammalian lineage, near the time of the divergence of marsupials.

Detection of hydrodynamic stimuli by the postcranial body of Florida manatees (Trichechus manatus latirostris)

Manatees live in shallow, frequently turbid waters. The sensory means by which they navigate in these conditions are unknown. Poor visual acuity, lack of echolocation, and modest chemosensation suggest that other modalities play an important role. Rich innervation of sensory hairs that cover the entire body and enlarged somatosensory areas of the brain suggest that tactile senses are good candidates. Previous tests of detection of underwater vibratory stimuli indicated that they use passive movement of the hairs to detect particle displacements in the vicinity of a micron or less for frequencies from 10 to 150 Hz. In the current study, hydrodynamic stimuli were created by a sinusoidally oscillating sphere that generated a dipole field at frequencies from 5 to 150 Hz. Go/no-go tests of manatee postcranial mechanoreception of hydrodynamic stimuli indicated excellent sensitivity but about an order of magnitude less than the facial region. When the vibrissae were trimmed, detection thresholds were elevated, suggesting that the vibrissae were an important means by which detection occurred. Manatees were also highly accurate in two-choice directional discrimination: greater than 90% correct at all frequencies tested. We hypothesize that manatees utilize vibrissae as a three-dimensional array to detect and localize low-frequency hydrodynamic stimuli.

Golgi Analysis of Neuron Morphology in the Presumptive Somatosensory Cortex and Visual Cortex of the Florida Manatee (Trichechus manatus latirostris)

The current study investigates neuron morphology in presumptive primary somatosensory (S1) and primary visual (V1) cortices of the Florida manatee (Trichechus manatus latirostris) as revealed by Golgi impregnation. Sirenians, including manatees, have an aquatic lifestyle, a large body size, and a relatively large lissencephalic brain. The present study examines neuron morphology in 3 cortical areas: in S1, dorsolateral cortex area 1 (DL1) and cluster cortex area 2 (CL2) and in V1, dorsolateral cortex area 4 (DL4). Neurons exhibited a variety of morphological types, with pyramidal neurons being the most common. The large variety of neuron types present in the manatee cortex was comparable to that seen in other eutherian mammals, except for rodents and primates, where pyramid-shaped neurons predominate. A comparison between pyramidal neurons in S1 and V1 indicated relatively greater dendritic branching in S1. Across all 3 areas, the dendritic arborization pattern of pyramidal neurons was also similar to that observed previously in the afrotherian rock hyrax, cetartiodactyls, opossums, and echidnas but did not resemble the widely bifurcated dendrites seen in the large-brained African elephant. Despite adaptations for an aquatic environment, manatees did not share specific neuron types such as tritufted and star-like neurons that have been found in cetaceans. Manatees exhibit an evolutionarily primitive pattern of cortical neuron morphology shared with most other mammals and do not appear to have neuronal specializations for an aquatic niche.

Neuron Types in the Presumptive Primary Somatosensory Cortex of the Florida Manatee (Trichechus manatus latirostris)

Within afrotherians, sirenians are unusual due to their aquatic lifestyle, large body size and relatively large lissencephalic brain. However, little is known about the neuron type distributions of the cerebral cortex in sirenians within the context of other afrotherians and aquatic mammals. The present study investigated two cortical regions, dorsolateral cortex area 1 (DL1) and cluster cortex area 2 (CL2), in the presumptive primary somatosensory cortex (S1) in Florida manatees (Trichechus manatus latirostris) to characterize cyto- and chemoarchitecture. The mean neuron density for both cortical regions was 35,617 neurons/mm(3) and fell within the 95% prediction intervals relative to brain mass based on a reference group of afrotherians and xenarthrans. Densities of inhibitory interneuron subtypes labeled against calcium-binding proteins and neuropeptide Y were relatively low compared to afrotherians and xenarthrans and also formed a small percentage of the overall population of inhibitory interneurons as revealed by GAD67 immunoreactivity. Nonphosphorylated neurofilament protein-immunoreactive (NPNFP-ir) neurons comprised a mean of 60% of neurons in layer V across DL1 and CL2. DL1 contained a higher percentage of NPNFP-ir neurons than CL2, although CL2 had a higher variety of morphological types. The mean percentage of NPNFP-ir neurons in the two regions of the presumptive S1 were low compared to other afrotherians and xenarthrans but were within the 95% prediction intervals relative to brain mass, and their morphologies were comparable to those found in other afrotherians and xenarthrans. Although this specific pattern of neuron types and densities sets the manatee apart from other afrotherians and xenarthrans, the manatee isocortex does not appear to be explicitly adapted for an aquatic habitat. Many of the features that are shared between manatees and cetaceans are also shared with a diverse array of terrestrial mammals and likely represent highly conserved neural features. A comparative study across manatees and dugongs is necessary to determine whether these traits are specific to one or more of the manatee species, or can be generalized to all sirenians.

Elaboration and Innervation of the Vibrissal System in the Rock Hyrax (Procavia capensis)

Mammalian tactile hairs are commonly found on specific, restricted regions of the body, but Florida manatees represent a unique exception, exhibiting follicle-sinus complexes (FSCs, also known as vibrissae or tactile hairs) on their entire body. The orders Sirenia (including manatees and dugongs) and Hyracoidea (hyraxes) are thought to have diverged approximately 60 million years ago, yet hyraxes are among the closest relatives to sirenians. We investigated the possibility that hyraxes, like manatees, are tactile specialists with vibrissae that cover the entire postfacial body. Previous studies suggested that rock hyraxes possess postfacial vibrissae in addition to pelage hair, but this observation was not verified through histological examination. Using a detailed immunohistochemical analysis, we characterized the gross morphology, innervation and mechanoreceptors present in FSCs sampled from facial and postfacial vibrissae body regions to determine that the long postfacial hairs on the hyrax body are in fact true vibrissae. The types and relative densities of mechanoreceptors associated with each FSC also appeared to be relatively consistent between facial and postfacial FSCs. The presence of vibrissae covering the hyrax body presumably facilitates navigation in the dark caves and rocky crevices of the hyrax's environment where visual cues are limited, and may alert the animal to predatory or conspecific threats approaching the body. Furthermore, the presence of vibrissae on the postfacial body in both manatees and hyraxes indicates that this distribution may represent the ancestral condition for the supraorder Paenungulata.

Manatee vibrissae: evidence for a "lateral line" function

Aquatic mammals use vibrissae to detect hydrodynamic stimuli over a range from 5 to 150 Hz, similar to the range detected by lateral line systems in fishes and amphibians. Manatees possess ∼5,300 vibrissae distributed over the body, innervated by ∼209,000 axons. This extensive innervation devoted to vibrissae follicles is reflected in enlarged, elaborate somatosensory regions of the gracile, cuneate, and Bischoff's brain-stem nuclei, ventrobasal thalamus, and presumptive somatosensory cortex. Our preliminary psychophysical testing indicates that in Florida and Antillean manatees the Weber fraction for detection thresholds for grating textures ranges from 0.025 to 0.14. At the lower end of this range, sensitivity is comparable to human index finger thresholds. For hydrodynamic stimuli of 5-150 Hz, detection threshold levels for manatees using facial or postfacial vibrissae were substantially lower than those reported for harbor seals and similar to reports of sensitivity for the lateral line systems of some fish. Our findings suggest that the facial and postfacial vibrissae are used to detect hydrodynamic stimuli, whereas only the facial vibrissae are used for direct contact investigation.

The neocortex of cetaceans: cytoarchitecture and comparison with other aquatic and terrestrial species

The evolutionary process of readaptation to the aquatic environment was accompanied by extreme anatomical and physiological changes in the brain. This review discusses cortical specializations in the three major lineages of marine mammals in comparison to related terrestrial and semiaquatic species. Different groups of marine mammals adopted a wide range of strategies to cope with the challenges of aquatic living. Cetaceans and hippopotamids possess a completely agranular neocortex in contrast to phocids and sirenians; vertical modules are observed in deep layers V and VI in manatees, cetaceans, phocids, and hippopotamids, but in different cortical areas; and clustering in layer II appears in the insular cortex of hippopotamids, phocids, and cetaceans. Finally, von Economo neurons are present in cetaceans, hippopotamids, sirenians, and some phocids, with specific, yet different, cortical distributions. The interpretation of the evolutionary and functional significance of such specializations, and their relationships with the degrees of adaptation to the aquatic environment and phylogeny, remain difficult to trace, at least until comprehensive data, including representative species from all of the major mammalian families, become available.

Neocortical neuron types in Xenarthra and Afrotheria: implications for brain evolution in mammals

Interpreting the evolution of neuronal types in the cerebral cortex of mammals requires information from a diversity of species. However, there is currently a paucity of data from the Xenarthra and Afrotheria, two major phylogenetic groups that diverged close to the base of the eutherian mammal adaptive radiation. In this study, we used immunohistochemistry to examine the distribution and morphology of neocortical neurons stained for nonphosphorylated neurofilament protein, calbindin, calretinin, parvalbumin, and neuropeptide Y in three xenarthran species-the giant anteater (Myrmecophaga tridactyla), the lesser anteater (Tamandua tetradactyla), and the two-toed sloth (Choloepus didactylus)-and two afrotherian species-the rock hyrax (Procavia capensis) and the black and rufous giant elephant shrew (Rhynchocyon petersi). We also studied the distribution and morphology of astrocytes using glial fibrillary acidic protein as a marker. In all of these species, nonphosphorylated neurofilament protein-immunoreactive neurons predominated in layer V. These neurons exhibited diverse morphologies with regional variation. Specifically, high proportions of atypical neurofilament-enriched neuron classes were observed, including extraverted neurons, inverted pyramidal neurons, fusiform neurons, and other multipolar types. In addition, many projection neurons in layers II-III were found to contain calbindin. Among interneurons, parvalbumin- and calbindin-expressing cells were generally denser compared to calretinin-immunoreactive cells. We traced the evolution of certain cortical architectural traits using phylogenetic analysis. Based on our reconstruction of character evolution, we found that the living xenarthrans and afrotherians show many similarities to the stem eutherian mammal, whereas other eutherian lineages display a greater number of derived traits.

Somatosensory nuclei of the manatee brainstem and thalamus

Florida manatees have an extensive, well-developed system of vibrissae distributed over their entire bodies and especially concentrated on the face. Although behavioral and anatomical assessments support the manatee's reliance on somatosensation, a systematic analysis of the manatee thalamus and brainstem areas dedicated to tactile input has never been completed. Using histochemical and histological techniques (including stains for myelin, Nissl, cytochrome oxidase, and acetylcholinesterase), we characterized the relative size, extent, and specializations of somatosensory regions of the brainstem and thalamus. The principal somatosensory regions of the brainstem (trigeminal, cuneate, gracile, and Bischoff's nucleus) and the thalamus (ventroposterior nucleus) were disproportionately large relative to nuclei dedicated to other sensory modalities, providing neuroanatomical evidence that supports the manatee's reliance on somatosensation. In fact, areas of the thalamus related to somatosensation (the ventroposterior and posterior nuclei) and audition (the medial geniculate nucleus) appeared to displace the lateral geniculate nucleus dedicated to the subordinate visual modality. Furthermore, it is noteworthy that, although the manatee cortex contains Rindenkerne (barrel-like cortical nuclei located in layer VI), no corresponding cell clusters were located in the brainstem ("barrelettes") or thalamus ("barreloids").

Topographical organization of the facial motor nucleus in Florida manatees (Trichechus manatus latirostris)

Florida manatees (Trichechus manatus latirostris) possess modified vibrissae that are used in conjunction with specialized perioral musculature to manipulate vegetation for ingestion, and aid in the tactile exploration of their environment. Therefore it is expected that manatees possess a large facial motor nucleus that exhibits a complex organization relative to other taxa. The topographical organization of the facial motor nucleus of five adult Florida manatees was analyzed using neuroanatomical methods. Cresyl violet and hematoxylin staining were used to localize the rostrocaudal extent of the facial motor nucleus as well as the organization and location of subdivisions within this nucleus. Differences in size, length, and organization of the facial motor nucleus among mammals correspond to the functional importance of the superficial facial muscles, including perioral musculature involved in the movement of mystacial vibrissae. The facial motor nucleus of Florida manatees was divided into seven subnuclei. The mean rostrocaudal length, width, and height of the entire Florida manatee facial motor nucleus was 6.6 mm (SD 8 0.51; range: 6.2-7.5 mm), 4.7 mm (SD 8 0.65; range: 4.0-5.6 mm), and 3.9 mm (SD 8 0.26; range: 3.5-4.2 mm), respectively. It is speculated that manatees could possess direct descending corticomotorneuron projections to the facial motornucleus. This conjecture is based on recent data for rodents, similiarities in the rodent and sirenian muscular-vibrissal complex, and the analogous nature of the sirenian cortical Rindenkerne system with the rodent barrel system.

Adaptations in the structure and innervation of follicle-sinus complexes to an aquatic environment as seen in the Florida manatee (Trichechus manatus latirostris)

Florida manatees are large-bodied aquatic herbivores that use large tactile vibrissae for several purposes. Facial vibrissae are used to forage in a turbid water environment, and the largest perioral vibrissae can also grasp and manipulate objects. Other vibrissae distributed over the entire postfacial body appear to function as a lateral line system. All manatee vibrissae emanate from densely innervated follicle-sinus complexes (FSCs) like those in other mammals, although proportionately larger commensurate with the caliber of the vibrissae. As revealed by immunofluorescence, all manatee FSCs have many types of C, Adelta and Abeta innervation including Merkel, club, and longitudinal lanceolate endings at the level of the ring sinus, but they lack other types such as reticular and spiny endings at the level of the cavernous sinus. As in non-whisking terrestrial species, the inner conical bodies of facial FSCs are well innervated but lack Abeta-fiber terminals. Importantly, manatee FSCs have two unique types of Abeta-fiber endings. First, all of the FSCs have exceptionally large-caliber axons that branch to terminate as novel, gigantic spindle-like endings located at the upper ring sinus. Second, facial FSCs have smaller caliber Abeta fibers that terminate in the trabeculae of the cavernous sinus as an ending that resembles a Golgi tendon organ. In addition, the largest perioral vibrissae, which are used for grasping, have exceptionally well-developed medullary cores that have a structure and dense small-fiber innervation resembling that of tooth pulp. Other features of the epidermis and upper dermis structure and innervation differ from that seen in terrestrial mammals.