Higher neuron densities in the cerebral cortex and larger cerebellums may limit dive times of delphinids compared to deep-diving toothed whales

The putative role of the habenula in animal migration

BACKGROUND

When an addicted animal seeks a specific substance, it is based on the perception of internal and external cues that strongly motivate to pursue the acquisition of that compound. In essence, a similar process acts out when an animal leaves its present area to begin its circannual migration. This review article examines the existence of scientific evidence for possible relatedness of migration and addiction by influencing Dorsal Diencephalic Conduction System (DDCS) including the habenula.

METHODS

For this review especially the databases of Pubmed and Embase were frequently and non-systematically searched.

RESULTS

The mechanisms of bird migration have been thoroughly investigated. Especially the mechanism of the circannual biorhythm and its associated endocrine regulation has been well elucidated. A typical behavior called "Zugunruhe" marks the moment of leaving in migratory birds. The role of magnetoreception in navigation has also been clarified in recent years. However, how bird migration is regulated at the neuronal level in the forebrain is not well understood. Among mammals, marine mammals are most similar to birds. They use terrestrial magnetic field when navigating and often bridge long distances between breeding and foraging areas. Population migration is further often seen among the large hoofed mammals in different parts of the world. Importantly, learning processes and social interactions with conspecifics play a major role in these ungulates. Considering the evolutionary development of the forebrain in vertebrates, it can be postulated that the DDCS plays a central role in regulating the readiness and intensity of essential (emotional) behaviors. There is manifold evidence that this DDCS plays an important role in relapse to abuse after prolonged periods of abstinence from addictive behavior. It is also possible that the DDCS plays a role in navigation.

CONCLUSIONS

The role of the DDCS in the neurobiological regulation of bird migration has hardly been investigated. The involvement of this system in relapse to addiction in mammals might suggest to change this. It is recommended that particularly during "Zugunruhe" the role of neuronal regulation via the DDCS will be further investigated.

Age-related changes in the primary auditory cortex of newborn, adults and aging bottlenose dolphins (Tursiops truncatus) are located in the upper cortical layers

Introduction

The auditory system of dolphins and whales allows them to dive in dark waters, hunt for prey well below the limit of solar light absorption, and to communicate with their conspecific. These complex behaviors require specific and sufficient functional circuitry in the neocortex, and vicarious learning capacities. Dolphins are also precocious animals that can hold their breath and swim within minutes after birth. However, diving and hunting behaviors are likely not innate and need to be learned. Our hypothesis is that the organization of the auditory cortex of dolphins grows and mature not only in the early phases of life, but also in adults and aging individuals. These changes may be subtle and involve sub-populations of cells specificall linked to some circuits.

Methods

In the primary auditory cortex of 11 bottlenose dolphins belonging to three age groups (calves, adults, and old animals), neuronal cell shapes were analyzed separately and by cortical layer using custom computer vision and multivariate statistical analysis, to determine potential minute morphological differences across these age groups.

Results

The results show definite changes in interneurons, characterized by round and ellipsoid shapes predominantly located in upper cortical layers. Notably, neonates interneurons exhibited a pattern of being closer together and smaller, developing into a more dispersed and diverse set of shapes in adulthood.

Discussion

This trend persisted in older animals, suggesting a continuous development of connections throughout the life of these marine animals. Our findings further support the proposition that thalamic input reach upper layers in cetaceans, at least within a cortical area critical for their survival. Moreover, our results indicate the likelihood of changes in cell populations occurring in adult animals, prompting the need for characterization.

Report on the brain of the monk seal (Monachus monachus, Hermann, 1779)

The Mediterranean monk seal (Monachus monachus, Hermann, 1779) is an endangered species of pinniped endemic to few areas of the Mediterranean Sea. Extensive hunting and poaching over the last two centuries have rendered it a rare sight, scattered mainly in the Aegean Sea and the western coast of North Africa. In a rare event, a female monk seal calf stranded and died in southern Italy (Brindisi, Puglia). During due necropsy, the brain was extracted and fixed. The present report is the first of a monk seal brain. The features reported are remarkably typical of a true seal brain, with some specific characteristics. The brain cortical circonvolutions, main fissures and the external parts are described, and an EQ was calculated. Overall, this carnivore adapted to aquatic life shares some aspects of its neuroanatomy and physiology with other seemingly distant aquatic mammals.

The Hypothalamus of the Beaked Whales: The Paraventricular, Supraoptic, and Suprachiasmatic Nuclei

The hypothalamus is the body's control coordinating center. It is responsible for maintaining the body's homeostasis by directly influencing the autonomic nervous system or managing hormones. Beaked whales are the longest divers among cetaceans and their brains are rarely available for study. Complete hypothalamic samples from a female Cuvier's beaked whale and a male Blainville's beaked whale were processed to investigate the paraventricular (PVN) and supraoptic (SON) nuclei, using immunohistochemical staining against vasopressin. The PVN occupied the preoptic region, where it reached its maximum size, and then regressed in the anterior or suprachiasmatic region. The SON was located from the preoptic to the tuberal hypothalamic region, encompassing the optical structures. It was composed of a retrochiasmatic region (SONr), which bordered and infiltrated the optic tracts, and a principal region (SONp), positioned more medially and dorsally. A third vasopressin-positive nucleus was also detected, i.e., the suprachiasmatic nucleus (SCN), which marked the end of the SON. This is the first description of the aforementioned nuclei in beaked whales-and in any marine mammals-as well as their rostro-caudal extent and immunoreactivity. Moreover, the SCN has been recognized for the first time in any marine mammal species.

How smart dinosaurs?

This commentary discusses the main points made in Reiner's article on the prospect that some theropod dinosaurs could have given rise to a lineage that achieved a human level of intelligence, and those made in Herculano-Houzel's article on the potentially monkey-like numbers of neurons in the pallium of large theropods, and the implications of this for their intelligence.

Relating enhancer genetic variation across mammals to complex phenotypes using machine learning

Protein-coding differences between species often fail to explain phenotypic diversity, suggesting the involvement of genomic elements that regulate gene expression such as enhancers. Identifying associations between enhancers and phenotypes is challenging because enhancer activity can be tissue-dependent and functionally conserved despite low sequence conservation. We developed the Tissue-Aware Conservation Inference Toolkit (TACIT) to associate candidate enhancers with species' phenotypes using predictions from machine learning models trained on specific tissues. Applying TACIT to associate motor cortex and parvalbumin-positive interneuron enhancers with neurological phenotypes revealed dozens of enhancer-phenotype associations, including brain size-associated enhancers that interact with genes implicated in microcephaly or macrocephaly. TACIT provides a foundation for identifying enhancers associated with the evolution of any convergently evolved phenotype in any large group of species with aligned genomes.

Role of intracortical neuropil growth in the gyrification of the primate cerebral cortex

The convolutions of the mammalian cerebral cortex allow the enlargement of its surface and addition of novel functional areas during evolution while minimizing expansion of the cranium. Cognitive neurodevelopmental disorders in humans, including microcephaly and lissencephaly, are often associated with impaired gyrification. In the classical model of gyrification, surface area is initially set by the number of radial units, and the forces driving cortical folding include neuronal growth, formation of neuropil, glial cell intercalation, and the patterned growth of subcortical white matter. An alternative model proposes that specified neurogenic hotspots in the outer subventricular zone (oSVZ) produce larger numbers of neurons that generate convexities in the cortex. This directly contradicts reports showing that cortical neurogenesis and settling of neurons into the cortical plate in primates, including humans, are completed well prior to the formation of secondary and tertiary gyri and indeed most primary gyri. In addition, during the main period of gyrification, the oSVZ produces mainly astrocytes and oligodendrocytes. Here we describe how rapid growth of intracortical neuropil, addition of glial cells, and enlargement of subcortical white matter in primates are the primary forces responsible for the post-neurogenic expansion of the cortical surface and formation of gyri during fetal development. Using immunohistochemistry for markers of proliferation and glial and neuronal progenitors combined with transcriptomic analysis, we show that neurogenesis in the ventricular zone and oSVZ is phased out and transitions to gliogenesis prior to gyral development. In summary, our data support the classical model of gyrification and provide insight into the pathogenesis of congenital cortical malformations.

Multimodal Assessment of Bottlenose Dolphin Auditory Nuclei Using 7-Tesla MRI, Immunohistochemistry and Stereology

The importance of assessing neurochemical processes in the cetacean brain as a tool for monitoring their cognitive health and to indirectly model human neurodegenerative conditions is increasingly evident, although available data are largely semiquantitative. High-resolution MRI for post-mortem brains and stereology allow for quantitative assessments of the cetacean brain. In this study, we scanned two brains of bottlenose dolphins in a 7-Tesla (7T) MR scanner and assessed the connectivity of the inferior colliculi and ventral cochlear nuclei using diffusion tensor imaging (DTI). Serial thick sections were investigated stereologically in one of the dolphins to generate rigorous quantitative estimates of identifiable cell types according to their morphology and expression of molecular markers, yielding reliable cell counts with most coefficients of error <10%. Fibronectin immunoreactivity in the dolphin resembled the pattern in a human chronic traumatic encephalopathy brain, suggesting that neurochemical compensation for insults such as hypoxia may constitute a noxious response in humans, while being physiological in dolphins. These data contribute to a growing body of knowledge on the morphological and neurochemical properties of the dolphin brain and highlight a stereological and neuroimaging workflow that may enable quantitative and translational assessment of pathological processes in the dolphin brain in the future.

Volumetric and connectivity assessment of the caudate nucleus in California sea lions and coyotes

In addition to a large (chimpanzee-sized) and heavily convoluted brain, one of the most striking neurobiological features in pinnipeds is the large size of the head of the caudate nucleus, which dwarfs the rest of the striatum. Although previous research has suggested carnivore striatum is small in comparison to that of primates, there are limited volumetric data on separate striatal structures in carnivores. Therefore, the apparent functional implication of a potentially hypertrophic caudate to carnivores has not been discussed. Here, for the first time, we obtained separate volumetric measurements of caudate and putamen in California sea lions and coyotes. Exemplars of both species had very large caudate nuclei, approximately 1/75th of total brain volume. In both the sea lion and coyote, the caudate dwarfed the putamen at a ratio of 13 to 1 or greater, a finding in strong contrast to measurements showing larger putamen than caudate in primates. In addition, using post-mortem diffusion tensor brain imaging, we mapped and compared white matter connections between the dorsal caudate and the motor, premotor and frontopolar, and orbitofrontal cortices in healthy adult sea lions and healthy adult coyotes. The sea lions showed some evidence of greater premotor and frontopolar connectivity. These findings bear on previously underexplored striatal characteristics of large carnivores, and we discuss potential interpretations related to cognitive flexibility and sensorimotor transformation.

Systematic validation and assessment of immunohistochemical markers for central nervous system pathology in cetaceans, with emphasis on auditory pathways

Cetacean neuropathology is a developing field that aims to assess structural and neurochemical changes involved in neurodegenerative, infectious and traumatic processes, however markers used previously in cetaceans have rarely undergone systematic validation. This is a prerequisite to investigating the potential damage inflicted on the cetacean auditory system by anthropogenic noise. In order to assess apoptotic, neuroinflammatory and structural aberrations on a protein level, the baseline expression of biomarker proteins has to be characterized, implementing a systematic approach to validate the use of anti-human and anti-laboratory animal antibodies in dolphin tissues. This approach was taken to study 12 different antibodies associated with hypoxic-ischemic, inflammatory, plastic and excitatory-inhibitory changes implicated in acoustic trauma within the ventral cochlear nuclei and inferior colliculi of 20 bottlenose dolphins (Tursiops truncatus). Out of the 12 tested antibodies, pro-apoptotic protease factor 1 (Apaf-1), diacylglycerolkinase-ζ (DGK-ζ), B-cell lymphoma related protein 2 (Bcl-2), amyloid-β peptide (Aβ) and neurofilament 200 (NF200) were validated employing Western blot analyses and immunohistochemistry (IHC). The results of the validation process indicate specific patterns of immunoreactivity that are comparable to those reported in other mammals, thus suggesting a key panel of IHC biomarkers of pathological processes in the cetacean brain. As a consequence, the antibodies tested in this study may constitute a valid tool for supporting existing diagnostic methods in neurological diseases. The approach of systematic validation of IHC markers in cetaceans is proposed as a standard practice, in order for results to be transparent, reliable and comparable.

Methodology and Neuromarkers for Cetaceans’ Brains

Cetacean brain sampling may be an arduous task due to the difficulty of collecting and histologically preparing such rare and large specimens. Thus, one of the main challenges of working with cetaceans’ brains is to establish a valid methodology for an optimal manipulation and fixation of the brain tissue, which allows the samples to be viable for neuroanatomical and neuropathological studies. With this in view, we validated a methodology in order to preserve the quality of such large brains (neuroanatomy/neuropathology) and at the same time to obtain fresh brain samples for toxicological, virological, and microbiological analysis (neuropathology). A fixation protocol adapted to brains, of equal or even three times the size of human brains, was studied and tested. Finally, we investigated the usefulness of a panel of 20 antibodies (neuromarkers) associated with the normal structure and function of the brain, pathogens, age-related, and/or functional variations. The sampling protocol and some of the 20 neuromarkers have been thought to explore neurodegenerative diseases in these long-lived animals. To conclude, many of the typical measures used to evaluate neuropathological changes do not tell us if meaningful cellular changes have occurred. Having a wide panel of antibodies and histochemical techniques available allows for delving into the specific behavior of the neuronal population of the brain nuclei and to get a “fingerprint” of their real status.

Comparative morphology of the spinal cord and associated vasculature in shallow versus deep diving cetaceans

The cetacean vertebral canal houses the spinal cord and arterial supply to and venous drainage from the entire central nervous system (CNS). Thus, unlike terrestrial mammals, the cetacean spinal cord lies within a highly vascularized space. We compared spinal cord size and vascular volumes within the vertebral canal across a sample of shallow and deep diving odontocetes. We predicted that the (a) spinal cord, a metabolically expensive tissue, would be relatively small, while (b) volumes of vascular structures would be relatively large, in deep versus shallow divers. Our sample included the shallow diving Tursiops truncatus (n = 2) and Delphinus delphis (n = 3), and deep diving Kogia breviceps (n = 2), Mesoplodon europaeus (n = 2), and Ziphius cavirostris (n = 1). Whole, frozen vertebral columns were cross-sectioned at each intervertebral disc, scaled photographs of vertebral canal contents acquired, and cross-sectional areas of structures digitally measured. Areas were multiplied by vertebral body lengths and summed to calculated volumes of neural and vascular structures. Allometric analyses revealed that the spinal cord scaled with negative allometry (b = 0.51 ± 0.13) with total body mass (TBM), and at a rate significantly lower than that of terrestrial mammals. As predicted, the spinal cord represented a smaller percentage of the total vertebral canal volume in the deep divers relative to shallow divers studied, as low as 2.8% in Z. cavirostris. Vascular volume scaled with positive allometry (b = 1.2 ± 0.22) with TBM and represented up to 96.1% (Z. cavirostris) of the total vertebral canal volume. The extreme deep diving beaked whales possessed 22-35 times more vascular volume than spinal cord volume within the vertebral canal, compared with the 6-10 ratio in the shallow diving delphinids. These data offer new insights into morphological specializations of neural and vascular structures that may contribute to differential diving capabilities across odontocete cetaceans.

The Relevance of Ecological Transitions to Intelligence in Marine Mammals

Macphail's comparative approach to intelligence focused on associative processes, an orientation inconsistent with more multifaceted lay and scientific understandings of the term. His ultimate emphasis on associative processes indicated few differences in intelligence among vertebrates. We explore options more attuned to common definitions by considering intelligence in terms of richness of representations of the world, the interconnectivity of those representations, the ability to flexibly change those connections, and knowledge. We focus on marine mammals, represented by the amphibious pinnipeds and the aquatic cetaceans and sirenians, as animals that transitioned from a terrestrial existence to an aquatic one, experiencing major changes in ecological pressures. They adapted with morphological transformations related to streamlining the body, physiological changes in respiration and thermoregulation, and sensory/perceptual changes, including echolocation capabilities and diminished olfaction in many cetaceans, both in-air and underwater visual focus, and enhanced senses of touch in pinnipeds and sirenians. Having a terrestrial foundation on which aquatic capacities were overlaid likely affected their cognitive abilities, especially as a new reliance on sound and touch, and the need to surface to breath changed their interactions with the world. Vocal and behavioral observational learning capabilities in the wild and in laboratory experiments suggest versatility in group coordination. Empirical reports on aspects of intelligent behavior like problem-solving, spatial learning, and concept learning by various species of cetaceans and pinnipeds suggest rich cognitive abilities. The high energy demands of the brain suggest that brain-intelligence relationships might be fruitful areas for study when specific hypotheses are considered, e.g., brain mapping indicates hypertrophy of specific sensory areas in marine mammals. Modern neuroimaging techniques provide ways to study neural connectivity, and the patterns of connections between sensory, motor, and other cortical regions provide a biological framework for exploring how animals represent and flexibly use information in navigating and learning about their environment. At this stage of marine mammal research, it would still be prudent to follow Macphail's caution that it is premature to make strong comparative statements without more empirical evidence, but an approach that includes learning more about how animals flexibly link information across multiple representations could be a productive way of comparing species by allowing them to use their specific strengths within comparative tasks.