| Summary: | This research is part of a larger effort to understand the evolution of the mammalian brain. The studies in Chapters 2 – 4 address questions regarding the evolution of the visual system in primates and other non-primate large-brained mammals. Pinnipeds are an ideal subject for comparison, as pinnipeds are a diverse clade of carnivorous animals and consist of three families: Odobenidae (walrus), Otariidae (eared seals: sea lions and fur seals), and Phocidae (earless seals: true seals). In this research, we examined the cortical and subcortical visual components of two pinniped species with large brains, the California sea lion (Zalophus californianus) and the northern elephant seal (Mirounga angustirostris). To best examine how these species gained and require such large brains, it is important to simultaneously look to primate species that, independently of the pinniped lineage, came to also evolve large brains. As such, we examined the structure and components of the visual system in select primate species: a chimpanzee (Pan troglodytes) and macaques (Macaca nemestrina, Macaca mulatta, and Macaca radiata). Primates are notable for their large brains, and every primate species has an average brain size larger than then 0.4 g mouse brain, the animal model most commonly used in laboratory studies. Primate brains range considerably in size from 1.7 grams (mouse lemur) to over 1,300 grams (human), and share many common brain features despite species-specific specializations that develop across evolutionary history. These species are chosen for two important reasons: 1) Each has a highly developed visual system, and 2) All are members of the same phylogenetic mammalian radiation known as the Euarchontoglires. The primary goal of this research was to expand on our extensive knowledge of the primate visual system while also extending out to explore the visual system of under-studied species that similarly have large, complexly folded brains. A secondary goal of this research was to compare non-primates with large brains to primate brains, to ultimately emerge with a better understanding of how the visual system can evolve and function differently or similarly in the context of a large brain. The specific aims were to: Aim 1) To use the isotropic and flow fractionator techniques to understand the basic neuron and cell content of various primate species (Chapters 2 and 3). Neurons are the building blocks of neocortex, and an accurate estimate of the total number of neurons in a brain can reveal information concerning the specializations of cortex. In this collaborative effort, we determined the total numbers of cells and neurons within the neocortex of the adult chimpanzee and macaque brains. These are species that have relatively large brains compared to most mammals, and to the proposed brain size of our earliest mammalian ancestors. This data was compared to previously reported results in similarly prepared species, such as the baboon and New World monkeys. We found the same pattern of overall neuronal density described in all other primate species, in which primary visual cortex and primary somatosensory cortex contain higher-than-average neuron densities and primary motor cortex contains lower-than-average neuron densities. Aim 2) To describe the basic components of the visual systems of large-brained non-primate mammals (the California sea lion and northern elephant seal) (Chapter 4). Given their highly gyrified neocortex, similar studies of cell and neuron counts are not possible in seals and sea lion brains easily using the same methodologies as per Aim 1. We previously described the neuroanatomy of the somatosensory system at the level of the midbrain, thalamus, and cortex in pinnipeds (Sawyer et al., 2016), and this represents the first effort to describe the neuroanatomy of the lateral geniculate nucleus, superior colliculus, and primary visual cortex in the northern elephant seal and California sea lion. We examined the visual neuroanatomy in these two species using immunohistochemistry in coronal sections and other reconstruction methods. We found that the visual neuroanatomy is more similar in structure to other carnivores, such as cats, as opposed to primates, which is expected given these species’ phylogenetic position within the Carnivora order. This research will contribute to the small amount of neuroscientific research in pinnipeds, and we seek to encourage similar research in other pinniped species for comparative studies related to brain evolution.
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