Transitions in temporal niche have occurred many times over the course of mammalian evolution and have been associated with changes in sensory stimuli available to animals. This is particularly true of visual cues because levels of light are so much higher during the day than night. For this reason, evolutionary transitions between diurnal, nocturnal, and cathemeral lifestyles are expected to be accompanied by modifications of sensory systems to optimize the ability of animals to receive, process, and react to important stimuli in the environment. In chapter one, I examine the influence of temporal niche on investment in sensory brain tissue of diurnal and nocturnal rodent species. To do this, I measured the size of five sensory brain regions that process olfactory information (olfactory bulbs), visual information (lateral geniculate nucleus, superior colliculus) and auditory information (medial geniculate nucleus, and inferior colliculus). A phylogenetic framework was used to assess the influence of temporal niche on the relative sizes of these brain structures. Compared to nocturnal species, diurnal species had larger visual regions, whereas nocturnal species had larger olfactory bulbs than their diurnal counterparts. Of the two auditory structures examined, one (medial geniculate nucleus) was larger in diurnal species, while the other (inferior colliculus) did not differ significantly with temporal niche. Our results suggest possible tradeoffs of investment between olfactory and visual areas of the brain, with diurnal species investing more in processing visual information and nocturnal species investing more in processing olfactory information. In chapter two, I investigate investment in sensory brain tissue of cathemeral species by measuring the size of five sensory brain regions that process olfactory information (olfactory bulbs), visual information (lateral geniculate nucleus, superior colliculus) and auditory information (medial geniculate nucleus and inferior colliculus). Using a phylogenetic framework, I assessed the influence of temporal niche on the relative sizes of these brain structures between diurnal, nocturnal, and cathemeral rodents. At a very general level, my data reveal that sensory structures in the brains of cathemeral rodents are not simply intermediate in size between those of diurnal and nocturnal rodents. Rather, cathemeral species were either distinctly nocturnal-like or diurnal-like. Cathemeral species had olfactory bulbs similar in size to diurnal species, and smaller than nocturnal species. One visual structure, the superior colliculus, was not influenced by temporal niche. The other visual structure, the lateral geniculate nucleus, was larger in diurnal species compared to both nocturnal and cathemeral species. The two auditory structures showed different patterns of investment. The inferior colliculus of the cathemeral and nocturnal species was similar in size, both of which were significantly smaller than diurnal species. The medial geniculate nucleus was similar in size between diurnal and cathemeral species, both of which were larger than that of nocturnal species. These results suggest a more complicated scenario than simply partitioning investment to accommodate activity in both day and night. In chapter 3, I carry out a more refined assessment of the lateral geniculate nucleus (LGN) in diurnal, nocturnal, and cathemeral rodents. In chapters one and two, I found LGN to be largest in diurnal rodents, compared to nocturnal and cathemeral rodents. The LGN is subdivided into three regions which carry out specific functions involved in visual processing and modulation of circadian rhythms. The subregions of the LGN were significantly larger in diurnal species, suggesting increased investment in regions that carry out visual processing and circadian functions. When comparing the ratio of the dorsal and ventral LGN, there was no influence of temporal niche. This suggests that factors other than temporal niche impact the sizes of these two substructures in relation to one another.
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