| Summary: | Adult neurogenesis in mammals is predominantly restricted to two brain regions, thedentate gyrus of the hippocampus and the olfactory bulb, suggesting that these twobrain regions uniquely share functions that mediate its adaptive significance. Benefits ofadult neurogenesis across these two regions appear to converge on increased neuronaland structural plasticity that subserves coding of novel, complex, and fine-grainedinformation, usually with contextual components that include spatial positioning. Bycontrast, costs of adult neurogenesis appear to center on potential for dysregulationresulting in higher risk of brain cancer or psychological dysfunctions, but such costshave yet to be quantified directly. The three main hypotheses for the proximate functionsand adaptive significance of adult neurogenesis, pattern separation, memoryconsolidation, and olfactory spatial, are not mutually exclusive and can be reconciledinto a simple general model amenable to targeted experimental and comparative tests.Comparative analysis of brain region sizes across two major social-ecological groups ofprimates, gregarious (mainly diurnal haplorhines, visually-oriented, and in large socialgroups) and solitary (mainly noctural, territorial, and highly reliant on olfaction, as in mostrodents) suggest that solitary species, but not gregarious species, show positiveassociations of population densities and home range sizes with sizes of both thehippocampus and olfactory bulb, implicating their functions in social-territorial systemsmediated by olfactory cues. Integrated analyses of the adaptive significance of adultneurogenesis will benefit from experimental studies motivated and structured byecologically and socially valid selective contexts.
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