Nonapeptide circuits are an evolutionarily ancient component of the brain, and they exhibit numerous anatomical and functional features that are strongly conserved across the vertebrate classes. Nonetheless, certain features are evolutionarily plastic, particularly receptor distributions, which allow the nonapeptides to influence behaviour in a species-specific manner. At least in some cases, evolution in nonapeptide circuits appears to take a predictable course, given that various anatomical and functional features have evolved both divergently and convergently in relation to mating system in mammals and sociality in birds. Evolutionary modifications to nonapeptide signalling may take a variety of forms, all of which likely produce species-specific patterns of neuromodulation across brain regions. Importantly, these evolutionary modifications may adjust the relative influence of the various nonapeptide cell groups, which can be functionally distinct and even functionally opposed.
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