The brain and spinal cord exhibit several advanced characteristics in reptiles relative to amphibians, including larger size and greater definition of structural divisions and greater development of the cerebral cortex. Neural connections between the olfactory bulbs, the corpus striatum, and several other subcortical structures have become clearly established in reptiles, and these connections have been conserved in subsequent evolution such that they are present in mammals, including humans. This set of connections is sometimes referred to as the "reptilian brain" or "R-complex" and is thought to represent a neural circuit necessary for the mediation of basic functions such as predation and mating as well as the affective concomitants associated with social behaviors ranging from cooperation to aggression. In the study of mammals, we speak of the regulation of emotion by components of the reptilian brain. Herpetologists are generally reluctant to speak of emotion in their animals, but they have no difficulty recognizing the existence of the neural circuit in question and in understanding that it contributes to social and reproductive activities. Whether this contribution is limited to the organization of motor patterns or whether emotion also is involved remains an open question.

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