Abstract
The amygdala consists of a heterogeneous group of nuclei in the temporal lobe that control species-specific behaviors, including feeding, fighting, flight and reproduction, as well as forming and processing emotion-associated memories and reactions. Thus, the amygdala is critical for the regulation of complex social behaviors. The amygdalar nuclei can be subdivided into four main groups that are recognizable in most vertebrates. They have similar embryological origins, connections and functions. The four groups consist of (1) the =basolateral' nuclei, which have substantial connections with the sensory thalamus and sensory association areas of the cortex, and are important for processing sensory and emotional associations; (2) the medial extended amygdalar group, which together with (3) the superficial cortex-like amygdalar region, receives input from either the olfactory or accessory olfactory system, have numerous cells that concentrate steroidal hormones and influence sexual behavior; and (4) the central extended amygdalar nuclei, which receive substantial input from other amygdalar nuclei and send projections to the hypothalamus and brainstem, regulating the autonomic system, including fear responses. Although their shape and size differ, each of these four amygdala groups appear to have similar connections and functions in mammals, reptiles, birds, amphibians and, although less well-studied, in fish. The sensory amygdalar nuclei regulate the ventromedial hypothalamus and the autonomic system through different pathways. These pathways are present in amphibians and reptiles, as well as in mammals. The evolutionary conservation of these pathways suggests they are critical for the survival of species and individuals. The amygdala plays a critical role in social behaviors, and the primate amygdala appears to have increased in size in relation to the pressures of an increasingly complex social life. The volume of the basolateral amygdalar group is related to the extent of social behaviors in nonhuman primates and lizards, reflecting its importance in social interactions. Further comparative studies in additional taxonomic groups are needed to expand our understanding of the interaction between amygdalar function and social behaviors.