RESUMEN
The extreme virilization of the female spotted hyena raises interesting questions with respect to sexual differentiation of the brain and behavior. Females are larger and more aggressive than adult, non-natal males and dominate them in social encounters; their external genitalia also are highly masculinized. In many vertebrates, the arginine vasopressin (VP) innervation of the forebrain, particularly that of the lateral septum, is associated with social behaviors such as aggression and dominance. Here, we used immunohistochemistry to examine the distribution of VP cells and fibers in the forebrains of adult spotted hyenas. We find the expected densely staining VP immunoreactive (VP-ir) neurons in the paraventricular and supraoptic nuclei, as well as an unusually extensive distribution of magnocelluar VP-ir neurons in accessory regions. A small number of VP-ir cell bodies are present in the suprachiasmatic nucleus and bed nucleus of the stria terminalis; however, there are extensive VP-ir fiber networks in presumed projection areas of these nuclei, for example, the subparaventricular zone and lateral septum, respectively. No significant sex differences were detected in the density of VP-ir fibers in any area examined. In the lateral septum, however, marked variability was observed. Intact females exhibited a dense fiber network, as did two of the four males examined; the two other males had almost no VP-ir septal fibers. This contrasts with findings in many other vertebrate species, in which VP innervation of the lateral septum is consistently greater in males than in females.