A sex difference in the hypothalamus of the spotted hyena

The female spotted hyena (Crocuta crocuta) presents the most highly 'masculinized' external genitalia of any extant mammal; the female has a large, fully erectile clitoris through which she gives birth, urinates and copulates. Contemporary understanding suggests that naturally circulating androgens, during critical stages of development, account for this unusual morphology. Androgens found in the female spotted hyena might also be expected to 'masculinize' the brain, particulary in regions linked with sexual behavior and found to be dimorphic in many other species, the medial preoptic area and adjacent anterior hypothalomus (mPOA/AH). However, the sexual behavior of female spotted hyenas does not indicate hypothalamic masculinization. Within the hyena mPOA/AH we found a sexually dimorphic nucleus (hSDN) that was twofold larger in males. Compared to the sex differences in other mammals this difference is modest; androgen exposure may partly, but not completely, masculinize the hypothalamus of female spotted hyenas.