Hyenas
The four, extant species of hyenids (aardwolves, striped hyenas, brown hyenas, and spotted hyenas) are compared and contrasted. Despite belonging to a small family of carnivorans, these species show a wide range of social systems (from solitary to gregarious), mating systems (from monogamy to promiscuity), and feeding ecologies (from myrmecophagy, through solitary scrounging, to group hunting). The most remarkable differences, however, concern their reproductive biology: Whereas the first three species show the typical mammalian pattern of sexual dimorphism, the fourth - the spotted hyena - shows the most extreme form of sexual monomorphism evidenced by any mammal. The female spotted hyena is behaviorally and morphologically “masculinized,” being larger than the male, socially dominant over the male, and possessing external reproductive anatomy that bears striking resemblance to that of the male. Notably, the female has no “external” vagina; instead, the urogenital canal passes through a peniform clitoris, providing the female spotted hyena with a singular opening through which she urinates, copulates, and gives birth. Decades of experimental, endocrine, morphological, and histological studies have been aimed at demystifying this evolutionary puzzle. The developmental trajectory of female, and male, reproductive structures suggest that a combination of genetic and endocrine factors, including androgen production by the fetal ovary and placental metabolism of androstenedione, operate in tandem during sexual differentiation of the spotted hyena. But, formation of the “masculine” genitalia of female Crocuta appears to result from a unique mechanism that is unknown at this time.
