The ancient world generally exhibited more interest in the hyena than this entry might suggest. With the exception of Aristotle (HA 5799b 15-30, GA 757a 2-13) most writers believed that the hyena changes sex, from male to female and back again, at intervals of six months or a year: Aesop 242 & 243 Perry, p.416; Aelian Nat.An. 1.25, Ps-Clement Recog. 8.25, Oppian Cyn. 3.288-92, Ovid Met. 15.408-10, Photius Bibl. 244.379a18, Pliny N.H. 8.44, Tertullian de Pallio 3.2. In fact, adult male and female hyenas are indistinguishable visually, short of dissection.