In this species, males lack an intromittent, or penis-like, organ and females have developed a penis-like structure called gynosoma , which is used to penetrate the body of the males to collect so-called spermatophores. Sex-role reversal But examples of sex-role reversal — when females compete more intensely than males to obtain mates — are not rare in nature. Parental investment was proposed in by the American evolutionary biologist Robert Trivers as a key factor determining which sex is under higher sexual selection pressure. This is the result of an elongation of the clitoris due to a hormonal boost during the final stage of cub development. In some species, such as in some damselflies, males even remove the sperm transferred to the female by previous males. This implies males can produce a much larger number of gametes — the cells that merge during sex — than females do, which, in turn, entails important consequences for differences between the sexes. In some cases, the evolution of this so-called role reversal comes with stunning adaptations. A most amazing process in regards to physiological changes associated to sex-role reversal is found in fishes, such as the hermaphrodite gilt-head bream Sparus aurata. All the individuals are males when they hatch but, when they reach a certain age, they can become females, depending on weight, hormones and social factors. Among vertebrates, the females of the spotted hyena Crocuta crocuta have developed a pseudo-penis structure. What leads to the development of this structure? Instead, the way each sex behaves depends on several factors such as asymmetries in parental investment, sex-ratio or the availability of mates. There are a variety of examples in nature in which males are the caregivers or females compete for access to mates. In the extraordinary case of Neotrogla, females are pulling the sperm out of the male body using their innovative and exclusive penis-like organ.