elephants in South Africa sexually coerced and killed rhinoceroses. This interpretation of the elephants' behaviour was disputed by one of the original study's authors, who said there was "nothing sexual about these attacks". Parthenogenesis Parthenogenesis is a form of asexual reproduction in which growth and development of embryos occur without fertilisation. Technically, parthenogenesis is not a behaviour, however, sexual behaviours may be involved. Whip-tailed lizard females have the ability to reproduce through parthenogenesis and as such males are rare and sexual breeding non-standard. Females engage in "pseudocopulation" to stimulate ovulation, with their behaviour following their hormonal cycles; during low levels of oestrogen, these (female) lizards engage in "masculine" sexual roles. Those animals with currently high oestrogen levels assume "feminine" sexual roles. Lizards that perform the courtship ritual have greater fecundity than those kept in isolation due to an increase in hormones triggered by the sexual behaviours. So, even though asexual whiptail lizards populations lack males, sexual stimuli still increase reproductive success. From an evolutionary standpoint these females are passing their full genetic code to all of their offspring rather than the 50% of genes that would be passed in sexual reproduction. It is rare to find true parthenogenesis in fishes, where females produce female offspring with no input from males. All-female species include the Texas silverside, Menidia clarkhubbsi and a complex of Mexican mollies. Parthenogenesis has been recorded in 70 vertebrate species including hammerhead sharks, blacktip sharks, amphibians and lizards. Unisexuality Unisexuality occurs when a species is all-male or all-female. Unisexuality occurs in some fish species and can take complex forms. Squalius alburnoides, a minnow found in several river basins in Portugal and Spain, appears to be an all-male species. The existence of this species illustrates the potential complexity of mating systems in fish. The species originated as a hybrid between two species and is diploid but not hermaphroditic. It can have triploid and tetraploid forms, including all-female forms that reproduce mainly through hybridogenesis. Others Interbreeding: Hybrid offspring can result from the mating of two organisms of distinct but closely related parent species, although the resulting offspring is not always fertile. According to Alfred Kinsey, genetic studies on wild animal populations have shown a "large number" of inter-species hybrids. Prostitution: There are reports that animals occasionally engage in prostitution. A small number of pair-bonded females within a group of penguins took nesting material (stones) after copulating with a non-partner male.