Animal sexual behaviour
Animal sexual behaviour takes many different forms, even within the same species. Common mating or reproductively motivated systems include monogamy, polyandry, polygamy, and promiscuity. Other sexual behaviour may be reproductively motivated (e.g. sex apparently due to duress or coercion and situational sexual behaviour) or non-reproductively motivated (e.g. interspecific sexuality, sexual arousal from objects or places, sex with dead animals, homosexual sexual behaviour, bisexual sexual behaviour, and a range of other behaviours).When animal sexual behaviour is reproductively motivated, it is often termed mating or copulation; for most non-human mammals, mating and copulation occur at estrus (the most fertile period in the mammalian female's reproductive cycle), which increases the chances of successful impregnation. Some animal sexual behaviour involves competition, sometimes fighting, between multiple males. Females often select males for mating only if they appear strong and able to protect themselves. The male that wins a fight may also have the chance to mate with a larger number of females and will therefore pass on his genes to their offspring.Historically, it was believed that only humans and a small number of other species performed sexual acts other than for reproduction, and that animals' sexuality was instinctive and a simple ""stimulus-response"" behaviour. A range of species masturbate and may use objects as tools to help them do so.