Common mating or reproductively motivated systems include monogamy, polygyny, polyandry, polygamy and promiscuity. Various types of mating systems include monogamy, polygyny, polyandry, promiscuity, and polygamy. Other mating systems, including polygyny, polyandry, polygamy, polygynandry, and promiscuity, also occur. In most tinamou species, the males practice simultaneous polygyny and the females sequential polyandry. Their mating systems are highly variable and can include monogamy, polygyny, and polyandry. The Muslim personal law allows Polygamy but not Polyandry. They are monogamous with a slight tendency toward social polyandry. They practiced both fraternal and sequential polyandry. Fraternal polyandry was a common custom among Saka. The female equivalent of fraternal polyandry is sororate marriage. In Tibet, those husbands are often brothers; "fraternal polyandry". Guppies have the mating system called polyandry, where females mate with multiple males. An even rarer subtype called cooperative simultaneous polyandry also exists in some species. There is some evidence in ancient sources that the Spartans practiced polyandry. This type of mating is sequential polyandry.