Marriage is to be heterosexual and monogamous. In humans, social monogamy takes the form of monogamous marriage. All sexual relations outside the confines of monogamous, heterosexual marriage are regarded as immoral. Anthropologists tend to believe humans are neither completely monogamous nor completely polygamous. Similarly, scientists disagree whether Lower Paleolithic humans were largely monogamous or polygynous. Domestic dogs are polygamous in contrast to wolves that are generally monogamous. Dogs are polygamous in contrast to wolves that are generally monogamous. Two-spirited people also can take part in marriages, either monogamous and polygamous ones. Gilmartin likewise found no differences in marital satisfaction between sexually open and sexually monogamous couples. They took multiple sexual partners in serially monogamous relationships. These females have 60% less offspring than females that are in a monogamous relationship. American crows are socially monogamous cooperative breeding birds. Bigamy is a crime in most countries that recognise only monogamous marriages. However, both monogamous and polyandrous groups are found in south Sumatra. Marriages are required to be monogamous and legally registered.