<span>Compared to other vertebrates, where a species usually has a single mating system, human display great variety. Humans also differ by having formal marriages which in many cultures involve negotiation and arrangement between elder relatives. Regarding sexual dimorphism (see the section about animals above), humans are in the intermediate group with moderate sex differences in body size but with relatively large testes. This indicates relatively frequent sperm competition which is supported by reports of extrapair paternity of 2–22% in socially monogamous and polygynous human societies. One estimate is that 83% of human societies are polygynous, 0.05% are polyandrous, and the rest are monogamous. Even the last group may at least in part be genetically polygynous.[4]
Polygyny is associated with an increased sharing of subsistence provided by women. This is consistent with the theory that if women raise the children alone, men can concentrate on the mating effort. Polygyny is also associated with greater environmental variability in the form of variability of rainfall. This may increase the differences in the resources available to men. An important association is that polygyny is associated with a higher pathogen load in an area which may make having good genes in a male increasingly important. A high pathogen load also decreases the relative importance of sororal polygyny which may be because it becomes increasingly important to have genetic variability in the offspring (See Major histocompatibility complex and sexual selection).[4]
Virtually all the terms used to describe animal mating systems were adopted from social anthropology, where they had been devised to describe systems of marriage. This shows that human sexual behavior is unusually flexible since, in most animal species, one mating system dominates. While there are close analogies between animal mating systems and human marriage institutions, these analogies should not be pressed too far, because in human societies, marriages typically have to be recognized by the entire social group in some way, and there is no equivalent process in animal societies. The temptation to draw conclusions about what is "natural" for human sexual behavior from observations of animal mating systems should be resisted: a socio-biologist observing the kinds of behavior shown by humans in any other species would conclude that all known mating systems were natural for that species, depending on the circumstances or on individual differences.[citation needed]
As culture increasingly affects human mating choices, ascertaining what is the 'natural' mating system of the human animal from a zoological perspective becomes increasingly difficult. Some clues can be taken from human anatomy, which is essentially unchanged from the prehistoric past:
humans have a large relative size of testes to body mass in comparison to most primates;
humans have a large ejaculate volume and sperm count in comparison to other primates;
as compared to most primates, humans spend more time in copulation;
as compared to most primates, humans copulate with greater frequency;
the outward signs of estrous in women (i.e. higher body temperature, breast swelling, sugar cravings, etc.), are often perceived to be less obvious in comparison to the outward signs of ovulation in most other mammals;
for most mammals, the estrous cycle and its outward signs bring on mating activity; the majority of female-initiated matings in humans coincides with estrous[citation needed], but humans copulate throughout the reproductive cycle;
after ejaculation/orgasm in males and females, humans release a hormone that has a sedative effect; however human females may remain sexually receptive and may remain in the plateau stage of orgasm if their orgasm has not been completed.
Some have suggested that these anatomical factors signify some degree of sperm competition, though as levels of genetic and societal promiscuity are highly varied across cultures,[5] this evidence is far from conclusive.</span>
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