Sexual selection in humans

Sexual selection in humans concerns the concept of sexual selection, introduced by Charles Darwin as an element of his theory of natural selection, as it affects humans. The role of sexual selection in human evolution has not been firmly established although neoteny has been cited as being caused by human sexual selection. It has been suggested that sexual selection played a part in the evolution of the anatomically modern human brain, i.e. the structures responsible for social intelligence underwent positive selection as a sexual ornamentation to be used in courtship rather than for survival itself, and that it has developed in ways outlined by Ronald Fisher in the Fisherian runaway model. Fisher also stated that the development of sexual selection was 'more favourable' in humans. Sexual selection in humans concerns the concept of sexual selection, introduced by Charles Darwin as an element of his theory of natural selection, as it affects humans. The role of sexual selection in human evolution has not been firmly established although neoteny has been cited as being caused by human sexual selection. It has been suggested that sexual selection played a part in the evolution of the anatomically modern human brain, i.e. the structures responsible for social intelligence underwent positive selection as a sexual ornamentation to be used in courtship rather than for survival itself, and that it has developed in ways outlined by Ronald Fisher in the Fisherian runaway model. Fisher also stated that the development of sexual selection was 'more favourable' in humans. Some hypotheses about the evolution of the human brain argue that it is a sexually selected trait, as it would not confer enough fitness in itself relative to its high maintenance costs (a quarter to a fifth of the energy and oxygen consumed by a human). Current consensus about the evolutionary development of the human brain accepts sexual selection as a potential contributing factor but maintains that human intelligence and the ability to store and share cultural knowledge would have likely carried high survival value as well. Sexual selection's role in human evolution cannot be definitively established, as features may result from an equilibrium among competing selective pressures, some involving sexual selection, others natural selection, and others pleiotropy. Richard Dawkins argued that Charles Darwin described sexual selection as depending on 'the advantage which certain individuals have over others of the same sex and species, solely in respect of reproduction'. Darwin noted that sexual selection is of two kinds and concluded that both kinds had operated on humans: 'The sexual struggle is of two kinds; in the one it is between the individuals of the same sex, generally the male sex, in order to drive away or kill their rivals, the females remaining passive; whilst in the other, the struggle is likewise between the individuals of the same sex, in order to excite or charm those of the opposite sex, generally the females, which no longer remain passive, but select the more agreeable partners.' Charles Darwin conjectured that the male beard, as well as the hairlessness of humans compared to nearly all other mammals, were results of sexual selection. He reasoned that since the bodies of females are more nearly hairless, the loss of fur was due to sexual selection of females at a remote prehistoric time when males had overwhelming selective power, and that it nonetheless affected males due to genetic correlation between the sexes. He also hypothesized that contrasts in sexual selection acting along with natural selection were significant factors in the geographical differentiation in human appearance of some isolated groups, as he did not believe that natural selection alone provided a satisfactory answer. Although not explicit, his observation that in Khoisan women 'the posterior part of the body projects in a most wonderful manner' (known as steatopygia) implies sexual selection for this characteristic. In The Descent of Man, and Selection in Relation to Sex, Darwin viewed many physical traits which vary around the world as being so trivial to survival that he concluded some input from sexual selection was required to account for their presence. He noted that variation in these features among the various peoples of the world meant human mate-choice criteria would also have to be quite different if the focus was similar, and he himself doubted that, citing reports indicating that ideals of beauty did not, in fact, vary in this way around the world. Men are generally hairier than women, and Darwin was of the opinion that hairlessness was related to sexual selection; however, several other explanations have been advanced to explain human hairlessness, a leading one is loss of body hair to facilitate sweating. This idea closely relates to that of the suggested need for increased photoprotection and is part of the most-commonly-accepted scientific explanation for the evolution of pigmentary traits. Indicating that a trait is under sexual selection can be difficult to prove through correlational methods, as characters may result from different selective pressures, some involving sexual selection, others natural selection, and some may be accidental and due to pleiotropy. For example, monogamous primates are known to typically exhibit little sexual dimorphism such as particularly large males armed with huge canines; however, powerful big-toothed males can provide protection against predators and may be bigger for that reason, rather than in order to win confrontations over females. Males and females differing in size can specialize in, and more fully exploit, different food resources while avoiding competing with each other; furthermore, body size can be useful in avoiding predators and may also be of assistance in securing a mate. This is further complicated by the consideration that with larger body size, the skeleton of mammals becomes much more robust and massive (relatively speaking). Bearing these caveats in mind, levels of sexual dimorphism are generally seen as a marker of sexual selection. Studies have shown the earliest homininae were highly dimorphic and that this tendency lessened over the course of human evolution, suggesting humans have become more monogamous. In contrast, gorillas living in harems exhibit a much stronger sexual dimorphism (see: homininae). The theory of sexual selection has been used to explain a number of human anatomical features. These include rounded breasts, facial hair, pubic hair and penis size. The breasts of primates are flat, yet are able to produce sufficient milk for feeding their young. The breasts of non-lactating human females are filled with fatty tissue and not milk. Thus it has been suggested the rounded female breasts are signals of fertility. Richard Dawkins has speculated that the loss of the penis bone in humans, when it is present in other primates, may be due to sexual selection by females looking for a clear sign of good health in prospective mates. Since a human erection relies on a hydraulic pumping system, erection failure is a sensitive early warning of certain kinds of physical and mental ill health. Homo has a thicker penis than the other great apes, though it is on average no longer than the chimpanzee's. It has been suggested the evolution of the human penis towards larger size was the result of female choice rather than sperm competition, which generally favors large testicles. However, penis size may have been subject to natural selection, rather than sexual selection, due to a larger penis' efficiency in displacing the sperm of rival males during sexual intercourse. A model study showed displacement of semen was directly proportional to the depth of pelvic thrusting, as an efficient semen displacement device.