Semen displacement as a sperm competition strategy in humans. Sperm competition occurs when the sperm of two or more males simultaneously occupy the reproductive tract of a female and compete to fertilize an egg. Greater choice and variety of mates increases a females chance to produce more viable offspring. There is a very simple primal answer to why your husband enjoys this so. Masturbation simple english wikipedia, the free encyclopedia. Human males appear to have evolved mechanisms for anticipating risk of partner extrapair copulation and associated sperm competition.
The low degree of human sperm competition birkhead 2000. Mate retention, semen displacement, and human sperm. My husband wants to watch me with another man and im. Such is the case with human sperm competition no doubt it occurs, but how often and whether it has shaped human evolution still needs clarification. Psychological adaptation to human sperm competition 2002. Sperm from one copulation appeared to hinder the retention of sperm at the next copulation for up to 8 days. I therefore first want to clarify exactly what sperm competition is, and explain why we think it is an important factor for the readership of molecular human reproduction to consider.
The book is a pioneering analysis of the evolutionary biology of human sexuality, proposing that all aspects have been shaped by the phenomenon of sperm. Human sperm competition shackelford major reference. Sperm competition results when multiple males mate with the same female, the winners sperm fertilizing her eggs. Behavioural ecologists and evolutionary biologists have for a number of years been interested in the biological implications of sperm from different males competing for fertilization of the egg in the female tract. Sperm competition and its evolutionary consequences in humans. There are, however, striking differences in penis morphology between different species see birkhead, 2000. Sperm competition and the evolution of reproductive systems. Pdf on jan 1, 2017, robin baker and others published human sperm competition find, read and cite all the research you need on. However, as a guy interested in both the practical application of science and the science itself, i would have liked an inbetween version. This book discusses these implications for human sexual behaviour and human infertility problems. We used a questionnaire to investigate psychological responses to the risk of sperm competition for 194 men in committed, sexual relationships in the united states and in germany. Mechanisms of human sexual selection sexual selection can operate through multiple mechanisms, including contests, mate choice, scrambles, sexual coercion, and sperm competition table 1.
Selection from sperm competition is expected to favor opposing adaptations that function either in the avoidance of or engagement in sperm competition. The fossil record suggests a human pattern of low bodysize sexual di. With the recognition, afforded by recent evolutionary science, that female infidelity was a recurrent feature of modern humans evolutionary history has come the development of a unique area in the study of human mating. We examine some of the implications of the possibility that the human penis may have evolved to compete with sperm from other males by displacing rival semen from the cervical end of the vagina prior to ejaculation. Sperm competition in humans classic and contemporary. In some avian species, the tactics are used concurrently, with the result that mate guarding behaviors and ipc. Evolution and human sexuality wiley online library. This sperm competition is probably both a lottery and a race, but more than anything it could also be a war, with sperm of different morphologies playing different roles. This, unsurprisingly, sparked off a rivalry with the team running the human genome project and a race to be the first to sequence the human genome. Cuckoldry is an adaptive problem faced by parentally investing males of socially monogamous species e. Sperm competition human female human male cervical mucus subordinate male these keywords were added by machine and not by the authors. The human penis may have been selected to displace seminal fluids implanted in the female reproductive tract by a rival male. Sperm competition and the evolution of reproductive. Competition can occur when females have multiple potential mating partners.
Semen displacement as a sperm competition strategy in humans double mating for semen displacement to be adaptive it presupposes situations in which human females have sex with multiple two or more males in fairly close successiontemporal proximity to. Mate retention, semen displacement, and human sperm competition. Semen displacement as a sperm competition strategy in. Human sperm competition ebook by robin baker 9788461686506. This ejaculation is usually accompanied by a powerful, pleasurable feeling called an orgasm. In addition to the ostensible impact of female choice on the evolution of more elaborate male genitalia eberhard, 1996, there is reason to believe that sperm competition played a role in shaping the human penis. Sperm competition theory argues that the number of sperm inseminated into a female by a male is a tradeoff between two opposing pressures. Mate guarding and frequent inpair copulation in humans. Males have adapted to battle with competing sperm date.
In 1995, dr mark bellis and i published a book called human sperm competition. But until human sperm competition there had never been a discussion of the phenomenon for the human species in book form, despite its relevance for a full understanding of human reproduction. Sperm competition occurs between sperm from two or more rival males when they make an attempt to fertilize a female within a sufficiently short period of time. In humans, female physical attractiveness and personality characteristics that attract rival males may increase the likelihood of female sexual infidelity and, therefore, place. However, multiple mates for a female means each individual male has. Pdf sperm competition occurs when a female copulates with two or more males within a sufficiently brief time period, resulting in the sperm of. However, multiple mates for a female means an individual male has. When sperm competition occurs, fertilization success may be determined, in part, by the outcome of direct interactions between sperm from rival males.
Each of these mechanisms of sexual selection has probably played a role over human evolution, but contest competition the use. A very interesting essay from the journal of evolutionary psychology on sperm competition and its role in shaping the human penis, size, length, width, forceful ejaculation, evolutionary strategies for successful fertilization i. Sep 24, 2014 where males can successfully avoid sperm competition from rivals, one might expect a decrease in their expenditure on tactics for the engagement in sperm competition and vice versa. On the one hand, the risk that sperm may find themselves in competition with the sperm from another male favours the male inseminating more sperm. Sperm competition in animals is thought to be an involuntary action, like the beating of a heart or the mobilization of the immune system or, well, that thing that happens upon delivery of said sperm. Before the church and modern morals females were free to mate with as many males as they cared to. Semen displacement as a sperm competition strategy in humans double mating for semen displacement to be adaptive it presupposes situations in which human females have sex with multiple two or more males in fairly close successiontemporal proximity to one another. Human sperm competition by robin baker overdrive rakuten. Sperm competition occurs whenever sperm from two or more males compete to fertilize a given set of ova parker, 1970, 1998. In species where females mate with multiple males, the sperm from these males must compete to fertilise available ova. Adaptations for the avoidance of sperm competition can include the use of mate guarding or anti.
In this study, we examine the relationship between mate guarding and ejaculate quality using humans as an empirical model. On the other hand, ejaculates are costly to produce and males are favoured who economize over the number of. We used ejaculates from human donors to compare mixes of semen in vitro from the same or different males. Number of sperm in human ejaculates varies in accordance with sperm competition theory 1989. Sperm competition is the competitive process between spermatozoa of two or more different males to fertilize the same egg during sexual reproduction. Masturbation for males is the act of stimulating the penis manually to provide selfpleasure. The reader will not find reliable answers to these questions in baker and bellis book. This hypothesis states that sperm from different males interact to incapacitate each other in a variety of ways. Over the past decade, there has been a vast amount of interest in the subject of human sperm competition.
Pdf psychological adaptation to human sperm competition. For older teens and adults, masturbation usually ends in what is called an ejaculation the release of liquid called semen which contains sperm cells. It was good for the species, whether the females mate liked it or not. For example, the shape of the human penis may have been selectively shaped by sperm competition.
Sperm competition also likely explains variation in the duration of spermatogenesis. A comparative evolutionary analysis article pdf available in animal behavior and cognition. Goetz florida atlantic university abstractwith the recognition, afforded by recent evolutionary science, that female in. In species where females mate with multiple males, the sperm of two or more males must compete to fertilise available ova. These values suggest that the risk of sperm competition in humans is. Mate guarding and frequent inpair copulation ipc may have evolved as anticuckoldry tactics in avian species and in humans. Bellis, journalanimal behaviour, year1993, volume46, pages887909. Sexual selection from sperm competition is expected to favor opposing adaptations in males that function either in the avoidance of sperm competition by guarding females from rival males or in the engagement in sperm competition by increased expenditure on the. The risk of sperm competition has been argued to shape the evolution of almost every aspect of human sexuality. The penis evolved as an internal fertilization device.
Human sperm competition sets out the thesis that adopting an evolutionary approach to human reproduction exposes the subtle and sophisticated ways in which human sexual anatomy, physiology and behaviour are designed to interact. Sperm competition and the evolution of spermatogenesis. In this chapter, we present the evidence of sperm competition in humans. Pdf adaptation to sperm competition in humans aaron. Copulation, masturbation, and infidelity springerlink. Throughout the book, reference is made to work on the sexual behavioural ecologists and evolutionary biologists have for a number of years been interested in the biological implications of sperm from different males competing for fertilization of. One evolutionary response to sperm competition is the variety in penis morphology of many species. Dixson 1998 is consistent with predominantly singlemale mating.
This process is experimental and the keywords may be updated as the learning algorithm improves. Psychological and physiological adaptations to sperm. Sperm competition favors large testes and rapid evolution of proteins associated with ejaculate. This volume brings together, in one place, a key set of classic and contemporary papers that have examined possible adaptations to sperm competition in humans. Information about the openaccess article human sperm competition. Sperm competition occurs when a female copulates with two or more males within a sufficiently brief time period, resulting in the sperm of the different males competing to fertilize ova. Sexual selection from sperm competition is expected to favor opposing adaptations in males that function either in the avoidance of sperm competition by guarding females from rival males or in the engagement in sperm competition by increased expenditure on the ejaculate. The efficiency of the block declined with time after copulation but was fixed at its current level by an intercopulatory orgasm which. Psychological adaptation to human sperm competition. Sperm competition is a form of postcopulatory sexual selection whereby male ejaculates simultaneously physically compete to fertilize a single ovum. This results primarily as a consequence of polyandrous mating systems, or due to extra. Dec 07, 1999 this study examines one of the possible mechanisms of sperm competition, i. No evidence for killer sperm or other selective interactions. Lamunyon d a department of psychology, florida atlantic university, 2912 college avenue, davie, fl 33314, usa.
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