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. 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 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 ipc may have evolved as anticuckoldry tactics in avian species and in humans. No evidence for killer sperm or other selective interactions. For example, the shape of the human penis may have been selectively shaped by sperm competition. 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. Sperm competition and the evolution of reproductive. 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.
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. 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. Evolution and human sexuality wiley online library. 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. Psychological and physiological adaptations to sperm. 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. 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. 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. Dixson 1998 is consistent with predominantly singlemale mating. Bellis, journalanimal behaviour, year1993, volume46, pages887909. On the other hand, ejaculates are costly to produce and males are favoured who economize over the number of.
Mate retention, semen displacement, and human sperm competition. There is a very simple primal answer to why your husband enjoys this so. 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 an individual male has. Although lloyd doubts this, significant sperm competition has probably occurred over human evolution. Mate retention, semen displacement, and human sperm. Goetz florida atlantic university abstractwith the recognition, afforded by recent evolutionary science, that female in. The reader will not find reliable answers to these questions in baker and bellis book.
When sperm competition occurs, fertilization success may be determined, in part, by the outcome of direct interactions between sperm from rival males. Pdf sperm competition occurs when a female copulates with two or more males within a sufficiently brief time period, resulting in the sperm of. 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. Competition can occur when females have multiple potential mating partners. In this study, we examine the relationship between mate guarding and ejaculate quality using humans as an empirical model. Mate guarding and frequent inpair copulation in humans. We used ejaculates from human donors to compare mixes of semen in vitro from the same or different males. Human sperm competition shackelford major reference. Psychological adaptation to human sperm competition 2002.
One evolutionary response to sperm competition is the variety in penis morphology of many species. Before the church and modern morals females were free to mate with as many males as they cared to. Sperm competition and the evolution of reproductive systems. In species where females mate with multiple males, the sperm of two or more males must compete to fertilise available ova. Selection from sperm competition is expected to favor opposing adaptations that function either in the avoidance of or engagement in 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. Semen displacement as a sperm competition strategy in. The fossil record suggests a human pattern of low bodysize sexual di. Sperm competition and its evolutionary consequences in humans. 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. 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. Dec 07, 1999 this study examines one of the possible mechanisms of sperm competition, i. Males have adapted to battle with competing sperm date. The risk of sperm competition has been argued to shape the evolution of almost every aspect of human sexuality.
This hypothesis states that sperm from different males interact to incapacitate each other in a variety of ways. Sperm from one copulation appeared to hinder the retention of sperm at the next copulation for up to 8 days. Human sperm competition ebook by robin baker 9788461686506. Pdf on jan 1, 2017, robin baker and others published human sperm competition find, read and cite all the research you need on. 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. Pdf adaptation to sperm competition in humans aaron. It was good for the species, whether the females mate liked it or not. Mechanisms of human sexual selection sexual selection can operate through multiple mechanisms, including contests, mate choice, scrambles, sexual coercion, and sperm competition table 1. 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. Semen displacement as a sperm competition strategy in humans. Sperm competition and the evolution of spermatogenesis. 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.
A comparative evolutionary analysis article pdf available in animal behavior and cognition. In some avian species, the tactics are used concurrently, with the result that mate guarding behaviors and ipc. 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 human penis may have been selected to displace seminal fluids implanted in the female reproductive tract by a rival male. Each of these mechanisms of sexual selection has probably played a role over human evolution, but contest competition the use. Human sperm competition by robin baker overdrive rakuten. 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. 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. Cuckoldry is an adaptive problem faced by parentally investing males of socially monogamous species e. Sperm competition occurs whenever sperm from two or more males compete to fertilize a given set of ova parker, 1970, 1998. Lamunyon d a department of psychology, florida atlantic university, 2912 college avenue, davie, fl 33314, usa.
The efficiency of the block declined with time after copulation but was fixed at its current level by an intercopulatory orgasm which. 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. 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. For older teens and adults, masturbation usually ends in what is called an ejaculation the release of liquid called semen which contains sperm cells. This process is experimental and the keywords may be updated as the learning algorithm improves. 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. In this chapter, we present the evidence of sperm competition in humans. 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. Copulation, masturbation, and infidelity springerlink. Sperm competition also likely explains variation in the duration of spermatogenesis. The low degree of human sperm competition birkhead 2000. Over the past decade, there has been a vast amount of interest in the subject of human sperm competition. Adaptations for the avoidance of sperm competition can include the use of mate guarding or anti.
In 1995, dr mark bellis and i published a book called human sperm competition. My husband wants to watch me with another man and im. Human males appear to have evolved mechanisms for anticipating risk of partner extrapair copulation and associated sperm competition. There are, however, striking differences in penis morphology between different species see birkhead, 2000. Sperm competition in humans classic and contemporary. 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. Sperm competition favors large testes and rapid evolution of proteins associated with ejaculate. Pdf psychological adaptation to human sperm competition. These values suggest that the risk of sperm competition in humans is. Sperm competition human female human male cervical mucus subordinate male these keywords were added by machine and not by the authors. In 1998, craig venter, founder of the institute for genomic research in maryland, usa, announced that he had formed a new private company later to become celera genomics to take on the task of sequencing the human genome. This results primarily as a consequence of polyandrous mating systems, or due to extra.
However, as a guy interested in both the practical application of science and the science itself, i would have liked an inbetween version. 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. Greater choice and variety of mates increases a females chance to produce more viable offspring. Psychological adaptation to human sperm competition. This book discusses these implications for human sexual behaviour and human infertility problems. Masturbation simple english wikipedia, the free encyclopedia. The semen displacement hypothesis integrates considerable information about genital morphology and human. This ejaculation is usually accompanied by a powerful, pleasurable feeling called an orgasm. Information about the openaccess article human sperm competition.