Sperm competition

Dean, M.D., Ardlie, K.G. and Nachman, M.W. (2006) The frequency of multiple paternity suggests that sperm competition is common in house mice (Mus domesticus). Mol. Ecol. 15,4141—4151. 

Wedell, N., Gage, M.J.G. and Parker, G.A. (2002) Sperm competition, male prudence and sperm-limited females. Trends Ecol. Evol. 17, 313-320. 

Baker, R. R. Beilis, M. A. 1995. Human sperm competition. Copulation, masturbation and infidelity. London Chapman and Hall. 

Smith (1984) suggested that sperm competition would occur whenever a woman had sex with two different men within 7—9 days, this being the maximum active life of human sperm once inside the cervix. He was probably correct (Baker and Beilis 1995), but to be conservative, I have in the past used a 5-day criterion, using the fertile rather than the active life of sperm (Baker and Beilis 1995). Other authors (e.g. Gomendio and Roldan 1993) would prefer to be even more conservative and use a criterion of only 2-3 days but, as shown previously (Baker and Beilis 1995), even this actually makes very little difference to estimates of the level of sperm competition. If a woman is going to have sex with two different men within the space of 7 days, she almost always does so within 2-3 days, perhaps precisely so as to promote the most active sperm competition (Beilis and Baker 1990). Estimates for the UK suggest that about 4% of children are the result of sperm competition, each being conceived while their mother contains sperm in her reproductive tract from two or more different men (Baker and Beilis 1995). 

Over the past few years, research at Manchester has led to a relatively detailed hypothesis of the way that sperm competition may have shaped the sexual strategies of both men and women (Baker and Beilis 1995 Baker 1996). The various suggestions are outlined at different points in this paper, alongside results from new experimental investigations. 

The main theme of this rather multi-faceted paper is that the importance of sperm competition to human sexuality varies not only from person to person but also from one stage of sexual ontogeny to another. 

COPULATION, MASTURBATION AND INFIDELITY SPERM COMPETITION AND FEMALE SEXUAL STRATEGY... 

One of the themes of this paper is that the importance of sperm competition varies at different stages in both male and female sexual ontogeny. This section is concerned with the role of sperm competition in female sexual strategy. 

The second phase (Phase II) begins when the female (ideally) has found and attracted a male who is an acceptable compromise in terms of the genes and resources offered. Reproduction eventually begins. The woman s second child is the most likely of all her children to have been fathered by her partner, the first sometimes having been fathered by one of her previous male partners (towards the end of Phase I) (Schacht and Ger-showitz 1963 Baker and Beilis 1995). In the middle of Phase II, fidelity is at its greatest and the risk of sperm competition at its lowest. 

Sperm Competition and Female Behaviour from Phase I to Phase III of Reproductive Ontogeny... 

The indication was, therefore, that for women in Phase I of their reproductive ontogeny, EPCs (and any sperm competition that might incidentally result) were part of a shopping around for resources not genes. In contrast, the proportion of copulations that were EPCs by the Phase II or III females in the Company survey increased as the probability of conception increased (Fig 2a). The indication was, therefore, that EPCs and any sperm competition that resulted were part of this population s shopping around for genes. 

If women are using infidelity, and hence sperm competition, in different ways at different stages in their lives, we might also expect men to adjust their ejaculates in different ways at different stages in their lives. However, before we can examine the ways that men adapt to sperm competition at their different ontogenetic stages, it is necessary first to know what other factors influence ejaculate size. 

SPERM COMPETITION AND SPERM NUMBERS DURING IN-PAIR COPULATION... 

During this relationship, the man will routinely inseminate his partner at more or less frequent intervals, depending on many factors. During this routine in-pair sexual activity, the risk of sperm competition will vary from one copulation to another, again depending on many factors. 

Sperm Competition Theory has long predicted that when the risk of sperm competition is higher, males should inseminate more sperm (Parker 1982 Parker 1990). This prediction has now been supported by many different studies of many different animals, and the principle has been found to apply across species, within species and from insemination to insemination by the same male (Harvey and Harcourt 1984 Beilis et al. 1990 Gage and Baker 1991). 

One of the first successes in the study of human sperm competition was the demonstration that the number of sperm in a man s ejaculate during In-Pair Copulation (IPC) with an established partner was a function of the risk of that partner already containing sperm from another man (Baker and Beilis 1989b). Later, it was shown that individual males made this adjustment from one IPC to the next (Baker and Beilis 1993a, 1995). 

The parameter used in those studies to measure the risk of sperm competition was the percentage of time the couple had spent together since their last copulation (or in the last ten days, whichever was the shorter). This parameter has since been verified as a good index of the risk of sperm competition (Baker and Beilis 1995). 

COPULATION AND INFIDELITY SPERM COMPETITION SPECIALISTS AMONG MEN ... 

Figure 4. The number of sperm inseminated during In-Pair Copulation (IPC) increases with the risk that the female contains sperm from another man. Risk of sperm competition is measured as the percent of time the couple have spent together since their last IPC (or the last 10 days, whichever is the shorter). Lower percent times together are associated with higher risks of sperm competition. Residual sperm numbers calculated from the parameters listed in Table 1 modified by exclusion of percent time together. All of the parameters listed are therefore statistically controlled (including hours since last ejaculation). Pseudo-replication of data is avoided by including only the first IPC inseminate per couple. Number of couples per data point as shown, F, 47 =5.7, P=0.022.

I hypothesised (Baker and Beilis 1995 Baker 1996) that the within-population frequency distribution of testes size reflects a balanced polymorphism. In this hypothesis, men with larger testes were programmed to spend less time with partners, ejaculate often, and expose their sperm more often to sperm competition. Men with smaller testes were programmed to invest more in mate guarding and only infrequently to expose their sperm to competition. Between the two extremes, the intermediate men were programmed to pursue a more mixed strategy. 

For the moment, therefore, the hypothesis that men with larger testes are a morph genetically programmed for a greater involvement in sperm competition continues to be supported. 

Bisexuals begin their sexual explorations at an earlier age, have more sexual partners (both male and female), and are at a greater risk to disease than heterosexuals (Kinsey et al. 1948 Johnson et al. 1994). Whereas bisexuals probably have fewer children than heterosexuals within long-term relationships, they probably have them younger (like lesbian bisexuals), and have more via short-term sexual liaisons and sperm competition (see review in Baker and Beilis 1995),... 

The prediction was, therefore, that bisexuals should produce ejaculates that are more competitive and more prepared for sperm competition than heterosexuals. On the basis of Sperm Competition Theory, it was predicted that bisexuals should inseminate women with ejaculates containing more sperm than heterosexuals. 

At first sight, therefore, there is no support from a study of bisexuals ejaculates for the hypothesis that bisexuality is a syndrome adapted to sperm competition. The situation is discussed further below. 

Thus far in this paper, I have established that once men reach the stage in their sexual ontogeny when they have a long-term partner, they meet the expectation of Sperm Competition Theory and inseminate their partner with more sperm when the risk of sperm competition is higher. Some men, particularly those with large testes and/or with greater symmetry, seem more likely to behave in a way that will involve them in sperm competi-... 

Table 2. A test of the prediction by Sperm Competition Theory that a male should inseminate more sperm during Extra-Pair Copulations than during copulations with an established partner. The prediction is not supported. Data from Baker Bainbridge (unpublished)...

The current data set contains 20 ejaculates (from 11 different men with 13 different women) which were collected under circumstances other than within a long-term partnership. There were three main situations (see Material and Methods). All were outside of a partnership (and thus qualify as EPCs) and all were situations in which the man would have had every reason, consciously and/or subconsciously, to assume that the woman already contained sperm from another man. Sperm competition theory, therefore, would predict that the male should inseminate an above average number of sperm (Parker 1990). However, neither actual sperm numbers, numbers controlled for hours since last ejaculation, nor numbers controlled for all of the factors in Table 1, supported the prediction (Table 2). On the contrary, males seemed to inseminate fewer sperm into women who were not their established partner. 

It seems, therefore, that men respond to an elevated risk of sperm competition during IPCs by inseminating their long-term partners with more sperm but to an elevated risk during EPCs by inseminating their short-term partners with fewer sperm. Why - and is it in some way due to a link between masturbation and sperm competition ... 

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