Masturbation

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

Following Smith (1984) I have argued that the risk of sperm warfare has helped to shape the evolution of just about every aspect of human sexuality - from testis size to penis shape, and from the wet sheet phenomenon to masturbation and the female orgasm (Baker and Beilis 1995 Baker 1996). 

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

Short gap since last insemination Masturbation, Nocturnal or other orgasm... 

Previous studies of the number of sperm men ejaculate have had medical origins and have almost universally involved samples collected via masturbation. The multiple regression equations that are the basis of the single variable analyses presented in this paper are given in Table 1. The variables of greatest interest to ethologists are discussed in later sec-... 

Table 1. Multiple regression equations for the number of sperm inseminated during copulation or ejaculated during masturbation. Also shown are the variables entered but subsequently removed from the equations because P>0.05. The equations are based on the total data sets with missing values for independent variables replaced by means as explained in Materials and Methods. Both equations are a very significant fit to the data (P<0.001) and all variables have P<0.05. However, because of pseudo-replication, the regression equations are a tool rather than ends in themselves...

Table 3. Number of sperm ejaculated during IPC and masturbation by men who did and did not contribute EPC ejaculates The indication is that it is EPC ejaculates that are different, not the men in the Manchester whole-ejaculate study who contributed them. Analysis of first sample of each type per couple. Data from Baker Bainbridge (unpublished)...

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 ... 

Analysis of the first sample of each type produced by the male of each pair shows that there is a significant heterogeneity in the time to last ejaculation for the different ejaculates (ANOVA F2 144 = 3.33 P = 0.038) with EPCs following more quickly after a recent ejaculate than IPCs and masturbations (EPCs 25 + 6 hrs after the previous ejaculation IPCs 53 7 masturbations 41 3 N = 15, 60 and 72 respectively). Moreover, the recent ejaculation was more likely to be a masturbation (60%, 12/20 before EPC 33%, 116/350 before IPC y2 = 6.031 P = 0.014). 

It is not, however, that the males who contributed EPC ejaculates simply had a higher masturbation rate. Although the sample size for IPCs from these men is small and the heterogeneity not significant (P = 0.24), the men who contributed EPCs show a pattern of abstinence times similar to that by the total contributors (EPCs 22 5 h IPCs 39+12 h Masturbations 46 + 9 h N = 20, 10 and 44 respectively). Moreover, the masturbation rate for EPC contributors (every 46 h) is about the same as for men without partners (every 42 4 h N = 27). Bisexuals, however, maintain a higher rate (every 28 + 7 h N = 5) with an abstinence interval about the same as that which precedes EPCs. 

Other authors have proposed that masturbation occurs because sperm have a limited shelf life (Levin 1975). Here, however, the hypothesis is that masturbation produces a more competitive EPC ejaculate. If so, it could imply that, because of their high ejaculation rate, bisexuals are permanently prepared for EPCs whereas heterosexuals anticipate each event opportunistically. In which case it remains possible that, as hypothesised, bisexuals are a morph adapted to an above average involvement in sperm competition (section 5.2). 

Figure 7. Influence of the time interval between a male masturbating and then inseminating a female on the number of sperm inseminated and the number of spenn retained. Although longer abstinence times (masturbation to copulation) increases the number of sperm ejaculated into the woman, the number of sperm she retains does not increase (Baker and Beilis 1993a,b). If the competitiveness of an ejaculate is a trade-off between the age and number of sperm, ejaculates preceded by masturbation a day beforehand may well be the most competitive for an EPC male (Baker 1996).

Men inseminate fewer sperm if they have masturbated beforehand, the amount depending on how recently the masturbation occurred (Baker and Beilis 1993a). The flow-back study shows, however, that the number retained by the female does not decrease (Fig. 7). Inseminates preceded by masturbation are retained in higher proportions and similar numbers as inseminates not so preceded. Yet the sperm inseminated are younger. They should therefore live longer inside the female and, whether the sperm competition proceeds as a lottery, a race or as warfare, could be at a competitive advantage. 

If this is the explanation, men with large testes and more-symmetrical men would gain a double advantage. Because they seem to manufacture more sperm all together (ejaculating more sperm during both copulation and masturbation), they can inseminate... 

As Sperm Competition Theory is applied to more and more organisms, its early promise as a paradigm for the understanding of sexual behaviour has been maintained. I suggest that human sexuality is no exception. As I hope this paper continues to show, there are few aspects of human copulation, masturbation and infidelity that have not been shaped during evolution, at least in part, by the threat of sperm competition. 

In contrast to this ever-nearer dead-end for the direct investigation of ejaculates, the possibilities for investigating the way that testis size and symmetry influence male (and, for symmetry, female) reproductive strategy at different stages of their lives seem to be widening. There is still much more to discover about human copulation, masturbation and infidelity than is discussed in this interim description of the state-of-the-art. 

Levin, R. J. (1975). Masturbation and nocturnal emissions - possible mechanisms for minimising teratozoospermie and hyperspermie in man. Medical Hypotheses 1, 130-131. 

Perhaps all this dated alchemical terminology seems much too richerche to you. Instead, perhaps you have read, and even agreed with, those scholars who say that this apparent self-portrait of the naked young Marcel in a train really represents him masturbating. If you believe that, then you may also... 

---