Are factors apart from sperm functional parameters important in assessing potential male fertility?

In the last two decades more emphasis has been placed on sperm functional parameters and their predictive value in male fertility potential. Some major functional parameters or approaches to sperm functionality are graphically depicted in Fig. 1 [after van der Horst and du Plessis (2017)]. Most of these sperm functional parameters relate to the challenges the sperm must overcome inside the female reproductive system (particularly the cervix and the oviduct) in order to fertilize the oocyte.

Fig. 1. Basic and functional semen tests. AR – acrosome reaction; CASA – computer-aided sperm analysis; HBA – hyaluronan binding assay; HOS test – hypo-osmotic swelling test; HZA – hemizona assay; ROS – reactive oxygen species; SDF – sperm DNA fragmentation.

Males with very good semen parameters as well as good sperm functionality (Table 1 and Fig. 1) may still struggle to have offspring and in many cases there does not appear to be any problem in terms of the fertility of their partner. In this instance the male is often considered to suffer from some or other idiopathic factor. A question that arises then are there any other factors that compromise fertility or not allowing conception in the female that are independent of good semen parameters and sperm functionality as indicated above?

Stress

Stress seems to be one of the major factors that influence male fertility. Current theories suggest that it may operate at many levels. It causes the release of glucocorticoids from the adrenal glands and affects metabolism at many levels and among others decrease testosterone levels. The decrease in testosterone results in lower libido but testosterone is also important in spermatogenesis and accordingly results in a decrease in sperm quality. Stress could also trigger oxidative stress whereby un-neutralized free radicals affect semen quality and fertility.

The psychological pressure in the male to try to conceive or because of the forced timing of intercourse around the woman’s ovulatory cycle seem to further enhance stress.

Frequency of sexual intercourse

A common mistake is the belief that long abstinence is required prior to the so called five-day window in which conception may be more likely in humans. It is now well established that a longer than five-day abstinence is associated with poor sperm quality. Furthermore, it has recently been determined by laboratories in Cape Town, Denmark and London that a very short abstinence period (2 to 4 hours) results in sperm with better functionality such as a high percentage sperm motility, progressive sperm motility, less ROS and fragmentation among others than during three days of abstinence. There is evidence to suggest that couples that have sex every day have a slight advantage over couples that have sex every other day in terms of satisfaction and conception. Accordingly, the so called five-day window period of ovulation is over emphasized as a sex restricted period preceded by the now outdated long abstinence period. Theoretically sperm reserves cannot be exhausted due to the extremely high rates of sperm production (an average human male produces 1500 sperm every second!!! Or 5.4 million sperm every hour!!!).

Quality of intercourse and what have we learnt from the animal world?

We have very little knowledge of this aspect in humans and can only extrapolate from particularly the domestic animals such as merino rams. At the University of Stellenbosch merinos have been bred for high fertility (one ram with 30 ewes will produce on average 31 offspring – also twins) and merinos bred for lower fertility (one ram with 30 ewes will produce on average 24 lambs). We analysed the semen parameters of these two groups of rams using SCA CASA and by and large the semen quality and several sperm functional parameters were similar in the two groups. So which factor(s) resulted in the lower lamb rates in the lower fertility group? An Australian study actually showed that in high fertility rams ejaculation occurred with every copulation (as many as 30 times per day) but was considerably less in rams with lower fertility (less offspring). This equates to the quality of copulation as an important factor that seems to be independent of sperm quality and may even play a role in humans?

Cryptic female choice (CFC)

Cryptic female choice (CFC) has only in the last three decades received major attention. «Sexual selection by cryptic female choice can result from a female-controlled process or structure that selectively favours paternity by conspecific males with a particular trait over that of others that lack the trait when the female has copulated with both types» (Eberhard 1996)”. A simpler definition of CFC is a form of mate choice which occurs both in pre and post copulatory circumstances when females of some species use physical or chemical mechanisms to control a male’s success of fertilizing their oocytes.

In ducks for example there is forceful copulation by several males but the female has a valve that allows her to squirt out sperm of the male that she apparently “dislikes”. It may be the male that has poor sperm traits or poor survival genes that is “somehow sensed” by the female duck?

It is interesting to speculate on these mechanisms in humans. One study has shown that the most important reason a woman “intuitively” (CFC?) selects a specific male to have children with is that he is the one that will care best for their future children. If this can be verified it seems to underpin yet another evolutionary force for the survival of the species.

We still have a very poor understanding of these mechanisms but at least this blog has demonstrated that many other factors apart from sperm quality determines live birth outcome.

Image 1: Distribution of semen examination results from men in couples starting a pregnancy within one year of unprotected sexual intercourse leading to a natural conception. From Campbell et al. (5); fifth percentile given with variability (95% confidence interval) and as presented in the WHO 6 (2021) manual.

Prof Gerhard van der Horst (PhD, PhD)
Senior Consultant
MICROPTIC S.L.