Sperm number

Most of the aspects of sexuality studied here (e.g. sperm numbers attractiveness) are influenced by many variables which cannot be controlled experimentally. Instead, they are controlled statistically, using the following procedure. 

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

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.

Such analyses use mean values for testis size, sperm numbers and other parameters for the different species. Yet within species, differences between individuals can often be as extreme as differences in means across species. Moreover, these inter-individual differences are often heritable. 

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. 

This does not mean that the earliest predictions of Sperm Competition Theory have all been supported. In particular, it now seems that there are many more elements to a competitive ejaculate than simply sperm number, with sperm size, sperm morphology and now sperm age being just three that have so far been implicated. It also seems, from this paper, that the most strategic ejaculate may well be different for the IPC male and the EPC male. 

Sperm evaluation11 Sperm number (count, concentration), quality (morphology, motility), chromosomal defects... 

Sperm evaluations routinely include data on sperm number, morphology and motility (Berman etal., 1996). Testicular spermatid count or cauda epididymal weight are other useful measures of sperm production, but no surrogate measures are adequate to evaluate sperm morphology or motility. A number of other indices of sperm function have been employed (e.g., sperm-zona interactions, acrosome reaction, sperm swelling studies), but these have been of limited utility and infrequently used and have not been incorporated into any testing guidelines. 

In test species, sperm number or sperm concentration can be evaluated in ejaculated, epididymal or testicular samples (Seed et al.,... 

The number of sperm in an ejaculate is influenced by several variables, including the length of abstinence and the ability to obtain the entire ejaculate. Intra- and interindividual variability are often high, but less variability is observed in ejaculates collected at regular intervals from the same male (Williams et al., 1990). Because sperm contribute to epididymal weight in experimental animals, expression of results as a ratio of sperm counts to epididymal weight may actually mask a decline in sperm number, and absolute sperm counts can improve resolution. 

Bird et al. (1983) found that when captive American kestrels Falco sparverius) were fed 8 mg kg-1 of mirex for 69 days, there was a decline in sperm concentration and a slight increase in semen volume. An overall net decrease of 70% in sperm number was observed. The investigators... 

Sommer RJ, Ippolito DL, Peterson RE. 1996. In utero and lactational exposure of the Male Holtzman rat to 2,3,7,8-tetrachlorodibenzo-p-dioxin decreased epididymal and ejaculated sperm numbers without alterations in sperm transit rate. Toxicol Appl Pharmacol 140 146-153. 

The assessment of sperm quality includes measures of sperm number, anomalies, and motility, but the ability of the sperm to fertilize the ovum is not assessed. In order to increase the sensitivity for detection of fertility effects, investigation of in vitro fertilization could be considered. Another potentially more sensitive option may be the continuous breeding protocol, where the animals produce several litters instead of only one litter per pair. 

Depending on dose, external irradiation may lead to different types of reproductive toxicity in the male companion animal (Scialli et al, 1995, cited in Ellington and Wilker, 2006 De Celis et al, 1996, cited in Ellington and Wilker, 2006). At very high doses, it may lead to permanent aspermia. At intermediate doses it may lead to reduction in sperm numbers. Finally, at lower doses external irradiation may lead to DNA alterations in sperm cells (Scialli et al, 1995, cited in Ellington and Wilker, 2006 De Celis et al, 1996, cited in Ellington and Wilker, 2006). 

Reproductive Effects. Studies regarding possible reproductive effects in humans exposed to formaldehyde are restricted to a study that found no evidence for effects on sperm numbers or morphology in a small number of pathologists (Ward et al. 1984) and a study that found no evidence for increased rates of miscarriage among a group of 275 persons with presumed residential exposure to formaldehyde (Garry et al. 1980). 

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