Assessment of sperm quality traits in relation to fertility in boar semen

Myo-inositol improves the viability of boar sperm during liquid storage

Introduction

Liquid preservation of boar semen is a highly preferred method for semen preservation in pig production. However, oxidative stress is the main challenge during the liquid preservation of boar semen in a time dependent manner. Therefore, supplementation of sperm with antioxidants during storage to protect them from oxidative stress has been the focus of recent research. Myo-inositol (Myo-Ins), the most active form of inositol, which belongs to the vitamin (Vit.) (B1 group has been shown to improve semen quality) (1). This study aimed to investigate whether Myo-Ins supplementation protects boar sperm in liquid preservation against oxidative stress and determine the appropriate concentration of Myo-Ins to be used in this regard.

Methods

Boar sperm was diluted with a semen extender with different concentrations of Myo-Ins (2, 4, 6, and 8 mg/mL) depending on the previous studies (1, 24). Sperm motility and viability, plasma membrane and acrosome integrity, mitochondrial membrane potential (MMP), semen time survival, and gene expression were measured and analyzed on days 0, 1, 3, 5, and 7 for the different samples.

Results

Different concentrations of Myo-Ins exerted different protective effects on the boar sperm quality. The addition of 2 mg/mL Myo-Ins resulted in higher sperm motility and viability, plasma membrane and acrosome integrity, MMP, and effective survival time. Investigation of mRNA expression patterns via qRT-PCR suggested that the 2 mg/mL Myo-Ins sample had increased expression of antioxidative genes.

Conclusion

The addition of Myo-Ins to semen extender improved the boar semen quality by decreasing the effects of oxidative stress during liquid preservation at 17°C. Additionally, 2 mg/mL is the optimum inclusion concentration of Myo-Ins for semen preservation.

Effect of Poria cocos Mushroom Polysaccharides (PCPs) on the Quality and DNA Methylation of Cryopreserved Shanghai White Pig Spermatozoa

In this study, we explore the effects of Poria cocos mushroom polysaccharides (PCPs) on the quality and DNA methylation of the cryopreserved spermatozoa of Shanghai white pigs. A total of 24 ejaculates (three ejaculate samples per boar) from eight Shanghai white pigs were manually collected. The pooled semen was diluted with a based extender supplemented with different concentrations of PCPs (0, 300, 600, 900, 1200, and 1500 μg/mL). Once thawed, the quality of the spermatozoa and their antioxidant function were assessed. In the meantime, the effect of spermatozoa DNA methylation was also analyzed. The results show that compared with the control group, 600 μg/mL of PCPs significantly improves the spermatozoa viability (p < 0.05). The motility and plasma membrane integrity of the frozen-thawed spermatozoa are significantly higher after treatment with 600, 900, and 1200 μg/mL of PCPs compared with the control group (p < 0.05). In comparison with the control group, the percentages of acrosome integrity and mitochondrial activity are significantly enhanced after the application of 600 and 900 μg/mL PCPs (p < 0.05). The reactive oxygen species (ROS), the malondialdehyde (MDA) levels, and the glutathione peroxidase (GSH-Px) activity, in comparison with the control group, are significantly decreased in all groups with PCPs (all p < 0.05). The enzymatic activity of superoxide dismutase (SOD) in spermatozoa is significantly higher in the treatment with 600 μg/mL of PCPs than in the other groups (p < 0.05). As compared with the control group, a significant increase in the catalase (CAT) level is found in the groups with PCPs at 300, 600, 900, and 1200 μg/mL (all p < 0.05). In comparison with the control group, the 5-methylcytosine (5-mC) levels are significantly decreased in all groups with PCPs (all p < 0.05). As a result of these findings, a certain amount of PCPs (600-900 μg/mL) added to the cryodiluent can significantly improve the quality of Shanghai white pig spermatozoa and can also reduce the methylation of spermatozoa DNA caused by cryopreservation. This treatment strategy may establish a foundation for the cryopreservation of semen from pigs.

Effectiveness of non-pharmaceutical intervention on sperm quality: a systematic review and network meta-analysis

Infertility affects about 10% of the world's population and has been recognized by the WHO as a global public health problem. The aim of this network meta-analysis was to investigate the efficacy of non-pharmaceutical interventions on sperm quality. All randomized clinical trials (RCTs) from the PubMed, MEDLINE, Embase, China national knowledge infrastructure (CNKI), Wanfang database, and Cochrane Library databases evaluating the effectiveness of non-pharmaceutical interventions on semen parameters using network meta-analyses. Results of the ω-3 fatty acid, lycopene, acupuncture, and vitamin suggested evident advantages in improving sperm concentration (MD, 9.93 (95% CI, 7.21 to 12.65)), (MD, 8.79 (95% CI, 2.67 to 14.91)), (MD, 5.40 (95% CI, 2.32 to 8.49)) and (MD, 3.82 (95% CI, 0.70 to 6.94) respectively). Acupuncture has a significant advantage over placebo in improving sperm total motility (MD, 17.81 (95% CI, 10.32 to 25.29)), and the effect of lycopene was obviously greater than that of placebo (MD, 19.91 (95% CI, 2.99 to 36.83)). Lycopene, Coenzyme Q10 (CoQ10), acupuncture, ω-3 fatty acid, and vitamin suggested significant advantages in improving sperm forward motility (MD, 8.64 (95% CI, 1.15 to 16.13), MD, 5.28 (95% CI, 2.70 to 7.86), MD, 3.95 (95% CI, 3.23 to 4.67), MD, 3.50 (95% CI, 2.21 to 4.79)) and (MD, 2.38 (95% CI, 0.96 to 3.80) respectively). This review establishes that non-pharmaceutical interventions, particularly acupuncture, exercise, lycopene, ω-3 fatty acids, CoQ10, zinc, vitamins, selenium, carnitine, or foods rich in these supplements, profitably improve sperm quality that may be used to treat male infertility.

The Interaction of NO and H2S in Boar Spermatozoa under Oxidative Stress

Various recent studies dedicated to the role of nitric oxide (NO) and hydrogen sulfide (H2S) in somatic cells provide evidence for an interaction of the two gasotransmitters. In the case of male gametes, only the action of a single donor of each gasotransmitter has been investigated up until today. It has been demonstrated that, at low concentrations, both gasotransmitters alone exert a positive effect on sperm quality parameters. Moreover, the activity of gaseous cellular messengers may be affected by the presence of oxidative stress, an underlying condition of several male reproductive disorders. In this study, we explored the effect of the combination of two donors SNP and NaHS (NO and H2S donors, respectively) on boar spermatozoa under oxidative stress. We applied NaHS, SNP, and their combination (DD) at 100 nM concentration in boar spermatozoa samples treated with Fe2+/ascorbate system. After 90 min of incubation at 38 °C, we have observed that progressive motility (PMot) and plasma membrane integrity (PMI) were improved (p < 0.05) in DD treatment compared to the Ctr sample under oxidative stress (CtrOX). Moreover, the PMot of DD treatment was higher (p < 0.05) than that of NaHS. Similar to NaHS, SNP treatment did not overcome the PMot and PMI of CtrOX. In conclusion, for the first time, we provide evidence that the combination of SNP and NaHS surmounts the effect of single-donor application in terms of PMot and PMI in porcine spermatozoa under oxidative stress.

Integrity of Sperm Cell Membrane in the Semen of Crossbred and Purebred Boars during Storage at 17 °C: Heterosis Effects

Simple Summary

The cell membrane of spermatozoa is the main structural element of these gametes. In boars, due to its structure, it is most susceptible to various types of damage induced by various factors. Artificial insemination in pigs mainly involves the use of liquid semen preserved at 17 °C. Thus, it is important to monitor this semen during its storage. In practice, the changes that can take place in sperm during the preservation and storage of boar semen are not analysed. Furthermore, considerable variation is observed in the characteristics of boar semen, which may depend on the breed or crossbreeding variant of the boar. Crossbred boars are often used in artificial insemination, because they not only easily produce ejaculates with good parameters, but also have good libido characteristics. However, despite the benefits of artificial insemination with semen of crossbred boars, there is insufficient knowledge of the sensitivity of cell structures to conditions associated with semen storage in comparison with boars of the parent breeds. For this reason, a study was conducted to analyse changes in the integrity of sperm cell membranes taking place during the storage of semen collected from Duroc × Pietrain crossbred boars and purebred boars of the parent breeds. The sperm of Duroc × Pietrain crossbred boars were found to be less sensitive to the conditions of semen storage and to better retain cell membrane integrity than the sperm of purebred males, which was confirmed by calculating the heterosis effects for semen assessed at different hours of storage at 17 °C.

Abstract

The aim of the study was to assess changes in the integrity of sperm cell membranes during the storage of semen collected from Duroc × Pietrain crossbred boars and purebred boars of the component breeds. To compare the cell membrane integrity of sperm heads in crossbred and purebred boars, heterosis effects were estimated. The study was conducted on 48 ejaculates collected from Duroc × Pietrain crossbred boars and from purebred Duroc and Pietrain boars used for artificial insemination. Microscope slides were prepared from each ejaculate for the evaluation of the cell membrane integrity of the sperm, at 1, 24, 48, 72, and 96 h after collection of the ejaculate. Diluted ejaculates were stored at 17 °C. Sperm membrane integrity was analysed by two methods: SYBR-14/PI and eosin–nigrosin. Our results showed that the cell membrane integrity of sperm heads changed with storage time, but the extent of the changes varied depending on the genetic group of boars. The semen of Duroc × Pietrain crossbreds was clearly seen to be less sensitive to storage conditions than that of boars of the parent breeds, which was confirmed by the calculated heterosis effects. The percentage of sperm with an intact cell membrane was higher in crossbred boars than in purebred boars (p ≤ 0.05). In addition, significantly fewer moribund sperm spermatozoa and spermatozoa with a damaged cell membrane were observed in crossbred boars (p ≤ 0.05). In the semen of purebred Duroc and Pietrain boars, the cell membrane integrity of the sperm should be assessed more often during storage than in the semen of Duroc × Pietrain crossbred boars. This study provides valuable information for the development and implementation of semen quality monitoring in crossbred boars and boars of the parent breeds during storage at 17 °C with respect to the cell membrane structure of sperm heads. The evaluation methods used effectively identify damage to the cell membranes of the sperm during semen storage.

Effects of breed and ejaculate volume on sperm morphology and semen parameters of boars

The aim of the study was to determine the relation between the semen quality, frequency of sperm defects, sperm dimensions and shape, and the ejaculate volume of Large White and Landrace boars. A total of 648 ejaculates collected from 31 Large White and 30 Landrace boars were divided into three groups according to the criterion of the ejaculate volume. In this study Landrace boars produced ejaculates with higher volume, sperm concentration, and total numbers of spermatozoa than Large White boars. Landrace boars also showed a lower frequency of sperm with morphological abnormalities (P < 0.05). Landrace boars sperm had larger heads, which were by 0.15 μm longer, and by a larger perimeter and area (P < 0.05). Landrace boar spermatozoa also had a longer flagellum and were generally larger and by 2.07 μm longer than Large White boar sperm (P < 0.05). Significant differences were also found in the shape of sperm of the two breeds (P < 0.05). Landrace boars sperm had more elongated heads, and the ratio of head size to flagellum length was lower than in Large White boars sperm (P < 0.05). Sperm from ejaculates with low volume had a shorter flagellum and a greater head length/flagellum length ratio than sperm from medium- and high-volume ejaculates (P < 0.05).

Relationship between Fertility Traits and Kinematics in Clusters of Boar Ejaculates

Simple Summary

Swine reproduction efficiency is determined by the fertility potential of the sow and sperm quality. The objective of this study is to compare boar sperm motility and kinematic features to evaluate their relationships with reproductive success after artificial insemination (AI). In this study, the movement patterns of boar ejaculates were analyzed by a computer-assisted semen analysis (CASA)-Mot system, and the kinematic values of ejaculate clusters were assessed. The semen of the Pietrain boars showed more linear trajectory of the spermatozoa, while curvilinear velocity and oscillatory movement characterized the semen of the Duroc × Pietrain boars. The offspring of sows inseminated with Pietrain boars showed significantly lower number of stillbirths. In addition, ejaculate grouping into clusters did not have a predictive capacity on litter size variables. Nevertheless, the kinematic variables of the ejaculate may have a predictive, albeit reduced, capacity regarding litter size variables. The results of this study therefore open up possibilities for future assessments of fertility.

Abstract

The aim was to determine the relationship between kinematic parameters of boar spermatozoa and fertility rates of sow, as well as to assess the effect of sperm clusters on the fertility capacity of the ejaculate. Semen samples were collected from 11 sexually mature boars. Samples were analyzed by an ISAS^®v1 CASA-Mot system for eight kinematic parameters. Ejaculate clusters were characterized using multivariate procedures, such as principal factors (PFs) analysis and clustering methods (the k-means model). Four different ejaculate clusters were identified from two kinematic PFs which involved linear trajectory and velocity. There were differences (p < 0.05) between the sperm kinematic variables by sire line. There was no statistical difference (p > 0.05) between dam lines and ejaculate clusters in fertility variables. The discriminant ability of the different kinematics of sperm variables to predict litter size fertility was analyzed using receiver operating characteristics (ROC) curve analysis. Curvilinear velocity (VCL), average path velocity (VAP), amplitude of lateral head displacement (ALH), and beat-cross frequency (BCF) showed significant, albeit limited, predictive capacity for litter size fertility variables (range: 0.55–0.58 area under curve, AUC). The kinematic analysis of the ejaculates in clusters did not have a predictive capacity for litter size variables.

Measurement of membrane integrity in canine spermatozoa using a fluorescent computer-assisted spermatozoal quantification method after SYBR-14/PI staining compared with manual counting after CFDA/PI staining

The aim of this study was to compare measurements of spermatozoal membrane status in dogs using computer-assisted spermatozoal quantification (CASQ) after staining with SYBR-14 and propidium iodide (PI) with manual counting after CFDA/PI staining. CASQ was performed on fresh (n = 11) and thawed cryopreserved canine semen (n = 91) using (1) a red long-pass (LP) filter on an untreated sample (membrane-disrupted spermatozoa, MDS count) and in a sample with all cellular membranes disrupted (total spermatozoal count, TC), (2) green LP filter for a TC and the red filter for an MDS count and (3) a green short-pass filter to obtain a membrane-intact spermatozoa (MIS) count and the red filter to obtain the MDS count, which were added to give a TC (red-green filter CASQ, n = 50). Spermatozoa were also stained with CFDA/PI, manually examined and classified as MIS or MDS. All measurements were performed in duplicate. The percentage of membrane-intact spermatozoa (MIS) was calculated. The percentage of progressively motile spermatozoa (PMS) was determined subjectively. The data were analysed to measure the agreement between the CASQ and CFDA/PI methods, repeatability of the methods and correlation between the MIS and PMS percentage. Compared with the CFDA/PI method, the agreement of MIS percentage with red filter CASQ was -12% to 34%, green LP filter CASQ -42% to 47% and red-green filter CASQ -23% to 29%. The repeatability of the CFDA/PI and red-green filter CASQ methods were the highest. The MIS and PMS percentages were always correlated (p < .05). Measurement of MIS percentage using red and red-green filter CASQ appeared to be the most reliable automated methods.

Measurement of membrane integrity in canine spermatozoa using a fluorescent computer-assisted spermatozoal quantification method and manual counting after eosin-nigrosin staining compared with manual counting after CFDA/PI staining

The aim of this study was to compare measurement of spermatozoal membrane status using computer-assisted spermatozoal quantification (CASQ) and eosin-nigrosin (EN) staining with manual counting after CFDA/PI staining. Analysis was performed on both fresh and thawed cryopreserved canine semen. Membrane-disrupted spermatozoa (MDS) were counted using CASQ (n = 311) in an untreated sample and a completely membrane-disrupted sample, and the percentage of membrane-intact spermatozoa (MIS) calculated: (Total count - Untreated sample count) ÷ Total count × 100. Spermatozoa were stained with a one-step EN stain (n = 501), and then, at least 100 spermatozoa were manually examined under ×1,000 magnification and classified as MDS (stained with eosin) or MIS (non-stained). Spermatozoa from the same samples were also stained with CFDA/PI, and then, at least 200 spermatozoa were manually examined under ×1,000 magnification and classified as MIS (completely stained by CFDA) or MDS. The percentage of progressively motile spermatozoa (PMS) was determined by both computer-assisted semen analysis (CASA) and subjective methodologies, and the data were subsequently analysed to measure the agreement between the CASQ and EN methods with the CFDA/PI technique using Bland-Altman methodology. Pearson's correlation was measured between the MIS and PMS percentage samples and correlation coefficients compared. The mean MIS percentage was lower for CASQ and higher for EN than in CFDA/PI for all comparisons. The agreement of MIS percentage between CASQ and CFDA/PI was -20.2% to 32.0%, and between EN and CFDA/PI was -32.9% to 14.9%. In all methods, the MIS and PMS percentages were correlated (p < .001). Measurement of CFDA/PI appeared to be the most reliable and accurate method of determining MIS percentage in dogs. Further investigation is required to determine whether the CASQ technique can be improved. Eosin-nigrosin staining also appeared to be unreliable at MIS <80% and overestimated the MIS percentage.

Effects of Isatis root polysaccharide on boar sperm quality during liquid storage and in vitro fertilization

Liquid preservation of boar semen is a preferred method in pig husbandry, and antioxidants to protect against sperm oxidative stress during periods of storage have become the focus of recent research. Through its antioxidant activity, Isatis root polysaccharide (IRPS), a plant extract, can effectively reduce the cellular lipid peroxidation caused by the accumulation of reactive oxygen species inside mitochondria. In the present study, there was examination of the effects of no supplementation (Control) of a semen extender with or supplementation in different concentrations of IRPS (0.2, 0.4, 0.6, 0.8, and 1.2 mg/mL) on sperm quality variables and antioxidant capacity during liquid storage. The results indicate that after prolonged storage (≥ 3 days), the sperm motility was greater in the group supplemented with 0.6 mg/mL IRPS than in the other groups (P <  0.05). The use of this IRPS concentration also resulted in maintanence of acrosome integrity, plasma membrane integrity, mitochondrial membrane potential, and antioxidant capacity of the sperm (P <  0.05). Furthermore, the results of an in vitro fertilization study indicate IRPS at 0.6 mg/mL markedly increased the sperm fertilization capacity (P <  0.01) and embryonic development to the blastocyst stage (P <  0.05). The addition of 0.6 mg/mL IRPS enhanced the antioxidant capacity of boar sperm, resulting in greater preservation of sperm motility and fertilization capacity during liquid storage. These findings indicate that IRPS has the potential to be used as a component of a semen-preserving diluent to maintain sperm quality during storage.

Assessment of chosen semen characteristics of two colour morphs of the Arctic fox Alopex lagopus L

The aim of the study was to evaluate semen quality in the two most popular colour morphs of the Arctic fox Alopex lagopus L., blue and white, based on ejaculate parameters, acrosin activity and analysis of sperm morphology. The research material consisted of ejaculates collected once by manual stimulation from 20 one-year-old male Arctic foxes (10 individuals of the blue morph and 10 of the white morph). Ejaculates were evaluated in terms of volume, sperm concentration, total number of spermatozoa and the percentage of spermatozoa with major and minor defects. The study revealed that male blue Arctic foxes produce ejaculates with much higher concentration (148.75 × 10⁶ /ml) and total number of spermatozoa (98.16 × 10⁶ ) compared to white Arctic foxes (42.88 × 10⁶ /ml and 35.2 × 10⁶ respectively). The level of acrosin activity from white foxes seemed to be higher compared to blue foxes but the difference was not statistically confirmed. Semen from Arctic foxes is characterized by high inter-individual variability in sperm morphology. The frequency of morphological changes in sperm from Arctic foxes does not significantly depend on ejaculate volume, sperm concentration or the total number of spermatozoa in the ejaculate, but is associated with acrosin activity.

Effect of long-term storage in Safe Cell+ extender on boar sperm DNA integrity and other key sperm parameters

BACKGROUND

There is some controversy about the extent of changes in different sperm cell features in stored boar semen, especially regarding the potential role of the DNA fragmentation assay for assessment of sperm fertilizing ability. The aim of this study was to assess the effect of time of storage and the dynamic changes in sperm cell characteristics in normospermic boar semen stored in long-term extender, in order to determine the susceptibility to damage of particular structures of spermatozoa during cooling and storage at 17 °C for 240 h post collection. The study included five ejaculates from each of seven boars of the Polish Large White breed (n = 35 ejaculates). The sperm characteristics were assessed using a flow cytometer and a computer assisted sperm analyzer on samples at 0, 48, 96, 168 and 240 h post collection.

RESULTS

The sperm chromatin structure assay (SCSA) showed a significant abrupt increase (P < 0.01) in the DNA fragmentation index (%DFI) after 48 h of semen storage with only subtle changes thereafter, not exceeding 5% on average after 240 h of storage. The use of a combination of SYBR-14/PI stains did not reveal any significant changes in the percentage of live sperm cells up to 168 h of semen storage. A significant (P < 0.01) decrease in the percentage of live spermatozoa with intact acrosomes was observed after prolonged semen storage (168 h). A significant and progressive decrease in sperm motility was recorded during the whole period of semen storage.

CONCLUSIONS

Storage of boar semen extended in long-term diluent at 17 °C for 48 h initially induced a decrease in the integrity of sperm DNA. This suggests that the structure of boar sperm DNA is susceptible to damage, especially during semen extension and at the beginning of sperm storage. These findings support the opinion that the SCSA test has only a low potential for routine assessment of boar semen preserved in the liquid state and for assessment of sperm quality changes during 10 days of semen preservation. Remarkably, the integrity of acrosomes and plasma membranes remained nearly unchanged for 7 days.

Effects of different concentrations of Pseudomonas aeruginosa on boar sperm quality

Bacteriospermia in boar ejaculates is a frequent finding that compromises the sperm quality and, consequently, causes economic losses in swine industry. The present study sought to evaluate the effect of different concentrations of Pseudomonas aeruginosa on boar sperm quality over a storing period of 11 days at 15-17 ° C. Ten commercial seminal doses coming from post-pubertal and healthy boars were artificially inoculated with different infective concentrations of P. aeruginosa, ranging from 2 × 10(8) to 2 × 10(4)cfu/mL. Negative controls were non-inoculated doses. Sperm quality, assessed as sperm motility (CASA), sperm viability, acrosome integrity and pH, as well as the bacterial growth, were checked after 0, 1, 2, 4, 7, 9 and 11 days of storage at 15-17 ° C. Results obtained showed significant decreases in the percentages of total and progressive sperm motility, sperm viability and acrosome integrity in the greatest infective concentrations (2 × 10(7) and 2 × 10(8)cfu/mL), when compared to the negative control. In contrast, there was no effect on seminal pH throughout the experiment. Results indicate the presence of P. aeruginosa in boar semen, apart from being a potential source for the spread of infectious diseases and harmful impact on sows, negatively affects the longevity and fertilizing ability of boar sperm when present in high concentrations. Thus, P. aeruginosa causes deleterious effects on boar sperm quality during liquid storage at 15-17 ° C, thus strict hygienic measures must be implemented in boar studs to minimize bacterial concentration of semen doses.

Effects of Enterobacter cloacae on boar sperm quality during liquid storage at 17°C

Contamination of fresh and extended boar sperm often occurs in farms and artificial insemination (AI) centres during semen collection, processing and storage. The presence of bacteria produces detrimental effects on boar sperm quality, which may cause economic losses in reproductive centres. The present study has evaluated for the first time how the presence of Enterobacter cloacae affects the preservation of boar spermatozoa in liquid storage at 15-17 °C for an 11-day period. With this purpose, extended semen samples from seven healthy post-pubertal boars were artificially contaminated with different sperm:bacterium ratios (2:1; 1:1; 1:5 and 1:10) of E. cloacae. The 1:0 ratio (non-inoculated) served as a negative control. The most infective ratios (i.e. 1:5 and 1:10) significantly damaged sperm motility and membrane integrity, increased sperm agglutination, and decreased the osmotic resistance of spermatozoa. In contrast, the negative impact that the lowest bacterial concentration (2:1) had on boar sperm quality was clearly lower. In addition, other parameters such as pH were also more affected at the highest infective ratios (i.e. 1:5 and 1:10), despite no damage being observed on sperm morphology. In conclusion, the present work shows that damage inflicted by the presence of E. cloacae in boar sperm during liquid storage at 15-17 °C compromises the longevity and fertilising ability of seminal doses when bacterial concentration is higher than a 1:1 ratio. Further research is warranted to address by which mechanism E. cloacae impairs boar sperm quality.

Sperm wars and the evolution of male fertility

Females frequently mate with several males, whose sperm then compete to fertilize available ova. Sperm competition represents a potent selective force that is expected to shape male expenditure on the ejaculate. Here, we review empirical data that illustrate the evolutionary consequences of sperm competition. Sperm competition favors the evolution of increased testes size and sperm production. In some species, males appear capable of adjusting the number of sperm ejaculated, depending on the perceived levels of sperm competition. Selection is also expected to act on sperm form and function, although the evidence for this remains equivocal. Comparative studies suggest that sperm length and swimming speed may increase in response to selection from sperm competition. However, the mechanisms driving this pattern remain unclear. Evidence that sperm length influences sperm swimming speed is mixed and fertilization trials performed across a broad range of species demonstrate inconsistent relationships between sperm form and function. This ambiguity may in part reflect the important role that seminal fluid proteins (sfps) play in affecting sperm function. There is good evidence that sfps are subject to selection from sperm competition, and recent work is pointing to an ability of males to adjust their seminal fluid chemistry in response to sperm competition from rival males. We argue that future research must consider sperm and seminal fluid components of the ejaculate as a functional unity. Research at the genomic level will identify the genes that ultimately control male fertility.