Postcopulatory sexual selection results in spermatozoa with more uniform head and flagellum sizes in rodents

Selection on sperm size in response to promiscuity and variation in female sperm storage organs

Sperm cells are exceptionally morphologically diverse across taxa. However, morphology can be quite uniform within species, particularly for species where females copulate with many males per reproductive bout. Strong sexual selection in these promiscuous species is widely hypothesized to reduce intraspecific sperm variation. Conversely, we hypothesize that intraspecific sperm size variation may be maintained by high among-female variation in the size of sperm storage organs, assuming that paternity success improves when sperm are compatible in size with the sperm storage organ. We use individual-based simulations and an analytical model to evaluate how selection on sperm size depends on promiscuity level and variation in sperm storage organ size (hereafter, female preference variation). Simulations of high promiscuity (10 mates per female) showed stabilizing selection on sperm when female preference variation was low, and disruptive selection when female preference variation was high, consistent with the analytical model results. With low promiscuity (2-3 mates per female), selection on sperm was stabilizing for all levels of female preference variation in the simulations, contrasting with the analytical model. Promiscuity level, or mate sampling, thus has a strong impact on the selection resulting from female preferences. Furthermore, when promiscuity is low, disruptive selection on male traits will occur under much more limited circumstances (i.e. only with higher among-female variation) than many previous models suggest. Variation in female sperm storage organs likely has strong implications for intraspecific sperm variation in highly promiscuous species, but likely does not explain differences in intraspecific sperm variation for less promiscuous taxa.

What is the importance of sperm subpopulations?

The study of sperm subpopulations spans three decades. The origin, meaning, and practical significance, however, are less clear. Current technology for assessing sperm morphology (CASA-Morph) and motility (CASA-Mot) has enabled the accurate evaluation of these features, and there are many options for data classification. Subpopulations could occur as a result of the stage of development of each spermatozoon in the subpopulation. Spermatogenesis might contribute to the production of these subpopulations. Insights from evolutionary biology and recent molecular research are indicative of the diversity among male gametes that could occur from unequal sharing of transcripts and other elements through cytoplasmic bridges between spermatids. Sperm cohorts exiting the gonads would contain different RNA and protein contents, affecting the spermatozoon physiology and associations with the surrounding environmental milieu. Subsequently, these differences could affect how spermatozoa interact with the environmental milieu (maturation, mixing with seminal plasma, and interacting with the environmental milieu, or female genital tract and female gamete). The emergence of sperm subpopulations as an outcome of evolution, related to the reproductive strategies of the species, genital tract structures, and copulatory and fertilization processes. This kind of approach in determining the importance of sperm subpopulations in fertilization capacity should have a practical impact for conducting reproductive technologies, inspiring and enabling new ways for the more efficient use of spermatozoa in the medical, animal breeding, and conservation fields. This manuscript is a contribution to the Special Issue in memory of Dr. Duane Garner.

Intraspecific variation in testis organisation and sperm head morphology of the delicate mouse (Pseudomys delicatulus): its possible causes and consequences

The delicate mouse (Pseudomys delicatulus) ranges from Queensland to northern Western Australia. In this study the morphology of the cellular organisation of the testis and cauda epididymal spermatozoa are compared. Individuals from the mainland of the Northern Territory and Western Australia invariably have only 60–70% of the testes composed of seminiferous tubules with the interstitial tissue between the tubules containing abundant lipid rich Leydig cells, whereas the cauda epididymal sperm have highly polymorphic, often pear-shaped, heads and basally attached tails. In individuals from Queensland the seminiferous tubules make up approximately 80% of the testis, whereas in the cauda epididymides the sperm populations are generally less variable and have bilaterally flattened heads with the tail attached to the lower concave surface. These differences in the morphology of the delicate mouse testis and spermatozoa in these two geographic regions suggest differences in intensity of intermale sperm competition, with individuals from northern Western Australia and the mainland of the Northern Territory exhibiting monogamy whereas those from Queensland may exhibit some degree of intermale sperm competition and hence possibly have a polyandrous or promiscuous mating system. These findings support the suggestion that P. delicatulus, as currently recognised, contains at least one cryptic species.

The social shape of sperm: using an integrative machine-learning approach to examine sperm ultrastructure and collective motility

Sperm is one of the most morphologically diverse cell types in nature, yet they also exhibit remarkable behavioural variation, including the formation of collective groups of cells that swim together for motility or transport through the female reproductive tract. Here, we take advantage of natural variation in sperm traits observed across Peromyscus mice to test the hypothesis that the morphology of the sperm head influences their sperm aggregation behaviour. Using both manual and automated morphometric approaches to quantify their complex shapes, and then statistical modelling and machine learning to analyse their features, we show that the aspect ratio of the sperm head is the most distinguishing morphological trait and statistically associates with collective sperm movements obtained from in vitro observations. We then successfully use neural network analysis to predict the size of sperm aggregates from sperm head morphology and show that species with relatively wider sperm heads form larger aggregates, which is consistent with the theoretical prediction that an adhesive region around the equatorial region of the sperm head mediates these unique gametic interactions. Together these findings advance our understanding of how even subtle variation in sperm design can drive differences in sperm function and performance.

Highly variable sperm morphology in the masked finch (Poephila personata) and other estrildid finches

Spermatozoa exhibit remarkable levels of morphological diversification among and within species. Among the passerine birds, the zebra finch (Taeniopygia guttata) has become a model system for studies of sperm biology, yet studies of closely related Estrildidae finches remain scarce. Here, we examine sperm morphology in the masked finch (Poephila personata) and place the data into the broader context of passerine sperm morphology using data for an additional 189 species. The masked finch exhibited high levels of within- and among-male variation in total sperm length and in specific sperm components. Furthermore, among-male variance in sperm length was significantly greater in estrildid (N = 12) compared with non-estrildid species (N = 178). We suggest that the high variation in sperm morphology in the masked finch and other estrildid species is likely to be linked to low levels of sperm competition, hence relaxed or weak selection on sperm length, in the clade. Our findings highlight that the highly variable sperm of the masked finch and widely studied zebra finch are ‘typical’ for estrildid species and stress the relevance of studying groups of closely related species. Finally, we suggest that further studies of Estrildidae will enhance our understanding of sperm diversity and avian diversity more generally.

Predictive Capacity of Boar Sperm Morphometry and Morphometric Sub-Populations on Reproductive Success after Artificial Insemination

Simple Summary

The efficiency of swine production measured as litter size influences the profitability of the pig industry. Furthermore, sow fertility potential depends in part on the boar semen quality and reproductive efficiency. The objective of this study is to compare boar sperm head size and morphometric features of shape to evaluate their relationships with reproductive success after artificial insemination (AI). A morphometric analysis of boar ejaculate reveals morphometrically separate sub-populations. The differences between sub-populations are displayed for sperm head size. In addition, sperm clustering into sub-populations did not have a predictive capacity on litter size variables. Nevertheless, the morphometric variables of the sperm 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 of the study was to compare the morphometric features of sperm head size and shape from the Pietrain line and the Duroc × Pietrain boar crossbred terminal lines, and to evaluate their relationship with reproductive success after artificial insemination of sows produced from crossbreeding the York, Landrace and Pietrain breeds. Semen samples were collected from 11 sexually mature boars. Only ejaculates with greater than 70% motility rate and <15% of abnormal sperm were used for artificial inseminations (AI) and included in the study. Samples were analyzed using an ISAS^®v1 computer-assisted sperm analysis system for eight morphometric parameters of head shape and size (CASA-Morph). Sub-populations of morphometric ejaculates were characterized using multivariate procedures, such as principal component (PC) analysis and clustering methods (k-means model). Four different ejaculate sub-populations were identified from two PCs that involved the head shape and size of the spermatozoa. The discriminant ability of the different morphometric sperm variables to predict sow litter size was analyzed using a receiver operating characteristics (ROC) curve analysis. Sperm head length, ellipticity, elongation, and regularity showed significant predictive capacity on litter size (0.59, 0.59, 0.60, and 0.56 area under curve (AUC), respectively). The morphometric sperm sub-populations were not related to sow litter size.

Assessments of sperm quality integrating morphology, swimming patterns, bioenergetics and cell signalling

Spermatozoa are diverse in form and function and these differences impact on their fertilizing capacity. Because of considerable inter-male and inter-species differences in sperm traits, assessments of sperm quality demand that we consider variations at different levels. We should thus pay attention not only to average values but also intra- and inter-sperm population variations and subpopulation structure. Sperm shape and size evolve in reponse to postcopulatory sexual selection. Assessments of morphological variation, with conventional microscopy or with computer-assisted systems, should bear this in mind. In rodents sperm head shape is asymmetric so it requires more complex tools, such as geometric morphometrics. Sperm function also evolves under postcopulatory sexual selection and this could be used as a basis to assess sperm performance. Sperm cells swim actively to overcome barriers in the female tract and develop a peculiar motility pattern in the final stages prior to and during fertilization. Both types of movement can be analyzed by computer-assisted microscopy systems. Sperm have high energetic demands for cell homeostasis, motility, and signalling. Bioenergetics can be analyzed by various means, including extracellular flux analyses to characterize glycolysis and mitochondrial respiration. Finally, cell signalling during capacitation has received much attention and can be assessed by microscopy (conventional or computer-assisted) or flow cytometry. Recent advances in image-flow cytometry affords analyses of high cell numbers with spatial localization of subcellular changes, which will have a big impact in the development of functional tests for the andrology clinic and in sperm preservation and use in artificial insemination.

Sexual selection and the evolution of sperm morphology in sharks

Post-copulatory sexual selection, and sperm competition in particular, is a powerful selective force shaping the evolution of sperm morphology. Although mounting evidence suggests that post-copulatory sexual selection influences the evolution of sperm morphology among species, recent evidence also suggests that sperm competition influences variation in sperm morphology at the intraspecific level. However, contradictory empirical results and limited taxonomic scope have led to difficulty in assessing the generality of sperm morphological responses to variation in the strength of sperm competition. Here, we use phylogenetically controlled analyses to explore the effects of sperm competition on sperm morphology and variance in sharks, a basal vertebrate group characterized by wide variation in rates of multiple mating by females, and consequently sperm competition risk. Our analyses reveal that shark species experiencing greater levels of sperm competition produce sperm with longer flagella and that sperm flagellum length is less variable in species under higher sperm competition risk. In contrast, neither the length of the sperm head and midpiece nor variation in sperm head and midpiece length was associated with sperm competition risk. Our findings demonstrate that selection influences both the inter- and intraspecific variation in sperm morphology and suggest that the flagellum is an important target of sexual selection in sharks. These findings provide important insight into patterns of selection on the ejaculate in a basal vertebrate lineage.

Evolution of female promiscuity in Passerides songbirds

Background

Female promiscuity is highly variable among birds, and particularly among songbirds. Comparative work has identified several patterns of covariation with social, sexual, ecological and life history traits. However, it is unclear whether these patterns reflect causes or consequences of female promiscuity, or if they are byproducts of some unknown evolutionary drivers. Moreover, factors that explain promiscuity at the deep nodes in the phylogenetic tree may be different from those important at the tips, i.e. among closely related species. Here we examine the relationships between female promiscuity and a broad set of predictor variables in a comprehensive data set (N = 202 species) of Passerides songbirds, which is a highly diversified infraorder of the Passeriformes exhibiting significant variation in female promiscuity.

Results

Female promiscuity was highly variable in all major clades of the Passerides phylogeny and also among closely related species. We found several significant associations with female promiscuity, albeit with fairly small effect sizes (all R² ≤ 0.08). More promiscuous species had: 1) less male parental care, particularly during the early stages of the nesting cycle (nest building and incubation), 2) more short-term pair bonds, 3) greater degree of sexual dichromatism, primarily because females were drabber, 4) more migratory behaviour, and 5) stronger pre-mating sexual selection. In a multivariate model, however, the effect of sexual selection disappeared, while the other four variables showed additive effects and together explained about 16% of the total variance in female promiscuity. Female promiscuity showed no relationship with body size, life history variation, latitude or cooperative breeding.

Conclusions

We found that multiple traits were associated with female promiscuity, but these associations were generally weak. Some traits, such as reduced parental care in males and more cryptic plumage in females, might even be responses to, rather than causes of, variation in female promiscuity. Hence, the high variation in female promiscuity among Passerides species remains enigmatic. Female promiscuity seems to be a rapidly evolving trait that often diverges between species with similar ecologies and breeding systems. A future challenge is therefore to understand what drives within-lineage variation in female promiscuity over microevolutionary time scales.

Electronic supplementary material

The online version of this article (10.1186/s12862-019-1493-1) contains supplementary material, which is available to authorized users.

Moderate heritability and low evolvability of sperm morphology in a species with high risk of sperm competition, the collared flycatcher Ficedula albicollis

Spermatozoa represent the morphologically most diverse type of animal cells and show remarkable variation in size across and also within species. To understand the evolution of this diversity, it is important to reveal to what degree this variation is genetic or environmental in origin and whether this depends on species' life histories. Here we applied quantitative genetic methods to a pedigreed multigenerational data set of the collared flycatcher Ficedula albicollis, a passerine bird with high levels of extra-pair paternity, to partition genetic and environmental sources of phenotypic variation in sperm dimensions for the first time in a natural population. Narrow-sense heritability (h² ) of total sperm length amounted to 0.44 ± 0.14 SE, whereas the corresponding figure for evolvability (estimated as coefficient of additive genetic variation, CV_a ) was 0.02 ± 0.003 SE. We also found an increase in total sperm length within individual males between the arrival and nestling period. This seasonal variation may reflect constraints in the production of fully elongated spermatozoa shortly after arrival at the breeding grounds. There was no evidence of an effect of male age on sperm dimensions. In many previous studies on laboratory populations of several insect, mammal and avian species, heritabilities of sperm morphology were higher, whereas evolvabilities were similar. Explanations for the differences in heritability may include variation in the environment (laboratory vs. wild), intensity of sexual selection via sperm competition (high vs. low) and genetic architecture that involves unusual linkage disequilibrium coupled with overdominance in one of the studied species.

Phylogeography of a widespread sub-Saharan murid rodent Aethomys chrysophilus: the role of geographic barriers and paleoclimate in the Zambezian bioregion

Murid rodents of the genus Aethomys are one of the most common rodents in drier habitats in sub-Saharan Africa. Among them, the red veld rat Aethomys chrysophilus is the most widespread species with the core distribution located in the Zambezian bioregion. In this study, we describe phylogeographic structure of the species and estimate its age from a time-calibrated phylogeny of the genus. Seven parapatric clades were identified in the mitochondrial cytochrome b phylogeny, where some of the distributions of these clades have been separated by previously described biogeographical divides (Zambezi-Kafue river system, Rukwa Rift and the Eastern Arc Mountains). One internal clade corresponded to populations previously described as a distinct species, Aethomys ineptus. The whole A. chrysophilus complex was estimated to be 1.3 (0.5–2.4) Mya old, with A. ineptus originating 0.7 (0.1–1.4) Mya before present. The internal position of A. ineptus was also recovered in phylogenetic reconstruction based on two nuclear genes and thus it is not a consequence of mitochondrial introgression. In addition, we analyzed skull form variation across the species’ distributional range and found no significant difference between A. ineptus and the rest of A. chrysophilus complex.

A trade-off between thickness and length in the zebra finch sperm mid-piece

The sperm mid-piece has traditionally been considered to be the engine that powers sperm. Larger mid-pieces have therefore been assumed to provide greater energetic capacity. However, in the zebra finch Taeniopygia guttata, a recent study showed a surprising negative relationship between mid-piece length and sperm energy content. Using a multi-dimensional approach to study mid-piece structure, we tested whether this unexpected relationship can be explained by a trade-off between mid-piece length and mid-piece thickness and/or cristae density inside the mitochondrial helix. We used selective plane illumination microscopy to study mid-piece structure from three-dimensional images of zebra finch sperm and used high-resolution transmission electron microscopy to quantify mitochondrial density. Contrary to the assumption that longer mid-pieces are larger and therefore produce or contain a greater amount of energy, our results indicate that the amount of mitochondrial material is consistent across mid-pieces of varying lengths, and longer mid-pieces are simply proportionately ‘thinner’.

Experimental evolution reveals that sperm competition intensity selects for longer, more costly sperm

It is the differences between sperm and eggs that fundamentally underpin the differences between the sexes within reproduction. For males, it is theorized that widespread sperm competition leads to selection for investment in sperm numbers, achieved by minimizing sperm size within limited resources for spermatogenesis in the testis. Here, we empirically examine how sperm competition shapes sperm size, after more than 77 generations of experimental selection of replicate lines under either high or low sperm competition intensities in the promiscuous flour beetle Tribolium castaneum. After this experimental evolution, populations had diverged significantly in their sperm competitiveness, with sperm in ejaculates from males evolving under high sperm competition intensities gaining 20% greater paternity than sperm in ejaculates from males that had evolved under low sperm competition intensity. Males did not change their relative investment into sperm production following this experimental evolution, showing no difference in testis sizes between high and low intensity regimes. However, the more competitive males from high sperm competition intensity regimes had evolved significantly longer sperm and, across six independently selected lines, there was a significant association between the degree of divergence in sperm length and average sperm competitiveness. To determine whether such sperm elongation is costly, we used dietary restriction experiments, and revealed that protein-restricted males produced significantly shorter sperm. Our findings therefore demonstrate that sperm competition intensity can exert positive directional selection on sperm size, despite this being a costly reproductive trait.

The egg coat zona pellucida 3 glycoprotein - evolution of its putative sperm-binding region in Old World murine rodents (Rodentia: Muridae)

In eutherian mammals, before fertilisation can occur the spermatozoon has to bind to, and penetrate, the egg coat, the zona pellucida (ZP). In the laboratory mouse there is good evidence that the primary sperm-binding site is a protein region encoded by Exon 7 of the ZP3 gene and it has been proposed that binding is species specific and evolves by sexual selection. In the present study we investigate these hypotheses by comparing Exon 6 and 7 sequences of ZP3 in 28 species of murine rodents of eight different divisions from Asia, Africa and Australasia, in which a diverse array of sperm morphologies occurs. We found considerable nucleotide (and corresponding amino acid) sequence divergence in Exon 7, but not in Exon 6, across these species, with evidence for positive selection at five codon positions. This molecular divergence does not appear to be due to reinforcement to reduce hybridisation, nor does it correlate with divergence in sperm head morphology or tail length, thus it is unlikely to be driven by inter-male sperm competition. Other forms of post-copulatory sexual selection therefore appear to have resulted in the molecular divergence of this region of ZP3 in this highly speciose group of mammals.

The occurrence and relative abundance of small terrestrial mammals on Theda Station in the Northern Kimberley, Western Australia

Significant gaps in knowledge currently exist regarding the small mammal fauna of the Northern Kimberley (NOK) bioregion. Theda Station is a previously unsurveyed pastoral lease in the NOK. The aim of the current study was to determine the presence of small mammals (non-volant, <2 kg) on Theda Station and to compare these findings with those recently obtained on the adjacent Doongan Station. Between 2006 and 2014, 226 site surveys were conducted across 69 sites, with over 26 000 trap-nights encompassing a range of habitats. Thirteen of the 27 small mammal species known to occur in the NOK were detected. Four species (Pseudomys nanus, Rattus tunneyi, Zyzomys argurus and Sminthopsis virginiae) were common, five (Pseudomys delicatulus, Pseudantechinus ningbing, Dasyurus hallucatus, Isoodon macrourus and Petropseudes dahli) were detected less frequently, and four (Leggadina lakedownensis, Hydromys chrysogaster, Planigale maculata and Petaurus breviceps) were occasionally recorded. Our study provides important baseline data for small mammals in this region. It highlights the lack of detailed knowledge of both the presence of, and temporal fluctuations in, the region’s small mammal fauna. This study supports a non-uniform distribution of the small mammal fauna across the NOK, with Theda Station lying within a transition zone between the high rainfall rugged coastal and near-coastal areas and the lower rainfall areas of the east.

Sperm length variations in five species of cypridoidean non-marine ostracods (Crustacea)

Spermatozoa of the ostracod superfamily Cypridoidea include some of the longest in the animal kingdom, but unlike other so-called giant spermatozoa, they are aflagellate, probably evolved only once, and represent an exceptionally old trait. Sperm length variations within cypridoidean species remain poorly known, a lack that hinders the development of hypotheses to explain their length and variation. For this study, the lengths of 500 spermatozoa from each of five species of freshwater cypridoidean ostracods, Candonopsis tenuis (Brady, 1886), Fabaeformiscandona subacuta (Yang, 1982), Heterocypris rotundata (Bronshtein, 1928), Ilyocypris japonica Okubo, 1990, and Notodromas trulla Smith and Kamiya, 2014, were measured, including the lengths of the posterior and anterior regions. No overall pattern in sperm variation was discernible. Length variations between species, between males of the same species, and within individual males varied from low (Candonopsis tenuis) to extraordinarily large (Notodromas trulla and Fabaeformiscandona subacuta). Sperm competition, cryptic female choice, sperm heteromorphism, and testis size are unlikely to explain all of the variations observed. Age structures of the populations sampled might play a role in explaining some intraspecific variation. The differing amounts of variation in sperm characters revealed in this study suggest that multiple evolutionary trends and pressures shape sperm lengths in this superfamily.

Early diversification of sperm size in the evolutionary history of the old world leaf warblers (Phylloscopidae)

Sperm morphological traits are highly variable among species and are commonly thought to evolve by post-copulatory sexual selection. However, little is known about the evolutionary dynamics of sperm morphology, and whether rates of evolutionary change are variable over time and among taxonomic groups. Here, we examine sperm morphology from 21 species of Old World leaf warblers (Phylloscopidae), a group of generally dull, sexually monochromatic birds, which are known to have high levels of extra-pair paternity. We found that sperm length differs markedly across species, spanning about 40% of the range observed across a larger selection of passerine birds. Furthermore, we found strong support for an 'early-burst' model of trait evolution, implying that the majority of divergence in sperm length has occurred early in the evolutionary history of this clade with subsequent evolutionary stasis. This large early divergence matches the early divergence reported in ecological traits (i.e. body size and feeding behaviour). Our findings demonstrate that rates of evolution in sperm morphology can change over time in passerine taxa, and that evolutionary stasis in sperm traits can occur even in species exhibiting characteristics consistent with moderate-to-high levels of sperm competition. It remains a major challenge to identify the selection mechanisms and possible constraints responsible for these variable rates of sperm evolution.

Intramale variation in sperm size: functional significance in a polygynous mammal

Studies concerning the relationships between sperm size and velocity at the intraspecific level are quite limited and often yielded contradictory results across the animal kingdom. Intramale variation in sperm size may represent a meaningful factor to predict sperm velocity, due to its relationship with the level of sperm competition among related taxa. Because sperm phenotype is under post-copulatory sexual selection, we hypothesized that a reduced intramale variation in sperm size is associated with sperm competitiveness in red deer. Our results show that low variation in sperm size is strongly related to high sperm velocity and normal sperm morphology, which in turn are good predictors of male fertility in this species. Furthermore, it is well known that the red deer show high variability in testicular mass but there is limited knowledge concerning the significance of this phenomenon at intraspecific level, even though it may reveal interesting processes of sexual selection. Thereby, as a preliminary result, we found that absolute testes mass is negatively associated with intramale variation in sperm size. Our findings suggest that sperm size variation in red deer is under a strong selective force leading to increase sperm function efficiency, and reveal new insights into sexual selection mechanisms.

No evidence for a trade-off between sperm length and male premating weaponry

Male ornaments and armaments that mediate success in mate acquisition and ejaculate traits influencing competitive fertilization success are under intense sexual selection. However, relative investment in these pre- and post-copulatory traits depends on the relative importance of either selection episode and on the energetic costs and fitness gains of investing in these traits. Theoretical and empirical work has improved our understanding of how precopulatory sexual traits and investments in sperm production covary in this context. It has recently also been suggested that male weapon size may trade off with sperm length as another post-copulatory sexual trait, but the theoretical framework for this suggestion remains unclear. We evaluated the relationship between precopulatory armaments and sperm length, previously reported in ungulates, in five taxa as well as meta-analytically. Within and between taxa, we found no evidence for a negative or positive relationship between sperm length and male traits that are important in male-male contest competition. It is important to consider pre- and post-copulatory sexual selection together to understand fitness, and to study investments in different reproductive traits jointly rather than separately. A trade-off between pre- and post-copulatory sexual traits may not manifest itself in sperm length but rather in sperm number or function. Particularly in large-bodied taxa such as ungulates, sperm number is more variable interspecifically and likely to be under more intense selection than sperm length. We discuss our and the previous results in this context.

Morphometry of boar sperm head and flagellum in semen backflow after insemination

Once deposited in the female reproductive system, sperm begin their competition and undergo a selection to reach the site of fertilization. Little is known about the special characteristics of sperm that reach the oviduct and are able to fertilize, with even less information on the role of sperm dimension and shape in transport and fertilization. Here, we examine whether sperm morphometry could be involved in their journey within the uterus. For this purpose, sperm head dimension (length, width, area, and perimeter) and shape (shape factor, ellipticity, elongation, and regularity), and flagellum length were analyzed in the backflow at different times after insemination (0-15, 16-30, and 31-60 minutes). Sperm morphometry in the backflow was also analyzed taking into account the site of semen deposition (cervical vs. intrauterine). Finally, flagellum length was measured at the uterotubal junction. Sperm analyzed in the backflow were small (head and flagellum) with different head shapes compared with sperm observed in the dose before insemination. The site of deposition influenced head morphometry and tail size both being smaller in the backflow after cervical insemination compared with intrauterine insemination. Mean tail length of sperm collected in the backflow was smaller than that in the insemination dose and at the uterotubal junction. Overall, our results suggest that sperm size may be involved in sperm transport either because of environment or through sperm selection and competence on their way to encounter the female gamete.