Postcopulatory sexual selection is associated with reduced variation in sperm morphology

Sperm competition favours intermediate sperm size in a hermaphrodite1

Sperm competition is a potent mechanism of postcopulatory sexual selection that has been found to shape reproductive morphologies and behaviours in promiscuous animals. Especially sperm size has been argued to evolve in response to sperm competition through its effect on sperm longevity, sperm motility, the ability to displace competing sperm, and ultimately fertilization success. Additionally, sperm size has been observed to co-evolve with female reproductive morphology. Theoretical work predicts that sperm competition may select for longer sperm but may also favour shorter sperm if sperm size trades-off with number. In this study, we studied the relationship between sperm size and postmating success in the free-living flatworm, Macrostomum lignano. Specifically, we used inbred isolines of M. lignano that varied in sperm size to investigate how sperm size translated into the ability of worms to transfer and deposit sperm in a mating partner. Our results revealed a hump-shaped relationship with individuals producing sperm of intermediate size having the highest sperm competitiveness. This finding broadens our understanding of the evolution of sperm morphology by providing empirical support for stabilizing selection on sperm size under sperm competition.

How Female-Female Competition Affects Male-Male Competition: Insights into Postcopulatory Sexual Selection from Socially Polyandrous Species

AbstractSexual selection is a major driver of trait variation, and the intensity of male competition for mating opportunities has been linked with sperm size across diverse taxa. Mating competition among females may also shape the evolution of sperm traits, but the effect of the interplay between female-female competition and male-male competition on sperm morphology is not well understood. We evaluated variation in sperm morphology in two species with socially polyandrous mating systems, in which females compete to mate with multiple males. Northern jacanas (Jacana spinosa) and wattled jacanas (J. jacana) vary in their degree of social polyandry and sexual dimorphism, suggesting species differences in the intensity of sexual selection. We compared mean and variance in sperm head, midpiece, and tail length between species and breeding stages because these measures have been associated with the intensity of sperm competition. We found that the species with greater polyandry, northern jacana, has sperm with longer midpieces and tails as well as marginally lower intraejaculate variation in tail length. Intraejaculate variation was also significantly lower in copulating males than in incubating males, suggesting flexibility in sperm production as males cycle between breeding stages. Our results indicate that stronger female-female competition for mating opportunities may also shape more intense male-male competition by selecting for longer and less variable sperm traits. These findings extend frameworks developed in socially monogamous species to reveal that sperm competition may be an important evolutionary force layered atop female-female competition for mates.

Hidden impacts of conservation management on fertility of the critically endangered kākāpō

Background

Animal conservation often requires intensive management actions to improve reproductive output, yet any adverse effects of these may not be immediately apparent, particularly in threatened species with small populations and long lifespans. Hand-rearing is an example of a conservation management strategy which, while boosting populations, can cause long-term demographic and behavioural problems. It is used in the recovery of the critically endangered kākāpō (Strigops habroptilus), a flightless parrot endemic to New Zealand, to improve the slow population growth that is due to infrequent breeding, low fertility and low hatching success.

Methods

We applied Bayesian mixed models to examine whether hand-rearing and other factors were associated with clutch fertility in kākāpō. We used projection predictive variable selection to compare the relative contributions to fertility from the parents' rearing environment, their age and previous copulation experience, the parental kinship, and the number of mates and copulations for each clutch. We also explored how the incidence of repeated copulations and multiple mates varied with kākāpō density.

Results

The rearing status of the clutch father and the number of mates and copulations of the clutch mother were the dominant factors in predicting fertility. Clutches were less likely to be fertile if the father was hand-reared compared to wild-reared, but there was no similar effect for mothers. Clutches produced by females copulating with different males were more likely to be fertile than those from repeated copulations with one male, which in turn had a higher probability of fertility than those from a single copulation. The likelihood of multiple copulations and mates increased with female:male adult sex ratio, perhaps as a result of mate guarding by females. Parental kinship, copulation experience and age all had negligible associations with clutch fertility.

Conclusions

These results provide a rare assessment of factors affecting fertility in a wild threatened bird species, with implications for conservation management. The increased fertility due to multiple mates and copulations, combined with the evidence for mate guarding and previous results of kākāpō sperm morphology, suggests that an evolutionary mechanism exists to optimise fertility through sperm competition in kākāpō. The high frequency of clutches produced from single copulations in the contemporary population may therefore represent an unnatural state, perhaps due to too few females. This suggests that opportunity for sperm competition should be maximised by increasing population densities, optimising sex ratios, and using artificial insemination. The lower fertility of hand-reared males may result from behavioural defects due to lack of exposure to conspecifics at critical development stages, as seen in other taxa. This potential negative impact of hand-rearing must be balanced against the short-term benefits it provides.

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.

Flagellum tapering and midpiece volume in songbird spermatozoa

In contrast to numerous studies on spermatozoa length, relatively little work focuses on the width of spermatozoa, and particularly the width of the midpiece and flagellum. In flagellated spermatozoa, the flagellum provides forward thrust while energy may be provided via mitochondria in the midpiece and/or through glycolysis along the flagellum itself. Longer flagella may be able to provide greater thrust but may also require stronger structural features and more or larger mitochondria to supply sufficient energy. Here, we use scanning electron microscopy to investigate the ultrastructure of spermatozoa from 55 passerine species in 26 taxonomic families in the Passerides infraorder. Our data confirm the qualitative observation that the flagellum tapers along its length, and we show that longer flagella are wider at the neck. This pattern is similar to mammals, and likely reflects the need for longer cells to be stronger against shearing forces. We further estimate the volume of the mitochondrial helix and show that it correlates well with midpiece length, supporting the use of midpiece length as a proxy for mitochondrial volume, at least in between-species studies where midpiece length is highly variable. These results provide important context for understanding the evolutionary correlations among different sperm cell components and dimensions.

Sperm Numbers as a Paternity Guard in a Wild Bird

Sperm competition is thought to impose strong selection on males to produce competitive ejaculates to outcompete rival males under competitive mating conditions. Our understanding of how different sperm traits influence fertilization success, however, remains limited, especially in wild populations. Recent literature highlights the importance of incorporating multiple ejaculate traits and pre-copulatory sexually selected traits in analyses aimed at understanding how selection acts on sperm traits. However, variation in a male’s ability to gain fertilization success may also depend upon a range of social and ecological factors that determine the opportunity for mating events both within and outside of the social pair-bond. Here, we test for an effect of sperm quantity and sperm size on male reproductive success in the red-back fairy-wren (Malurus melanocephalus) while simultaneously accounting for pre-copulatory sexual selection and potential socio-ecological correlates of male mating success. We found that sperm number (i.e., cloacal protuberance volume), but not sperm morphology, was associated with reproductive success in male red-backed fairy-wrens. Most notably, males with large numbers of sperm available for copulation achieved greater within-pair paternity success. Our results suggest that males use large sperm numbers as a defensive strategy to guard within-pair paternity success in a system where there is a high risk of sperm competition and female control of copulation. Finally, our work highlights the importance of accounting for socio-ecological factors that may influence male mating opportunities when examining the role of sperm traits in determining male reproductive success.

Postcopulatory sexual selection is associated with sperm aggregate quality in Peromyscus mice

In some species, sperm form coordinated groups that are hypothesized to improve their swimming performance in competitive contexts or to navigate through the viscous fluids of the female reproductive tract. Here we investigate sperm aggregation across closely related species of Peromyscus mice that naturally vary by mating system to test the predictions that sperm aggregates 1) are faster than solitary sperm in species that females mate multiply to aid cells in sperm competition, and 2) outperform solitary sperm cells in viscous environments. We find significant variation in the size of sperm aggregates, which negatively associates with relative testis mass, a proxy for sperm competition risk, suggesting that postcopulatory sexual selection has a stabilizing effect on sperm group size. Moreover, our results show that sperm aggregates are faster than solitary sperm in some, but not all, species, and this can vary by fluid viscosity. Of the two species that produce the largest and most frequent groups, we find that sperm aggregates from the promiscuous P. maniculatus are faster than solitary sperm in every experimentally viscous environment but aggregation provides no such kinematic advantage under these same conditions for the monogamous P. polionotus. The reduced performance of P. polionotus aggregates is associated with less efficient aggregate geometry and the inclusion of immotile or morphological abnormal sperm. Our cross-species comparison yields insight into the evolution of sperm social behaviors, provides evidence of extensive variation in the Peromyscus lineage, and reveals that differences in sperm aggregate quality associate with postcopulatory sexual selection.

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 reproductive system of Ceracis cornifer (Mellié) and first description of sperm structure in a minute tree-fungus beetle (Tenebrionoidea: Ciidae)

Studies on the spermatogenesis of Tenebrionidae beetles (Tenebrionoidea) have shown an unusual organization of spermatozoa, in which they are arranged antiparallelly within the testicular cysts. Despite such works, many taxa of Tenebrionoidea remain to be studied, including the minute tree-fungus beetles (Ciidae). Among the challenges in the study of the internal morphology of ciids is their small body size (about 2 mm or less), which makes dissections and comparisons extremely arduous. Here, we provide, for the first time, an anatomical and histological study of the reproductive system (female and male) and a description of sperm structure of Ceracis cornifer Mellié (Ciidae), under light and transmission electron microscopes. We pointed out the growth of a single oocyte at a time in females, aside of evidences toward a constant copulatory activity that can provide a continuous supply of sperm in their small spermatheca. In males, sperm have an antiparallel organization within the cysts, a condition observed so far only in members of Tenebrionoidea. Furthermore, we observed sperm dimorphism in C. cornifer. We described, for the first time, this characteristic in a Tenebrionoidea taxon, and discussed the possible relationship between sperm dimorphism and antiparallel organization of sperm.

Context-dependent behavioural plasticity compromises disruptive selection of sperm traits in squid

Sperm morphology is generally uniform within a species due to selective pressures that act to achieve better fertilization outcomes under postcopulatory competitive circumstances. Therefore, polyandry that intensifies post-mating sperm competition should constrain intraspecific sperm polymorphism. Contrary to this paradigm, we previously found that a polyandrous squid, Heterololigo bleekeri, produces dimorphic eusperm (flagellum length dimorphism; FLD), which is closely associated with alternative reproductive tactics (ARTs); large males (consorts) transfer their spermatophores inside the female’s mantle cavity, while small males (sneakers) do so outside the mantle. Thus, FLD was considered as the consequence of different insemination strategies that arise from different modes of sperm competition, sperm storage and the fertilization environment. However, in other squid species showing ARTs, the choice of mating behaviour is rather conditional (i.e., switching mating tactic between consorts and sneakers), which poses the question of whether sperm FLD could have evolved. Here, we investigated five species in the family Loliginidae that exhibit ARTs and found that all species showed sneaker-biased FLD. However, in a species with conditional ARTs, we found FLD rather ambiguous and the testicular somatic index to be nearly continuous among individuals at transitional state, suggesting that plasticity in mating behaviour compromises the disruptive selection on a sperm morphological trait.

Longer Sperm Swim More Slowly in the Canary Islands Chiffchaff

Sperm swimming performance affects male fertilization success, particularly in species with high sperm competition. Understanding how sperm morphology impacts swimming performance is therefore important. Sperm swimming speed is hypothesized to increase with total sperm length, relative flagellum length (with the flagellum generating forward thrust), and relative midpiece length (as the midpiece contains the mitochondria). We tested these hypotheses and tested for divergence in sperm traits in five island populations of Canary Islands chiffchaff (Phylloscopus canariensis). We confirmed incipient mitochondrial DNA differentiation between Gran Canaria and the other islands. Sperm swimming speed correlated negatively with total sperm length, did not correlate with relative flagellum length, and correlated negatively with relative midpiece length (for Gran Canaria only). The proportion of motile cells increased with relative flagellum length on Gran Canaria only. Sperm morphology was similar across islands. We thus add to a growing number of studies on passerine birds that do not support sperm morphology-swimming speed hypotheses. We suggest that the swimming mechanics of passerine sperm are sufficiently different from mammalian sperm that predictions from mammalian hydrodynamic models should no longer be applied for this taxon. While both sperm morphology and sperm swimming speed are likely under selection in passerines, the relationship between them requires further elucidation.

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.

Weapons Evolve Faster Than Sperm in Bovids and Cervids

In polyandrous species, males face reproductive competition both before and after mating. Sexual selection thus shapes the evolution of both pre- and postcopulatory traits, creating competing demands on resource allocation to different reproductive episodes. Traits subject to strong selection exhibit accelerated rates of phenotypic divergence, and examining evolutionary rates may inform us about the relative importance and potential fitness consequences of investing in traits under either pre- or postcopulatory sexual selection. Here, we used a comparative approach to assess evolutionary rates of key competitive traits in two artiodactyl families, bovids (family Bovidae) and cervids (family Cervidae), where male–male competition can occur before and after mating. We quantified and compared evolutionary rates of male weaponry (horns and antlers), body size/mass, testes mass, and sperm morphometrics. We found that weapons evolve faster than sperm dimensions. In contrast, testes and body mass evolve at similar rates. These results suggest strong, but differential, selection on both pre- and postcopulatory traits in bovids and cervids. Furthermore, we documented distinct evolutionary rates among different sperm components, with sperm head and midpiece evolving faster than the flagellum. Finally, we demonstrate that, despite considerable differences in weapon development between bovids and cervids, the overall evolutionary patterns between these families were broadly consistent.

Measuring Pre- and Post-Copulatory Sexual Selection and Their Interaction in Socially Monogamous Species with Extra-Pair Paternity

When females copulate with multiple males, pre- and post-copulatory sexual selection may interact synergistically or in opposition. Studying this interaction in wild populations is complex and potentially biased, because copulation and fertilization success are often inferred from offspring parentage rather than being directly measured. Here, I simulated 15 species of socially monogamous birds with varying levels of extra-pair paternity, where I could independently cause a male secondary sexual trait to improve copulation success, and a sperm trait to improve fertilization success. By varying the degree of correlation between the male and sperm traits, I show that several common statistical approaches, including univariate selection gradients and paired t-tests comparing extra-pair males to the within-pair males they cuckolded, can give highly biased results for sperm traits. These tests should therefore be avoided for sperm traits in socially monogamous species with extra-pair paternity, unless the sperm trait is known to be uncorrelated with male trait(s) impacting copulation success. In contrast, multivariate selection analysis and a regression of the proportion of extra-pair brood(s) sired on the sperm trait of the extra-pair male (including only broods where the male sired ≥1 extra-pair offspring) were unbiased, and appear likely to be unbiased under a broad range of conditions for this mating system. In addition, I investigated whether the occurrence of pre-copulatory selection impacted the strength of post-copulatory selection, and vice versa. I found no evidence of an interaction under the conditions simulated, where the male trait impacted only copulation success and the sperm trait impacted only fertilization success. Instead, direct selection on each trait was independent of whether the other trait was under selection. Although pre- and post-copulatory selection strength was independent, selection on the two traits was positively correlated across species because selection on both traits increased with the frequency of extra-pair copulations in these socially monogamous species.

Penis morphology facilitates identification of cryptic African bat species

Africa hosts a high number of bat species, many of which have been poorly studied. Among African vesper bats (Vespertilionidae), some species are morphologically similar to each other, hampering identification in the field. Consequently, basic information on these species’ population dynamics, distributions, or behaviors is vague and/or incorrectly documented. Among some Vespertilionidae, variation in flaccid penis shape enables species identification. This variation in morphology is thought to have resulted from sexual selection operating on the erect penis. Furthermore, in the context of strong postcopulatory competition, divergence in sperm traits may have evolved among closely related species. These male reproductive characters have hitherto not been investigated in southern African vespertilionid bats. In this study, we present the morphology of flaccid penis, erect penis, and sperm, of six small vesper species, based on the prediction that these characters have evolved sufficiently toward different optima to allow species discrimination. Species identification based solely on the morphological characters of the penis entirely matched species identification based on traditional (craniodental) characters. We illustrate penis morphology of these species to assist with field identifications for future ecological, conservation, or behavioral studies. Sperm morphology was sufficient to discriminate between the different genera, but could not reliably separate Neoromicia nana, Ne. zuluensis, and Ne. capensis. Hence, we demonstrate that these difficult to identify species can be readily distinguished based on traits directly observable in the field and on living animals, which will lead to more focused field studies on these otherwise morphologically cryptic species. Finally, our comparisons of penis morphology support the suggestion that Hypsugo anchietae is better placed within the genus Neoromicia.

Three decades of sperm competition in birds

In the three decades, since Birkhead and Møller published Sperm competition in birds (1992, Academic Press) more than 1000 papers have been published on this topic, about half of these being empirical studies focused on extrapair paternity. Both technological innovations and theory have moved the field forward by facilitating the study of both the mechanisms underlying sperm competition in both sexes, and the ensuing behavioural and morphological adaptations. The proliferation of studies has been driven partly by the diversity of both behaviours and morphologies in birds that have been influenced by sperm competition, but also by the richness of the theory developed by Geoff Parker over the past 50 years. This article is part of the theme issue 'Fifty years of sperm competition'.

Body size and climate as predictors of plumage colouration and sexual dichromatism in parrots

Psittaciformes (parrots, cockatoos and lorikeets) comprise one of the most colourful clades of birds. Their unique pigments and safe cavity nesting habits are two potential explanations for their colourful character. However, plumage colour varies substantially between parrot species and sometimes also between males and females of the same species. Here, we use comparative analyses to evaluate what factors correlate with colour elaboration, colour diversity and sexual dichromatism. Specifically, we test the association between different aspects of parrot colouration and (a) the intensity of sexual selection and social interactions, (b) variation along the slow-fast life-history continuum and (c) climatic variation. We show that larger species and species that live in warm environments display more elaborated colours, yet smaller species have higher levels of sexual dichromatism. Larger parrots tend to have darker and more blue and red colours. Parrots that live in wetter environments are darker and redder, whereas species inhabiting warm regions have more blue plumage colours. In general, each of the variables we considered explain small to moderate amounts of variation in parrot colouration (up to 15%). Our data suggest that sexual selection may be acting more strongly on males in small, short-lived parrots leading to sexual dichromatism. More elaborate colouration in both males and females of the larger, long-lived species with slow tropical life histories suggests that mutual mate choice, social selection and reduced selection for crypsis may be important in these species, as has been shown for passerines.

Sperm Morphology and Male Age in Black-Throated Blue Warblers, an Ecological Model System

Extra-pair paternity may drive selection on spermatozoa and ejaculate characteristics through sperm competition and cryptic female choice. Here, we examine sperm morphology in the black-throated blue warbler (Setophaga caerulescens), an ecological model species where extra-pair paternity is frequent and is linked with male age. We test whether sperm morphology relates to several aspects of male phenotype known or suspected to affect extra-pair paternity success. Sperm morphology did not correlate with the size of the white wing spot, a social status signal, nor with the volume of the cloacal protuberance. However, older males tended to have longer sperm cells. Although the sample size was limited, this pattern is intriguing, as longer cells may be advantageous in post-copulatory sexual selection and older males have larger testes and higher extra-pair paternity success in this species. Changes in sperm morphology with age are not observed in other birds, though they have been observed in insects and fishes. More research on sperm morphology is needed to clarify its role in extra-pair fertilizations in this well-studied species.

Methodological considerations for examining the relationship between sperm morphology and motility

Sperm cells of all taxa share a common goal to reach and fertilize an ovum, yet sperm are one of the most diverse cell types in nature. While the structural diversity of these cells is well recognized, the functional significance of variation in sperm design remains elusive. An important function of spermatozoa is a need to migrate toward the ova, often over long distances in a foreign environment, which may include a complex and hostile female reproductive tract. Several comparative and experimental studies have attempted to address the link between sperm morphology and motility, yet the conclusions drawn from these studies are often inconsistent, even within the same taxa. Much of what we know about the functional significance of sperm design in internally fertilizing species has been gleaned from in vitro studies, for which experimental parameters often vary among studies. We propose that discordant results from these studies are in part due to a lack of consistency of methods, conditions that do not replicate those of the female reproductive tract, and the overuse of simple linear measures of sperm shape. Within this review, we provide a toolkit for imaging, quantifying, and analyzing sperm morphology and movement patterns for in vitro studies and discuss emerging approaches. Results from studies linking morphology to motility enhance our understanding of the evolution of adaptive sperm traits and the mechanisms that regulate fertility, thus offering new insights into methods used in assisted reproductive technologies in animal science, conservation and public health.

Interspecific diversity of testes mass and sperm morphology in the Philippine chrotomyine rodents: implications for differences in breeding systems across the species

The high diversity of native Philippine murid rodents includes an old endemic group, the chrotomyines, which are the sister group of the Australasian hydromyines. Herein we detail their interspecific diversity of relative testes mass (RTM) and sperm morphology. We find that in chrotomyines, as in the Australasian hydromyines, testes mass relative to body mass differs by an order of magnitude across the species and ranges from a large RTM in Soricomys and Chrotomys species to a small RTM in Apomys. Sperm morphology is associated with these findings, with individuals in species of Soricomys and Chrotomys producing relatively larger spermatozoa with a prominent apical hook and long tail, whereas, by contrast, the Apomys species have a sperm head that either has a very short or no apical hook and a shorter tail. These findings indicate coevolution of RTM with sperm morphological traits across the species, with the marked interspecific differences in RTM suggesting differences in the intensity of intermale sperm competition and hence breeding system. Thus, we hypothesise that species of Soricomys and Chrotomys that produce more streamlined spermatozoa with longer tails have a polyandrous or promiscuous mating system, whereas the Apomys species, which produce smaller and less streamlined spermatozoa, may exhibit monogamy.

Asymmetries in reproductive anatomy: insights from promiscuous songbirds

Directional asymmetry in gonad size is commonly observed in vertebrates and is particularly pronounced in birds, where the left testis is frequently larger than the right. The adaptive significance of directional asymmetry in testis size is poorly understood, and whether it extends beyond the testes (i.e. side-correspondent asymmetry along the reproductive tract) has rarely been considered. Using the Maluridae, a songbird family exhibiting variation in levels of sperm competition and directional testis asymmetry, yet similar in ecology and life history, we investigated the relative roles of side-correspondence and sperm competition on male reproductive tract asymmetry at both inter- and intraspecific levels. We found some evidence for side-correspondent asymmetry. Additionally, sperm competition influenced directional asymmetry at each end of the reproductive tract: species experiencing higher levels of sperm competition had a relatively larger right testis and relatively more sperm in the right seminal glomerus. Within red-backed fairy-wrens (Malurus melanocephalus), auxiliary males had relatively more sperm in the left seminal glomerus, in contrast to a right-bias asymmetry throughout the reproductive tract in breeding males. Given that the number of sperm is important for competitive fertilization success, our results suggest that sperm competition shapes reproductive asymmetries beyond testis size, with likely functional consequences for male reproductive success.

Natural Variation and Genetic Determinants of Caenorhabditis elegans Sperm Size

The diversity in sperm shape and size represents a powerful paradigm to understand how selection drives the evolutionary diversification of cell morphology. Experimental work on the sperm biology of the male-hermaphrodite nematode Caenorhabditis elegans has elucidated diverse factors important for sperm fertilization success, including the competitive superiority of larger sperm. Yet despite extensive research, the molecular mechanisms regulating C. elegans sperm size and the genetic basis underlying natural variation in sperm size remain unknown. To address these questions, we quantified male sperm size variation of a worldwide panel of 97 genetically distinct C. elegans strains, allowing us to uncover significant genetic variation in male sperm size. Aiming to characterize the molecular genetic basis of C. elegans male sperm size variation using a genome-wide association study, we did not detect any significant quantitative trait loci. We therefore focused on the genetic analysis of pronounced sperm size differences observed between recently diverged laboratory strains (N2 vs. LSJ1/2). Using mutants and quantitative complementation tests, we demonstrate that variation in the gene nurf-1 underlies the evolution of small sperm in the LSJ lineage. Given the previous discovery that this same nurf-1 variation was central for hermaphrodite laboratory adaptation, the evolution of reduced male sperm size in LSJ strains likely reflects a pleiotropic consequence. Together, our results provide a comprehensive quantification of natural variation in C. elegans sperm size and first insights into the genetic determinants of Caenorhabditis sperm size, pointing at an involvement of the NURF chromatin remodeling complex.

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.

Sperm morphology and evidence for sperm competition among parrots

Sperm competition is an important component of post‐copulatory sexual selection that has shaped the evolution of sperm morphology. Previous studies have reported that sperm competition has a concurrently directional and stabilizing effect on sperm size. For example, bird species that show higher levels of extrapair paternity and larger testes (proxies for the intensity of sperm competition) have longer sperm and lower coefficients of variation in sperm length, both within and between males. For this reason, these sperm traits have been proposed as indexes to estimate the level of sperm competition in species for which other measures are not available. The relationship between sperm competition and sperm morphology has been explored mostly for bird species that breed in temperate zones, with the main focus on passerine birds. We measured sperm morphology in 62 parrot species that breed mainly in the tropics and related variation in sperm length to life‐history traits potentially indicative of the level of sperm competition. We showed that sperm length negatively correlated with the within‐male coefficient of variation in sperm length and positively with testes mass. We also showed that sperm is longer in sexually dichromatic and in gregarious species. Our results support the general validity of the hypothesis that sperm competition drives variation in sperm morphology. Our analyses suggest that post‐copulatory sexual selection is also important in tropical species, with more intense sperm competition among sexually dichromatic species and among species that breed at higher densities.

Small, Variable Sperm in a Barn Swallow Population with Low Extra-Pair Paternity

Sperm competition can theoretically affect sperm morphology; however, it remains unclear whether and how sperm morphology tracks the intensity of sperm competition in each population. The barn swallow Hirundo rustica is a model species used in the study of sexual selection, and exhibits considerable variation in extra-pair paternity (percentage extra-pair young, ca. 3-30%) among populations. In the Joetsu population of the barn swallow, extra-pair paternity is virtually absent (< 3%), providing a rare opportunity to study sperm morphology under limited sperm competition, and to compare it with those reported in populations with frequent extra-pair paternity (>15%). We found that head, midpiece, and total sperm length were significantly shorter in the Joetsu population than in populations with frequent extra-pair paternity. Moreover, the variability in total sperm length, measured as the coefficient of variation in the Joetsu population, was twice as high as that of populations with frequent extra-pair paternity. These results are consistent with a positive, directional, and stabilizing effect of sperm competition on sperm morphology. Together with previous studies in populations with frequent extra-pair paternity, the current study provides one of few sets of evidence to show a link between the intensity of sperm competition and the mean and variance of sperm morphology within a wild bird species.

Rapid evolution of testis size relative to sperm morphology suggests that post-copulatory selection targets sperm number in Anolis lizards

Post-copulatory sexual selection is thought to be responsible for much of the extraordinary diversity in sperm morphology across metazoans. However, the extent to which post-copulatory selection targets sperm morphology versus sperm production is generally unknown. To address this issue, we simultaneously characterized the evolution of sperm morphology (length of the sperm head, midpiece and flagellum) and testis size (a proxy for sperm production) across 26 species of Anolis lizards, a group in which sperm competition is likely. We found that the length of the sperm midpiece has evolved 2-3 times faster than that of the sperm head or flagellum, suggesting that midpiece size may be the most important aspect of sperm morphology with respect to post-copulatory sexual selection. However, testis size has evolved faster than any aspect of sperm morphology or body size, supporting the hypothesis that post-copulatory sexual selection acts more strongly upon sperm production than upon sperm morphology. Likewise, evolutionary increases in testis size, which typically indicate increased sperm competition, are not associated with predictable changes in sperm morphology, suggesting that any effects of post-copulatory selection on sperm morphology are either weak or variable in direction across anoles. Collectively, our results suggest that sperm production is the primary target of post-copulatory sexual selection in this lineage.

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.

Postcopulatory sexual selection reduces Z-linked genetic variation and might contribute to the large Z effect in passerine birds

The X and Z sex chromosomes play a disproportionately large role in intrinsic postzygotic isolation. The underlying mechanisms of this large X/Z effect are, however, still poorly understood. Here we tested whether faster rates of molecular evolution caused by more intense positive selection or genetic drift on the Z chromosome could contribute to the large Z effect in two closely related passerine birds, the Common Nightingale (Luscinia megarhynchos) and the Thrush Nightingale (L. luscinia). We found that the two species differ in patterns of molecular evolution on the Z chromosome. The Z chromosome of L. megarhynchos showed lower levels of within-species polymorphism and an excess of non-synonymous polymorphisms relative to non-synonymous substitutions. This is consistent with increased levels of genetic drift on this chromosome and may be attributed to more intense postcopulatory sexual selection acting on L. megarhynchos males as was indicated by significantly longer sperm and higher between-male variation in sperm length in L. megarhynchos compared to L. luscinia. Interestingly, analysis of interspecific gene flow on the Z chromosome revealed relatively lower levels of introgression from L. megarhynchos to L. luscinia than vice versa, indicating that the Z chromosome of L. megarhynchos accumulated more hybrid incompatibilities. Our results are consistent with the view that postcopulatory sexual selection may reduce the effective population size of the Z chromosome and thus lead to stronger genetic drift on this chromosome in birds. This can result in relatively faster accumulation of hybrid incompatibilities on the Z and thus contribute to the large Z effect.

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’.

Sperm morphology of the Rattini – are the interspecific differences due to variation in intensity of intermale sperm competition?

It is widely accepted that in mammals a causal relationship exists between postcopulatory sexual selection and relative testes mass of the species concerned, but how much it determines sperm size and shape is debatable. Here we detailed for the largest murine rodent tribe, the Rattini, the interspecific differences in relative testes mass and sperm form. We found that residual testes mass correlates with sperm head apical hook length as well as its angle, together with tail length, and that within several lineages a few species have evolved highly divergent sperm morphology with a reduced or absent apical hook and shorter tail. Although most species have a relative testes mass of 1–4%, these derived sperm traits invariably co-occur in species with much smaller relative testes mass. We therefore suggest that high levels of intermale sperm competition maintain a sperm head with a long apical hook and long tail, whereas low levels of intermale sperm competition generally result in divergent sperm heads with a short or non-existent apical hook and shorter tail. We thus conclude that sexual selection is a major selective force in driving sperm head form and tail length in this large tribe of murine rodents.

Sperm morphology, adenosine triphosphate (ATP) concentration and swimming velocity: unexpected relationships in a passerine bird

The relationship between sperm energetics and sperm function is poorly known, but is central to our understanding of the evolution of sperm traits. The aim of this study was to examine how sperm morphology and ATP content affect sperm swimming velocity in the zebra finch Taeniopygia guttata. We exploited the high inter-male variation in this species and created extra experimental power by increasing the number of individuals with very long or short sperm through artificial selection. We found a pronounced quadratic relationship between total sperm length and swimming velocity, with velocity increasing with length up to a point, but declining in the very longest sperm. We also found an unexpected negative association between midpiece length and ATP content: sperm with a short midpiece generally contained the highest concentration of ATP. Low intracellular ATP is therefore unlikely to explain reduced swimming velocity among the very longest sperm (which tend to have a shorter midpiece).

A Simple Confocal Microscopy-based Method for Assessing Sperm Movement

In the field of reproductive medicine, assessment of sperm motility is a key factor for achieving successful artificial insemination, in vitro fertilization, or intracellular sperm injection. In this study, the motility of boar sperms was estimated using real-time imaging via confocal microscopy. To confirm this confocal imaging method, flagellar beats and whiplash-like movement angles were compared between fresh and low-temperature-preserved (17℃ for 24 h) porcine sperms. Low-temperature preservation reduced the number of flagellar beats from 11.0±2.3 beats/s (fresh sperm) to 5.7±1.8 beats/s and increased the flagellar bending angle from 19.8°±13.8° (fresh) to 30.6°±15.6°. These data suggest that sperm activity can be assessed using confocal microscopy. The observed motility patterns could be used to develop a sperm evaluation index and automated confocal microscopic sperm motility analysis techniques.

Method matters: Experimental evidence for shorter avian sperm in faecal compared to abdominal massage samples

Birds are model organisms in sperm biology. Previous work in zebra finches, suggested that sperm sampled from males' faeces and ejaculates do not differ in size. Here, we tested this assumption in a captive population of house sparrows, Passer domesticus. We compared sperm length in samples from three collection techniques: female dummy, faecal and abdominal massage samples. We found that sperm were significantly shorter in faecal than abdominal massage samples, which was explained by shorter heads and midpieces, but not flagella. This result might indicate that faecal sampled sperm could be less mature than sperm collected by abdominal massage. The female dummy method resulted in an insufficient number of experimental ejaculates because most males ignored it. In light of these results, we recommend using abdominal massage as a preferred method for avian sperm sampling. Where avian sperm cannot be collected by abdominal massage alone, we advise controlling for sperm sampling protocol statistically.

The effects of inbreeding on sperm morphometry of captive-bred endangered mammals

Captive breeding is used for the conservation of endangered species, but inbreeding can result when a small number of founders are used to establish populations. Inbreeding can reduce the proportion of normal sperm in an ejaculate, but may also have effects on sperm size and shape (morphometry). We investigated the effects of inbreeding on sperm morphometry of black-footed ferrets (Mustela nigripes (Audubon and Bachman, 1851)) and red wolves (Canis rufus Audubon and Bachman, 1851) from captive breeding programs to determine if more inbred males produced sperm of poor quality (bulky head, small midpiece, short tail). We measured sperm head length, head width, midpiece length, midpiece width, and tail length on 10 sperm from each male of both species. A negative relationship between variation in sperm tail length and inbreeding coefficient (f) was found in black-footed ferret, suggesting that more inbred individuals will have reduced genetic and phenotypic variation. Analyses indicated a negative relationship between sperm head width and f and a positive relationship between sperm tail length and f in red wolf, suggesting that more inbred male red wolves could have faster sperm. These results indicate that inbreeding affects functionally important aspects of sperm morphometry, but that these effects may not be entirely negative.

Social dominance explains within-ejaculate variation in sperm design in a passerine bird

Background

Comparative studies suggest that sperm competition exerts stabilizing selection towards an optimal sperm design – e.g., the relative size and covariation of different sperm sections or a quantitative measure of sperm shape - that maximizes male fertility, which results in reduced levels of within-male variation in sperm morphology. Yet, these studies also reveal substantial amounts of unexplained within-ejaculate variance, and the factors presiding to the maintenance of such within-male variation in sperm design at the population level still remain to be identified. Sperm competition models predict that males should progressively invest more resources in their germline as their mating costs increase, i.e., the soma/germline allocation trade-off hypothesis. When access to fertile females is determined by social dominance, the soma/germline allocation trade-off hypothesis predicts that dominant males should invest less in the control of spermatogenesis. Hence, dominance should positively correlate with within-male variance in sperm design.

Results

In support of this hypothesis, we found that dominant house sparrow males produce ejaculates with higher levels of within-ejaculate variation in sperm design compared to subordinate males. However, after experimentally manipulating male social status, this pattern was not maintained.

Conclusions

Our results suggest that males might control variation in sperm design according to their social status to some extent. Yet, it seems that such within-ejaculate variation in sperm design cannot be rapidly adjusted to a new status. While variation in sperm design could result from various non-exclusive sources, we discuss how strategic allocation of resources to the somatic vs. the germline functions could be an important process shaping the relationship between within-male variation in sperm design and social status.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-017-0914-2) contains supplementary material, which is available to authorized users.

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.

The genetic basis and fitness consequences of sperm midpiece size in deer mice

An extensive array of reproductive traits varies among species, yet the genetic mechanisms that enable divergence, often over short evolutionary timescales, remain elusive. Here we examine two sister-species of Peromyscus mice with divergent mating systems. We find that the promiscuous species produces sperm with longer midpiece than the monogamous species, and midpiece size correlates positively with competitive ability and swimming performance. Using forward genetics, we identify a gene associated with midpiece length: Prkar1a, which encodes the R1α regulatory subunit of PKA. R1α localizes to midpiece in Peromyscus and is differentially expressed in mature sperm of the two species yet is similarly abundant in the testis. We also show that genetic variation at this locus accurately predicts male reproductive success. Our findings suggest that rapid evolution of reproductive traits can occur through cell type-specific changes to ubiquitously expressed genes and have an important effect on fitness.

Across species there are large changes in sperm morphology, yet how and why is unclear. Here, the authors show in Peromyscus mice, that the sperm midpiece length influences swimming speed and male fertility, and is regulated by cell-type specific expression of a ubiquitously expressed gene, Prkar1a.

Using sperm morphometry and multivariate analysis to differentiate species of gray Mazama

There is genetic evidence that the two species of Brazilian gray Mazama, Mazama gouazoubira and Mazama nemorivaga, belong to different genera. This study identified significant differences that separated them into distinct groups, based on characteristics of the spermatozoa and ejaculate of both species. The characteristics that most clearly differentiated between the species were ejaculate colour, white for M. gouazoubira and reddish for M. nemorivaga, and sperm head dimensions. Multivariate analysis of sperm head dimension and format data accurately discriminated three groups for species with total percentage of misclassified of 0.71. The individual analysis, by animal, and the multivariate analysis have also discriminated correctly all five animals (total percentage of misclassified of 13.95%), and the canonical plot has shown three different clusters: Cluster 1, including individuals of M. nemorivaga; Cluster 2, including two individuals of M. gouazoubira; and Cluster 3, including a single individual of M. gouazoubira. The results obtained in this work corroborate the hypothesis of the formation of new genera and species for gray Mazama. Moreover, the easily applied method described herein can be used as an auxiliary tool to identify sibling species of other taxonomic groups.

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.