Condition-dependent expression of pre- and postcopulatory sexual traits in guppies

No evidence for phenotypic condition-dependent ejaculate allocation in response to sperm competition in a seed beetle

Sperm competition is known to favor the evolution of male traits that confer an advantage in gaining fertilizations when females mate multiply. Ejaculate production can be costly and the strategic allocation of sperm in relation to the sperm competition environment is a taxonomically widespread phenomenon. However, variation among males in their ability to adjust ejaculate allocation has rarely been explored. Here, we manipulated the phenotypic condition of male seed beetles, Callosobruchus maculatus, via larval diet quality and measured ejaculate allocation across varying levels of sperm competition manipulated using olfactory cues. Furthermore, we asked how strategic ejaculation was impacted by previous ejaculation. We found no variation in ejaculate allocation in response to experimentally manipulated cues to sperm competition. Ejaculate allocation was reduced by a male's previous mating history but was unaffected by the larval diets on which males were reared. We suggest that either male seed beetles are unable to adjust ejaculate size to the immediate competitive environment, or that sperm displacement strategies employed by males favor maximal investment at all mating events, especially when unmated females are infrequently encountered. As our study is one of few to examine condition dependence in strategic ejaculation, emphasis should be placed on future studies investigating this possibility across a wider range of taxa and animal mating systems.

Male crickets in poor condition engage in less same-sex sexual behavior

Same-sex sexual behavior (SSB) is widespread among animals and is often treated as an evolutionary anomaly or mistake. An alternative view is that SSB occurs because individuals have broader or more permissive "mating filters." A broader filter means directing courtship toward anything that resembles a potential mate, while a narrower filter means having stricter criteria about when to court. Broader filters help ensure animals do not miss out on mating opportunities. However, it may be advantageous for individuals to narrow their mating filters under some conditions, such as when the costs of courtship increase because courting individuals are in poor condition. We examined whether mating filters, and the expression of SSB, is condition dependent in the Pacific field cricket (Teleogryllus oceanicus). When males were in poor condition (because they were reared on a poor diet), they courted other males less. However, condition did not influence courtship toward females. This finding provides evidence that animals can decrease their expression of SSB by flexibly narrowing their mating filters when in poor condition. Our results demonstrate the benefits of a mating filters approach to studying SSB-rather than being a mistake or an anomaly in need of special explanation, SSB may be better viewed as a consequence of flexible, condition-dependent changes in mating behavior. Such an approach can be applied to other contexts where behavior is directed toward unintended targets. We emphasize that our results, like other studies of SSB in nonhuman animals, tell us nothing about sexual identities in humans.

Effects of inbreeding and elevated rearing temperatures on strategic sperm investment

Males often strategically adjust the number of available sperm based on the social context (i.e. sperm priming response), but it remains unclear how environmental and genetic factors shape this adjustment. In freshwater ecosystems, high ambient temperatures often lead to isolated pools of hotter water in which inbreeding occurs. Higher water temperatures and inbreeding can impair fish development, potentially disrupting sperm production. We used guppies (Poecilia reticulata) to investigate how developmental temperature (26 °C, 30 °C) and male inbreeding status (inbred, outbred) influence their sperm priming response. We also tested if sperm priming was affected by whether the female was a relative (sister) and whether she was inbred or outbred. There was no effect of rearing temperature; male inbreeding status alone determined the number of available sperm in response to female presence, her inbreeding status, and her relatedness. Inbred males produced significantly more sperm in the presence of an unrelated, outbred female than when no female was present. Conversely, outbred males did not alter the number of sperm available in response to female presence or relatedness. Moreover, inbred males produced marginally more sperm when exposed to an unrelated female that was outbred rather than inbred, but there was no difference when exposed to an inbred female that was unrelated versus related. Together, a sperm priming response was only observed in inbred males when exposed to an outbred female. Outbred females in our study were larger than inbred females, suggesting that inbred males strategically allocated ejaculate resources toward females in better condition.

Testing for age-dependent effects of dietary restriction on the strength of condition dependence in ejaculate traits in the guppy (Poecilia reticulata)

Ejaculates can be costly to produce and depend on an individual's condition, defined as the pool of resources allocated to fitness. A method for assessing condition dependence is to manipulate resource availability and test for a reduction in trait expression. Here, we assess the effects of dietary restriction on two determinants of reproductive fitness in the guppy Poecilia reticulata-sperm production and sperm motility. Importantly, we administered dietary restriction at distinct developmental stages to test: (1) whether dietary restriction, when applied exclusively to juveniles, compromised the ejaculates of newly mature males; (2) whether any observed effects of dietary restriction seen in (1) were reversible when fish returned to an unrestricted diet; and (3) whether dietary restriction applied exclusively to adults influenced ejaculates. We found detrimental effects of resource limitation on both traits, and these were consistent across the three developmental stages tested. Furthermore, dietary restriction reduced male body weight, but this was partially reversed when diet-stressed juveniles (i.e. group 2) returned to unrestricted diets. This latter result suggests that diet-stressed males may sacrifice growth in order to maintain their investment in ejaculates. Together these findings underscore the importance of resource acquisition in determining the expression of ejaculate traits.

Using fish to understand how cities affect sexual selection before and after mating

Urbanization transforms natural and agricultural areas into built landscapes. Such profound habitat alteration imposes strong pressure on phenotypic trait changes through processes related to natural and/or sexual selection. Evidence of how natural selection drives changes to traits in urban biota is increasing, but little is known about the role of sexual selection. In this study, we assessed the effect of urbanization on the expression and interaction of males' pre-mating traits (body size and color) and a post-mating trait (sperm load). We used a widespread invasive species, the guppy (Poecilia reticulata), which is a wellknown model for studying sexual selection, but have never been studied in urban systems for this purpose. We found that urbanization did not affect mean body size or condition, but it resulted in size-dependent reductions in the expression of orange and iridescent colors, as well as sperm load. The orange color was reduced in small urban guppies, while the iridescent colors were reduced in large urban guppies compared to non-urban guppies. The difference in sperm load was only found in large males, with lower sperm load in urban guppies. The relationship between orange color and sperm load was positive in urban guppies but negative in non-urban guppies, while the association between iridescent color and sperm load followed the opposite pattern. Our findings suggest that sexual selection on pre- and post-mating traits is weaker in urban than in non-urban systems and that interactions between such traits are context dependent. These responses can be related to the pollution and altered visual environment of urban systems and provide an opportunity to advance our understanding of the mechanisms determining adaptation in cities.

Resource-dependent investment in male sexual traits in a viviparous fish

Exaggerated and conspicuous sexually selected traits are often costly to produce and maintain. Costly traits are expected to show resource-dependent expression, since limited resources prevent animals from investing maximally in multiple traits simultaneously. However, there may be critical periods during an individual's life where the expression of traits is altered if resources are limited. Moreover, costly sexual traits may arise from sexual selection acting both before (pre-copulatory) and after mating (post-copulatory). Gaining a robust understanding of resource-dependent trait expression therefore requires an approach that examines both episodes of sexual selection after resource limitation during different times in an individual's life. Yet few studies have taken such an approach. Here, we examine how resource restriction influences a set of pre- and post-copulatory traits in male pygmy halfbeaks (Dermogenys collettei), which invest in sexual ornaments and routinely engage in male-male contests and sperm competition. Critically, we examined responses in males when resources were restricted during development and after reaching sexual maturity. Both pre- and post-copulatory traits are resource-dependent in male halfbeaks. Body size, beak size, courtship behavior, and testes size were reduced by diet restriction, while, unexpectedly, the restricted-diet group had a larger area of red color on the beak and fins after diet treatment. These patterns were generally consistent when resources were restricted during development and after reaching sexual maturity. The study reinforces the role of resource acquisition in maintaining variation among sexual traits.

Personality, sperm traits and a test for their combined dependence on male condition in guppies

There is evidence that animal personality can affect sexual selection, with studies reporting that male behavioural types are associated with success during pre- and post-copulatory sexual selection. Given these links between personality and sexual traits, and the accumulating evidence that their expression can depend on an individual's dietary status (i.e. condition), a novel prediction is that changes in a male's diet should alter both the average expression of personality and sexual traits, and their covariance. We tested these predictions using the guppy Poecilia reticulata, a species previously shown to exhibit strong condition dependence in ejaculate traits and a positive correlation between sperm production and individual variation in boldness. Contrary to expectation, we found that dietary restriction-when administered in mature adult males-did not affect the expression of either behavioural (boldness and activity) or ejaculate traits, although we did find that males subjected to dietary stress exhibited a positive association between sperm velocity and boldness that was not apparent in the unrestricted diet group. This latter finding points to possible context-dependent patterns of covariance between sexually selected traits and personalities, which may have implications for patterns of selection and evolutionary processes under fluctuating environmental conditions.

Personality, sperm traits and a test for their combined dependence on male condition in guppies

There is evidence that animal personality can affect sexual selection, with studies reporting that male behavioural types are associated with success during pre- and post-copulatory sexual selection. Given these links between personality and sexual traits, and the accumulating evidence that their expression can depend on an individual's dietary status (i.e. condition), a novel prediction is that changes in a male's diet should alter both the average expression of personality and sexual traits, and their covariance. We tested these predictions using the guppy Poecilia reticulata, a species previously shown to exhibit strong condition dependence in ejaculate traits and a positive correlation between sperm production and individual variation in boldness. Contrary to expectation, we found that dietary restriction-when administered in mature adult males-did not affect the expression of either behavioural (boldness and activity) or ejaculate traits, although we did find that males subjected to dietary stress exhibited a positive association between sperm velocity and boldness that was not apparent in the unrestricted diet group. This latter finding points to possible context-dependent patterns of covariance between sexually selected traits and personalities, which may have implications for patterns of selection and evolutionary processes under fluctuating environmental conditions.

Personality, sperm traits and a test for their combined dependence on male condition in guppies

There is evidence that animal personality can affect sexual selection, with studies reporting that male behavioural types are associated with success during pre- and post-copulatory sexual selection. Given these links between personality and sexual traits, and the accumulating evidence that their expression can depend on an individual's dietary status (i.e. condition), a novel prediction is that changes in a male's diet should alter both the average expression of personality and sexual traits, and their covariance. We tested these predictions using the guppy Poecilia reticulata, a species previously shown to exhibit strong condition dependence in ejaculate traits and a positive correlation between sperm production and individual variation in boldness. Contrary to expectation, we found that dietary restriction-when administered in mature adult males-did not affect the expression of either behavioural (boldness and activity) or ejaculate traits, although we did find that males subjected to dietary stress exhibited a positive association between sperm velocity and boldness that was not apparent in the unrestricted diet group. This latter finding points to possible context-dependent patterns of covariance between sexually selected traits and personalities, which may have implications for patterns of selection and evolutionary processes under fluctuating environmental conditions.

The impact of diet quality on sperm velocity, morphology and normality in the zebra finch Taeniopygia guttata

Sperm traits can influence fertilisation success, but there is still much we do not understand about sperm condition dependence, that is, how much sperm traits depend on the male's energy acquisition and allocation. This is especially pronounced in avian taxa, despite extensive observational studies and sampling in wild populations. In this study we collected sperm samples before and after experimentally reducing diet quality of wild-derived captive zebra finches in small mixed-sex groups, which we compared to individuals on a control diet. We measured the length of sperm components (head, midpiece, flagellum and total sperm length), the proportion of sperm with normal morphology, the proportion of sperm that were progressively motile, and swimming velocity (curvilinear velocity; VCL). The only sperm trait we found to be impacted by reduced diet quality was a significant decrease in sperm midpiece length. This is consistent with emerging evidence in other non-model systems, as well the fact that diet can alter mitochondrial density and structure in other tissue types. There was also a significant decrease in sperm velocity and the proportion of motile sperm over the course of the experiment for both experimental groups (i.e. unrelated to diet). That in the control group experienced this decrease in sperm velocity was marked by no change in any sperm morphological measure highlights the existence of other important determinants of sperm velocity in passerine birds, likely including variation in other components of the seminal fluid.

Sensitive periods during the development and expression of vertebrate sexual signals: A systematic review

Many sexually selected traits exhibit phenotypic plasticity. Despite a growing appreciation for the ecological context in which sexual selection occurs, and for the role of plasticity in shaping traits associated with local adaptation and divergence, there is an important gap in knowledge about the onset and duration of plasticity in sexual trait expression. Integrating this temporal dimension of plasticity into models of sexual selection informs our understanding of the information conveyed by sexual traits and our predictions related to trait evolution, and is critical in this time of unprecedented and rapid environmental change. We conducted a systematic review of 869 studies to ask how trait modalities (e.g., visual and chemical) relate to the onset and duration of plasticity in vertebrate sexual signals. We show that this literature is dominated by studies of coloration in birds and fish, and most studies take place during the breeding season. Where possible, we integrate results across studies to link physiology of specific trait modalities with the life stage (e.g., juvenile, breeding, or nonbreeding) during which plasticity occurs in well-studied traits. Limitations of our review included a lack of replication in our dataset, which precluded formal analysis. We argue that the timing of trait plasticity, in addition to environmental context, is critical for determining whether and how various communication signals are associated with ecological context, because plasticity may be ongoing or occur at only one point in an individual's lifetime, and determining a fixed trajectory of trait expression. We advocate for careful consideration of the onset and duration of plasticity when analyzing how environmental variation affects sexual trait expression and associated evolutionary outcomes.

Sperm depletion in relation to developmental nutrition and genotype in Drosophila melanogaster

Nutrient limitation during development can restrict the ability of adults to invest in costly fitness traits, and genotypes can vary in their sensitivity to developmental nutrition. However, little is known about how genotype and nutrition affect male ability to maintain ejaculate allocation and achieve fertilization across successive matings. Using 17 isogenic lines of Drosophila melanogaster, we investigated how variation in developmental nutrition affects males' abilities to mate, transfer sperm, and sire offspring when presented with successive virgin females. We found that, with each successive mating, males required longer to initiate copulation, transferred fewer sperm, and sired fewer offspring. Males reared on a low-nutrient diet transferred fewer sperm than those reared on nutritionally superior diets, but the rate at which males depleted their sperm, as well as their reproductive performance, was largely independent of diet. Genotype and the genotype × diet interaction explained little of the variation in these male reproductive traits. Our results show that sperm depletion can occur rapidly and impose substantial fitness costs for D. melanogaster males across multiple genotypes and developmental environments.

Food Limitation but Not Enhanced Rates of Ejaculate Production Imposes Reproductive and Survival Costs to Male Crickets

Estimating costs of ejaculate production is challenging. Metabolic investment in ejaculates may come at the expense of other physiological functions and may negatively affect future reproduction and/or survival. These trade-offs are especially likely to occur under constrained resource pools (e.g., poor nutrition). Here, we investigated costs of ejaculate production via trade-offs in the field cricket Gryllus bimaculatus. We experimentally increased rates of ejaculate production, while keeping an unmanipulated group, in adult males kept at high and low feeding regimes and tested the effects of our treatments on (i) somatic maintenance (i.e., changes in male body mass), (ii) future reproduction (i.e., the likelihood of producing a spermatophore and the viability of its sperm), and (iii) lifetime survival and longevity. We predicted investment in ejaculates to impinge upon all measured responses, especially in low-fed individuals. Instead, we only found negative effects of food limitation, suggesting low or undetectable costs of spermatophore production. High mating rates may select for males to maximize their capacity of ejaculate production, making ejaculate traits less prone to trade-offs with other fitness-related life history traits. Nevertheless, males were impaired due to nutrient deficiency in producing viable ejaculates, suggesting condition-dependent costs for ejaculate production.

Separating differential allocation by females from direct effects of male condition in a beetle

Differential allocation is the adjustment of reproductive allocation, typically by a female, in response to the quality of her male partner. A recent theoretical model suggests that differential allocation may influence trade-offs between reproductive traits within a breeding attempt. Furthermore, it is often difficult to distinguish differential allocation from direct effects of male condition. We address these gaps using a novel cross-fostering design to exclude direct effects of male condition and to test whether differential allocation affects trade-offs between and within breeding attempts. This design detects differential allocation as effects of a female’s mating partner and direct effects of male condition as effects of the larvae’s sire. We used the burying beetle Nicrophorus vespilloides, a species which adjusts reproductive allocation by culling some larvae after hatching. We used food deprivation to manipulate the nutritional condition of both the female’s mating partner and the larvae’s sire. We find clear evidence for differential allocation as females mating with food-deprived males had fewer larvae than females mating with control males. There was a trade-off between number and size of larvae when females mated with control males, but a positive relationship when females mated with food-deprived males. Thus, differential allocation influenced relationships between reproductive traits within a breeding attempt, but not necessarily through trade-offs. Instead, we suggest that there may be cryptic heterogeneity in quality among females or their mating partners that was only exposed when females mated with a male in poor condition.

Quantifying the costs of pre- and postcopulatory traits for males: Evidence that costs of ejaculation are minor relative to mating effort

Although it is widely stated that both mating behavior and sperm traits are energetically costly for males, we currently lack empirical estimates of the relative costs to males of pre‐ versus postcopulatory investments. Such estimates require the experimental separation of the act of mating from that of ejaculation, which is a nontrivial logistical challenge. Here, we overcome this challenge using a novel morphological manipulation (gonopodium tip ablation) in the eastern mosquitofish (Gambusia holbrooki) to tease apart investment in mating effort from that in sperm replenishment following ejaculation. We quantified the relative cumulative costs of investing in mating effort and ejaculation by comparing somatic traits and reproductive performance among three types of males: ablated males that could attempt to mate but not ejaculate; unablated males that could both mate and ejaculate; and control males that had no access to females. We show that, after eight weeks, mating investment significantly reduces both body growth and immunocompetence and results in a significant decline in mating effort. In contrast, cumulative investment into sperm replenishment following ejaculation has few detectable effects that are only apparent in smaller males. These minor costs occur despite the fact that G. holbrooki has very high levels of sperm competition and multiple mating by both sexes, which is usually associated with elevated levels of sperm production. Crucially, our study is the first, to our knowledge, to experimentally compare the relative costs of pre‐ and postcopulatory investment on components of male fitness in a vertebrate.

Meta-analysis reveals that animal sexual signalling behaviour is honest and resource based

Animals often need to signal to attract mates and behavioural signalling may impose substantial energetic and fitness costs to signallers. Consequently, individuals often strategically adjust signalling effort to maximize the fitness payoffs of signalling. An important determinant of these payoffs is individual state, which can influence the resources available to signallers, their likelihood of mating and their motivation to mate. However, empirical studies often find contradictory patterns of state-based signalling behaviour. For example, individuals in poor condition may signal less than those in good condition to conserve resources (ability-based signalling) or signal more to maximize short-term reproductive success (needs-based signalling). To clarify this relationship, I systematically searched for published studies examining animal sexual signalling behaviour in relation to six aspects of individual state: age, mated status, attractiveness, body size, condition and parasite load. Across 228 studies and 147 species, individuals (who were predominantly male) invested more into behavioural signalling when in good condition. Overall, this suggests that animal sexual signalling behaviour is generally honest and ability-based. However, the magnitude of state-dependent plasticity was small and there was a large amount of between-study heterogeneity that remains unexplained.

Latitudinal variation in sexual dimorphism in a freshwater fish group

Tropical animals are characterized by showy ornaments and conspicuous body colours as compared with their temperate relatives. Some recent studies have hypothesized that sexual selection pressures are stronger in the tropics than in the temperate zone. Although negative correlations between latitude and the degree of sexual dimorphism would support this hypothesis, phylogeny should be taken into account in such comparative studies. Comparisons of the degree of sexual dimorphism in body size and fin lengths among species of the Adrianichthyidae, a freshwater fish family having a wide geographical range throughout Southeast and East Asia, revealed that lower latitude species are sexually more dimorphic in all characters than higher latitude species. Phylogenetic generalized least squares analyses using a mitochondrial DNA phylogeny demonstrated that the negative correlations between latitude and the degree of sexual dimorphism become non-significant when phylogeny is considered, but that the variance in the degree of sexual dimorphism is explained not only by phylogeny but also almost equally by latitude. Ancestral state reconstruction indicated that sexual dimorphisms have evolved independently even within major clades. These findings are consistent with the view that tropical species are exposed to stronger sexual selection pressures than temperate species. We discuss possible causes of the latitudinal variation in sexual selection pressure.

Differential gene regulation in selected lines for high and low sperm production in male guppies

In species where females mate with more than one male during the same reproductive event, males typically increase the number of sperm produced to boost their fertilization share. Sperm is not limitless, however, and theory predicts that their production will come at the cost of other fitness-related traits, such as body growth or immunocompetence, although these evolutionary trade-offs are notoriously difficult to highlight. To this end, we combined artificial selection for sperm production with a transcriptome analysis using Poecilia reticulata, a fish characterized by intense sperm competition in which the number of sperm transferred during mating is the most important predictor of fertilization success, yet sperm production is highly variable among males. We compared the brain and testes transcriptome in male guppies of lines artificially selected for high and low sperm production by identifying pivotal differentially expressed gene sets that may regulate spermatogenesis and immune function in this species. Despite the small differences in single genes' expression, gene set enrichment analysis showed coordinated gene expression differences associated with several pathways differentially regulated in the two selection lines. High sperm production males showed an upregulation of pathways related to immunosuppression and development of spermatozoa indicating a possible immunological cost of sperm production.

Effects of condition and sperm competition risk on sperm allocation and storage in neriid flies

Ejaculate traits can be sexually selected and often exhibit heightened condition-dependence. However, the influence of sperm competition risk in tandem with condition-dependent ejaculate allocation strategies is relatively unstudied. Because ejaculates are costly to produce, high-condition males may be expected to invest more in ejaculates when sperm competition risk is greater. We examined the condition-dependence of ejaculate size by manipulating nutrient concentration in the juvenile (larval) diet of the neriid fly Telostylinus angusticollis. Using a fully factorial design we also examined the effects of perceived sperm competition risk (manipulated by allowing males to mate first or second) on the quantity of ejaculate transferred and stored in the three spermathecae of the female reproductive tract. To differentiate male ejaculates, we fed males nontoxic rhodamine fluorophores (which bind to proteins in the body) prior to mating, labeling their sperm red or green. We found that high-condition males initiated mating more quickly and, when mating second, transferred more ejaculate to both of the female’s posterior spermathecae. This suggests that males allocate ejaculates strategically, with high-condition males elevating their ejaculate investment only when facing sperm competition. More broadly, our findings suggest that ejaculate allocation strategies can incorporate variation in both condition and perceived risk of sperm competition.

Effects of nutrient limitation on sperm and seminal fluid: a systematic review and meta-analysis

Theory predicts that costly sexual traits should be reduced when individuals are in poor condition (i.e. traits should exhibit condition-dependent expression). It is therefore widely expected that male ejaculate traits, such as sperm and seminal fluid, will exhibit reduced quantity and quality when dietary nutrients are limited. However, reported patterns of ejaculate condition dependence are highly variable, and there has been no comprehensive synthesis of underlying sources of such variation in condition-dependent responses. In particular, it remains unclear whether all ejaculate traits are equally sensitive to nutrient intake, and whether such traits are particularly sensitive to certain dietary nutrients, respond more strongly to nutrients during specific life stages, or respond more strongly in some taxonomic groups. We systematically reviewed these potential sources of variation through a meta-analysis across 50 species of arthropods and vertebrates (from 71 papers and 348 effect sizes). We found that overall, ejaculate traits are moderately reduced when dietary nutrients are limited, but we also detected substantial variation in responses. Seminal fluid quantity was strongly and consistently condition dependent, while sperm quantity was moderately condition dependent. By contrast, aspects of sperm quality (particularly sperm viability and morphology) were less consistently reduced under nutrient limitation. Ejaculate traits tended to respond in a condition-dependent manner to a wide range of dietary manipulations, especially to caloric and protein restriction. Finally, while all major taxa for which sufficient data exist (i.e. arthropods, mammals, fish) showed condition dependence of ejaculate traits, we detected some taxonomic differences in the life stage that is most sensitive to nutrient limitation, and in the degree of condition dependence of specific ejaculate traits. Together, these biologically relevant factors accounted for nearly 20% of the total variance in ejaculate responses to nutrient limitation. Interestingly, body size showed considerably stronger condition-dependent responses compared to ejaculate traits, suggesting that ejaculate trait expression may be strongly canalised to protect important reproductive functions, or that the cost of producing an ejaculate is relatively low. Taken together, our findings show that condition-dependence of ejaculate traits is taxonomically widespread, but there are also many interesting, biologically relevant sources of variation that require further investigation. In particular, further research is needed to understand the differences in selective pressures that result in differential patterns of ejaculate condition dependence across taxa and ejaculate traits.

Context-specific behavioural changes induced by exposure to an androgenic endocrine disruptor

Pharmaceutical contaminants are being detected with increased frequency in organisms and ecosystems worldwide. This represents a major environmental concern given that various pharmaceuticals act on drug targets that are evolutionarily conserved across diverse taxa, are often persistent in the environment, and can bioconcentrate in organisms and bioaccumulate in food chains. Despite this, relatively little is known about the potential for pharmaceutical contaminants to affect animal behaviour, especially across multiple fitness-related contexts. Here, we investigated impacts of 21-day exposure of wild-caught male eastern mosquitofish (Gambusia holbrooki) to a field-realistic level of the veterinary pharmaceutical 17β-trenbolone-a growth-promoting steroid used extensively in beef production worldwide and a potent androgenic endocrine disruptor repeatedly detected in surface waters affected by livestock effluent run-off. First, we examined male boldness, activity, and exploratory behaviour in a novel environment (maze arena) and found no significant effect of 17β-trenbolone exposure. Second, the same males were tested in a reproductive assay for their tendency to associate with a stimulus (unexposed) female behind a partition. Exposed males exhibited reduced association behaviour, taking longer to first associate with, and spending less time within close proximity to, a female. Third, all males were assayed for sperm function (computer-assisted sperm analysis, sperm viability) or quantity (total sperm count) and, although no significant main effects of 17β-trenbolone were seen on sperm traits, exposure altered the relationship between male morphology and sperm function. Lastly, morphological traits were assessed and exposed males were found to have, on average, increased mass relative to length. In combination, these results demonstrate that exposure to a field-realistic level of 17β-trenbolone can produce subtle but important trait alterations in male fish-including context-specific behavioural changes, disruption of key sperm function trade-offs, and altered morphology-with potential impacts on exposed wildlife.

Ejaculate-mediated paternal effects: evidence, mechanisms and evolutionary implications

Despite serving the primary objective of ensuring that at least one sperm cell reaches and fertilises an ovum, the male ejaculate (i.e. spermatozoa and seminal fluid) is a compositionally complex 'trait' that can respond phenotypically to subtle changes in conditions. In particular, recent research has shown that environmentally and genetically induced changes to ejaculates can have implications for offspring traits that are independent of the DNA sequence encoded into the sperm's haploid genome. In this review, we compile evidence from several disciplines and numerous taxonomic systems to reveal the extent of such ejaculate-mediated paternal effects (EMPEs). We consider a number of environmental and genetic factors that have been shown to impact offspring phenotypes via ejaculates, and where possible, we highlight the putative mechanistic pathways by which ejaculates can act as conduits for paternal effects. We also highlight how females themselves can influence EMPEs, and in some cases, how maternally derived sources of variance may confound attempts to test for EMPEs. Finally, we consider a range of putative evolutionary implications of EMPEs and suggest a number of potentially useful approaches for exploring these further. Overall, our review confirms that EMPEs are both widespread and varied in their effects, although studies reporting their evolutionary effects are still in their infancy.

Developmental diet irreversibly shapes male post-copulatory traits in the neriid fly Telostylinus angusticollis

Nutrient availability has been shown to influence investment in many fitness-related traits, including male reproductive success. Many studies have demonstrated that a reduction in nutrient availability alters male post-copulatory trait expression, with some studies demonstrating an effect of developmental nutrients and others, an effect of adult nutrients. However, few studies have manipulated both developmental and adult nutrients in the same experiment. Therefore, it is not clear what life-stage has the greatest effect on post-copulatory trait expression, and if the effects of developmental and adult nutrients can interact. Here, we investigate effects of developmental and adult nutrition on male testes and accessory gland size, sperm movement within the female reproductive tract and sperm length in the neriid fly, Telostylinus angusticollis. We found that males fed a nutrient-poor developmental diet produced sperm with a reduced tail beat frequency and had smaller testes and accessory glands compared to males fed a nutrient-rich developmental diet. In contrast, we found no effects of adult nutrition on any traits measured, although sperm length was correlated with body size and male age but unaffected by nutrition at any stage. Therefore, investment in adult post-copulatory traits is determined early on by developmental nutrients in male neriid flies, and this effect is not altered by adult nutrient availability.

Multiple biological mechanisms result in correlations between pre- and post-mating traits that differ among versus within individuals and genotypes

Reproductive traits involved in mate acquisition (pre-mating traits) are predicted to covary with those involved in fertilization success (post-mating traits). Variation in male quality may give rise to positive, and resource allocation trade-offs to negative, covariances between pre- and post-mating traits. Empirical studies have yielded mixed results. Progress is hampered as researchers often fail to appreciate that mentioned biological mechanisms can act simultaneously but at different hierarchical levels of biological variation: genetic correlations may, for example, be negative due to genetic trade-offs but environmental correlations may instead be positive due to individual variation in resource acquisition. We measured pre-mating (aggression, body weight) and post-mating (ejaculate size) reproductive traits in a pedigreed population of southern field crickets (Gryllus bimaculatus). To create environmental variation, crickets were raised on either a low or a high nymphal density treatment. We estimated genetic and environmental sources of correlations between pre- and post-mating traits. We found positive genetic correlations between pre- and post-mating traits, implying the existence of genetic variation in male quality. Over repeated trials of the same individual (testing order), positive changes in one trait were matched with negative changes in other traits, suggesting energy allocating trade-offs within individuals among days. These findings demonstrate the need for research on pre- and post-mating traits to consider the hierarchical structure of trait correlations. Only by doing so was our study able to conclude that multiple mechanisms jointly shape phenotypic associations between pre- and post-mating traits in crickets.

Sperm form and function: what do we know about the role of sexual selection?

Sperm morphological variation has attracted considerable interest and generated a wealth of predominantly descriptive studies over the past three centuries. Yet, apart from biophysical studies linking sperm morphology to swimming velocity, surprisingly little is known about the adaptive significance of sperm form and the selective processes underlying its tremendous diversification throughout the animal kingdom. Here, we first discuss the challenges of examining sperm morphology in an evolutionary context and why our understanding of it is far from complete. Then, we review empirical evidence for how sexual selection theory applies to the evolution of sperm form and function, including putative secondary sexual traits borne by sperm.

Sperm priming response to perceived mating opportunities is reduced in male guppies with high baseline sperm production

Producing sperm is costly and males have been selected to strategically adjust their sperm production and/or expenditure according to the fitness return associated with a specific mating. For example, males respond to fluctuations in the mating opportunities by adjusting the number of "ready" sperm. This phenomenon is known as "sperm priming" and is interpreted as a strategy to economize the investment in sperm. The cost and benefits of the sperm priming response, however, are expected to depend on a male's baseline sperm production (BSP) in the absence of females, because of the different risk of sperm depletion and the nonlinearly increasing costs of sperm production. We tested this prediction in 2 replicated lines of male guppies Poecilia reticulata that were artificially selected for high and low BSP. BSP has a large genetic variance and a high sire heritability in guppies, and males respond to the perceived mating opportunities by increasing the number of "ready" sperm. We investigated whether males with a different BSP differed in their sperm priming response. We found that when the perceived mating opportunities increased, males from low-sperm lines had a stronger sperm priming response than those from high-sperm lines. This result suggests that adaptive plasticity in sperm priming has the potential to evolve in response to different levels of BSP. The comparison between guppy populations with different levels of sperm production would allow to test whether the pattern reported here is also observed at the interpopulation level.

Population demography and heterozygosity-fitness correlations in natural guppy populations: An examination using sexually selected fitness traits

Heterozygosity-fitness correlations (HFCs) have been examined in a wide diversity of contexts, and the results are often used to infer the role of inbreeding in natural populations. Although population demography, reflected in population-level genetic parameters such as allelic diversity or identity disequilibrium, is expected to play a role in the emergence and detectability of HFCs, direct comparisons of variation in HFCs across many populations of the same species, with different genetic histories, are rare. Here, we examined the relationship between individual microsatellite heterozygosity and a range of sexually selected traits in 660 male guppies from 22 natural populations in Trinidad. Similar to previous studies, observed HFCs were weak overall. However, variation in HFCs among populations was high for some traits (although these variances were not statistically different from zero). Population-level genetic parameters, specifically genetic diversity levels (number of alleles, observed/expected heterozygosity) and measures of identity disequilibrium (g2 and heterozygosity-heterozygosity correlations), were not associated with variation in population-level HFCs. This latter result indicates that these metrics do not necessarily provide a reliable predictor of HFC effect sizes across populations. Importantly, diversity and identity disequilibrium statistics were not correlated, providing empirical evidence that these metrics capture different essential characteristics of populations. A complex genetic architecture likely underpins multiple fitness traits, including those associated with male fitness, which may have reduced our ability to detect HFCs in guppy populations. Further advances in this field would benefit from additional research to determine the demographic contexts in which HFCs are most likely to occur.

Pre- and postcopulatory sexual selection favor aggressive, young males in polyandrous groups of red junglefowl

A challenge in evolutionary biology is to understand the operation of sexual selection on males in polyandrous groups, where sexual selection occurs before and after mating. Here, we combine fine-grained behavioral information (>41,000 interactions) with molecular parentage data to study sexual selection in replicated, age-structured groups of polyandrous red junglefowl, Gallus gallus. Male reproductive success was determined by the number of females mated (precopulatory sexual selection) and his paternity share, which was driven by the polyandry of his female partners (postcopulatory sexual selection). Pre- and postcopulatory components of male reproductive success covaried positively; males with high mating success also had high paternity share. Two male phenotypes affected male pre- and postcopulatory performance: average aggressiveness toward rival males and age. Aggressive males mated with more females and more often with individual females, resulting in higher sexual exclusivity. Similarly, younger males mated with more females and more often with individual females, suffering less intense sperm competition than older males. Older males had a lower paternity share even allowing for their limited sexual exclusivity, indicating they may produce less competitive ejaculates. These results show that-in these populations-postcopulatory sexual selection reinforces precopulatory sexual selection, consistently promoting younger and more aggressive males.

Are sexually selected traits affected by a poor environment early in life?

Background

Challenging conditions experienced early in life, such as a restricted diet, can detrimentally affect key life-history traits. Individuals can reduce these costs by delaying their sexual maturation, albeit at the price of the later onset of breeding, to eventually reach the same adult size as individuals that grow up in a benevolent environment. Delayed maturation can, however, still lead to other detrimental morphological and physiological changes that become apparent later in adulthood (e.g. shorter lifespan, faster senescence). In general, research focuses on the naturally selected costs of a poor early diet. In mosquitofish (Gambusia holbrooki), males with limited food intake early in life delay maturation to reach a similar adult body size to their well-fed counterparts (‘catch-up growth’). Here we tested whether a poor early diet is costly due to the reduced expression of sexually selected male characters, namely genital size and ejaculate traits.

Results

We found that a male’s diet early in life significantly influenced his sperm reserves and sperm replenishment rate. Shortly after maturation males with a restricted early diet had significantly lower sperm reserves and slower replenishment rates than control diet males, but this dietary difference was no longer detectable in older males.

Conclusions

Although delaying maturation to reach the same body size as well fed juveniles can ameliorate some costs of a poor start in life, our findings suggest that costs might still arise because of sexual selection against these males. It should be noted, however, that the observed effects are modest (Hedges’ g = 0.20–0.36), and the assumption that lower sperm production translates into a decline in fitness under sperm competition remains unconfirmed.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-016-0838-2) contains supplementary material, which is available to authorized users.

The total opportunity for sexual selection and the integration of pre- and post-mating episodes of sexual selection in a complex world

It is well known that sexual selection can target reproductive traits during successive pre- and post-mating episodes of selection. A key focus of recent studies has been to understand and quantify how these episodes of sexual selection interact to determine overall variance in reproductive success. In this article, we review empirical developments in this field but also highlight the considerable variability in patterns of pre- and post-mating sexual selection, attributable to variation in patterns of resource acquisition and allocation, ecological and social factors, genotype-by-environment interaction and possible methodological factors that might obscure such patterns. Our aim is to highlight how (co)variances in pre- and post-mating sexually selected traits can be sensitive to changes in a range of ecological and environmental variables. We argue that failure to capture this variation when quantifying the opportunity for sexual selection may lead to erroneous conclusions about the strength, direction or form of sexual selection operating on pre- and post-mating traits. Overall, we advocate for approaches that combine measures of pre- and post-mating selection across contrasting environmental or ecological gradients to better understand the dynamics of sexual selection in polyandrous species. We also discuss some directions for future research in this area.

Dynamic resource allocation between pre- and postcopulatory episodes of sexual selection determines competitive fertilization success

In polyandrous mating systems, male reproductive success depends on both mate-acquisition traits (precopulatory) and sperm competitive abilities (postcopulatory). Empirical data on the interaction between these traits are inconsistent; revealing positive, negative or no relationships. It is generally expected that the investment in pre- and postcopulatory traits is mediated by environmental conditions. To test how dietary resource availability affects sexual ornamentation, sperm quality and their interrelationship in three-spined sticklebacks (Gasterosteus aculeatus), full-sibling groups were raised under three conditions differing in food quantity and/or quality (i.e. carotenoid content): (i) high-quantity/high-quality, (ii) high-quantity/low-quality or (iii) low-quantity/low-quality. After 1 year of feeding, food-restricted males developed a more intense breeding coloration and faster sperm compared with their well-fed brothers, indicating that they allocated relatively more in pre- and postcopulatory traits. Moreover, they outcompeted their well-fed, carotenoid-supplemented brothers in sperm competition trials with equal numbers of competing sperm, suggesting that food-restricted males maximize their present reproductive success. This may result in reduced future reproductive opportunities as food-restricted males suffered from a higher mortality, had an overall reduced body size, and sperm number available for fertilization. In accordance with theory, a trade-off between the investment in pre- and postcopulatory traits was observed in food-restricted males, whereas well-fed males were able to allocate to both traits resulting in a significantly positive relationship.

Environmentally induced (co)variance in sperm and offspring phenotypes as a source of epigenetic effects

Traditionally, it has been assumed that sperm are a vehicle for genes and nothing more. As such, the only source of variance in offspring phenotype via the paternal line has been genetic effects. More recently, however, it has been shown that the phenotype or environment of fathers can affect the phenotype of offspring, challenging traditional theory with implications for evolution, ecology and human in vitro fertilisation. Here, I review sources of non-genetic variation in the sperm phenotype and evidence for co-variation between sperm and offspring phenotypes. I distinguish between two environmental sources of variation in sperm phenotype: the pre-release environment and the post-release environment. Pre-release, sperm phenotypes can vary within species according to male phenotype (e.g. body size) and according to local conditions such as the threat of sperm competition. Post-release, the physicochemical conditions that sperm experience, either when freely spawned or when released into the female reproductive tract, can further filter or modify sperm phenotypes. I find evidence that both pre- and post-release sperm environments can affect offspring phenotype; fertilisation is not a new beginning – rather, the experiences of sperm with the father and upon release can drive variation in the phenotype of the offspring. Interestingly, there was some evidence for co-variation between the stress resistance of sperm and the stress resistance of offspring, though more studies are needed to determine whether such effects are widespread. Overall, it appears that environmentally induced covariation between sperm and offspring phenotypes is non-negligible and further work is needed to determine their prevalence and strength.

Male sperm storage compromises sperm motility in guppies

Sperm senescence can have important evolutionary implications due to its deleterious effects on sperm quality and offspring performance. Consequently, it has been argued that polyandry (female multiple mating) may facilitate the selection of younger, and therefore competitively superior, sperm when ejaculates from multiple males compete for fertilization. Surprisingly, however, unequivocal evidence that sperm ageing influences traits that underlie sperm competitiveness is lacking. Here, we used a paired experimental design that compares sperm quality between 'old' and 'young' ejaculates from individual male guppies (Poecilia reticulata). We show that older sperm exhibit significant reductions in sperm velocity compared with younger sperm from the same males. We found no evidence that the brightness of the male's orange (carotenoid) spots, which are thought to signal resistance to oxidative stress (and thus age-related declines in sperm fitness), signals a male's ability to withstand the deleterious effects of sperm ageing. Instead, polyandry may be a more effective strategy for females to minimize the likelihood of being fertilized by aged sperm.

Experimental reduction in dietary omega-3 polyunsaturated fatty acids depresses sperm competitiveness

The health benefits of diets containing rich sources of long-chain omega-3 polyunsaturated fatty acids (n-3 LC-PUFA) are well documented and include reductions in the risk of several diseases typical of Western societies. The dietary intake of n-3 LC-PUFA has also been linked to fertility, and there is abundant evidence that a range of ejaculate traits linked to fertility in humans, livestock and other animals depend on an adequate intake of n-3 LC-PUFA from dietary sources. However, relatively few studies have explored how n-3 LC-PUFA influence reproductive fitness, particularly in the context of sexual selection. Here, we show that experimental reduction in the level of n-3 LC-PUFA in the diet of guppies (Poecilia reticulata) depresses a male's share of paternity when sperm compete for fertilization, confirming that the currently observed trend for reduced n-3 LC-PUFA in western diets has important implications for individual reproductive fitness.