Male mating success in a North American pitviper: influence of body size,testosterone, and spatial metrics

Males with enhanced traits relative to conspecifics often show increased mating and reproductive success and thus have a fitness advantage. The opportunity or potential for sexual selection is predicted to occur under these conditions. Here, we investigated proximate determinants of mating success in male copperhead snakes (Agkistrodon contortrix), a medium‐sized pitviper of North America. Specifically, we investigated the relationships of body size (snout‐vent length, body mass), body condition index, spatial metrics (total distance moved, home range size), and plasma testosterone concentration on mating success in males. The single mating season lasts from August through September. We compared a set of candidate linear mixed models and selected the best‐fitting one using the adjusted Akaike Information Criterion (AICc). The AICc‐selected model (model 2), with testosterone, body condition index, and home range size as predictor variables, showed that male mating success was positively correlated with testosterone. To our knowledge, this is the first report to show the relationship of testosterone and individual mating success in any snake species. A parallel study conducted on male fitness in A. contortrix of the same population used microsatellite markers to assign parentage of fathers (known mothers). Unlike our study, they found that snout‐vent length was positively correlated with reproductive success and that males were experiencing greater sexual selection. This relationship has been detected under natural conditions in other species of snakes. Although behavioural data are important in any mating system analysis, they should not stand alone to infer parentage, relationships or selection metrics (e.g. Bateman gradients). Long‐term sperm storage by females, female cryptic choice, and other factors contribute to the complexity of mating success of males. Accordingly, we thus conclude that estimates of reproductive success and fitness in cryptic species, such as copperheads and other snakes, require robust molecular methods to draw accurate conclusions regarding proximate and evolutionary responses. © 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 115 , 185–194.     

Seasonal change in the opportunity for sexual selection

Environmental and population parameters that influence the strength of sexual selection may vary considerably over the course of the reproductive season. However, the potential for sexual selection frequently fails to translate into variation in reproductive success among individuals. We investigated seasonal changes in variation in reproductive success, measured as the opportunity for sexual selection, using parentage analysis in 20 experimental populations of the European bitterling (Rhodeus amarus, Cyprinidae), a small freshwater fish with a promiscuous, resource-based mating system. We showed that although the largest males sired most offspring over the entire reproductive season, variation in reproductive success and hence the opportunity for sexual selection was low at the start of the season but increased significantly at its end. This seasonal difference probably arose from the superior competitive endurance of large males and from a higher temporal clustering of reproductively active females at the start of the breeding season than later in the season. The spatial distribution of oviposition sites had a negligible effect on the variation in reproductive success. We discuss the potential implications of our results for the importance and strength of sexual selection in natural populations.     

Field evidence challenges the often‐presumed relationship between early male maturation and female‐biased sexual size dimorphism

Female‐biased sexual size dimorphism (SSD) is often considered an epiphenomenon of selection for the increased mating opportunities provided by early male maturation (i.e., protandry). Empirical evidence of the adaptive significance of protandry remains nonetheless fairly scarce. We use field data collected throughout the reproductive season of an SSD crab spider, Mecaphesa celer, to test two hypotheses: Protandry provides fitness benefits to males, leading to female‐biased SSD, or protandry is an indirect consequence of selection for small male size/large female size. Using field‐collected data, we modeled the probability of mating success for females and males according to their timing of maturation. We found that males matured earlier than females and the proportion of virgin females decreased abruptly early in the season, but unexpectedly increased afterward. Timing of female maturation was not related to clutch size, but large females tended to have more offspring than small females. Timing of female and male maturation was inversely related to size at adulthood, as early‐maturing individuals were larger than late‐maturing ones, suggesting that both sexes exhibit some plasticity in their developmental trajectories. Such plasticity indicates that protandry could co‐occur with any degree and direction of SSD. Our calculation of the probability of mating success along the season shows multiple male maturation time points with similar predicted mating success. This suggests that males follow multiple strategies with equal success, trading‐off access to virgin females with intensity of male–male competition. Our results challenge classic hypotheses linking protandry and female‐biased SSD, and emphasize the importance of directly testing the often‐assumed relationships between co‐occurring animal traits.     

Temporal and microspatial variation in the intensities of natural and sexual selection in the yellow dung fly Scathophaga stercoraria

Studies of phenotypic selection in natural populations often concentrate only on short time periods and do not quantify selection intensities. We quantified temporal and microspatial variation in the intensities of natural and sexual selection for body size in the yellow dung fly over 2 years. Female fecundity selection intensity remained approximately constant over the season with an overall mean ± SE of 0.187 ± 0.014. Selection intensity for male reproductive success, defined as eggs obtained by mating males, did not differ from zero, indicating there was no assortative mating by size. Sexual selection intensity for male mating success favouring large males was variable but overall strong in the two years (0.499 ± 0.053 and 0.510 ± 0.051). As theoretically expected for male–male competition, sexual selection intensity increased with competitor density and reached an asymptote at about 250 males per pat; it also decreased with time in spring and increased again in autumn as a function of density. Small males had the best chance of obtaining a female at very low male densities. Greater selection intensity for large size in males than females is consistent with, and might be responsible for, the observed sexual size dimorphism in this species, as males are larger. The seasonal pattern of mean male body size (smallest at the beginning and end of the season) most likely reflects mere environmental (primarily temperature) influences on phenotypic size.     

Sexual selection and mating patterns in a mammal with female-biased sexual size dimorphism

In mammals, species with highly male-biased sexual size dimorphismtend to have high variance in male reproductive success. However,little information is available on patterns of sexual selection,variation in male and female reproductive success, and bodysize and mating success in species with female-biased size dimorphism.We used parentage data from microsatellite DNA loci to examinethese issues in the yellow-pine chipmunk (Tamias amoenus), asmall ground squirrel with female-biased sexual size dimorphism.Chipmunks were monitored over 3 years in the Kananaskis Valley,Alberta, Canada. We found evidence of high levels of multiplepaternity within litters. Variation in male and female reproductivesuccess was equal, and the opportunity for sexual selectionwas only marginally higher in males than females. Male and femalereproductive success both depended on mating success. We foundno evidence that the number of genetic mates a male had dependedon body size. Our results are consistent with a promiscuousmating system in which males and female mate with multiple partners.Low variation in male reproductive success may be a generalfeature of mammalian species in which females are larger thanmales.     

Proximate mechanisms of the differences in reproductive success of males bearing different alleles of Pgdh – a gene involved in a sexual conflict in bulb mite

Enzyme polymorphism in phosphogluconate dehydrogenase (Pgdh) is a striking example of single gene polymorphism involved in sexual conflict in bulb mite Rhizoglyphus robini. Males homozygous for the S Pgdh allele were shown to achieve higher reproductive success than FF homozygous males, while negatively influencing fecundity of their female partners. Here, we investigate proximate mechanisms responsible for the increased reproductive success of SS males and find that the S allele is associated with shorter time until copulation, higher copulation frequency and increased sperm production. We also show that Pgdh alleles are probably codominant, with SS males gaining the highest reproductive success, FF males – the lowest – and FS‐heterozygous males taking an intermediate position in all fitness parameters differentiating males of different genotypes. Additionally, we confirm the negative effect that S‐bearing males impose on the fecundity of females they mate with, showing a clear pattern of interlocus sexual conflict. We discuss that this effect is probably associated with increased copulation frequency. Whereas, contrary to what we have predicted, the S allele does not cause increased general male mobility, we speculate that the S allele‐bearing males are more efficient in forcing copulation and/or detecting females.     

Sexual selection without sexual dimorphism: Bateman gradients in a simultaneous hermaphrodite

One of the most general patterns in sexual selection is stronger selection on mating activity in males than in females. This asymmetry is thought to result from the higher energetic cost of producing one female compared to one male gamete (anisogamy). However, most studies focused on gonochoric species with strong sexual dimorphism, in which males and females are necessarily under different selection regimes. The question remains whether anisogamy alone would suffice to produce such differences. In simultaneous hermaphrodites one can compare sexual selection on the male and female functions in the absence of sexual dimorphism. Here we quantify sexual selection in the hermaphroditic freshwater snail Physa acuta under laboratory conditions. We combine exhaustive behavioral records of mating activity in mating groups and molecular paternity assignment to measure the mating success and reproductive success of 120 individuals. Our results validate the prediction of stronger selection to gain mating partners in the male than in the female function. Moreover, we did not detect cross‐sex effects on fitness, or correlations between male and female production of offspring over the course of our experiment. We conclude that with respect to sexual selection P. acuta is comparable to gonochorists, confirming that anisogamy is a sufficient explanation for the differences in sexual selection regimes between sexes.     

Quantifying sexual selection: a comparison of competing indices with mating system data from a terrestrially breeding salamander

Calculations for quantifying the potential for sexual selection remain controversial. Many indices have been promoted in the literature, but each has unique sets of advantages and disadvantages. Using marbled salamanders, I evaluated the performance of several measures by manipulating intensity of sexual selection in experimental breeding replicates of varying operational sex ratio. Theory predicts that sexual selection among males will be higher when sex ratio is male‐biased and lower when female‐biased. I used microsatellite data to assign hatchling parentage, estimate adult fitness, and calculate several indices of inequality for quantifying sexual selection. Opportunity for selection and Morisita index always conformed to theoretical expectations, which was not the case for index of resource monopolization, standardized Morisita index, or binomial skew index. Although I conclude that opportunity for selection is advantageous in sexual selection studies because of its link to formal theory, this should be tested against the null hypothesis of random variation in ambiguous cases. In the present study, although variation in both reproductive and mating success was high when quantified using opportunities for selection, it was only significantly greater than random expectations for reproductive success. This study provides further empirical support for the continued use of opportunity for selection in sexual selection studies. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 99 , 73–83.     

Patterns of male reproductive success in a highly promiscuous whale species: the endangered North Atlantic right whale

Parentage analyses of baleen whales are rare, and although mating systems have been hypothesized for some species, little data on realized male reproductive success are available and the patterns of male reproductive success have remained elusive for most species. Here we combine over 20 years of photo-identification data with high-resolution genetic data for the majority of individual North Atlantic right whales to assess paternity in this endangered species. There was significant skew in male reproductive success compared to what would be expected if mating was random (P < 0.001). The difference was due to an excess of males assigned zero paternities, a deficiency of males assigned one paternity, and an excess of males assigned as fathers for multiple calves. The variance in male reproductive success was high relative to other aquatically mating marine mammals, but was low relative to mammals where the mating system is based on resource- and/or mate-defence polygyny. These results are consistent with previous data suggesting that the right whale mating system represents one of the most intense examples of sperm competition in mammals, but that sperm competition on its own does not allow for the same degree of polygyny as systems where males can control access to resources and/or mates. The age distribution of assigned fathers was significantly biased towards older males (P < 0.05), with males not obtaining their first paternity until approximately 15 years of age, which is almost twice the average age of first fertilization in females (8 years), suggesting that mate competition is preventing younger males from reproducing. The uneven distribution of paternities results in a lower effective population size in this species that already has one of the lowest reported levels of genetic diversity, which may further inhibit reproductive success through mate incompatibility of genetically similar individuals.     

Genetic variation but weak genetic covariation between pre‐ and post‐copulatory episodes of sexual selection in Drosophila melanogaster

When females mate polyandrously, male reproductive success depends both on the male's ability to attain matings and on his ability to outcompete rival males in the fertilization of ova post‐copulation. Increased investment in  ejaculate components may trade off with investment in precopulatory traits due to resource allocation. Alternatively, pre‐ and post‐copulatory traits could be positively related if individuals can afford to invest heavily in traits advantageous at both episodes of selection. There is empirical evidence for both positive and negative associations between pre‐ and post‐copulatory episodes, but little is known about the genetic basis of these correlations. In this study, we measured morphological, chemical and behavioural precopulatory male traits and investigated their relationship with measures of male fitness (male mating success, remating inhibition and offensive sperm competitiveness) across 40 isofemale lines of Drosophila melanogaster. We found significant variation among isofemale lines, indicating a genetic basis for most of the traits investigated. However, we found weak evidence for genetic correlations between precopulatory traits and our indices of male fitness. Moreover, pre‐ and post‐copulatory episodes of selection were uncorrelated, suggesting selection may act independently at the different episodes to maximize male reproductive success.     

Role of sexual and natural selection in evolution of body size and shape: a phylogenetic study of morphological radiation in grouse

We use standardized independent contrasts (SICs) to elucidate the effect of ecology and mating systems on morphological radiation in grouse. The analysis of SICs for 38 skeletal measurements from 20 taxa, showed that changes in mating system had a significant effect on body size of both sexes. Sexual size dimorphism in grouse is consistent with Rensch's rule; the slope of the regression of male vs. female size SICs was 1.4, significantly >1. Changes in habitat were associated with accelerated rates of evolution of body proportions. SICs for male and female scores of size independent factors were directly proportional to each other (slope = 1), indicating extreme similarities between male and female ecology. Females, however, were better adapted to longer, more energy efficient flight than males. Size independent morphological differences among grouse are adaptive and are related to the differences in habitat and foraging behaviour among the species.     

Impact of male mating history on the postmating resumption of sexual receptivity and lifetime reproductive success in Choristoneura rosaceana females

Abstract.  The first objective of the present study is to test the hypothesis that the decrease in the number of eupyrene spermatozoa in the spermatheca is directly associated with the resumption of sexual receptivity in female moths, an aspect that has not been examined in previous studies. The obliquebanded leafroller, Choristoneura rosaceana , is used and females mated with previously mated males have a shorter refractory period than those mated with virgins. This difference is associated with a faster rate of movement of sperm from the spermatheca. Overall, the length of the female refractory period coincides with the mean time required for the number of eupyrene sperm in the spermatheca to drop to approximately 3000, regardless of male mating history. Although such a decline in sperm numbers may be a factor responsible for the resumption of sexual receptivity, this is clearly not the only one because more than 40% of females remate even though sperm numbers in the spermatheca are well above this threshold. Virgin males do not vary the mass or the content of their ejaculate as a function of the female's reproductive status and this may increase the risk of sperm competition if the female is previously mated. The second objective of this study is to examine the effect of previous male mating history on female reproductive potential. Females mated with previously mated males have a significantly lower fecundity than those mated with virgin males. However, in all treatments, remating increases both female longevity and lifetime fecundity. There is also a significant effect of female mass on the length of the refractory period and on lifetime fecundity, with large females resuming sexual receptivity sooner and laying more eggs than small ones, regardless of male mating history.     

成虫体型大小驱动六斑月瓢虫性间相互选择(英文)

【目的】配偶选择是性选择的中心过程,当繁殖成本发生波动和配偶质量存在差异时,传统的性别作用也随之不同。为了验证在某些条件下雄性也可以像雌性一样进行选择,我们研究了雄性和雌性六斑月瓢虫Menochilus sexmaculatus(Fabricius)的交配成功率与作为适合度指标的成虫体型大小之间的关系。【方法】从初始种群中随机选择体型小和体型大的两性成虫,进行雄性和雌性配偶选择试验。并对成功交配的各个配对组的后代也进行配偶选择试验,测试其交配成功率。【结果】配偶选择试验结果表明,在雄性和雌性交配试验中交配的成功取决于体型大小。雄性根据其品质即体型大小这一繁殖力指标对雌性进行区分。同样,在雌性交配试验中体型大的雄虫比体型小的雄虫交配成功率高。无论亲本体型如何,体型大的雌虫都比体型小的雌虫产卵量高;不过,体型大小不影响卵的存活力。体型大的亲本的后代比体型小的亲本的后代发育更快,寿命更长,但是将其与来自初始种群的后代进行配偶选择时,它们在性合意(sexual desirability)或竞争力方面没有表现任何优势。【结论】六斑月瓢虫中存在以体型大小为目的性状的相互配偶选择。     

Natural selection on body size traits in a long‐lived bird,the snow petrel Pagodroma nivea

I consider the possibility of selection favouring large body size in a population of snow petrels (Pagodroma nivea), a long‐lived seabird species. I measured natural selection on body size traits in a population from 1987 to 1998. There was evidence of selection on body size associated with fecundity and survival. Directional selection on bill length and stabilizing selection on tarsus length associated with reproductive success were detected among males. Selection associated with survival favoured males with longer bills. However, selection was weak in all cases. No evidence of selection acting on female body size traits was detected. Offspring–parents regression suggested that bill length and tarsus length were heritable. Although I was able to identify the targets of selection in this population, I could not demonstrate the ecological implications of both tarsus length and bill length variation. The selection on male, but not on female, body size traits suggests factors such as intrasexual competition for nests and/or mates rather than factors such as feeding efficiency as mechanisms of selection on bill size.     

Within‐season variation in sexual selection in a fish with dynamic sex roles

The strength of sexual selection may vary between species, among populations and within populations over time. While there is growing evidence that sexual selection may vary between years, less is known about variation in sexual selection within a season. Here, we investigate within‐season variation in sexual selection in male two‐spotted gobies (Gobiusculus flavescens). This marine fish experiences a seasonal change in the operational sex ratio from male‐ to female‐biased, resulting in a dramatic decrease in male mating competition over the breeding season. We therefore expected stronger sexual selection on males early in the season. We sampled nests and nest‐holding males early and late in the breeding season and used microsatellite markers to determine male mating and reproductive success. We first analysed sexual selection associated with the acquisition of nests by comparing nest‐holding males to population samples. Among nest‐holders, we calculated the potential strength of sexual selection and selection on phenotypic traits. We found remarkable within‐season variation in sexual selection. Selection on male body size related to nest acquisition changed from positive to negative over the season. The opportunity for sexual selection among nest‐holders was significantly greater early in the season rather than late in the season, partly due to more unmated males. Overall, our study documents a within‐season change in sexual selection that corresponds with a predictable change in the operational sex ratio. We suggest that many species may experience within‐season changes in sexual selection and that such dynamics are important for understanding how sexual selection operates in the wild.     

Effects of variation at the flower-colour A locus on mating system parameters in Ipomoea purpurea

Although alleles at both the W and A loci in the common morning glory, Ipomoea purpurea, produce similar white-flowered phenotypes, these alleles differ by over an order of magnitude in average frequency. In this initial attempt to determine the causes of this difference, we employed artificial arrays of plants to estimate mating system characteristics (total siring success, selfing rates and contribution to the outcross pollen pool) for the homozygous pigmented and white-flowered genotypes at the A locus. This experiment demonstrates that: (1) at both low and high frequencies, white-flowered plants were visited by pollinators at the same rate as plants with pigmented flowers; (2) at both frequencies, the a allele exhibited a greater total siring success (self and outcross pollen) than the A allele; (3) individuals of both genotypes contributed equally to the outcross pollen pool; and (4) aa plants may have a higher selfing rate than AA plants. Coupled with minimal inbreeding depression in I. purpurea, these observations indicate that the allele producing white flowers enjoys a transmission advantage that would tend to cause this allele to increase in frequency. This transmission advantage is very similar to that shown previously to be operating on the white-flowered allele at the W locus, although the specific causes of the advantage appear to differ between loci. The frequency difference between the two alleles is thus not likely to be due to differences in the effect of flower-colour variation on transmission. Rather, substantially greater deleterious pleiotropic effects associated with the white-flower a allele is likely to be the primary cause of the frequency difference.     

The evolution of mammal body sizes: responses to Cenozoic climate change in North American mammals

Explanations for the evolution of body size in mammals have remained surprisingly elusive despite the central importance of body size in evolutionary biology. Here, we present a model which argues that the body sizes of Nearctic mammals were moulded by Cenozoic climate and vegetation changes. Following the early Eocene Climate Optimum, forests retreated and gave way to open woodland and savannah landscapes, followed later by grasslands. Many herbivores that radiated in these new landscapes underwent a switch from browsing to grazing associated with increased unguligrade cursoriality and body size, the latter driven by the energetics and constraints of cellulose digestion (fermentation). Carnivores also increased in size and digitigrade, cursorial capacity to occupy a size distribution allowing the capture of prey of the widest range of body sizes. With the emergence of larger, faster carnivores, plantigrade mammals were constrained from evolving to large body sizes and most remained smaller than 1 kg throughout the middle Cenozoic. We find no consistent support for either Cope's Rule or Bergmann's Rule in plantigrade mammals, the largest locomotor guild (n = 1186, 59% of species in the database). Some cold‐specialist plantigrade mammals, such as beavers and marmots, showed dramatic increases in body mass following the Miocene Climate Optimum which may, however, be partially explained by Bergmann's rule. This study reemphasizes the necessity of considering the evolutionary history and resultant form and function of mammalian morphotypes when attempting to understand contemporary mammalian body size distributions.     

Sexual size dimorphism and selection in the wild in the waterstrider Aquarius remigis: Body size,components of body size and male mating success

Sexual size dimorphism is assumed to be adaptive and is expected to evolve in response to a difference in the net selection pressures on the sexes. Although a demonstration of sexual selection is neither necessary nor sufficient to explain the evolution of sexual size dimorphism, sexual selection is generally assumed to be a major evolutionary force. If contemporary sexual selection is important in the evolution and maintenance of sexual size dimorphism then we expect to see concordance between patterns of sexual selection and patterns of sexual dimorphism. We examined sexual selection in the wild, acting on male body size, and components of body size, in the waterstrider Aquarius remigis, as part of a long term study examining net selection pressures on the two sexes in this species. Selection was estimated on both a daily and annual basis. Since our measure of fitness (mating success) was behavioral, we estimated reliabilities to determine if males perform consistently. Reliabilities were measured as ? statistics and range from fair to perfect agreement with substantial agreement overall. We found significant univariate sexual selection favoring larger total length in the first year of our study but not in the second. Multivariate analysis of components of body size revealed that sexual selection for larger males was not acting directly on total length but on genital length. Sexual selection for larger male body size was opposed by direct selection favoring smaller midfemoral lengths. While males of this species are smaller than females, they have longer genital segments and wider forefemora. Patterns of contemporary sexual selection and sexual size dimorphism agree only for genital length. For total length, and all other components of body size examined, contemporary sexual selection was either nonsignificant or opposed the pattern of size dimporhism. Thus, while the net pressures of contemporary selection for the species may still act to maintain sexual size dimorphism, sexual selection alone does not.     

Sexual and lifetime selection on body size in a marine fish: the importance of life-history trade-offs

Many field measurements of viability and sexual selection on body size indicate that large size is favoured. However, life-history theory predicts that body size may be optimized and that patterns of selection may often be stabilizing rather than directional. One reason for this discrepancy may be that field estimates of selection tend to focus on limited components of fitness and may not fully measure life-history trade-offs. We use an 8-year, demographic field study to examine both sexual selection and lifetime selection on body size of a coral reef fish (the bicolour damselfish, Stegastes partitus). Selection via reproductive success of adults was very strong (standardized selection differential=1.04). However, this effect was balanced by trade-offs between large adult size and reduced cumulative survival during the juvenile phase. When we measured lifetime fitness (net reproductive rate), selection was strongly stabilizing and only weakly directional, consistent with predictions from life-history theory.     

Lifetime selection on a hypoallometric size trait in the spotted hyena

Size-related traits are common targets of natural selection, yet there is a relative paucity of data on selection among mammals, particularly from studies measuring lifetime reproductive success (LRS). We present the first phenotypic selection analysis using LRS on size-related traits in a large terrestrial carnivore, the spotted hyena, which displays a rare pattern of female-biased sexual size dimorphism (SSD). Using path analysis, we investigate the operation of selection to address hypotheses proposed to explain SSD in spotted hyenas. Ideal size measures are elusive, and allometric variation often obfuscates interpretation of size proxies. We adopt a novel approach integrating two common methods of assessing size, and demonstrate lifetime selection on size-related traits that scale hypoallometrically with overall body size. Our data support selection on hypoallometric traits in hyenas, but not on traits exhibiting isometric or hyperallometric scaling relationships, or on commonly used measures of overall body size. Our results represent the first estimate of lifetime selection on a large carnivore, and suggest a possible route for maintenance of female-biased SSD in spotted hyenas. Finally, our results highlight the importance of choosing appropriate measures when estimating animal body size, and suggest caution in interpreting selection on size-related traits as selection on size itself.