Semen quantity and quality correlate with bank vole males’ social status

Laboratory studies reveal that in several rodent species the females prefer dominant males as mating partners. Here we investigate the correlation between bank vole males’ social rank and their sperm quality and quantity. We used agonistic encounters to determine males’ social status. Sperm quality was assessed by its motility, viability, maturity, morphology and sperm tail membrane integrity. Relatively more dominant males were heavier than males of lower social status. The males’ social position affected the testes, seminal vesicles and coagulation gland development. The weights of these reproductive organs were significantly higher in more dominant males than in more subordinate males. Sperm counts and the values of the other parameters describing sperm quality were higher in high-ranking males than in subordinates. Our results suggest that bank vole females benefit from choosing and mating with high-ranking males by obtaining more and better-quality sperm.     

Body size and symbiotic status influence gonad development in <Emphasis Type="Italic">Aiptasia pallida</Emphasis> anemones

Pale anemones (Aiptasia pallida) coexist with dinoflagellates (primarily Symbiodinium minutum) in a mutualistic relationship. The purpose of this study was to investigate the role of these symbionts in gonad development of anemone hosts. Symbiotic and aposymbiotic anemones were subjected to light cycles that induced gametogenesis. These anemones were then sampled weekly for nine weeks, and gonad development was analyzed histologically. Anemone size was measured as mean body column diameter, and oocytes or sperm follicles were counted for each anemone. Generalized linear models were used to evaluate the influence of body size and symbiotic status on whether gonads were present and on the number of oocytes or sperm follicles produced. Body size predicted whether gonads were present, with larger anemones being more likely than smaller anemones to develop gonads. Both body size and symbiotic status predicted gonad size, such that larger and symbiotic anemones produced more oocytes and sperm follicles than smaller and aposymbiotic anemones. Overall, only 22 % of aposymbiotic females produced oocytes, whereas 63 % of symbiotic females produced oocytes. Similarly, 6 % of aposymbiotic males produced sperm follicles, whereas 60 % of symbiotic males produced sperm follicles. Thus, while gonads were present in 62 % of symbiotic anemones, they were present in only 11 % of aposymbiotic anemones. These results indicate that dinoflagellate symbionts influence gonad development and thus sexual maturation in both female and male Aiptasia pallida anemones. This finding substantiates and expands our current understanding of the importance of symbionts in the development and physiology of cnidarian hosts.     

Delayed sexual maturity in males of Vespa velutina

Vespa velutina var nigrithorax (Lepelletier, 1835) is an invasive predator of bees accidentally introduced in France in 2004, and it is having a serious impact on apiculture and ecosystems. Studying the reproduction of an invasive species is key to assess its population dynamic. This study explores the sexual maturation of V. velutina males and the evolution of their fertility. The main studied parameters were physiologic (spermiogenesis, spermatogenesis) and anatomic (testes size and structure, head width). Two populations of males were described based on their emergence period: early males in early summer or classic males in autumn. Each testis has an average of 108 testicular follicles. Spermatogenesis is synchronous, with only 1 sperm production wave, and completed, on average, at 10.3 d after emergence with the degeneration of the testes. The sperm counts in seminal vesicles of mature males are 3 × 10⁶ in October/November and 0.8 × 10⁶ in June. In comparison, females store 0.1 × 10⁶ sperm in their spermathecae. The early males emerged from colonies made by fertilized queens. The reproductive potential of these early males seemed limited, and their function in the colony is discussed. The sperm stock evolution in autumn males suggests the occurrence of a reproductive pattern of male competition for the access to females and a single copulation per male. The synchronicity of male and foundress emergences and sexual maturation is of primary importance for the mating success and the future colony development.     

Prolonged use of letrozole causes morphological changes on gonads in Galea spixii

Letrozole is used as a therapeutic agent in reproductive disorders caused by high estrogen levels. Letrozole inhibits cytochrome P450 aromatase and reduces estrogen levels. However, the effects of long-term use on reproductive traits are unknown. The aim of this study was to evaluate the prolonged use of letrozole in the gonads of rodents (Spix''s yellow-toothed cavy; Galea spixii). Forty-eight rodents (24 males and 24 females) were randomly divided into the treated and control groups. Letrozole administration started at 15 days of age and continued weekly until 30, 45, 90, and 120 days of age. The body, testis, and ovary weights were analyzed, as well as the morphological progression of spermatogenesis and folliculogenesis. Macroscopically, body weight gain and gonads weight were increased in the letrozole group. Microscopically, the ovaries of treated females showed stratified epithelium and a cellular disorder of the tunica albuginea. In the testes of treated males, the development of seminiferous tubules was delayed and sperm was absent. The collective findings indicate that the prolonged use of letrozole alters secondary sexual characteristics, and causes weight gain, reproductive changes, and male infertility.     

Sperm maturation and male tactic-specific differences in ejaculates in the plainfin midshipman fish Porichthys notatus

Using the plainfin midshipman fish Porichthys notatus, a species with alternative reproductive tactics (ARTs), we investigated how sperm maturation shapes sperm competitive abilities. We compared sperm performance and morphology before and after final sperm maturation by sampling sperm from the testes and stripped ejaculates of guarders and sneakers. In accordance with sperm competition risk theory, ejaculates from sneaker males had three times as much sperm as ejaculates from guarder males and sneaker males produced faster swimming sperm than guarder males, but this was only the case after final sperm maturation had occurred. Additionally, fully mature sperm found in ejaculates had larger heads and midpieces than sperm found in the testes. These results emphasize the important role played by non-sperm components of an ejaculate in mediating sperm performance and potentially also morphology.     

Size Dependent Male Reproductive Tactic in the Two-Spotted Goby (Gobiusculus flavescens)

Male investment in testes and sperm duct gland in the polygamous nest breeding two-spotted goby Gobiusculus flavescens (Fabricius) was investigated in relation to time in reproductive season and individual physical parameters. This small teleost fish is most likely the most abundant species found along the rocky shores of the North East Atlantic. The two-spotted goby has a single reproductive season, during which nest-caring males can raise several clutches of offspring. According to the literature the males are on average larger than the females. Here we report for the first time a population showing a reversal of this trend, with males on average being smaller than females, a difference likely caused by a large proportion of small males. Early in the breeding season these small males have typical sneaker characters, with relatively large testes and small seminal duct glands compared to the larger dominant territorial males. The presence of these two alternative male reproductive tactics is confirmed by histological studies, which shows the presence of sperm in the sperm duct glands (SDG) of smaller males, but not in the SDG of intermediate and larger males. To our knowledge, males with typical sneaker characters have not been reported in earlier studied populations of two-spotted goby. Interestingly we found that testes investment declined significantly over the course of the breeding season, and that this reduction was significantly more pronounced in small compared to the large males. Further, a significant increase in seminal duct gland (SDG) mass was observed for the smaller males over the breeding season. We propose that this indicates a possible shift in mating tactic by smaller males from a parasitic to a nest-holding tactic over the course of the breeding season. Thus, the observed size dependent plasticity in investment in SDG over time suggests that the reproductive tactic of G. flavescens is conditional, and possibly influenced by mate availability and male—male competition.     

Changes in sperm stores, ejaculate size, fertilization success, and sexual motivation over repeated matings in the common toad, Bufo bufo (Anura: Bufonidae)

Fertilization is of central importance in the determination of reproductive success for both males and females. In species where males have the chance to mate repeatedly within a short period of time, sperm stocks may become depleted and males may have to carefully economize on their sperm reserves. Also, intensive intrasexual competition for females and repeated matings may lead to exhaustion on the behavioural level. To determine whether the reproductive potential of males is limited and if such a limitation is due to behavioural exhaustion or sperm depletion, we experimentally investigated changes in sperm stores, sperm expenditure, fertilization success, and sexual motivation over three repeated matings in the common toad, Bufo bufo , where the breeding season is short and sequential polygyny occurs. At the end of the breeding season, the number of sperm stored in the testes of males mated repeatedly was close to 50% lower than in testes of unmated males. Ejaculate size, which was estimated by applying a novel method allowing direct quantification, decreased by 88% from first to third matings. We also observed a drop in fertilization success from the first two to third matings by 65%, which was largest in males that had started the reproductive season in bad body condition. Some of these males also showed a decreased interest in females in the third mating round. Our results suggest that sperm depletion and loss of sexual motivation may together set a limit to the reproductive potential of common toad males. The present study draws attention to a limitation in reproductive potential, which may occur more often than currently anticipated and has the potential to strongly influence several aspects of reproductive behaviour.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 96 , 361–371.     

Temperature-dependent ovariole and testis maturation in the yellow dung fly

Temperature is one of the abiotic environmental factors most strongly affecting animal behaviour, physiology, and life history. In insects, lower temperatures generally slow down most physiological processes, reducing growth rate and prolonging the juvenile period. Here, we investigate temperature‐dependent ovariole and testis maturation in the anautogenous yellow dung fly, Scathophaga stercoraria L. (Diptera: Scathophagidae), and relate it to corresponding temperature effects on pre‐adult development time and the adult pre‐reproductive period. Flies were reared in the laboratory at three constant temperatures (18, 22, and 26 °C), and the size of the developing ovarioles and testes (reflecting sperm production) was measured over time (i.e., age). Ovariole size increased asymptotically over the first 12 days of adult life, while the testes continued to fill after day 10. In accordance with the temperature‐size rule, warmer temperatures resulted in smaller ovarioles (eggs) and smaller testes, independent of body size. Warmer temperatures also greatly reduced pre‐adult development time by more than half, from 12 to 25 °C, the larger males always taking 1–3 days longer than the females. Corresponding temperature effects on the adult pre‐reproductive period were small (<1 day between 15 and 25 °C), with males taking 5–6 days and females 10–13 days to first reproduction. Time lost by males during the pre‐adult stage, when ovaries and testes are produced, can thus be more than compensated‐for by time gained during the pre‐reproductive period, when eggs and sperm are produced, so males can nevertheless start reproducing sooner than females.     

Mating success of male guppy <Emphasis Type="Italic">Poecilia reticulata</Emphasis> in the wild

To understand how the diversity among male sexual traits is maintained despite directional female choice for attractive traits, it is important to study male reproductive success in each reproductive episode. Male reproductive success is indeed the comprehensive result of several processes such as mate choice, mating, and fertilization. In this study, male mating success was estimated in a wild guppy Poecilia reticulata population of Hiji River, Okinawa Island, in southern Japan. The mating success of males was estimated as the percentage of sperm transferred in relation to the total sperm reserve. In a preliminary experiment, it was confirmed that the sperm reserves of males were partly replenished after 12 h and fully replenished after 72 h, when all sperm were released. In addition, the results of investigation in the wild population revealed that approximately 20% of males had no sperm reserves at all, although their sperm reserves should have been replenished by sexual isolation. Male sexual traits such as body size or color pattern did not affect the percentage of sperm transferred. Results from this study provide important insights into male sperm transfer ability and sperm production in this population. However, the high frequency of male sperm-transfer success estimated in this study should to be taken with caution, and whether it is due to differences between wild and laboratory conditions, sex ratio or the rates of multiple matings by females in the wild population remain to be fully explored.     

3-Monochloropropane-1,2-diol (alpha-chlorohydrin) disrupts spermatogenesis and causes spermatotoxicity in males of the Egyptian fruit-bat (Rousettus aegyptiacus)

We evaluated the sterilizing effect of 3-monochloropropane-1, 2-diol (3-MCPD) in male Egyptian fruit bats (Rousettus aegyptiacus). We used three groups. One was treated with 70 mg/kg 3-MCPD for 4 days. The second group was treated with 3-MCPD as a bait formulation (known concentration of 3-MCPD mixed with a known amount of food). The third group was untreated controls. We compared the weights of the reproductive organs, histology of the testes, occurrence of spermatogenesis, and the count, motility and abnormalities of epididymal sperm of treated males with those of the untreated control group. 3-MCPD caused significantly decreased weights of reproductive organs, several testicular histological alterations and spermatogenic arrest accompanied by significant decreases in sperm count and motility, and significantly increased number of abnormal sperm. 3-MCPD bait was readily accepted by the animals. 3-MCPD, even in low doses and after limited exposure, disrupted spermatogenesis in males of the Egyptian fruit bat. Our findings have potential value for public health and agricultural authorities, and for vertebrate pest managers. 3-MCPD may have application for control of this pest.     

Testosterone production and spermatogenesis in free-ranging Eurasian lynx (Lynx lynx) throughout the year

Seasonal variation in reproduction is common in mammals as an adaptation to annual changes in the habitat. In lynx, male reproduction activity is of special interest because female lynxes are monoestric with an unusual narrow (about 1 month) breeding season. In Eurasian lynx, mating occurs between January and April depending on the latitude. To characterize the seasonal pattern of sperm and testosterone production in free-ranging Eurasian lynxes, long-term frozen-stored testis material obtained postmortem from 74 hunted or road-killed lynxes in Sweden was used to analyze annual changes in testis mass, testicular testosterone content, and spermatogenetic activity. Values of most gonadal parameters obtained in subadult lynxes were significantly different from the values observed in adult males. In adult lynxes, a moderate annual fluctuation of gonadal parameters was found which was most profound for testis weight and testicular testosterone concentration reaching highest values in March (median of 2.18 g and 2.67 μg/g tissue respectively). Grouping the data of pre-/breeding (January–April) and postbreeding season (May–September) revealed significant changes in testis weight and testosterone concentration. The relative spermatogenetic activity remained high in postbreeding testes. However, net sperm production decreased according to reduction of testis mass and a tendency to lower cauda epididymal sperm numbers in the postbreeding period was observed. Our results demonstrate that it is possible to analyze the gonadal activity of frozen testis/epididymis tissue postmortem and that male Eurasian lynxes show—opposite to the females—only moderate seasonal changes in their reproductive capacity.     

Lack of effects on male fertility from a quadrivalent HPV vaccine in Sprague‐Dawley rats

BACKGROUND: Human papillomavirus (HPV) infection is one of the most common sexually transmitted diseases in both men and women. A recently developed quadrivalent HPV vaccine, Gardasil^®, has been shown to be highly effective in the prevention of several HPV‐mediated diseases. The objective of the present study was to evaluate the potential effects of the vaccine on male fertility including reproductive performance, sperm evaluations, and histology of the testes. In addition, anti‐HPV antibodies were measured during the study. METHODS: Group 1 (30 male rats) received the full human dose of vaccine (0.5 mL, ～200‐fold excess based on body weight) by intramuscular injection at 6 weeks, 3 weeks, and 3 days prior to cohabitation. Group 2 males received only 1 dose at 3 days prior to cohabitation. Additional groups (20 male rats each) were administered PBS or Merck Aluminum Adjuvant similarly to Group 1. Ten males in the vaccine‐treated groups were bled for immunogenicity assays after each dose. Twenty males per group were mated to untreated female rats. Cesarean sections were performed on Gestation Day 15 or 16. Cohabited males were necropsied and sperm count and motility were evaluated. RESULTS: There were no unscheduled deaths during the study and no evidence of toxicity in vaccine‐treated male rats. The vaccine induced a specific antibody response to the 4 HPV types after each injection. There were no effects on the cesarean‐section parameters of females or reproductive parameters of the cohabited male rats, including histomorphology of testes and epididymis, sperm count, and sperm motility. CONCLUSIONS: These results demonstrate that this quadrivalent HPV vaccine had no detectable adverse effects on routine measures of male fertility in rats. Birth Defects Res (Part B) 89:376–381, 2010. © 2010 Wiley‐Liss, Inc.     

Seasonal reproduction of yellowish myotis,Myotis levis (chiroptera: Vespertilionidae), from a Neotropical highland

With a nearly global distribution the vespertilionid bat Myotis represents one of the most exceptional examples of adaptive radiation among mammals. We investigated the reproductive activity of the vespertilionid bat yellowish myotis, Myotis levis, from a highland area in Southeastern Brazil. The data were obtained through histological analyses of the male and female genital systems from February 2010 to May 2011. The testes of the adult yellowish myotis showed seasonal morphological characteristics which were categorized in the following stages: rest, maturing, mature, and mating. Rest and maturing males were recorded throughout the rainy season (October‐March). In the rest stage no spermatogenesis was observed and the epididymal duct was devoid of spermatozoa. Maturing individuals had started spermatogenesis and few spermatozoa were found in the epididymal duct. Mature males were found toward the end (February‐March) of the rainy season, when full spermatogenic activity was recorded and spermatozoa were packed in the epididymal duct. Although not recorded, mating probably occurred in the middle of the dry season (April–September) when the cauda epididymis was enlarged and packed with sperm. The spermatozoa remained stored in the cauda epididymis for at least three months when the testes entered into regression. The ovaries showed all types of ovarian follicles throughout the study period except mature follicles which were registered only in July (mid‐dry season). Lactating females were captured in the beginning of the rainy season. The seasonal reproductive characteristics of the yellowish myotis from this Neotropical highland area were similar those of epididymal sperm‐storing temperate vespertilionids. J. Morphol. 274:1230–1238, 2013. © 2013 Wiley Periodicals, Inc.     

Effect of host size on male fitness in the parasitoid wasp Dinarmus basalis

The effect of host size on male fitness was tested in the parasitoid wasp Dinarmus basalis (Hymenoptera, Pteromalidae) using hosts of different fresh weight. Fitness was measured as the sperm stock in seminal vesicles, and the ability to access females in single or competition situations. Both body size and sperm in seminal vesicles increased with host fresh weight. Males from small hosts had a reduced size and sperm stock compared to those from larger hosts. In single situations, males from both small and large hosts had similar reproductive capacities, whereas in multiple mating or competition situations, males from small hosts were at a disadvantage, inseminating fewer females and copulating less frequently. However, females did not appear to choose between males, and no effect on sperm stored in the spermatheca was observed. Being small does not prevent a D. basalis male mating and producing progeny in single situations, although more offspring could be expected from larger males because of their better competitive abilities.     

Reproductive biology of Rioraja agassizi from the coastal southwestern Atlantic ecosystem between northern Uruguay (34°S) and northern Argentina (42°S)

A total of 552 individuals of Rioraja agassizi (257 females and 295 males) were collected by bottom-trawl during research cruises. Sexual dimorphism was observed with females are heavier than males for a given total length (TL). Using logistic regression, it was determined that TL at 50% maturity of males was 475 mm TL and of females 520 mm TL. This estimation agrees with the morphological parameters measured. Although not statistically significant, a peak in reproductive activity was observed in males during late spring and summer. The monthly variation in the gonadosomatic index and oviducal gland width, together with the largest diameter of ovarian follicles, suggests that R. agassizi females have a partially defined annual reproductive cycle with two peaks, one from November (spring) to February (summer) and another in July (winter).     

Paternity following multiple mating in ladybird <Emphasis Type="Italic">Harmonia axyridis</Emphasis>

We investigated factors which may determine realised paternity following multiple mating in a promiscuous ladybird Harmonia axyridis (Pallas) (Coleoptera: Coccinellidae), namely the mating order and ability of males of different morphs to fertilize females. In experiments using males of two colour morphs, we found that the first male had a reproductive advantage over the second male (sperm precedence P2 = 0.38). Males of the melanic morph spectabilis had a reproductive advantage over males of the semi-melanic morph axyridis (P2 = 0.35). Analysis of paternity using microsatellites applied to young larvae, which avoids possible differential mortality of progeny, confirmed the precedence of the first male and higher paternity rate of colour morphs that are more frequent in the wild. Hatchability of eggs increased from 65% after the first mating to 85% after the second mating delayed by five days. Average duration of mating was 157 min and did not differ between the first and second mating and between the colour morphs of males.     

Effects of larval exposure to sublethal concentrations of the ecdysteroid agonists RH-5849 and tebufenozide (RH-5992) on male reproductive physiology in Spodoptera litura

Sublethal concentrations of the bisacylhydrazine moulting hormone agonists, RH-5849, and tebufenozide (RH-5992) were fed to sixth (final) instar larvae of Spodoptera litura. Both RH-5849 and tebufenozide adversely affected the mating success of S. litura when the surviving treated males were crossed with normal females. The ecdysone agonists decreased the longevity of treated males and of untreated females when crossed with treated males. The number of eggs laid by untreated females mated to treated males was decreased, and the fertility (percentage of hatching success) of the resulting eggs was reduced. These effects on male reproductive success were at least in part explained by a reduction in the number of sperm transferred during mating. The adverse effects of tebufenozide on male reproductive function were qualitatively the same as those of RH-5849, but tebufenozide was active at lower concentrations. To understand the reason for these adverse effects on male reproduction, we investigated the effects of the insecticides on male reproductive physiology. Male reproductive tract development and testicular volume of resulting adult moths were adversely affected by sublethal larval exposure to the ecdysone agonists. Dose-dependent reductions occurred in the production of eupyrene and apyrene spermatozoa in the adult testes, and in the number of spermatozoa released from the testes into the male reproductive tract. The descent into the male tract of both eupyrene and apyrene sperm was found to start at the normal stage of development in both normal and treated insects, but the daily rhythm of sperm descent was subsequently disturbed in the insecticide-treated moths. This affected the numbers of sperm in the upper vas deferens (UVD), seminal vesicle (SV), and duplex (duplex). Injections of RH-5849 given to pharate adult or newly emerged adult S. litura also caused drastic reduction in the number of sperm in the upper regions of the male tract, when measured 24 h after injection. The possible importance of pest population reduction through the sublethal anti-reproductive effects of insecticides is discussed.     

Reproductive ecology of Odontostilbe pequira (Steindachner, 1882) (Characidae, Cheirodontinae) in the Paraguay River, southern Pantanal, Brazil

The reproductive biology of Odontostilbe pequira was studied aiming to determining differences in population structure, reproductive tactics and correlating the reproductive period with rainfall, temperature and level of the Paraguay River, in the southern Pantanal, Brazil. Data were obtained for 623 individuals (366 females and 257 males), and of these, 253 females and 126 males were dissected for reproductive analysis. No significant variation was observed in the distribution of standard length and total weight between the sexes. The sex ratio was 1.42:1 (female: male), but the ratio did not differ over most months and between most length classes. The reproductive period was long (10 months). No correlation was found between the gonadosomatic index (GSI) of both sexes with water temperature and rainfall over the months analyzed. Males showed no significant association between the GSI and river level, but a marginally significant correlation was observed for females. Moreover, an effect of the mean historical river level on GSI was observed in both sexes, indicating that the flooding regime drive the reproductive activity, which proportions spawnings even when rainfall and temperature levels are low. Length at first maturity of the females was 24.2 mm and of the males 22.2 mm, with a significant difference between the sexes. The mean absolute fecundity was 181.4 oocytes/female, while mean relative fecundity was 0.544 oocytes/mg. Absolute fecundity was positively related to total weight, gonad weight and standard length. The mean diameter of the mature oocytes was 0.46 mm and the frequency distribution of the diameters showed various modes, indicating a multiple spawning. Thus, the reproductive tactics of O. pequira was characterized as “opportunistic strategist”, with reproductive activity strongly associated with the flood pulse.     

Effects of age and photoperiod on reproduction and the spleen in the marsh rice rat (Oryzomys palustris)

To examine the interactions between age and photoperiod on reproduction and spleen weights, we exposed adult male and female rice rats of various ages to photoperiods of 16:8-h light-dark photoperiods (16L:8D) or 12L:12D. After 10 wk, animals were killed and the following data were recorded: weights of testes, seminal vesicles, uterus, ovaries, body, and spleen and, in addition, vaginal patency. Young adult males displayed a greater degree of testicular and seminal vesicle regression in short photoperiods than did older males; the testes of most older males did not regress in response to short photoperiods. Spleen weight was unresponsive to short photoperiods in all males, but was affected by age. Females, however, exhibited reproductive organ regression and decreased vaginal patency in response to short photoperiods at all ages examined. Body weights were affected by photoperiod in young females, and, as in males, photoperiod had no effect on spleen weights. These data suggest that the reproductive response to photoperiod in adult male rice rats declines with age, whereas in adult females it does not.     

Males' evolutionary responses to experimental removal of sexual selection

We evaluated the influence of pre- and post-copulatory sexual selection upon male reproductive traits in a naturally promiscuous species, Drosophila melanogaster. Sexual selection was removed in two replicate populations through enforced monogamous mating with random mate assignment or retained in polyandrous controls. Monogamous mating eliminates all opportunities for mate competition, mate discrimination, sperm competition, cryptic female choice and, hence, sexual conflict. Levels of divergence between lines in sperm production and male fitness traits were quantified after 38-81 generations of selection. Three a priori predictions were tested: (i) male investment in spermatogenesis will be lower in monogamy-line males due to the absence of sperm competition selection, (ii) due to the evolution of increased male benevolence, the fitness of females paired with monogamy-line males will be higher than that of females paired with control-line males, and (iii) monogamy-line males will exhibit decreased competitive reproductive success relative to control-line males. The first two predictions were supported, whereas the third prediction was not. Monogamy males evolved a smaller body size and the size of their testes and the number of sperm within the testes were disproportionately further reduced. In contrast, the fitness of monogamous males (and their mates) was greater when reproducing in a non-competitive context: females mated once with monogamous males produced offspring at a faster rate and produced a greater total number of surviving progeny than did females mated to control males. The results indicate that sexual selection favours the production of increased numbers of sperm in D. melanogaster and that sexual selection favours some male traits conferring a direct cost to the fecundity of females.