Multiple mating and repeated copulations: effects on male reproductive success in red flour beetles

In many insects, both sexes mate multiple times and females use stored sperm for fertilizations. While males frequently engage in two distinct behaviours, multiple mating (with different females) and repeated copulations (with the same female), the reproductive consequences of these behaviours for males have been quantified for only a few species. In this study, males of the red flour beetle, Tribolium castaneum, were found to be capable of mating with as many as seven different virgin females within 15 min. Across sequential copulations with virgin females, there was no decline in either male insemination success or average female progeny production over 48 h. However, when males copulated with previously mated females, there was a significant decline in male paternity success across sequential copulations, possibly due to male sperm depletion. In separate experiments, T. castaneum males were found to engage in two to six repeated copulations with the same, individually marked female. These repeated copulations did not increase male insemination success, short-term female fecundity, or male paternity success. Repeated copulations may possibly play a role in sperm defence. This study indicates that males may frequently engage in multiple matings, but these additional matings may lead to diminishing male reproductive returns.     

Hypothalamic receptors of sex hormones in the mechanism of the sexual differentiation of the brain in rats

Studies have been made on the content of receptors of estradiol (E2) and testosterone (T) in cytoplasmic and nuclear fractions of the hypothalamus of male and female rats during neonatal development, as well as in adult females after androgenization in neonatal period and adult males castrated within 3 days of postnatal life. It was shown that both E2 and T are present in the blood serum of male and female newborn rats. In female hypothalamus, only E2 receptors were found, whereas in males both types of receptors were revealed, their content being higher than in females. In adult animals subjected to changes in the level of sex hormones in the blood during early neonatal period, changes in concentration of the receptors in the hypothalamic centres of regulation of tonic and cyclic secretion of gonadotropins were found. The data obtained presumably reveal the role of receptors of sex hormones in sex differentiation of the brain.     

Neural Control of Muscle Androgen Receptors

The number of cytosolic androgen receptors in rat skeletal muscle increases following denervation and disuse. This increase was postulated to represent altered intracellular distribution and consequent diminished sensitivity of skeletal muscle to androgens. To test this hypothesis, we measured total (homogenate) androgen receptor levels after denervation. Total (homogenate) androgen receptor binding did not change in response to denervation of leg muscles from adult male rats. An increase in cytosolic receptor number with no increase in total (homogenate) receptor levels supports the hypothesis of altered intracellular distribution of androgen receptors in denervated muscle. Cytosolic androgen receptor binding in muscle from male rats increased by 40% after denervation, whereas in females the increase was 17%. These increases could not be altered by endocrine manipulations of males or females.     

Sexual incentive motivation, olfactory preference, and activation of the vomeronasal projection pathway by sexually relevant cues in non-copulating and naive male rats

There are some apparently healthy male rats that fail to mate after repeated testing with receptive females. We have previously shown that these "non-copulator (NC)" males show no partner preference for a receptive female when given the opportunity to physically interact with a sexually receptive female or a sexually active male. We also demonstrated that although NC males prefer odors from estrous females to odors from anestrous females, this preference is significantly reduced in comparison to the preference displayed by copulating (C) males. The aim of the present study was to evaluate in NC males sexual incentive motivation, that is, the approach behavior of male rats to either a sexually receptive female or a sexually active male in a test where the subjects can smell, hear, and see the stimulus animal but prevents their physical interaction. In addition, we determined whether NC rats have alterations in their ability to detect odors from conspecifics or odors related to food. In the detection of odors from conspecifics, we determined if these NC males are sexually attracted toward odors from receptive females or sexually active males. For food-related odors, we quantified the time it took the subjects to locate a hidden a piece of apple. Finally, using the induction of Fos-immunoreactivity (Fos-IR) as an index of neuronal activation, we compared the response of the vomeronasal projection pathway (VN pathway) of C and NC male rats exposed to estrous bedding. Males without sexual experience (WSE) were included in all experiments to determine the importance of previous heterosexual experience in the different behavioral tests and in the activity of the VN pathway. In the sexual incentive motivation test, we found that C and WSE male rats have a clear preference for estrous females over sexually active males, whereas NC male rats showed no preference. In odor tests, our results showed that C males had a clear preference for odors from estrous females as opposed to odors from sexually active males. Although NC and WSE male rats showed a preference for estrous female odors, this preference was significantly reduced compared to that shown by C males. No differences were found between WSE, C, and NC males in the detection of stimuli associated with food-related odors. A significant increase in Fos-IR was observed in the mitral cell layer of the accessory olfactory bulb in all groups when exposed to estrous bedding. However, only the C male rats exposed to estrous female bedding showed an increase Fos-IR in all structures of the VN pathway. An increase in Fos-IR was observed in the medial preoptic area (MPOA) of WSE males exposed to estrous bedding. No increases in Fos-IR were detected along the VN pathway in NC male rats. We proposed that NC male rats do not display sexual behavior due to a reduced sexual motivation that could be caused by alterations in the neuronal activity of the VN pathway during the processing of estrous odors.     

Glutamate AMPA/kainate receptors, not GABA(A) receptors, mediate estradiol-induced sex differences in the hypothalamus

Sex differences in brain morphology underlie physiological and behavioral differences between males and females. During the critical perinatal period for sexual differentiation in the rat, gonadal steroids act in a regionally specific manner to alter neuronal morphology. Using Golgi-Cox impregnation, we examined several parameters of neuronal morphology in postnatal day 2 (PN2) rats. We found that in the ventromedial nucleus of the hypothalamus (VMN) and in areas just dorsal and just lateral to the VMN that there was a sex difference in total dendritic spine number (males greater) that was abolished by treating female neonates with exogenous testosterone. Dendritic branching was similarly sexually differentiated and hormonally modulated in the VMN and dorsal to the VMN. We then used spinophilin, a protein that positively correlates with the amount of dendritic spines, to investigate the mechanisms underlying these sex differences. Estradiol, which mediates most aspects of masculinization and is the aromatized product of testosterone, increased spinophilin levels in female PN2 rats to that of males. Muscimol, an agonist at GABA(A) receptors, did not affect spinophilin protein levels in either male or female neonates. Kainic acid, an agonist at glutamatergic AMPA/kainate receptors, mimicked the effect of estradiol in females. Antagonizing AMPA/kainate receptors with NBQX prevented the estradiol-induced increase in spinophilin in females but did not affect spinophilin level in males.     

Endocannabinoids: Friends and foes of reproduction

Endocannabinoids are fatty acid amides like anandamide (AEA), and monoacylglycerols like 2-arachidonoylglycerol, that bind to cannabinoid, vanilloid and peroxisome proliferator-activated receptors. Their biological actions are controlled through not yet fully characterized cellular mechanisms. These compounds, together with their related enzymes, that include key proteins for the synthesis and degradation of endocannabinoids, cannabinoid and non-cannabinoid receptors, and purported membrane transporter(s), form the “endocannabinoid system (ECS)”. In the past few years AEA and related ECS elements have emerged as essential players in various aspects of human reproduction, both for males and females. Here, the key features of the ECS and the potential of its components to direct human fertility towards a positive or negative end will be reviewed. In particular, the involvement of AEA and related ECS elements in regulating embryo oviductal transport, blastocyst implantation and placental development (in females), and sperm survival, motility, capacitation and acrosome reaction (in males) will be addressed, as well as the role of endocannabinoids in sperm–oviduct interactions. Additionally, the possibility that blood AEA and its hydrolase FAAH may represent reliable diagnostic markers of natural and assisted reproduction in humans will be discussed, along with the therapeutic exploitation of ECS-oriented drugs as useful fertility enhancers.     

Spatial distributions of male and female strawberry poison frogs and their relation to female reproductive resources

In many species with a resource-based mating system, males defend resources to increase their attractiveness to females. In the strawberry poison frog, Dendrobates pumilio, suitable tadpole-rearing sites appear to be a limited resource for females. Territorial males have been suggested to defend tadpole-rearing sites to increase their access to females. In this study we investigate the spatial association between tadpole-rearing sites and the sexes as well as the spatial association of males and females. If strawberry poison frogs have resource defense polygyny, we expect males and females to be associated with tadpole-rearing sites and that females will deposit their offspring in tadpole-rearing sites inside the territories of their mates. To test this hypothesis, home range and core area sizes were calculated for both sexes and the association patterns were compared in two areas that differed in their abundance of tadpole-rearing sites. Home ranges and core areas of females were much larger than male home ranges. Females showed a clumped distribution in the vicinity of tadpole-rearing sites. Males were not clumped and were less associated with tadpole-rearing sites. Females generally did not use tadpole-rearing sites in the territory of their mates and we therefore conclude that males did not defend tadpole-rearing sites for females. Our data are consistent with the general assumption that female distribution is influenced by resource distribution and that male distribution depends on female distribution. Nevertheless, the distribution of D. pumilio females was also influenced by male spacing patterns. Males probably initially establish their core areas where female density is high and then females move among territories to sample males. Males compete vigorously for places with high female density, the defense of which is likely important for enhancing their mating success. In general, the spacing patterns did not differ between populations but the sex ratio was strongly female biased in the habitat with more tadpole-rearing sites, reflecting the direct reliance of females on these resources.     

Effect of pairing in vitro on the glutathione level of male Schistosoma mansoni

The effect of in vitro incubation on the level of the intracellular nucleophile, glutathione (GSH), in adult Schistosoma mansoni was investigated. The GSH levels of freshly collected adult male and female parasites were 8.5 +/- 2.5 and 2.7 +/- 0.7 nmol/10 worms, respectively, as determined by an enzymatic assay. Twenty-four-hour incubation of unpaired males in RPMI-1640 medium at 37 C resulted in a 1.7-fold increase (P less than 0.001) in GSH level that remained elevated for at least 7 days. The increase was dependent on exogenous L-cystine, suggesting that it was due to biosynthesis of GSH. Biosynthesis in male S. mansoni was confirmed by isolating [3H] GSH from parasites incubated in medium containing L-[3H] cystine or [3H] glycine. In contrast to unpaired males, the GSH level of paired males as well as that of unpaired or paired females did not increase after 24 hr in vitro. When males that had been incubated unpaired for 24 hr were allowed to couple in vitro with freshly collected females, their GSH level fell to that of continuously paired males. These observations provide evidence that in vitro female schistosomes can influence the physiology of the male.     

Effect of electroacupuncture on electroconvulsive shock--an autoradiographic study for opioid receptors

Radioreceptor-binding assay combined with autoradiography technique in the brain slices of rats was used to investigate cerebral areas related to electroconvulsive shock (ECS) and to analyze changes of opioid receptors of these areas in electroacupuncture (EA) anticonvulsion. The results from image processing and detection of optic density (O.D.) showed that repeated ECS resulted in an increase of opioid receptor densities in the caudate nucleus, hippocampus, habenulae nucleus and amygdala. In the group of EA plus ECS, however, the opioid receptor densities in the hippocampus, habenula nucleus and amygdala decreased as compared with that in ECS group. In addition, the receptor density in the caudate nucleus showed a decrease tendency. The results provided morphological evidence of the involvement of opioid receptors in these cerebral areas in the process of ECS and EA anticonvulsion.     

Non‐Conceptive Sexual Behavior in Spiders: A Form of Play Associated with Body Condition,Personality Type,and Male Intrasexual Selection

In the socially polymorphic spider Anelosimus studiosus, males mature early in the reproductive season and recruit to the webs of juvenile females and guard them until they mature. During the period before females mature, males and females engage in repeated bouts of non‐conceptive (play) sexual behavior, where the pair courts and engages in mock copulation; both males and females gain performance‐enhancing experience via these encounters. In this study, we examined the factors that underlie individual variation in the tendency to engage in non‐conceptive mating and determine whether it impacts male–male competition for females. We found that docile females, being less resistant to mating in general, are more likely to accept male courtship and non‐conceptive copulation as juveniles. Personality type influenced the exhibition of non‐conceptive sexual behavior in males as well. High body condition males of the aggressive phenotype were more likely to engage in non‐conceptive sexual behavior than males with lower body condition. Body condition did not influence docile males’ propensity to engage in non‐conceptive sexual behavior, but female size did. Docile males engaged in more non‐conceptive sexual displays with larger females. Engaging in non‐conceptive sexual displays negatively impacted male performance in staged male–male contests for access to females. This cost was greatest for males of the aggressive phenotype, which are otherwise favored in male–male contests. Our findings indicate expression of non‐conceptive sexual displays is linked to personality and results in reproductive performance trade‐offs for male A. studiosus.     

Aggressive behavior in female golden hamsters: development and the effect of repeated social stress

In male golden hamsters, agonistic behavior matures during puberty, changing from play fighting to adult-like aggression. In addition, this transition is accelerated by repeated social subjugation early in puberty. However, little is known about the development of agonistic behavior in females. In the present study, we compared the development of agonistic behavior in male and female golden hamsters. Furthermore, we also tested the effects of repeated social subjugation on the development of agonistic behavior during puberty. Hamsters were tested for agonistic behavior in the presence of a smaller intruder at different intervals during puberty. Several observations were made. First, the frequency of attacks remained stable in females, while varying in males. Second, the transition from play fighting to adult-like aggression occurred at earlier time periods in females than in males. Finally, a clear transitional period marked by attacks focused on the flanks was observable in males around mid-puberty. However, this transitional period was not apparent in females. In addition, juvenile females were exposed to aggressive adult males or females. In both cases, repeated exposure to stress had no statistically significant effect on the development of agonistic behavior. After 2 weeks of subjugation, exposure to aggressive adults had no effect on serum cortisol levels, indicating that juvenile females habituate to repeated social stress. These data show significant sex differences in the development of agonistic behavior and adaptation to repeated stress in juvenile golden hamsters.     

Size-dependent male mating behaviour and sexual harassment in a population of Atlantic mollies (Poecilia mexicana) from a sulphur creek

The Atlantic molly Poecilia mexicana inhabits a variety of habitats. In previous studies, small males from clear-water populations showed considerably more sexual behaviours than large males. Males from a sulphur creek, the El Azufre, or from a sulphur cave (cave molly) showed comparatively low sexual activity, and typical size-dependent mating behaviour was either absent or large males showed even more sexual behaviour than smaller ones. Sexually deprived cave molly males increased their sexual activity, but small males still did not show more sexual behaviour than large ones, suggesting that the genetic basis for typical size-dependent mating behaviour has been lost. Likely, the extreme abiotic conditions (hypoxia and toxic hydrogen sulphide) have selected against energetically costly behaviours. In this paper, we examined how El Azufre males would respond to sexual deprivation. We found these males to also increase their sexual activity after sexual deprivation but, unlike cave mollies, small males now showed more sexual behaviour than large ones, suggesting that the genetic basis for this behaviour is not lost in this population. Differences between populations may be due to the less harsh characteristics of El Azufre (e.g. less H₂S), resulting in weaker selection against energy-consuming behaviours. Furthermore, we tested for potential costs of male sexual harassment for females in terms of reduced feeding efficiency in the presence of a male, as was shown for P. mexicana from a clear-water population, but was not found in cave mollies or the El Azufre fish. Test females were either starved for 1 week to increase hunger levels or male sexual deprivation and female starvation were combined; however, we found no evidence for male sexual harassment even after these treatments. Our results parallel previous findings in the cave molly and suggest that El Azufre males have lost specific behavioural traits that cause sexual harassment.     

Mate choice and sexual conflict in <Emphasis Type="Italic">Brachionus plicatilis</Emphasis> (Rotifera)

Male mate choice in Brachionus plicatilis is based on information from coronal receptors and contact chemoreception of a glycoprotein signal on the body surface of females. Male mating behavior is modulated by mating signal quality and strength, which determines male mating persistence and circling intensity. We probe the sensory abilities of males by better resolving their discrimination of female age. Males preferentially initiate mating with young females, but not too young. Males circle 0.2 h old females just hatched from resting eggs only about 1/2 as frequently as 12 h old females. Males are even more discriminating of females with whom they copulate, preferring 3 h old females significantly more than 0.2 h or 6 h old females. In contrast, males cannot distinguish between virgin females and those who have already copulated. Similarly, males circled and copulated with females hatching from resting eggs with equal frequency as those hatching from amictic eggs. The counterpoint to male mate choice is female resistance to mating. When males contact females, the females respond with one of four behaviors: no response, acceleration, foot flipping, or coronal retraction. In 65% of male–female encounters, there was no initial response by females. However, when males began circling females, females accelerated 11.1 times more often than when males were absent. The second type of evasive female behavior was foot-flipping, which tends to knock off males attempting to circle the female. In the presence of circling males, females performed foot-flipping behavior three times more often than in the absence of males. Coronal retraction, where they stop swimming and withdraw their corona, was observed less frequently than acceleration or foot-flipping, and there was no difference in the presence or absence of males. These data are interpreted in the context of sexual conflict, where the behaviors that optimize male and female fitness differ. Guest editors: S. S. S. Sarma, R. D. Gulati, R. L. Wallace, S. Nandini, H. J. Dumont & R. Rico-Martínez Advances in Rotifer Research     

桉小卷蛾触角感器的超微结构

[目的] 桉小卷蛾是桉树、番石榴等林木和果树的一种重要害虫。为确定桉小卷蛾成虫触角上的感器对寻找生存环境及对外界环境的各种刺激所起的作用,研究了桉小卷蛾触角感器的种类、形态和分布。[方法] 利用扫描电镜对桉小卷蛾成虫触角感器进行了详细观察和研究。[结果] 桉小卷蛾雌、雄蛾触角由柄节、梗节和鞭节组成,触角的外侧面覆盖有鳞片;雄性触角略短于雌虫,且鞭节近1/4处有一凹陷。共发现7种感器,即鳞形感器、毛形感器、锥形感器、腔锥形感器、耳形感器、刺形感器和Böhm氏鬃毛,其中以毛形感器数量最多。[结论] 桉小卷蛾雌雄成虫触角的感器种类、分布、数量有一定的规律但又有差异。建议应用气相-触角电位联用技术GC-EAD和触角电位EGA等技术进一步解析触角感器的功能。     

Analysis of the Gut Microbiota by High-Throughput Sequencing of the V5–V6 Regions of the 16S rRNA Gene in Donkey

Considerable evidence suggests that the gut microbiota is complex in many mammals and gut bacteria communities are essential for maintaining gut homeostasis. To date the research on the gut microbiota of donkey is surprisingly scarce. Therefore, we performed high-throughput sequencing of the 16S rRNA genes V5–V6 hypervariable regions from gut fecal material to characterize the gut microbiota of healthy donkeys and compare the difference of gut microbiota between male and female donkeys. Sixty healthy donkeys (30 males and 30 females) were enrolled in the study, a total of 915,691 validated reads were obtained, and the bacteria found belonged to 21 phyla and 183 genera. At the phylum level, the bacterial community composition was similar for the male and female donkeys and predominated by Firmicutes (64 % males and 64 % females) and Bacteroidetes (23 % males and 21 % females), followed by Verrucomicrobia, Euryarchaeota, Spirochaetes, and Proteobacteria. At the genus level, Akkermansia was the most abundant genus (23 % males and 17 % females), followed by Sporobacter, Methanobrevibacter, and Treponema, detected in higher distribution proportion in males than in females. On the contrary, Acinetobacter and Lysinibacillus were lower in males than in females. In addition, six phyla and 15 genera were significantly different between the male and female donkeys for species abundance. These findings provide previously unknown information about the gut microbiota of donkeys and also provide a foundation for future investigations of gut bacterial factors that may influence the development and progression of gastrointestinal disease in donkey and other animals.     

Vasopressin regulates social recognition in juvenile and adult rats of both sexes, but in sex- and age-specific ways

In adult male rats, vasopressin (AVP) facilitates social recognition via activation of V1a receptors within the lateral septum. Much less is known about how AVP affects social recognition in adult females or in juvenile animals of either sex. We found that administration of the specific V1a receptor antagonist d(CH(2))(5)[Tyr(Me)(2)]AVP into the lateral septum of adult rats impaired, whereas AVP extended, social discrimination in both sexes. In juveniles, however, we detected a sex difference, such that males but not females showed social discrimination. Interestingly, administration of the V1a receptor antagonist to juveniles (either intracerebroventricularly or locally in the lateral septum) did not prevent social discrimination, but instead significantly decreased the investigation of a novel as opposed to a familiar animal in both sexes, with stronger effects in males. V1a receptors were found to be abundantly expressed in the lateral septum with higher binding density in females than in males. These findings demonstrate that activation of V1a receptors in the lateral septum is important for social recognition in both sexes, and that the roles of septal V1a receptors in social recognition change during development.     

Aggression and arginine vasopressin immunoreactivity regulation by androgen receptor and estrogen receptor alpha

In the following study, we asked which steroid receptors regulate aggression and arginine vasopressin (AVP) immunoreactivity (– ir) in several limbic regions. Using spontaneous mutant and knockout mice, we generated a novel cross of mice whose offspring lacked estrogen receptor α (ERα), androgen receptor (AR) or both ERα and AR. The wild-type (WT) males and females were compared with ERα knockout (ERαKO) male, mutated AR (Tfm) male and ERαKO/Tfm (double knockout; DKO) male littermates. Animals were gonadectomized and treated with 17β-estradiol (E2) prior to resident-intruder aggression tests. WT and Tfm males showed aggression whereas WT females, ERαKO and DKO males did not. In the lateral septum, WT and Tfm male brains had significantly denser AVP-ir as compared with WT females and DKO males. ERαKO male brains were intermediate in the amount of AVP-ir present. In the medial amygdala, brains from all genotypes had equivalent AVP-ir, except DKO males, which had significantly less AVP-ir. Overall, the expression of aggressive behavior coincided with AVP-ir in WT, Tfm and DKO males. However, in ERαKO males and WT females, the amount of AVP-ir was not associated with resident-intruder aggression. In sum we have shown that E2 acts via ERα to regulate aggression in male mice. In contrast both ERα and AR contribute to AVP-ir in limbic brain regions.     

Sex difference in glucocorticoid binding in rat pituitary is estrogen dependent

Sex-dependent differences in corticosteroid binding were assessed in individual pituitaries from adult male and female rats that had been adrenalectomized 12 h before sacrifice. Soluble binding was assayed in duplicate on LH-20 columns. Gonadally intact females showed significantly less 3H-dexamethasone binding than did intact males (p less than 0.01). This difference was confirmed in a second study (p less than .001). However, when ovariectomized females were compared with gondadectomized males, there was no difference in receptor concentration. Estrogen was able to reverse the effect of ovariectomy: ovariectomized females receiving estrogen (10 micrograms/rat/day) had significantly fewer receptors than intact males; p less than 0.01). Progesterone (500 micrograms/rat/day) did not antagonize the effect of estrogen in the pituitary. A sex difference was also found in the Type I (mineralocorticoid) receptor subpopulation which comprised approximately 10% of the total receptors, with females having fewer receptors than males. These results demonstrate that in the pituitary, the level of functional corticosteroid receptors is subject to a 20% down-regulation by circulating levels of estrogen. This raises the possibility that the lower number of receptors in females may act to reduce their sensitivity to the negative feedback effects of glucocorticoids at the level of the pituitary.     

Mating behavior and mate recognition pheromone blocking of male receptors in Brachionus plicatilis Müller (Rotifera)

Copulatory behavior of three S and three L type B. plicatilis strains from different geographic areas was analyzed. A 29 KD surface glycoprotein on females, characterized as a Mate Recognition Pheromone (MRP), binds to receptors in the corona of males and blocks mate recognition. Blocking was observed in all S and L strains even though the MRP was isolated from a single L-type strain. Binding was quantified using image analysis and a 20-fold difference was observed among strains. A direct relationship between the male discrimination of females and the intensity of MRP binding to male receptors was found. This relationship might be useful as a tool to examine variation in the mate recognition systems of other rotifer species.     

BREEDING COMPETITION IN A PACIFIC SALMON (COHO: ONCORHYNCHUS KISUTCH): MEASURES OF NATURAL AND SEXUAL SELECTION

In the breeding system of Pacific salmon, females compete for oviposition territories, and males compete to fertilize eggs. The natural selection in females and sexual selection in males likely has been responsible for their elaborate breeding morphologies and the dimorphism between the sexes. We quantified direct-selection intensities during breeding on mature coho salmon (Oncorhynchus kisutch), measured for seven phenotypic characters, including three secondary sexual characters. Wild and sea-ranched hatchery coho were used to enhance the range of phenotypes over which selection could be examined. The fish were allowed to breed in experimental arenas where we could quantify components of breeding success as well as estimate overall breeding success. We found that without competition, natural selection acts only on female body size for increased egg production; there is no detectable selection on males for the phenotypic distribution we used. Under competition, the opportunity for selection increased sixfold among females. Natural selection favored female body size and caudal-peduncle (tail) depth. Increased body size meant increased egg production and access to nesting territories. The caudal peduncle, used in burst swimming and nest digging, influenced both successful egg deposition and nest survival. Increasing density increased competition among females, though it did not significantly intensify natural selection on their characters. In males, competition increased the opportunity for selection 52-fold, which was nine times greater than for females. Sexual selection favored male body size and hooked snout length, both characters directly influencing male access to spawning opportunities. Selection on male body size was also affected significantly by breeding density. The ability of large males to control access to spawning females decreased at higher densities reflecting an increase in the operational sex ratio. Further, the relative success of small males, which could sneak access to spawning females, appeared to increase as that of intermediate-sized males decreased. Such disruptive selection may be responsible for the evolution of alternative reproductive tactics in salmon.