Female-biased Operational Sex Ratio Research Articles

Nephila spider male aggregation: preference for optimal female size and web clustering.

Sexual size dimorphism theory predicts biased operational sex ratios (OSRs) and an uneven distribution of males among certain females. We studied this phenomenon through a field census of the giant wood spider Nephila pilipes (family Nephilidae) in Singapore, a species where females are, on average, 6.9 times larger than males. Specifically, we tested two hypotheses concerning male distribution, given their tendency to aggregate in certain female webs. The optimal female size hypothesis predicts that males would predominantly occupy webs of intermediate-sized females. The web clustering hypothesis posits that more males would be found in webs closer together compared to those farther apart. Our snapshot census revealed a female-biased OSR (females: males = 1.85) with an uneven distribution of males in female webs. Most males were found in webs of intermediate-sized females aligning with the optimal female size hypothesis. Proximity among female webs was indicative of male presence, lending support to the web clustering hypothesis. While our study's limited sample size warrants caution, we conclude that in N. pilipes, male occupation of female webs is facilitated by the clustering of webs, and males prefer to cohabit with optimally sized, receptive females.

Unraveling the mating system of the burrowing shrimp Lepidophthalmus siriboia (Decapoda Callichiridae) based on life history traits

Natural history studies are important in helping to understand the origin and evolution of social organization as well as the evolution of specialized morphological structures linked to mating behavior of animals. Here we describe the burrow use pattern, sex ratio, and sexual dimorphism of the burrowing shrimp Lepidophthalmus siriboia to test a series of evolutionary hypotheses. To this end, a total of 259 individuals of L. siriboia were collected from the northeast region of Brazil. No shrimp pairs or burrows inhabited by more than one shrimp were detected during the study period. A solitary habit is a non-random behavior in this species because single shrimps were found with a greater frequency than expected by chance. All ovigerous females were found living solitarily, which suggests that males abandon these females shortly after insemination. Contrary to the expectations of an anisogamous species, L. siriboia produced a female-biased operational sex ratio (OSR), contradicting the hypothesis that only males compete for mates. The latter was supported by the existence of sexual dimorphism in cheliped size, a condition that argues in favor of female–female competition in this species. In the same line of reasoning, heterochely was present in adult males, although it was also present to a lesser degree in adult females. Female asymmetry of chelipeds suggests the evolution of an unconventional role in female major cheliped use. The major cheliped showed a positive allometric growth pattern through the ontogeny of both sexes. However, when growth patterns of the major and minor chelipeds were compared, the fitted regression lines for each sex had different slopes, indicating that the cheliped could be an appendage sexually selected by individuals of the opposite sex during mating.

Women’s Intrasexual Competition Results in Beautification

Psychology research focuses primarily on male competition. This research, however, investigates women’s competition for love and the ideal partner in the mating market and reveals one psychological consequence for women, that is, beautification. This is demonstrated with ecologically valid, real-world archive and online search query data, a quasi-experiment, and a series of controlled experiments with random assignments. Intrasexual competition, indexed by the operational sex ratio (OSR) and income inequality (GINI), predicts women’s beautification reflected by Google search queries for cosmetic surgery terms (Study 1) and the density of certificated plastic surgeons (Study 2). Female college students from faculties with female-biased OSRs exhibit greater appearance focus than women from male-biased faculties (Study 3). A causal relationship, between women’s intrasexual competition and beautification (and even self-objectification), is subsequently demonstrated in experiments (Studies 4–6). Additionally, self-objectification due to intrasexual competition leads to women’s preference for appearance-oriented products (Study 6). Implications are discussed.

Deer Indirectly Alter the Reproductive Strategy and Operational Sex Ratio of an Unpalatable Forest Perennial.

Environmental conditions impose restrictions and costs on reproduction. Multiple reproductive options exist when increased reproductive costs drive plant populations toward alternative reproductive strategies. Using 4 years of demographic data across a deer impact gradient, where deer alter the abiotic environment, we parameterize a size-dependent integral projection model for a sexually labile and unpalatable forest perennial to investigate the demographic processes driving differentiation in the operational sex ratio (OSR) of local populations. In addition to a relative increase in asexual reproduction, our results illustrate that nontrophic indirect effects by overabundant deer on this perennial result in delayed female sex expression to unsustainably large plant sizes and lead to more pronounced plant shrinkage following female sex expression, effectively increasing the cost of reproduction. Among plants of reproductive age, increased deer impact decreases the size-dependent probability of flowering and reduces reproductive consistency over time. This pattern in sex expression skews populations toward female-biased OSRs at low deer impact sites and male-biased OSRs at intermediate and high deer impact sites. While this shift toward a male-biased OSR may ameliorate pollen limitation, it also decreases the effective population size when coupled with increased asexual reproduction. The divergence of reproductive strategies and reduced lifetime fitness in response to indirect deer impacts illustrate the persistent long-term effects of overabundant herbivores on unpalatable understory perennials.

Calling behavior of males and females of a Bornean frog with male parental care and possible sex-role reversal

In many species that use acoustic signals for mate attraction, males are usually the most vocal sex. In frogs, females typically remain silent, while males produce advertisement calls to attract mates. In some species, females vocalize, but usually as a response to an initial male advertisement call. The smooth guardian frog (Limnonectes palavanensis), found on Borneo, has exclusive paternal care while the females mate and desert after laying the clutch. Males provide care to the eggs until hatching and then they transport the tadpoles to small bodies of water. The vocal repertoire of this species has never been described. Males have a distinctive advertisement call to attract females, but produce the call very infrequently. We found that females of L. palavanensis not only respond to male advertisement calls but also vocalize spontaneously, forming lek-like aggregations around a single male. Males may or may not respond to a particular female with a short courtship call, which is elicited only by the female call and not the male advertisement call. The calling rate of females is consistently higher throughout the night compared with the calling rate of males. These observations suggest that this species exhibits a reversal in calling behavior and possibly a sex-role-reversed mating system. Exceptional cases of species with a sex-role reversed mating system have been observed in fishes and birds, but not in frogs. For sex-role reversal to occur, there must be intense parental care by the males and a surplus of females. Additionally, females should exhibit characteristics that are usually observed in males in species with conventional sex roles. We found that in L. palavanensis, females are highly vocal, exhibiting higher calling rates compared with the calling rates of the males. This behavior, where females out-signal males has not been observed in anurans. This female calling behavior coupled with observations of several females approaching a male provides evidence of a female-biased operational sex ratio, a characteristic of a sex-role-reversed mating system. Thus, this study provides quantitative evidence that L. palavanensis exhibits various aspects consistent with a sex-role reversed mating system.

Sex-Specific Arrival Times on the Breeding Grounds: Hybridizing Migratory Skuas Provide Empirical Support for the Role of Sex Ratios.

In migratory animals, protandry (earlier arrival of males on the breeding grounds) prevails over protogyny (females preceding males). In theory, sex differences in timing of arrival should be driven by the operational sex ratio, shifting toward protogyny in female-biased populations. However, empirical support for this hypothesis is, to date, lacking. To test this hypothesis, we analyzed arrival data from three populations of the long-distance migratory south polar skua (Catharacta maccormicki). These populations differed in their operational sex ratio caused by the unidirectional hybridization of male south polar skuas with female brown skuas (Catharacta antarctica lonnbergi). We found that arrival times were protandrous in allopatry, shifting toward protogyny in female-biased populations when breeding in sympatry. This unique observation is consistent with theoretical predictions that sex-specific arrival times should be influenced by sex ratio and that protogyny should be observed in populations with female-biased operational sex ratio.

Effect of Female-Biased Sex Ratios on Female Homosexual Behavior in Japanese Macaques: Evidence for the "Bisexual Preference Hypothesis".

We aimed to explain the frequent and prevalent female homosexual behavior in the context of female-biased operational sex ratios (OSR) and qualified sex ratios (Q) in a free-ranging group of Japanese macaques (Macaca fuscata) living at Arashiyama-Kyoto, Japan. Our data included the average availability of sexually mature males during females' putative fertile period (OSR), the ratio of sexually mature males to sexually mature females (Q), as well as heterosexual and female homosexual solicitations and consortships collected during 13 mating seasons from 136 females. Our results did not support the "heterosexual deprivation hypothesis," which holds that female homosexual behavior is attributable to a shortage of male mates. Likewise, our results did not support the "lack of opposite-sex sexual competitor hypothesis," which holds that females have more access to female mates when male sexual rivals are scarce. Of the 11 predictions tested, only one yielded statistically significant results: we found that higher ratios of availability of preferred female partners to preferred male partners were associated with female homosexual consortships rather than female heterosexual consortships. This result supported the "bisexual preference hypothesis," which holds that female homosexual behavior is attributable to female preference for certain female mates relative to certain male mates. We conclude that when a female targets another female as a mate, it is an active choice for a female sexual partner over available male alternatives, rather than a by-default situation that occurs because males are not available as sexual partners, or because females are better able to access female sexual partners due to a scarcity of male sexual competitors.

Reproductive behaviour of female rosy bitterling Rhodeus ocellatus in response to a female-biased operational sex ratio

While the effect of Operational Sex Ratio (OSR) on reproductive behaviour of males has been studied extensively, little is known of the response of females facing a female-biased OSR. We investigated the effect of different OSRs on female reproductive behaviour using the rosy bitterling, Rhodeus ocellatus, a freshwater fish that lays its eggs inside the gills of living freshwater mussels. Three levels of OSR (male/female ratio 1:1, 1:3 and 1:5) were tested. We demonstrated that inspection of the mussel (spawning substrate) by individual females increased with increasingly female-biased OSR, but that the rate of following territorial male decreased. Aggression towards other females was not affected by the OSR. Interestingly, when a male bitterling led a non-dominant female towards the mussel, the dominant female would become aggressive to the male and chase the non-dominant female away. Aggression towards male followed a bell-shaped pattern and was highest at an OSR of 1:3. In both the female-biased OSRs examined, almost 50% of dominant females tended to chase away other females and defend the mussel, showing territoriality in a similar manner to males. These observations suggest that female reproductive behaviour is strongly affected by the OSR, and their reproductive tactics during courtship change from a passive role in courtship (following a male) to an active role in courtship (approaching a male), with presence of female territorial behaviour as the OSR becomes increasingly female-biased. This study provides strong evidence that a female-biased OSR has an important effect on female reproductive behaviour.

SPERM COMPETITIVE ABILITY EVOLVES IN RESPONSE TO EXPERIMENTAL ALTERATION OF OPERATIONAL SEX RATIO

In naturally polygamous organisms such as Drosophila, sperm competitive ability is one of the most important components of male fitness and is expected to evolve in response to varying degrees of male-male competition. Several studies have documented the existence of ample genetic variation in sperm competitive ability of males. However, many experimental evolution studies have found sperm competitive ability to be unresponsive to selection. Even direct selection for increased sperm competitive ability has failed to yield any measurable changes. Here we report the evolution of sperm competitive ability (sperm defense-P1, offense-P2) in a set of replicate populations of Drosophila melanogaster subjected to altered levels of male-male competition (generated by varying the operational sex ratio) for 55-60 generations. Males from populations with female-biased operational sex ratio evolved reduced P1 and P2, without any measurable change in the male reproductive behavior. Males in the male-biased regime evolved increased P1, but there was no significant change in P2. Increase in P1 was associated with an increase in copulation duration, possibly indicating greater ejaculate investment by these males. This study is one of the few to provide empirical evidence for the evolution of sperm competitive ability of males under different levels of male-male competition.

High mortality and female-biased operational sex ratio result in low encounter rates and moderate polyandry in a spider

Mating systems are shaped by ecological factors such as timing of maturation, spatiotemporal distribution of individuals, and the operational sex ratio. These factors influence male–female encounter rates, which shape the intensity and form of inter- and intrasexual competition. Whereas knowledge of natural encounter rates is vital for understanding the evolution of mating systems, data from the wild are difficult to obtain. We present data on the ecology and natural history of the subsocial spider Stegodyphus bicolor (Eresidae) with the aim of assessing its mating system. We investigated male mate search in relation to female spatial distribution, the timing of maturation of individuals, and the operational sex ratio, which may affect male–male competition and be used to predict mating strategies. We recorded male visits and cohabitation patterns with females of different reproductive state (immature and adults) to investigate evidence for mate choice as an indicator of sperm priority patterns. Finally, we used male visiting rates as a proxy for female natural mating rates. Both sexes matured synchronously: 51% of the females mated and received on average 1.2 visiting males. With increasing female availability male cohabitation time decreased. Preference for adult females, a decline in the occurrence of cohabitation with increasing female availability, and a lack of male protandry suggest that males do not employ strategies to protect paternity. Indeed, female-biased operational sex ratio throughout the mating season and high mortality may result in low male visitation rates. We suggest that ecological constraints shape a moderately polyandrous mating system in this species. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ••, ••–••.

Influences of orange spot patterns and operational sex ratio on male mating behaviors in the guppy Poecilia reticulata

The operational sex ratio (OSR) is an important factor that influences male mating behaviors in many animals. Several previous studies have demonstrated the effect of the OSR on mating behaviors in the guppy Poecilia reticulata, but the results are often inconsistent. In this study, we examined the effects of the OSR and exaggeration of a male sexual ornament (orange spot patterns) on mating behaviors in male guppies. We used a pair of males, one with large orange spots (bright male), an important criterion of female mate choice, and another with small orange spots (dull male). The OSR was adjusted based on the number of females. Both bright and dull males performed courtship displays and sneaking attempts more frequently under a female-biased OSR than those under a male-biased OSR. Bright males performed courtship displays more often than dull males, regardless of sex ratio. Under a male-biased OSR, females frequently responded to courtships from bright males, and bright males obtained more cooperative copulations than dull males. However, under a female-biased OSR, dull males frequently received female responses and copulated with females. These results indicate that mating behaviors of male guppies were affected by the OSR and males with large orange spots behaved differently than those with small orange spots. The results also imply that variations in local sex ratios may play a role in maintaining the diversity of orange spot patterns in male guppies by enhancing the mating opportunities of males with small orange spots in a female-biased environment.

Male, but not female, preference for an ornament expressed in both sexes of the polygynous mosquito Sabethes cyaneus

The evolution of female ornaments in species with a female-biased operational sex ratio (OSR) and intense female competition is well understood. In contrast, the adaptive value of female ornaments in species with a male-biased OSR and male competition remains largely unresolved. Mutual mate choice is one proposed explanation for the evolution of ornaments expressed in both males and females, a hypothesis supported by the increasing empirical evidence of mutual mate choice in species with a male-biased OSR. None the less, the evolution of female ornaments remains constrained, as investment in ornaments may detract from any direct benefits being signalled to males and females may fail to reap benefits sufficient to outweigh the costs of signalling. We used phenotypic engineering (i.e. manipulation of ornament size) to ask whether both sexes show a preference for sexually homologous ornaments in the polygynous mosquito, Sabethes cyaneus . We found a directional male preference for ornamented females, but no female preference for ornamented males. There was no evidence of assortative mating based on ornament size. We discuss these results within the framework of current sexual selection theory, addressing implications for both the evolution of male mate choice and the evolution of female ornaments.

The effect of nest aggregation on the reproductive behaviour of the peacock blenny Salaria pavo

The effect of nest aggregation in courtship behaviour was tested experimentally in an ecologically constrained, sex-role reversed population of the peacock blenny Salaria pavo. Mixed sex groups of eight males and eight females were tested in experimental tanks, containing eight potential nests either aggregated or dispersed. In the aggregated treatment, males spent more time inside their nests and monopolized other potential nests, causing a female-biased operational sex ratio (OSR). In the aggregated treatment, females also expressed more courtship behaviour. The results in general support the prediction that the aggregation of nests promotes male monopolization of potential nests, resulting in fewer nest-holding males and therefore a female-biased OSR that leads to the reversal of sex roles.

Conservation hotspots: implications of intense spatial area use by breeding male and female loggerheads at the Mediterranean’s largest rookery

ESR Endangered Species Research Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials ESR 10:191-202 (2009) - DOI: https://doi.org/10.3354/esr00137 Conservation hotspots: implications of intense spatial area use by breeding male and female loggerheads at the Mediterranean’s largest rookery Gail Schofield1,2,*, Martin K. S. Lilley2, Charles M. Bishop3, Peter Brown4, Kostas A. Katselidis1,5, Panayotis Dimopoulos1, John D. Pantis6, Graeme C. Hays2 1Department of Environmental & Natural Resources Management, University of Ioannina, G. Seferi 2, 30100 Agrinio, Greece 2Department Biological Sciences, Institute of Environmental Sustainability, Swansea University, Singleton Park, Swansea SA2 8PP, UK 3School of Biological Sciences, University of Bangor, Wales, Deiniol Road, Bangor, Gwynedd LL57 2UW, UK 4Navsys Ltd., Kirknewton, Edinburgh EH27 8DY, UK 5National Marine Park of Zakynthos, 1 El. Venizelou Street, 29100 Zakynthos, Greece 6Department of Ecology, School of Biology, UP Box 119, Aristotle University of Thessaloniki, 54006 Thessaloniki, Greece *Email: g.schof@gmail.com ABSTRACT: The implementation of appropriate protection measures for endangered species in protected areas requires knowledge of their fine-scale habitat use. In May and June of 2006 and 2007, we used GPS loggers (some linked to the Argos system) and a conventional Argos transmitter to track male and female loggerhead turtles Caretta caretta in the vicinity of the breeding area of Laganas Bay within the National Marine Park of Zakynthos, Greece. We obtained (1) 9681 useable locations (mean: 1383 locations ind.–1; range: 519 to 2198 locations) from Tracktag GPS loggers attached to 7 females for a mean duration of 34 d (range: 17 to 52 d); (2) 1245 useable locations (mean: 311 locations ind.–1; range: 38 to 1110 locations) from 4 males fitted with Fastloc Argos tags for a mean duration of 29 d (range: 3 to 51 d) and (3) 100 locations from 1 male fitted with a conventional Argos satellite tag tracked for 128 d. GPS data indicated that before the onset of nesting, both males and females primarily used an area within 500 m of the shore along a core 9 km stretch of coastline, where existing protective legislation requires strengthening. Our observations suggest that a 76.7% female-biased operational sex ratio, measured previously from in-water surveys, may represent a realistic sex ratio estimate in the period before nesting starts. In the first month following the onset of nesting, female spatial distribution remained similar, whereas most males departed for distant areas presumably to forage. Our study provides quantitative evidence of the need to improve the management planning and conservation measures to protect sea turtles in a coastal breeding area, and new insights on male turtle migration. KEY WORDS: Conservation management · Endangered species · Population density · Marine vertebrate · Migratory species · Multiple paternity · Wildlife telemetry Full text in pdf format Supplementary material PreviousNextCite this article as: Schofield G, Lilley MKS, Bishop CM, Brown P and others (2009) Conservation hotspots: implications of intense spatial area use by breeding male and female loggerheads at the Mediterranean’s largest rookery. Endang Species Res 10:191-202. https://doi.org/10.3354/esr00137Export citation Mail this link - Contents Mailing Lists - RSS Facebook - Tweet - linkedIn Cited by Published in ESR Vol. 10. Online publication date: January 28, 2009 Print ISSN: 1863-5407; Online ISSN: 1613-4796 Copyright © 2009 Inter-Research.

Biased sex ratio and low population density increase male mating success in the bug Nysius huttoni (Heteroptera: Lygaeidae)

Demographic factors such as operational sex ratio (OSR) and local population density (LPD) are temporally and spatially dynamic in the natural environment but the influence of these variables on male mating success and the mechanisms behind it are still poorly understood and highly controversial. Here, we manipulated the OSR and LPD of a seed bug, Nysius huttoni, and carried out a series of mating trials to test how these variables affected male mating success. The two demographic factors had no significant interactions, suggesting that they affect male mating success independently in N. huttoni. In this species male mating success was significantly higher in both male- and female-biased OSR than in even OSR. It is suggested that, in male-biased OSR, the increased intensity of competition and interference does not result in lower male mating success; rather, males may make more effort in courting and females may have more chance to encounter better males, resulting in higher male mating success. In female-biased OSR, females may become less choosy and less likely to reject male mating attempt, leading to the higher male mating success. Lower male mating success in N. huttoni in high LPD may be due to increased interference between males and/or delayed female receptiveness for mating. OSR had a stronger effect on male mating success than LPD in N. huttoni, suggesting that OSR and LPD affect mating success in different ways and intensities.

Females increase current reproductive effort when future access to males is uncertain

Trade-offs between current and future reproduction shape life histories of organisms, e.g. increased mortality selects for earlier reproductive effort, and mate limitation has been shown to shape male life histories. Here, we show that female life histories respond adaptively to mate limitation. Female common gobies (Pomatoschistus microps) respond to a female-biased operational sex ratio by strongly increasing the size of their first clutch. The plastic response is predicted by a model that assumes that females use the current competitive situation to predict future difficulties of securing a mating. Because female clutch size decisions are much more closely linked to population dynamics than male life-history traits, plastic responses to mate-finding limitations may be an underappreciated force in population dynamics.

Sex-role reversal of a monogamous pipefish without higher potential reproductive rate in females

In monogamous animals, males are usually the predominant competitors for mates. However, a strictly monogamous pipefish Corythoichthys haematopterus exceptionally exhibits a reversed sex role. To understand why its sex role is reversed, we measured the adult sex ratio and the potential reproductive rate (PRR), two principal factors influencing the operational sex ratio (OSR), in a natural population of southern Japan. The adult sex ratio was biased towards females throughout the breeding season, but the PRR, which increased with water temperature, did not show sexual difference. We found that an alternative index of the OSR (Sf/Sm: sex ratio of 'time in') calculated from the monthly data was consistently biased towards females. The female-biased OSR associated with sex-role reversal has been reported in some polyandrous or promiscuous pipefish, but factors biasing the OSR differed between these pipefish and C. haematopterus. We concluded that the similar PRR between the sexes in C. haematopterus does not confer reproductive benefit of polygamous mating on either sex, resulting in strict monogamous mating, and its female-biased adult sex ratio promotes female-female competition for a mate, resulting in sex-role reversal.

Reproductive success of male bank voles (Clethrionomys glareolus): the effect of operational sex ratio and body size

The operational sex ratio (OSR) may influence the intensity of competition for mates and mate choice and is therefore thought to be a major factor predicting the intensity and direction of sexual selection. We studied the opportunity for sexual selection, i.e., the variance in male reproductive success and the direction and intensity of sexual selection on male body mass in bank vole (Clethrionomys glareolus) enclosure populations with experimentally manipulated sex ratios. The opportunity for sexual selection was high among male-biased OSRs and decreased towards female-biased OSRs. Paradoxically, selection for large male body mass was strongest in female-biased OSRs and also considerable at intermediate OSRs, whereas at male-biased OSRs, only a weak relationship between male size and reproductive success was found. Litters in male-biased OSRs were more likely to be sired by multiple males than litters in female-biased OSRs. Our results suggest that the intensity and direction of sexual selection in males differs among different OSRs. Although the direction of sexual selection on male body mass was opposite than predicted, large body mass can be favored by sexual selection. Naturally varying OSRs may therefore contribute to maintain variation in male sexually selected traits.

Female pheromonal chorusing in an arctiid moth, Utetheisa ornatrix

We report an unusual case of communal sexual display in the arctiid moth Utetheisa ornatrix that we designate “female pheromonal chorusing.” As in most moths, female U. ornatrix release a long-distance sexual advertisement pheromone during a nightly activity period. We arranged U. ornatrix females in 2 types of signaling conditions: grouped and solitary. When the females were grouped with neighboring signaling females (grouped), they initiated pheromone release sooner, continued release with less interruption and over a longer total period, and performed the release with faster abdominal pumping than observed in isolated females (solitary). This differs from the usual form of sexual communication in moths: female (chemical) signalers, male receivers, and a general lack of interaction among females. At mating, male U. ornatrix transfer a large spermatophore that may enhance female reproductive success and which represents either mating effort or paternal investment. This action results in an extended postmating male refractory period leading to a female-biased operational sex ratio. We argue that this biased sex ratio generates intrasexual competition among females, to which they respond by elevating signaling effort such that the likelihood of at least matching their neighbors' signals is increased. In the field, U. ornatrix are clustered around patches of host plants, and we also explore the possibility that pheromonal chorusing is driven by cooperation among groups of related—or nonrelated—females.

Intrasexual competition and mate choice in assassin bugs with uniparental male and female care

Paternal care has the potential to lead to female-biased operational sex ratios and sex role reversal. Whereas this issue has been repeatedly investigated in fish, we present the first study on insects. We investigated two species of assassin bug: Rhinocoris tristis, which shows male care of eggs, and Rhinocoris carmelita, which shows female care of eggs. In both species, the potential reproductive rate of males exceeded that of females. Since the population sex ratio was unbiased and there was no difference in mortality rates between the sexes, we predicted that the operational sex ratio would be skewed in favour of males in both species and thus males would be the more competitive sex. Field and laboratory observations upheld this prediction, with males competing for females in both species. In R. tristis, males also competed for ownership of eggs, whereas females discriminated between males according to ownership of eggs. However, choosiness was not restricted to females in R. tristis. Observations suggested that R. tristis males discriminated between females according to their weight: heavier females, which were likely to oviposit sooner, were guarded for longer by males in a period of postcopulatory riding. We suggest that the primary function of postcopulatory riding is to prevent females from mating with conspecifics, thus reducing the possibility of sperm competition. This behaviour might also be an important prerequisite for the evolution of male care.