Operational Sex Ratio Research Articles

Phenology and Breeding Mechanisms of Calamus lobbianus Becc. and Calamus pygmaeus Becc.

The floral biology of Calamus is relatively unknown except for several species. In this study, Calamus lobbianus and Calamus pygmaeus were selected to represent the non-climbing rattan of the Sundaland's flagellate group. Observations on phenology, floral rewards and floral visitors as well as experiments on the breeding mechanism and operational sex ratio were performed for both species. For both species, anthesis started in the early morning for pistillate and staminate plants, ended by late noon (staminate plants) but lasted till the next day in pistillate plants for both species. Although both species appeared to be aseasonal in flowering, C. lobbianus exhibited a male-biased population while C. pygmaeus did not exhibit any sex bias. Nectar was observed to be extruded from the base of the petals of C. lobbianus of the staminate flowers and sterile staminate flowers of the pistillate plants. The concentration and volume of the nectar of the staminate flower and sterile staminate flower of C. lobbianus peaked at c. 11% and 9 μL around 1100 (Day 1) and c. 13% and 8 μL around 0930 (Day 1), respectively, but only appeared as a layer of glistening exudate in C. pygmaeus. Floral scent was not detected in any of the inflorescences of both species. Several insect visitors were observed to be the primary visitors of both Calamus species which include two species of Tetragonula, a species of Liostenogaster sp., and Stenodyneriellus sp. Experiments on breeding mechanism of C. lobbianus and C. pygmaeus indicate that both species are most likely apomictic. C. pygmaeus is capable of vegetative propagation through the rooting at the tip of inflorescence.

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.

Errors in estimating reproductive parameters with macroscopic methods: a case study on the protogynous blacktip grouper Epinephelus fasciatus (Forsskål 1775).

A size-based, histological analysis of the reproductive life history of the blacktip grouper, Epinephelus fasciatus (Forsskål 1775), was conducted in Indonesia to evaluate the error rate associated with macroscopic reproductive analysis. Histological results indicated that E. fasciatus was protogynous with female L50 at 13.4 cm total length (LT) and a size at sexual transition of 22.0 cm LT. The weight-length relationship for the species was W = 0.011 L3.13. Overall sex ratios were significantly female biased, operational sex ratios were significantly male biased, and sex ratios of mature individuals varied predictably with length from female to male dominance as size increased. No significant relationship between length and batch fecundity was found. The population has a spawning period from February to August. Overall, 54.4% of macroscopic evaluations were incorrect compared to histological results. Of the errors, 14.8% were a failure to detect ovotestes, 12.7% were classifying non-gonadal tissue as ovary or testis, 12.2% were misclassifying sex, and 12.7% were misclassifying maturity status. However, the largest source of error (47.7%) was from misclassifying both sex and maturity status. Of these, 92.9% were macroscopically classified as immature females, but were histologically confirmed to be mature males. Compared to histological results, the only accurate macroscopic results were the absence of a sex-based difference in weight-length relationship and spawning seasonality estimated by a gonadosomatic index (February-June). The use of macroscopic methods to estimate reproductive life-history parameters for sex-changing reefes fish may introduce significant inaccuracies and misinterpretations. Of the parameters estimated by histological methods, size at maturity, size-specific sex ratios, and spawning seasonality have the greatest potential to inform local fishery management policy.

Gender inequality in workloads explained by operational sex ratio

Ecological differences between human populations can affect the relative strength of sexual selection, and hence drive gender inequality. Here, we exploit the cultural diversity of southwestern China, where some village sex ratios are female-biased, in part due to a proportion of males entering monastic celibacy, to evaluate the role of sex ratio on the sexual division of labor. We used a detachable activity tracker to measure workload by step counts in both sexes among 561 individuals in 55 villages in six different areas. We show that a lower sex ratio and a higher prevalence of monasticism are associated with higher women's workloads and reduced men's workloads in the non-celibate population. As the operational sex ratio increases, gender inequality diminishes. This study offers valuable insights into the origins of gender disparities by examining the role of sex ratio on the sexual division of labor.

Drought Stress Might Induce Sexual Spatial Segregation in Dioecious Populus euphratica-Insights from Long-Term Water Use Efficiency and Growth Rates.

P. euphratica stands as the pioneering and dominant tree within desert riparian forests in arid and semi-arid regions. The aim of our work was to reveal why dioecious P. euphratica in natural desert riparian forests in the lower Tarim River exhibits sexual spatial distribution differences combined with field investigation, tree ring techniques, isotope analysis techniques, and statistical analyses. The results showed that P. euphratica was a male-biased population, with the operational sex ratio (OSR) exhibiting spatial distribution differences to variations in drought stress resulting from groundwater depth change. The highest OSR was observed under mild drought stress (groundwater depth of 6-7 m), and it was reduced under non-drought stress (groundwater depth below 6 m) or severe drought stress (groundwater depth exceeding 7 m). As drought stress escalated, the degradation and aging of the P. euphratica forest became more pronounced. Males exhibited significantly higher growth rates and WUEi than females under mild drought stress. However, under severe drought stress, males' growth rates significantly slowed down, accompanied by significantly lower WUEi than in females. This divergence determined the sexual spatial segregation of P. euphratica in the natural desert riparian forests of the lower Tarim River. Furthermore, the current ecological water conveyance project (EWCP) in the lower Tarim River was hard to fundamentally reverse the degradation and aging of the P. euphratica forest due to inadequate population regeneration. Consequently, we advocated for an optimized ecological water conveyance mode to restore, conserve, and rejuvenate natural P. euphratica forests.

Impact of pre-copulatory sexual cannibalism on genetic diversity and efficacy of selection.

Factors that increase reproductive variance among individuals act to reduce effective population size (Ne), which accelerates the loss of genetic diversity and decreases the efficacy of purifying selection. These factors include sexual cannibalism, offspring investment and mating system. Pre-copulatory sexual cannibalism, where the female consumes the male prior to mating, exacerbates this effect. We performed comparative transcriptomics in two spider species, the cannibalistic Trechaleoides biocellata and the non-cannibalistic T. keyserlingi, to generate genomic evidence to support these predictions. First, we estimated heterozygosity and found that genetic diversity is relatively lower in the cannibalistic species. Second, we calculated dN/dS ratios as a measure of purifying selection; a higher dN/dS ratio indicated relaxed purifying selection in the cannibalistic species. These results are consistent with the hypothesis that sexual cannibalism impacts operational sex ratio and demographic processes, which interact with evolutionary forces to shape the genetic structure of populations. However, other factors such as the mating system and life-history traits contribute to shaping Ne. Comparative analyses across multiple contrasting species pairs would be required to disentangle these effects. Our study highlights that extreme behaviours such as pre-copulatory cannibalism may have profound eco-evolutionary effects.

Female delayed mating has a limited impact on the reproductive output of Cerambyx welensii, a synovigenic longhorn beetle

AbstractIncreased female age at mating is considered a detrimental factor on reproductive output and fitness in insects, even if the impact is rather species specific. The effect of delayed mating on reproductive output has been widely studied in pest species controlled with mating disruption, as if the method is not fully effective in suppressing matings, it could still delay them, limiting female fitness and pest damage. Female mating delay, however, may also occur in natural habitats without invoking mating disruption. We studied the effect of female delayed mating in Cerambyx welensii (Küster) (Coleoptera: Cerambycidae), an oak‐living sapro‐xylophagous longhorn beetle considered a critical factor in oak decline. Several life history and ecological traits may potentially delay mating, including low abundance, (re)colonisation processes, sexual communication channel, host‐produced kairomones, operational sex ratio, reproductive interference and adverse weather conditions. We conducted laboratory tests to assess the impact of mating delays (0‐, 10‐, 20‐ and 30‐day post‐emergence) on reproductive output. Data showed that mating delay until at least 20 days of age had a limited effect on mating success, lifetime fecundity, longevity and fertility. The daily fecundity pattern depended on mating delay, and virgin females showed ovarian retention. We conclude that C. welensii females have evolved physiological adaptations to overcome mating delays and optimise fitness. We discuss our results from an evolutionary perspective, considering specifically the risk of early predation and egg‐laying time limitation. We hypothesise that unpredictable recurrent stochastic variation in male availability could act as an additional driver selecting for synovigeny in this longhorn species.

Mating System and Behavior of Steppe Rat Snake (<i>Elaphe dione</i>) in South Korea

The mating system affects the animal’s mating behavior, operational sex ratio (OSR), and the genetic information of paternity. It is generally known that many species in the order Serpent accept polyandry. We observed the mating behavior of steppe rat snakes three times from April 1 until September 30, 2020. In all cases, the sex ratio participating in mating was one female and two or more males. Moreover, in two cases, two males were observed to insert their hemipenes into one female’s cloaca at the same time, which was a unique finding. It was concluded that steppe rat snakes would be highly likely to adopt polyandry, a common mating system of snakes. It will be possible to understand the exact mating system of steppe rat snakes by conducting behavioral genetic studies on them.

Green, charismatic, microendemic, and threatened: breeding biology of the leaf frog Pithecopus rusticus (Anura: Hylidae: Phyllomedusinae)

ABSTRACT Pithecopus rusticus is a small and charismatic leaf frog species known only from the type locality, in the Brazilian Subtropical Highland Grasslands of the Atlantic Forest, southern Brazil. In this study, we describe aspects of the reproductive biology and populational size of P. rusticus, encompassing the characterization of breeding season, microhabitat use for vocalization and spawning, the variation in body size of males and females and operational sex ratio, as well as explore the influence of climate variables on the abundance of individuals. We collected information from 2015 to 2021, mainly in the breeding seasons of 2015/16 and 2017/18. We recorded 91 individuals of P. rusticus, with a recapture rate of 61% in the 2015/16 breeding season and 57% in the 2017/18 breeding season. The estimated population size was 28.9 individuals for the 2015/16 breeding season and 21 individuals for the 2017/18 breeding season. The temporal reproductive pattern was intermediate, being concentrated in spring to early summer months (October to January), and the male calling activity differed throughout the night, increasing from 9pm onwards. The abundance of P. rusticus was significantly related with relative air humidity. We found sexual size dimorphism, with females larger than males, and a male-skewed operational sex ratio. Males and females used Poaceae and Cyperaceae leaves located close to water. Spawns contained 43 ± 5 eggs, with a diameter of 3.26 ± 0.45 mm, and were deposited on leaves of Senecio (Asteraceae) and Eriocaulon (Eriocaulaceae). Information on natural history is essential to support the implementation and success of in situ and ex situ conservation strategies for this endangered species.

Sex ratio affects sexual selection against mutant alleles in a locus-specific way.

Higher male:female operational sex ratio (OSR) is often assumed to lead to stronger sexual selection on males. Yet, this premise has been directly tested by very few studies, with mixed outcomes. We investigated how OSR affects the strength of sexual selection against two deleterious alleles, a natural ebony mutant and a transgenic GFP insertion, in Drosophila melanogaster. To this end, we estimated the relative paternity share of homozygous mutant males competing against wild-type males under different OSRs (1:2, 1:1, 2:1). We also manipulated the mating pool density (18, 36, or 54 individuals) and assessed paternity over three consecutive days, during which the nature of sexual interaction changed. The strength of sexual selection against the ebony mutant increased with OSR, became weaker after the first day, and was little affected by density. In contrast, sexual selection against the GFP transgene was markedly affected by density: at the highest density, it increased with OSR, but at lower densities, it was strongest at 1:1 OSR, remaining strong throughout the experiment. Thus, while OSR can strongly affect the strength of sexual selection against "bad genes," it does not necessarily increase monotonically with male:female OSR. Furthermore, the pattern of relationship between OSR and the strength of sexual selection can be locus-specific, likely reflecting the specific phenotypic effects of the mutation.

Experimental evolution under varying sex ratio and behavioral plasticity in response to perceived competitive environment independently affect calling effort in male crickets.

The operational sex ratio (OSR) is a key component influencing the magnitude of sexual selection driving the evolution of male sexual traits, but males often also retain the ability to plastically modulate trait expression depending on the current environment. Here we employed an experimental evolution approach to determine whether the OSR affects the evolution of male calling effort in decorated crickets, a costly sexual trait, and whether plasticity in calling effort is altered by the OSR under which males have evolved. Calling effort of males from 2 selection regimes maintained at different OSRs over 18-20 generations (male vs. female biased) was recorded at 2 different levels of perceived competition, in the absence of rivals or in the presence of an experimentally muted competitor. The effect of the OSR on the evolution of male calling effort was modest, and in the opposite direction predicted by theory. Instead, the immediate competitive environment strongly influenced male calling effort as males called more in the presence of a rival, revealing considerable plasticity in this trait. This increased calling effort came at a cost, however, as males confined with a muted rival experienced significantly higher mortality.

Male-biased sex ratios increase the support for a strong leader: An evolutionary threat management perspective

An evolutionary threat management approach to leadership assumes that humans have an adaptive followership psychology that responds to specific ecological threats by selecting appropriate leaders to mitigate any risks. We hypothesize that having an imbalanced sex ratio in society (with either an abundance of adult men or women) increases the support for a strong leader, because people find such imbalances threatening to themselves and to society. We tested our hypothesis with five preregistered experimental studies and one archival study. In the five online scenario studies (total N=2249), we manipulated the operational sex ratios within a fictitious country and measured both explicit leader preferences (ideal leader traits) and implicit preferences (facial cues of leaders). The results showed that a male-biased sex ratio (abundance of men) robustly increased the preference for a strong leader across all studies, but the effects of a female-biased sex ratio (abundance of women) were smaller and not consistent. Furthermore, a male-biased sex ratio in society activated the impression that society is a competitive, dangerous place. Overall, the studies enhance our understanding of the impact of an ecological factor, the sex ratio in society, on the support for political leadership.

Mutual mate guarding with limited sexual conflict in a sex-role-reversed shorebird.

Mate guarding is typically considered a male strategy to protect paternity. However, under some circumstances, females might also benefit from guarding their mate. Female mate guarding might be particularly important in socially polyandrous species in which females compete for access to care-giving males. Because males also benefit from being near their partner to avoid paternity loss, pair members may have a mutual interest in mate guarding in polyandrous species. We studied the time spent together and movements that lead to separation, as behavioral measures of mate guarding, in the classically polyandrous red phalarope (Phalaropus fulicarius). We equipped 64 breeding pairs with miniaturized telemetry loggers with GPS to assess variation in mate-guarding intensity in relation to breeding phenology and season, nest attendance, and the occurrence of extrapair paternity. We show that red phalarope pairs were almost continuously together in the days before clutch initiation with no sex bias in separation movements, indicating mutual contribution to mate guarding. Our results suggest that in red phalaropes, both pair members guard their mate, with limited sexual conflict arising through biases in the operational sex ratio and a trade-off with male nest attendance. We found no clear relationship between mate-guarding intensity and the occurrence of extrapair paternity. In this non-territorial socially polyandrous species, mutual mate guarding might be the process underlying the evolution of a brief but strong social pair bond, with no other purpose than producing a clutch for a care-giving male.

Reproductive success of Bornean orangutan males: scattered in time but clustered in space

The social and mating systems of orangutans, one of our closest relatives, remain poorly understood. Orangutans (Pongo spp.) are highly sexually dimorphic and females are philopatric and maintain individual, but overlapping home ranges, whereas males disperse, are non-territorial and wide-ranging, and show bimaturism, with many years between reaching sexual maturity and attaining full secondary sexual characteristics (including cheek pads (flanges) and emitting long calls). We report on 21 assigned paternities, among 35 flanged and 15 unflanged, genotyped male Bornean orangutans (Pongo pygmaeus wurmbii), studied from 2003 to 2018 in Tuanan (Central Kalimantan, Indonesia). All 10 infants born since mid-2003 with an already identified sire were sired by flanged males. All adult males ranged well beyond the study area (c. 1000 ha), and their dominance relations fluctuated even within short periods. However, 5 of the 10 identified sires had multiple offspring within the monitored area. Several sired over a period of c. 10 years, which overlapped with siring periods of other males. The long-calling behavior of sires indicated they were not consistently dominant over other males in the area around the time of known conceptions. Instead, when they were seen in the area, the known sires spent most of their time within the home ranges of the females whose offspring they sired. Overall, successful sires were older and more often resident than others.Significance statementIt is difficult to assess reproductive success for individuals of long-lived species, especially for dispersing males, who cannot be monitored throughout their lives. Due to extremely long interbirth intervals, orangutans have highly male-skewed operational sex ratios and thus intensive male-male competition for every conception. Paternity analyses matched 21 immature Bornean orangutans with their most likely sire (only 10 of 50 genotyped males) in a natural population. Half of these identified sires had multiple offspring in the study area spread over periods of at least 10 years, despite frequently ranging outside this area. Dominance was a poor predictor of success, but, consistent with female mating tactics to reduce the risk of infanticide, known “sires” tended to have relatively high local presence, which seems to contribute to the males’ siring success. The results highlight the importance of large protected areas to enable a natural pattern of dispersal and ranging.

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.

Anuran swingers: misdirected mating attempts occurred early during anuran diversification

Abstract Promiscuity, explosive breeding and male-biased operational sex-ratios can affect the strength of male selectivity and may play major roles in the expression of misdirected matings (with conspecific males, other species, corpses or objects) in anuran amphibians. Yet, misdirected amplexus occur in multiple species diverging from these reproductive traits, suggesting that the expression of such behaviour is widespread, and may have occurred early during anuran diversification. Using two methods of ancestral-trait reconstruction which predict unknown states, we found a very high probability that most anuran genera express misdirected amplexus, suggesting that this by-product of anuran reproductive strategies is likely to have occurred early during anuran diversification. The persistence of misdirected amplexus suggests that its infrequent occurrence may outweigh individual costs of breeding attempts with unfit mates. We found a recent exponential increase in reported observations of misdirected amplexus, which, along with increased research effort and publication rate, may reveal an effect of environmental perturbations known to promote the occurrence of these behaviours.

The operational sex ratio experienced by mothers modulates the expression of sons’ alternative reproductive tactics in spider mites

The operational sex ratio experienced by mothers modulates the expression of sons’ alternative reproductive tactics in spider mites

Swarming and mate selection in Anopheles gambiae mosquitoes (Diptera: Culicidae).

Treating both male and female Anopheles gambiae as if they are "boids" (a computer program that mimics flocking in birds) explains much of the swarming and mating behavior in this important group of malaria vectors. It is suggested that species specific swarm sites act as the mate recognition system in anophelines and it is proposed that virgin females respond to the swarm site per se rather than the swarm itself. Given the high operational sex ratio and the inability of any male to dominate all females within the swarm, it is considered that chance, rather than sexual selection, is the most important determinant of mating. The male being in the swarm may be a sufficiently strong signal to the female of his fitness, so that more elaborate sexual selection is unnecessary. The possibility of alternative mechanisms for mating may also exist but need to be investigated further. Given the importance of swarms as the isolating mechanism between species, emphasis should be placed on determining the characteristics of swarm sites and markers between them.

Jumping spider invades an orb web to prey on a resident male

AbstractWhile surveying operational sex ratios of the giant golden orb weaverNephila pilipes(Fabricius, 1793) in Singapore, we documented a stunning case of predatory behavior of a jumping spiderViciria pavesiiThorell, 1877. A femaleV. pavesiiinvaded aN. pilipesorb web that was occupied by the resident female and four males, and successfully captured, killed, and removed one of theNephilamales. Whether araneophagy inV. pavesiiis opportunistic or a predatory ritual remains to be tested more precisely, but because the observed predatory event triggered an aggressive response by theN. pilipesalpha male, it is plausible thatV. pavesiimay engage in aggressive mimicry. We place our observation into the context of jumping spider cognition and behavioral tactics that are so far well understood only in a few spartaeine genera, notablyPortiaKarsch, 1878.ViciriaThorell, 1877, we argue, is another such jumping spider taxon worthy of behavioral scrutiny.

Persistence of a highly monoandrous mating system despite an extremely male-biased operational sex ratio in the firefly squid Watasenia scintillans

Persistence of a highly monoandrous mating system despite an extremely male-biased operational sex ratio in the firefly squid Watasenia scintillans