Synthesis of sexual selection: a systematic map of meta-analyses with bibliometric analysis

Sexual selection has been a popular subject within evolutionary biology because of its central role in explaining odd and counterintuitive traits observed in nature. Consequently, the literature associated with this field of study became vast. Meta-analytical studies attempting to draw inferences from this literature have now accumulated, varying in scope and quality, thus calling for a synthesis of these syntheses. We conducted a systematic literature search to create a systematic map with a report appraisal of meta-analyses on topics associated with sexual selection, aiming to identify the conceptual and methodological gaps in this secondary literature. We also conducted bibliometric analyses to explore whether these gaps are associated with the gender and origin of the authors of these meta-analyses. We included 152 meta-analytical studies in our systematic map. We found that most meta-analyses focused on males and on certain animal groups (e.g. birds), indicating severe sex and taxonomic biases. The topics in these studies varied greatly, from proximate (e.g. relationship of ornaments with other traits) to ultimate questions (e.g. formal estimates of sexual selection strength), although the former were more common. We also observed several common methodological issues in these studies, such as lack of detailed information regarding searches, screening, and analyses, which ultimately impairs the reliability of many of these meta-analyses. In addition, most of the meta-analyses' authors were men affiliated to institutions from developed countries, pointing to both gender and geographical authorship biases. Most importantly, we found that certain authorship aspects were associated with conceptual and methodological issues in meta-analytical studies. Many of our findings might simply reflect patterns in the current state of the primary literature and academia, suggesting that our study can serve as an indicator of issues within the field of sexual selection at large. Based on our findings, we provide both conceptual and analytical recommendations to improve future studies in the field of sexual selection.

A Slim Majority: The Influence of Sex Ratio on Women's Body Dissatisfaction and Weight Loss Motivations

The current investigation examined whether women's perceptions of the sex ratio (ratio of women to men) in the local population influence their body dissatisfaction and weight loss motivations. A higher ratio of women to men in a given population signifies a relative abundance of same-sex mating competitors, intensifying female intrasexual competition. Five studies (N = 1,776) tested the hypotheses that women's perceptions of a female-skewed sex ratio would correspond to increased feelings of intrasexual competitiveness and perceptions of unfavorable mating prospects, which would, in turn, be associated with heightened body dissatisfaction and weight loss motivations. Among university and community women (Studies 1and 2), perceptions of a female-skewed sex ratio corresponded to greater intrasexual competitiveness, increased body dissatisfaction, and increased dieting inclinations. Among single women, assessments of a female-skewed sex ratio corresponded to perceptions of unfavorable mating prospects, increased romantic pressure to alter their appearance, and higher body dissatisfaction (Study 3). Studies 4 and 5 experimentally manipulated perceived sex ratio. Women in the female-skewed condition felt less satisfied with their weights and shapes, but only if they believed the manipulation (Study 4). In Study 5, using a within-subjects design, women who evaluated a male-skewed (vs. female-skewed) dating profile array subsequently desired to lose less weight. Findings suggest women's perceptions of their social environments may contribute to body image and dieting motivations.

Sex roles and sex ratios in animals

In species with separate sexes, females and males often differ in their morphology, physiology and behaviour. Such sex-specific traits are functionally linked to variation in reproductive competition, mate choice and parental care, which have all been linked to sex roles. At the 150th anniversary of Darwin's theory on sexual selection, the question of why patterns of sex roles vary within and across species remains a key topic in behavioural and evolutionary ecology. New theoretical, experimental and comparative evidence suggests that variation in the adult sex ratio (ASR) is a key driver of variation in sex roles. Here, we first define and discuss the historical emergence of the sex role concept, including recent criticisms and rebuttals. Second, we review the various sex ratios with a focus on ASR, and explore its theoretical links to sex roles. Third, we explore the causes, and especially the consequences, of biased ASRs, focusing on the results of correlational and experimental studies of the effect of ASR variation on mate choice, sexual conflict, parental care and mating systems, social behaviour, hormone physiology and fitness. We present evidence that animals in diverse societies are sensitive to variation in local ASR, even on short timescales, and propose explanations for conflicting results. We conclude with an overview of open questions in this field integrating demography, life history and behaviour.

Perceived Mate Scarcity Leads to Increased Willingness to Mate Poach

Mate poaching, where an individual attempts to attract a pair-bonded individual, is a risky mating tactic. Yet, it is surprisingly common. Although many studies have investigated individual differences in mate poaching, few have examined potentially relevant ecological factors, such as mate availability. In this study, 254 unmated North American adults were primed with either perceived mate scarcity or abundance, and subsequently completed measures of fear of being single, intrasexual competitiveness, and attitudes toward mate poaching. Results from a sequential mediation model revealed that men and women primed with mate scarcity held a more positive attitude toward mate poaching relative to those primed with mate abundance, and that this link was mediated by an induced fear of being single and intrasexual competitiveness. Our results suggest that mate poaching is a facultative adaptation of human mating psychology driven by intrasexual competitiveness that is activated in response to environments of low mate availability. It highlights the need for researchers to consider ecological cues when studying individual variation in mate poaching behavior.

Mating competition and adult sex ratio in wild Trinidadian guppies

Most experimental tests of mating systems theory have been conducted in the laboratory, using operational sex ratios (ratio of ready-to-mate male to ready-to-mate female) that are often not representative of natural conditions. Here, we first measured the range of adult sex ratio (proportion of adult males to adult females; ASR) in two populations of Trinidadian guppies (Poecilia reticulata) differing in ambient predation risk (high vs. low). We then explored, under semi-wild conditions, the effect of ASR (i.e., 0.17, 0.50, 0.83) on mating competition patterns in these populations. ASR in the wild was female-biased and did not significantly differ between the two populations. The range of ASR in our experiment was representative of natural ASRs. As expected, we observed an increase in intrasexual aggression rates in both sexes as the relative abundance of competitors increased. In support of the risky competition hypothesis, all measured behaviors had lower rates in a high versus low predation-risk population, likely due to the costs of predation. In terms of mating tactics, a male-biased ASR did not lead males to favor forced mating over courtship, indicating that males did not compensate for the cost of competition by switching to a less costly alternative mating tactic. Overall, this study highlights the need for field experiments using natural ranges of ASRs to test the validity of mating systems theory in a more complex, ecologically relevant context.

Too Many Males or Too Many Females? Classroom Sex Ratio, Life History Strategies and Risk-Taking Behaviors

Prior research finds that sex ratio, defined as the proportion of males and females in a given context, is related to engagement in risk-taking behaviors. However, most research operationalizes sex ratio at a local context (e.g., regional or county), which fails to reflect with precision the sex ratios contexts of individuals at a closer level. Furthermore, the relationship between sex ratio and risk-taking behaviors may be affected by individuals’ life history strategy, with previous studies showing fast life history strategies linked to risk-taking behaviors, compared to slow life history strategies. The present study analyzes the relationship between classroom sex ratio and risk-taking behaviors and the interaction between classroom sex ratio and life history strategy in adolescents. The sample comprised 1214 participants nested in 57 classrooms, 49.75% females, 91.5% Spanish and a mean age of 16.15 years (SD = 1.23, range 14–21). Results from multilevel modeling showed a negative relation between classroom sex ratio and risk-taking behaviors in female adolescents with faster life history strategy. By contrast, classroom sex ratio in male adolescents related positively to risk-taking behaviors but did not interact with life history strategy. These findings underscore the importance of studying proximate sex ratio on risk-taking behaviors in adolescents and underline its potential influence in the development and expression of life history strategies.

Intra- and Intersexual Mate Competition in Two Cultures : A Comparison of Women's Response to Mate Competition with Women and Gender-Nonbinary Males in Samoa and among the Istmo Zapotec

The present study examined women's mate competition tactics in response to female and feminine-male rivals in two cultures in which competition against both occurs. In Samoa and the Istmo Zapotec (Southern Mexico), women not only compete with other women (intrasexually) but also compete with rival feminine males (intersexually) in order to access/retain the same masculine men as sexual/romantic partners. Using a mixed-method paradigm, women were asked about their experiences of intra- and intersexual mate competition, and these narratives were recorded. The tactics reportedly employed by participants, and those attributed to mate competitors, were categorized according to established taxonomies of mate competition tactics, and their frequencies compared. Within-culture, the likelihood that participant women had ever experienced intra- and intersexual mate competition did not differ. Furthermore, participants reported a similar pattern of behavioral tactics whether their rival was another woman or a feminine male. These included benefit provisioning tactics during mate acquisition and cost-inflicting tactics during mate retention. Similarly, the mate competition tactics attributed to rival women and rival feminine males bore a striking resemblance, focused on enticing target men. Results highlight the mate competition tactics employed by women outside of a Euro-American context, and the way cultural factors impact mating landscapes presumed to be exclusively heterosexual. The presence of feminine males, alongside masculine men's willingness to engage in sexual activity with them, induces women in such cultures to compete intersexually in comparable ways to intrasexual competition with rival women.

Adult sex ratio and male body condition affect alternative reproductive tactics in a spider

Biases in adult sex ratios can alter the intensity of sexual selection by enhancing competition for mates. Under intense competition males increase their investment in behaviors to outcompete rivals (e.g., fighting). Yet, given that in male-biased environments mating opportunities are rare males may alternatively reduce costly courtship and/or adopt alternative reproductive tactics (ARTs). Males of the spider Pisaura mirabilis adopt different mating tactics, offering females genuine nuptial gifts (prey), nutritionally worthless gifts (prey leftovers), or no gifts. To test whether behavioral shifts between gift tactics are triggered by changes in the competitive environment, we established replicate spider populations under natural conditions at varying adult sex ratios (male-biased, female-biased and equal) and sampled gift tactics repeatedly over time. We additionally explored how male individual traits, such as body size and condition, affect the expression of ARTs. In male-biased populations males produced more gifts but of low quality, suggesting competition to trigger increased mating effort to ensure mate acquisition and fertilizations, but through a worthless gift tactic. Production of gifts and of genuine gifts was favored by high body condition, pointing to energetic limitations as being central for male reproductive capacity. We hence highlight two co-existing mechanisms at play to explain ARTs in this system, the competitive social environment where expression of gift tactics is based on optimal-decision making to overcome competition, and a conditional strategy linked to the individual’s energetic state.

Testing Women's Trust in Other Women and Same-Sex Attracted Males in Three Cultures

Heterosexual women trust mating-relevant advice received from gay men more than that received from heterosexual women. This trust is predicated on women's perception that gay men lack ulterior sexual motives and romantically pursue other gay men. However, this trust may not hold in all cultures. For example, in both Samoa and the Istmo Zapotec of Southern Mexico, women take part in mate competition against feminine same-sex attracted males-referred to as fa'afafine and muxe, respectively-who regularly engage in sexual activity with masculine men. The present studies sought to replicate and extend research on women's trust in males who are same-sex attracted. Experiments were conducted in Canada, Samoa, and the Istmo Zapotec, with women randomly assigned to consider the likelihood of various mate-poaching behaviors performed by either a rival woman or a same-sex attracted male. In Canada, women were more trusting of cisgender gay men than other women. Similarly, Samoan women were more trusting of fa'afafine than other women. In the Istmo Zapotec, women were equally distrustful of women and feminine muxe gunaa, whereas more masculine muxe nguiiu were rated as more trustworthy than women and muxe gunaa. These results illustrate that women's trust in same-sex attracted males varies both between and within cultural contexts, perhaps impacted by the relative femininity of the male in question.

Effects of operational sex ratio, mating age, and male mating history on mating and reproductive behavior in Grapholita molesta

The short-lived polygamous moth Grapholita molesta (Busck) is an important fruit pest worldwide. Trapping males by synthetic female sex pheromones is not an effective reproductive control strategy. It is important to improve this technology by understanding the mating system of G. molesta. This study investigated mating opportunities and fertile egg production by altering the operational sex ratio, mating age, and male mating history in repeated single mating and multiple mating in the two sexes. Our results showed that the mating and reproductive parameters of virgin males were affected by the number and age of virgin females. Males preferred a female number ≤three-fifths of the male number or ≤2-day-old females, while they discriminated against a female number ≥three times of the male number or ≥5-day-old females. On the other hand, the mating and reproductive parameters of virgin females were affected by repeated single mating and especially multiple mating under different male mating histories. Females preferred once-mated males and discriminated against virgin males. These results indicated that mating systems including more and older virgin females for virgin males and different virgin males for virgin females may be suitable for suppressing G. molesta populations. Hence, these results revealed that preventing mating of virgin adults by synthetic female sex pheromones should be most effective in controlling G. molesta.

Male mating success is related to body condition and stress-induced leukocyte response in an anuran with scramble competition

Competition among males can be a decisive factor for successful mating in anuran explosive breeders with a male-biased population sex ratio. The Yungas Red-belly Toad (Melanophryniscus rubriventris (Vellard, 1947)) is an explosive breeder where males exhibit a classic scramble competition. We studied the body size, body condition, and stress-induced leukocyte response of M. rubriventris males under natural scenarios of scramble competition. We hand-captured adult males (categorised as single, in pairs, or in mating balls) and determined their body mass and length, body condition, and their leukocyte profile. We found that males in pairs had significantly higher body mass than single males and those in mating balls. We found the highest percentage of individuals with better body condition within the paired males who showed significantly higher body condition index value than singles or those in mating balls. Amplectant males (those collected in pairs or in mating balls) had a significantly lower proportion of lymphocytes and higher neutrophil–lymphocyte ratios than singles. Amplectant males showed significantly higher proportions of neutrophils or eosinophils than singles. Our results suggest that M. rubriventris males with better body condition can be more successful in acquiring mates, although scramble competition among males can trigger a stress response.

Understanding resource driven female-female competition: ovary and liver size in sand gobies

The operational sex ratio (OSR, ready-to-mate males to females) is a key factor determining mating competition. A shortage of a resource essential for reproduction of one sex can affect OSR and lead to competition within the opposite sex for resource-holding mates. In the sand goby (Pomatoschistus minutus), a fish with paternal care, male readiness to mate depends on acquiring a nest-site, whereas food abundance primarily impacts female egg production. Comparing body condition and gonadal investment of fish from two populations with different availability in resources (Baltic Sea: few nest-sites, more food; North Sea: many nest-sites, less food), we predicted females carrying more mature eggs in the Baltic Sea than in the North Sea. As predicted, ovaries were larger in Baltic Sea females, and so was the liver (storage of energy reserves and vitellogenic compounds) for both sexes, but particularly for females. More females were judged (based on roundness scores) to be ready to spawn in the Baltic Sea. Together with a nest colonization experiment confirming a previously documented difference between the two areas in nest-site availability, these results indicate a more female-biased OSR in the Baltic Sea population, compared to the North Sea, and generates a prediction that female-female competition for mating opportunities is stronger in the Baltic population. To our knowledge, this is the first time that female reproductive investment is discussed in relation to OSR using field data.

Male black widows parasitize mate-searching effort of rivals to find females faster

Mate-searching success is a critical precursor to mating, but there is a dearth of research on traits and tactics that confer a competitive advantage in finding potential mates. Theory and available empirical evidence suggest that males locate mates using mate-attraction signals produced by receptive females (personal information) and avoid inadvertently produced cues from rival males (social information) that indicate a female has probably already mated. Here, we show that western black widow males use both kinds of information to find females efficiently, parasitizing the searching effort of rivals in a way that guarantees competition over mating after reaching a female's web. This tactic may be adaptive because female receptivity is transient, and we show that (i) mate searching is risky (88% mortality) and (ii) a strongly male-biased operational sex ratio (from 1.2 : 1 to more than 10 : 1) makes competition inevitable. Males with access to rivals' silk trails moved at higher speeds than those with only personal information, and located females even when personal information was unreliable or absent. We show that following rivals can increase the potential for sexual selection on females as well as males and argue it may be more widespread in nature than is currently realized.

Competition for food in 2 populations of a wild-caught fish

Aggressive behavior when competing for resources is expected to increase as the ratio of competitors-to-resource ratio (CRR) units increases. Females are expected to be more aggressive than males when competing for food when body size is more strongly related to reproductive success in females than in males, whereas aggression is predicted to decrease under high ambient predation risk by natural selection. Under the risk allocation model, however, individuals under high ambient predation risk are expected to be more aggressive, and forage more in the absence of imminent risk than their low risk counterparts. An interaction between adult sex ratio (i.e., adult males/females), ambient predation risk (high vs. low), and sex on intrasexual competition for mates in Trinidadian guppies Poecilia reticulata has been shown. The interaction suggested an increase in aggression rates as CRR increased, except for males from the high predation population. To compare the patterns of competition for food versus mates, we replicated this study by using food patches. We allowed 4 male or 4 female guppies from high and low predation populations to compete for 5, 3, or 1 food patches. The foraging rate was higher in a high rather than low ambient predation risk population. Surprisingly, CRR, sex, and population of origin had no effect on aggression rates. Despite other environmental differences between the 2 populations, the effect of ambient predation risk may be a likely explanation for differences in foraging rates. These results highlight the importance for individuals to secure food despite the cost of competition and predation.

Noise can affect acoustic communication and subsequent spawning success in fish

There are substantial concerns that increasing levels of anthropogenic noise in the oceans may impact aquatic animals. Noise can affect animals physically, physiologically and behaviourally, but one of the most obvious effects is interference with acoustic communication. Acoustic communication often plays a crucial role in reproductive interactions and over 800 species of fish have been found to communicate acoustically. There is very little data on whether noise affects reproduction in aquatic animals, and none in relation to acoustic communication. In this study we tested the effect of continuous noise on courtship behaviour in two closely-related marine fishes: the two-spotted goby (Gobiusculus flavescens) and the painted goby (Pomatoschistus pictus) in aquarium experiments. Both species use visual and acoustic signals during courtship. In the two-spotted goby we used a repeated-measures design testing the same individuals in the noise and the control treatment, in alternating order. For the painted goby we allowed females to spawn, precluding a repeated-measures design, but permitting a test of the effect of noise on female spawning decisions. Males of both species reduced acoustic courtship, but only painted gobies also showed less visual courtship in the noise treatment compared to the control. Female painted gobies were less likely to spawn in the noise treatment. Thus, our results provide experimental evidence for negative effects of noise on acoustic communication and spawning success. Spawning is a crucial component of reproduction. Therefore, even though laboratory results should not be extrapolated directly to field populations, our results suggest that reproductive success may be sensitive to noise pollution, potentially reducing fitness.

Sex roles and sexual selection: lessons from a dynamic model system

Our understanding of sexual selection has greatly improved during the last decades. The focus is no longer solely on males, but also on how female competition and male mate choice shape ornamentation and other sexually selected traits in females. At the same time, the focus has shifted from documenting sexual selection to exploring variation and spatiotemporal dynamics of sexual selection, and their evolutionary consequences. Here, I review insights from a model system with exceptionally dynamic sexual selection, the two-spotted goby fish Gobiusculus flavescens. The species displays a complete reversal of sex roles over a 3-month breeding season. The reversal is driven by a dramatic change in the operational sex ratio, which is heavily male-biased at the start of the season and heavily female-biased late in the season. Early in the season, breeding-ready males outnumber mature females, causing males to be highly competitive, and leading to sexual selection on males. Late in the season, mating-ready females are in excess, engage more in courtship and aggression than males, and rarely reject mating opportunities. With typically many females simultaneously courting available males late in the season, males become selective and prefer more colorful females. This variable sexual selection regime likely explains why both male and female G. flavescens have ornamental colors. The G. flavescens model system reveals that sexual behavior and sexual selection can be astonishingly dynamic in response to short-term fluctuations in mating competition. Future work should explore whether sexual selection is equally dynamic on a spatial scale, and related spatiotemporal dynamics.

Impact of male alternative reproductive tactics on female costs of sexual conflict under variation in operational sex ratio and population density

Sexual conflict over mating rate is both pervasive and evolutionarily costly. For females, the lifetime reproductive fitness costs that arise through interactions with potential mates will be influenced by the frequency of such interactions, and the fitness cost of each interaction. Both of these factors are likely to be influenced by variation in operational sex ratio (OSR) and population density. Variation in OSR- and density-dependent male alternative reproductive tactics (ARTs) may be particularly important if the fitness costs that females experience vary with the reproductive tactics that males express. Using a simple model, we consider several examples of OSR- and/or density-dependent variation in male ARTs and the frequency of male-female interactions, and find that variation in the expression of male ARTs has the potential to augment or diminish the costs of frequent male interactions for females. Accurately documenting variation in the expression of male ARTs and associated female fitness costs will benefit future work in this area.

Temporal variation in the operational sex ratio and male mating behaviours in reindeer (Rangifer tarandus)

In polygynous species, sexual selection is mostly driven by male ability to monopolize access to females in oestrous. In ungulates, the operational sex ratio (OSR), i.e. the proportion of males to individuals ready to mate, varies throughout the peak rut, resulting from the temporal variation in the number of females in oestrous. But the way males adjust their mating tactics to maximise their access to females in oestrous (i.e. as OSR varies) is yet to be investigated. Using 15 years of behavioural observations in reindeer (Rangifer tarandus), we compared the relative importance of time within the rutting season (days to the peak-rut) and the OSR to explain the variation in the propensity (i.e. the frequency after controlling for the potential number of encounters) of young and adult dominant males to engage in four mating tactics: herding females, chasing other males, investigating female reproductive status, and courting females. Male-male agonistic behaviour was the most frequent mating behaviour, followed by herding. As predicted, dominant male mating tactics changed over the rutting season: first herding, then chasing other males, and finally investigating and courting females. In contrast to our prediction, we did not find support for the OSR theory. We noted some discrepancies in how young and adult dominant males adjusted their tactics during the mating season, adults being more efficient in timing and in performing their behaviour to maximise access to females in oestrous. The reported sequence of mating tactics may be more efficient than a static mating tactic to monopolize females in oestrous, regardless of the population composition.

Why (and When) Straight Women Trust Gay Men: Ulterior Mating Motives and Female Competition

Previous findings indicate that heterosexual women experience a greater sense of comfort and trust in their friendships with gay men than in their friendships with heterosexual individuals. In the present studies, we tested a hypothesis that not only explains why women exhibit increased trust in gay men but also yields novel predictions about when (i.e., in what contexts) this phenomenon is likely to occur. Specifically, we propose that gay men's lack of motives to mate with women or to compete with them for mates enhances women's trust in gay men and openness to befriend them. Study 1 demonstrated that women placed greater trust in a gay man's mating-but not non-mating (e.g., career) advice-than in the same advice given by heterosexual individuals. Study 2 showed that women perceived a gay man to be more sincere in scenarios relevant to sexual and competitive mating deception. In Study 3, exposing women to a visualization of increased mating competition enhanced their trust in gay men; when mating competition was salient, women's trust in mating information from a gay man was amplified. Study 4 showed that women who perceived higher levels of mating competition were more open to befriending gay men. Together, these converging findings support our central hypothesis, which not only provides a distal explanation for the trust that straight women place in gay men, but also provides novel insights into previously unidentified contexts that facilitate the formation and strengthening of this unique bond.

Sensory input attenuation allows predictive sexual response in yeast

Animals are known to adjust their sexual behaviour depending on mate competition. Here we report similar regulation for mating behaviour in a sexual unicellular eukaryote, the budding yeast Saccharomyces cerevisiae. We demonstrate that pheromone-based communication between the two mating types, coupled to input attenuation by recipient cells, enables yeast to robustly monitor relative mate abundance (sex ratio) within a mixed population and to adjust their commitment to sexual reproduction in proportion to their estimated chances of successful mating. The mechanism of sex-ratio sensing relies on the diffusible peptidase Bar1, which is known to degrade the pheromone signal produced by mating partners. We further show that such a response to sexual competition within a population can optimize the fitness trade-off between the costs and benefits of mating response induction. Our study thus provides an adaptive explanation for the known molecular mechanism of pheromone degradation in yeast.

Cells of the yeast Saccharomyces cerevisiae can mate with other cells of opposite mating type. Here, the authors show that the combination of a pheromone and a pheromone-degrading enzyme allows yeast cells to monitor relative mate abundance within a population and adjust their commitment to sexual reproduction.

The effect of size and sex ratio experiences on reproductive competition in Nicrophorus vespilloides burying beetles in the wild

Male parents face a choice: should they invest more in caring for offspring or in attempting to mate with other females? The most profitable course depends on the intensity of competition for mates, which is likely to vary with the population sex ratio. However, the balance of pay‐offs may vary among individual males depending on their competitive prowess or attractiveness. We tested the prediction that sex ratio and size of the resource holding male provide cues regarding the level of mating competition prior to breeding and therefore influence the duration of a male's biparental caring in association with a female. Male burying beetles, Nicrophorus vespilloides were reared, post‐eclosion, in groups that differed in sex ratio. Experimental males were subsequently translocated to the wild, provided with a breeding resource (carcass) and filmed. We found no evidence that sex ratio cues prior to breeding affected future parental care behaviour but males that experienced male‐biased sex ratios took longer to attract wild mating partners. Smaller males attracted a higher proportion of females than did larger males, securing significantly more monogamous breeding associations as a result. Smaller males thus avoided competitive male–male encounters more often than larger males. This has potential benefits for their female partners who avoid both intrasexual competition and direct costs of higher mating frequency associated with competing males.

Foraging competition in larger groups overrides harassment avoidance benefits in female reindeer (Rangifer tarandus)

Male harassment toward females during the breeding season may have a negative effect on their reproductive success by disturbing their foraging activity, thereby inducing somatic costs. Accordingly, it is predicted that females will choose mates based on their ability to provide protection or will aggregate into large groups to dilute per capita harassment level. Conversely, increasing group size may also lead to a decrease in foraging activity by increasing foraging competition, but this effect has rarely been considered in mating tactic studies. This study examined the importance of two non-exclusive hypotheses in explaining the variations of the female activity budget during the breeding season: the male harassment hypothesis, and the female foraging competition hypothesis. We used focal observations of female activity from known mating groups collected during the breeding season from a long-term (15 years) study on reindeer Rangifer tarandus. We found that females were more disturbed (i.e., spent less time feeding) in the presence of young dominant males, and marginally disturbed in the presence of satellite males, which supports the male harassment hypothesis. We also found that female disturbance level increased with group size, being independent of the adult sex ratio. Consequently, these results rejected the dilution effect, but strongly supported the foraging competition hypothesis. This study therefore highlights a potential conflict in female behaviour. Indeed, any gains from harassment protection were negated by an increase of 6-7 females, since adult males lead larger groups than young males.

The extent and consequences of p-hacking in science

A focus on novel, confirmatory, and statistically significant results leads to substantial bias in the scientific literature. One type of bias, known as "p-hacking," occurs when researchers collect or select data or statistical analyses until nonsignificant results become significant. Here, we use text-mining to demonstrate that p-hacking is widespread throughout science. We then illustrate how one can test for p-hacking when performing a meta-analysis and show that, while p-hacking is probably common, its effect seems to be weak relative to the real effect sizes being measured. This result suggests that p-hacking probably does not drastically alter scientific consensuses drawn from meta-analyses.

How populations differentiate despite gene flow: sexual and natural selection drive phenotypic divergence within a land fish, the Pacific leaping blenny

Background

Divergence between populations in reproductively important features is often vital for speciation. Many studies attempt to identify the cause of population differentiation in phenotype through the study of a specific selection pressure. Holistic studies that consider the interaction of several contrasting forms of selection are more rare. Most studies also fail to consider the history of connectivity among populations and the potential for genetic drift or gene flow to facilitate or limit phenotypic divergence. We examined the interacting effects of natural selection, sexual selection and the history of connectivity on phenotypic differentiation among five populations of the Pacific leaping blenny (Alticus arnoldorum), a land fish endemic to the island of Guam.

Results

We found key differences among populations in two male ornaments—the size of a prominent head crest and conspicuousness of a coloured dorsal fin—that reflected a trade-off between the intensity of sexual selection (male biased sex ratios) and natural selection (exposure to predators). This differentiation in ornamentation has occurred despite evidence suggesting extensive gene flow among populations, which implies that the change in ornament expression has been recent (and potentially plastic).

Conclusions

Our study provides an early snapshot of divergence in reproductively important features that, regardless of whether it reflects genetic or plastic changes in phenotype, could ultimately form a reproductive barrier among populations.

Breeding sex ratios in adult leatherback turtles (Dermochelys coriacea) may compensate for female-biased hatchling sex ratios

For vertebrates with temperature-dependent sex determination, primary (or hatchling) sex ratios are often skewed, an issue of particular relevance to concerns over effects of climate change on populations. However, the ratio of breeding males to females, or the operational sex ratio (OSR), is important to understand because it has consequences for population demographics and determines the capacity of a species to persist. The OSR also affects mating behaviors and mate choice, depending on the more abundant sex. For sea turtles, hatchling and juvenile sex ratios are generally female-biased, and with warming nesting beach temperatures, there is concern that populations may become feminized. Our purpose was to evaluate the breeding sex ratio for leatherback turtles at a nesting beach in St. Croix, USVI. In 2010, we sampled nesting females and later sampled their hatchlings as they emerged from nests. Total genomic DNA was extracted and all individuals were genotyped using 6 polymorphic microsatellite markers. We genotyped 662 hatchlings from 58 females, matching 55 females conclusively to their nests. Of the 55, 42 females mated with one male each, 9 mated with 2 males each and 4 mated with at least 3 males each, for a multiple paternity rate of 23.6%. Using GERUD1.0, we reconstructed parental genotypes, identifying 47 different males and 46 females for an estimated breeding sex ratio of 1.02 males for every female. Thus we demonstrate that there are as many actively breeding males as females in this population. Concerns about female-biased adult sex ratios may be premature, and mate choice or competition may play more of a role in sea turtle reproduction than previously thought. We recommend monitoring breeding sex ratios in the future to allow the integration of this demographic parameter in population models.

Influences of mating group composition on the behavioral time-budget of male and female Alpine Ibex (Capra ibex) during the rut

During the rut, polygynous ungulates gather in mixed groups of individuals of different sex and age. Group social composition, which may vary on a daily basis, is likely to have strong influences on individual's time-budget, with emerging properties at the group-level. To date, few studies have considered the influence of group composition on male and female behavioral time budget in mating groups. Focusing on a wild population of Alpine ibex, we investigated the influence of group composition (adult sex ratio, the proportion of dominant to subordinate males, and group size) on three behavioral axes obtained by Principal Components Analysis, describing male and female group time-budget. For both sexes, the first behavioral axis discerned a trade-off between grazing and standing/vigilance behavior. In females, group vigilance behavior increased with increasingly male-biased sex ratio, whereas in males, the effect of adult sex ratio on standing/vigilance behavior depended on the relative proportion of dominant males in the mating group. The second axis characterized courtship and male-male agonistic behavior in males, and moving and male-directed agonistic behavior in females. Mating group composition did not substantially influence this axis in males. However, moving and male-directed agonistic behavior increased at highly biased sex ratios (quadratic effect) in females. Finally, the third axis highlighted a trade-off between moving and lying behavior in males, and distinguished moving and female-female agonistic behavior from lying behavior in females. For males, those behaviors were influenced by a complex interaction between group size and adult sex ratio, whereas in females, moving and female-female agonistic behaviors increased in a quadratic fashion at highly biased sex ratios, and also increased with increasing group size. Our results reveal complex behavioral trade-offs depending on group composition in the Alpine ibex, and emphasize the importance of social factors in influencing behavioral time-budgets of wild ungulates during the rut.