Male-biased Adult Sex Ratio Research Articles

Large and interactive pipefish females display ornaments for longer with many males around

Why do animals display sexual ornaments – to attract mates, to compete for access to them, or both? In the broad-nosed pipefish (Syngnathus typhle), ornamented females commonly compete for access to males, whereas choosy males provide uniparental care. During courtship, females show a dynamic ornament, consisting of a row of dark B-shaped signs along the sides of their trunk. Here, we focus on the social context of female ornamentation and mating success (egg transfer) under experimentally female and male biased adult sex ratios. We found that the duration of female B-sign displays was not explained by adult sex ratios alone, but rather by sex ratios in combination with female body length and the number of female interactions. In detail, duration of B-sign display increased more steeply with the number of female-female interactions under the male biased adult sex ratio: that is, when more mating opportunities were available. Similarly, display duration increased with female body length, but only under the male biased adult sex ratio. In addition, successful egg transfer occurred more often in the male biased treatment and correlated significantly with female display duration in the same treatment. Taken together, these results suggest that in the context of the broad-nosed pipefish, the ornamental B-signs in females primarily serve as a sexually selected attractive signal. Our results highlight the importance of an interactive mating environment, male availability, and female size.

Eviction-driven infanticide and sexually selected adoption and infanticide in a neotropical parrot

Infanticide and adoption have been attributed to sexual selection, where an individual later reproduces with the parent whose offspring it killed or adopted. While sexually selected infanticide is well known, evidence for sexually selected adoption is anecdotal. We report on both behaviors at 346 nests over 27 y in green-rumped parrotlets (Forpus passerinus) in Venezuela. Parrotlets are monogamous with long-term pair bonds, exhibit a strongly male-biased adult sex ratio, and nest in cavities that are in short supply, creating intense competition for nest sites and mates. Infanticide attacks occurred at 256 nests in two distinct contexts: 1) Attacks were primarily committed by nonbreeding pairs (69%) attempting to evict parents from the cavity. Infanticide attacks per nest were positively correlated with population size and evicting pairs never adopted abandoned offspring. Competition for limited nest sites was a primary cause of eviction-driven infanticide, and 2) attacks occurred less frequently at nests where one mate died (31%), was perpetrated primarily by stepparents of both sexes, and was independent of population size. Thus, within a single species and mating system, infanticide occurred in multiple contexts due to multiple drivers. Nevertheless, 48% of stepparents of both sexes adopted offspring, and another 23% of stepfathers exhibited both infanticide and long-term care. Stepfathers were often young males who subsequently nested with widows, reaching earlier ages of first breeding than competitors and demonstrating sexually selected adoption. Adoption and infanticide conferred similar fitness benefits to stepfathers and appeared to be equivalent strategies driven by limited breeding opportunities, male-biased sex ratios, and long-term monogamy.

Mechanisms that can cause population decline under heavily skewed male-biased adult sex ratios.

While adult sex ratio (ASR) is a crucial component for population management, there is still a limited understanding of how its fluctuation affects population dynamics. To demonstrate mechanisms that hinder population growth under a biased ASR, we examined changes in reproductive success with ASR using a decapod crustacean exposed to female-selective harvesting. We examined the effect of ASR on the spawning success of females. A laboratory experiment showed that the number of eggs carried by females decreased as the proportion of males in the mating groups increased. Although the same result was not observed in data collected over 25 years in the wild, the negative effect of ASR was suggested when success in carrying eggs was considered as a spawning success. These results indicate that a surplus of males results in females failing to carry eggs, probably due to sexual coercion, and the negative effect of ASR can be detected at the population level only when the bias increases because failure in spawning success occurs in part of population. We experimentally examined how male-biased sex ratios affected the maintenance of genetic diversity in a population. The diversity of paternity in a clutch increased with the number of candidate fathers. However, over 50% of a clutch was fertilised by a single male regardless of the sex ratio, and the degree of diversity was less than half of the highest diversity expected in each mating group. We also experimentally examined the mating ability of males during the breeding season. The experiment showed that multiple mating by males could not compensate for the risk that their genotypes would be lost when multiple males competed for one female. These results suggest that a male-biased ASR could trigger a decline of genetic diversity in a population. We show that ASR skewed by female-selective harvesting decreases reproductive success not only of males that have few mating opportunities but also of females. We discuss that we may still underestimate the significance of ASR on population persistence due to the difficulty of revealing the effect of ASR.

Use of fermented spent coffee grounds as a substrate supplement for rearing black soldier fly larvae, Hermetia illucens (L), (Diptera: Stratiomyidae).

Spent coffee grounds (SCG), an increasingly abundant waste product with environmental disposal problems, has been used as a dietary supplement for many animals and have the potential to be used as a dietary supplement for black soldier fly (BSF) larvae; however, its effective use is still under scrutiny. To date, no studies have considered the use of SCG after microbial fermentation (fSCG) and its effects on BSF life history. A mixture of fruit and vegetable pulp residue supplemented with one of six different fSCG percentages (0%, 20%, 40%, 60%, 80%, and 100% by weight) were provided as a diet substrate in order to evaluate the effect of the fSCG quantity on BSF growth, yield, and conversion ability. The addition of fSCG to the pulp diet prolonged larval development times, while 100% fSCG affected the larval survival rate and resulted in a male-biased adult sex ratio. The 20-40% fSCG and 40-60% fSCG treatments supported the largest prepupal and mature larval sizes, respectively. The highest waste reduction efficiency and feed conversion rate by BSF larvae was found with 20% fSCG, similar to the control (0% fSCG). From the short rearing time, high yield, and high bioconversion efficiency, a 20% fSCG supplementation of the mixed pulp was recommended for rearing BSF larvae. These data are valuable for coffee by-product waste management in urban areas.

Saguinus mystax(Primates: Callitrichidae)

AbstractSaguinus mystax (Spix, 1823), a callitrichid (tamarin and marmoset family) commonly called the mustached tamarin, is 1 of 12 species in the genus Saguinus. In this large callitrichid (450–650 g), females are slightly heavier than males. It occurs in Brazil and Peru, south of Rio Amazonas from Río Ucayali in the west to Rio Purus in the east, in primary and secondary tierra firma rainforests. The diet consists of fruit pulp, insects, gums, nectar, and small vertebrates. It lives in groups of 3–10 individuals with a male-biased adult sex ratio and has a cooperative breeding system. In areas of sympatry, S. mystax forms mixed-species groups with saddle-back tamarins, Leontocebus. Although captured for the pet trade and sporadically hunted, it is considered “Least Concern” (LC) by the International Union for Conservation of Nature.

Sex-specific aging in bite force in a wild vertebrate.

The manifold differences between the sexes provide wide scope for sex differences in senescence. However, detecting physiological decline in old age and evaluating any sex difference in aging in a wild population can be challenging. This applies especially to long-lived species that require marking around birth in order to recognize elderly individuals, perhaps decades later. Here, we used bite force of known-age, long-lived Nazca boobies (Sula granti, a seabird) as a functional measure of muscle strength; surprisingly, only a single study has evaluated the possibility of senescent decline in muscle strength in a wild vertebrate. The male-biased adult sex ratio of this population constrains breeding opportunities across the lifespan for males, so we predicted that slower accumulation of reproductive costs would delay senescent decline in bite force in males compared to females, matching observed patterns in some non-muscle traits in this species. Data were collected from 349 adults using a force transducer at the start of the breeding season in November 2017 on Isla Española, Galápagos. Both sexes achieved less bite force in late life. The decline began at a later age in males, providing evidence of sex-specific schedules of decline in muscle function in a wild vertebrate.

Can an introduced predator select for adaptive sex allocation?

Most species produce equal numbers of sons and daughters, and sex differences in survival after parental care do not usually affect this pattern. Temporary overproduction of the scarcer sex can be adaptive when generations overlap, the sexes differ in life-history expectations, and parents can anticipate future mating opportunities. However, an alternative strategy of maximizing the competitiveness of the more abundant sex in these circumstances remains unexplored. We develop theory showing how mothers can maximize reproductive value when future mate competition will be high by producing more sons in the advantageous early hatching positions within their broods. Our model for optimal birth order was supported by long-term data of offspring sex in a parrot facing catastrophic female mortality caused by introduced predators. Swift parrots (Lathamus discolor) suffer high female mortality due to introduced sugar gliders (Petaurus breviceps) creating fluctuating male-biased adult sex ratios. Offspring hatched early within broods fledged in better condition, and in support of our model were more likely to be male in years with higher adult female mortality. We found a highly significant rank-order correlation between observed and predicted birth sex ratios. Our study shows the potential for mothers to maximize reproductive value via strategic biases in offspring sex depending on the advantages conferred by birth order and the predictability of future mate competition. Our long-term data support the predictions and appear to suggest that sex allocation strategies may evolve surprisingly quickly when anthropogenic pressures on populations are severe.

Regal Fritillary (Speyeria idalia) Sex Ratio in Tallgrass Prairie: Effects of Survey Timing and Management Regime

The regal fritillary, Speyeria idalia (Drury), was once a common inhabitant of North American grassland communities. Regal fritillary populations are commonly reported to have a male biased adult sex ratio (ASR) throughout their range. We assessed the observed ASR of regal fritillary throughout an annual flight period, investigated how the overall density of both sexes changed, and tested effects of prescribed fire, grazing and haying management treatments on male and female density. We found that regal fritillary exhibited an observed 2:1 male biased ASR across the entire emergence period. Our analysis also revealed that male density peaked earlier than female density in the flight period. Point estimates of density indicated sites that received prescribed burning at the moderate fire-return interval supported ≥1.3 times greater density of males and ≥5.6 times greater density of females versus sites burned with short and long fire-return intervals. Additionally, this effect was enhanced when combined with grazing which showed males were ≥1.9 times and females had ≥1.2 times greater point estimates of density in sites that were grazed and burned at a moderate fire-return interval versus other sites. The relatively stable status of regal fritillary within our study region suggests that a 2:1 male to female ASR may be considered the model composition of populations throughout their range. Likewise, the dynamic nature of the ASR throughout the flight period highlights the importance of conducting surveys across the flight period. Finally, these results corroborate an increasing number of research results that reveal common prairie management practices, such as prescribed fire can be applied within sites that contain regal fritillary and continue to support stable populations.

Causes and consequences of an unusually male-biased adult sex ratio in an unmanaged feral horse population.

The adult sex ratio (ASR) is important within ecology due to its predicted effects on behaviour, demography and evolution, but research examining the causes and consequences of ASR bias have lagged behind the studies of sex ratios at earlier life stages. Although ungulate ASR is relatively well-studied, exceptions to the usual female-biased ASR challenge our understanding of the underlying drivers of biased ASR and provide an opportunity to better understand its consequences. Some feral ungulate populations, including multiple horse populations, exhibit unusually male-biased ASR. For example, research suggests that the feral horse Equus ferus caballus population on Sable Island, Nova Scotia, Canada may exhibit a male-biased ASR. Such exceptions to the rule provide a valuable opportunity to reveal the contributions of environmental context and trait differences to ASR bias. We aimed to test for bias in Sable Island horse ASR, identify the demographic drivers of bias, and explore its demographic and social consequences. To do this, we used life history, movement and group membership information for hundreds of horses followed through a long-term individual-based study between 2007 and 2018. Sable Island horse ASR is male biased and this skew has increased over time, reaching 62% male in 2018. Our life table response experiment suggested that ASR skew was driven predominantly by male-biased adult survival. Further analyses pointed to sex-biased survival being driven by reduced female survival post-reproduction. Male-biased ASR was associated with reduced harem sizes, an increase in the number of social groups on the island, and reduced reproduction in young females. Our results support the idea that male-biased ASR in feral ungulate populations may be caused by a combination of high population density and high reproductive output. We suggest that female-biased mortality may be caused by females continuing to reproduce at high density, and thus being more susceptible to resource shortages. Thus, our results highlight the strong context dependence of ASR. Furthermore, our work indicates the potential for ASR to substantially alter a population's social organisation. Such changes in social structure could have knock-on consequences for demography by altering the formation/stability of social relationships, or competition for matings.

Male-biased adult sex ratio in non-breeding areas of an extreme long-distance migratory shorebird population

Migratory species often spend the non-breeding season in areas that differ in both distance from the breeding grounds and habitat quality, which can cause a segregation of the population. In sexually dimorphic species, differential energetic requirements and foraging strategies of each sex can lead to the use of different non-breeding grounds. However, because migratory birds use very distant and often hard to access habitats, it is often difficult to systematically assess a population's adult sex ratio (ASR) over time and across sites to get insights into sexual segregation. Here we study a long-distance migratory shorebird, the Hudsonian Godwit Limosa haemastica, to explore the ASR in two separated bays at Chiloé Archipelago (41°S, Chile) and evaluate potential segregation of the sexes within this population during the non-breeding season. Using a molecular approach, we developed a standardised criterion to accurately sex the individuals based on their bill length. With this criterion we were able to sex 92% of the sample (n = 265), with the remaining corresponding to birds in the overlapping range of bill lengths that needs to be sexed molecularly. We found that this godwit population, which breeds in Alaska, does not present a large-scale sexual segregation between non-breeding sites but is overall male-biased (ASR = 0.59). Interestingly however, there were significant differences in ASR between bays that were maintained throughout three consecutive non-breeding seasons. Further studies are needed to estimate potential consequences of these local differences in ASR on the population dynamics of this interhemispheric migratory species.

Swift declines predicted following mating system changes driven by an introduced predator.

In Focus: Heinsohn, R., Olah, G., Webb, M., Peakall, R., & Stojanovic, D. (2019). Sex ratio bias and shared paternity reduce individual fitness and population viability in a critically endangered parrot. Journal of Animal Ecology, 88, 502-510. While the effects of variation in the sex ratio of offspring have been thoroughly explored over the last century, the sex ratio of adults has received far less attention. A paper by Heinsohn and colleagues in this issue shows that changes in the adult sex ratio can have striking effects on mating systems, reproductive success and population viability. These impacts are all the more dramatic because they occur in a critically endangered species, the swift parrot, Lathamus discolor, of Tasmania. This species suffers heavy predation from the introduced sugar glider, Petaurus breviceps, which kills nesting females and their clutches, resulting in strongly male-biased adult sex ratios. The authors combined demographic and genetic data to show that, at sites with heavier predation, the remaining females were more likely to mate with multiple males. This shift in the mating system also led to lower overall nesting success, with fewer chicks fledged per nest at sites with higher levels of mixed paternity. Population viability models based on these data predicted steep population declines, with models using the highest observed rates of mixed paternity showing the sharpest declines. These results demonstrate that changes in the adult sex ratio can have far-reaching impacts, including on the fitness of populations themselves.

Natal habitat and sex-specific survival rates result in a male-biased adult sex ratio.

The adult sex ratio (ASR) is a crucial component of the ecological and evolutionary forces shaping the dynamics of a population. Although in many declining populations ASRs have been reported to be skewed, empirical studies exploring the demographic factors shaping ASRs are still rare. In this study of the socially monogamous and sexually dimorphic Black-tailed Godwit (Limosa limosa limosa), we aim to evaluate the sex ratio of chicks at hatch and the subsequent sex-specific survival differences occurring over 3 subsequent life stages. We found that, at hatch, the sex ratio did not deviate from parity. However, the survival of pre-fledged females was 15–30% lower than that of males and the sex bias in survival was higher in low-quality habitat. Additionally, survival of adult females was almost 5% lower than that of adult males. Because survival rates of males and females did not differ during other life-history stages, the ASR in the population was biased toward males. Because females are larger than males, food limitations during development or sex-specific differences in the duration of development may explain the lower survival of female chicks. Differences among adults are less obvious and suggest previously unknown sex-related selection pressures. Irrespective of the underlying causes, by reducing the available number of females in this socially monogamous species, a male-biased ASR is likely to contribute to the ongoing decline of the Dutch godwit population.

Elevated potential for intraspecific competition in territorial carnivores occupying fragmented landscapes

The distribution of mammals is determined by a suite of endogenous and exogenous factors. In territorial, polygynous species like tigers (Panthera tigris), males often center their space-use around female territories, repelling competitors from these areas. Competition among males for females leads to increased mortality of both sexes and infanticide of unrelated cubs, which can lead to population declines. We hypothesized that increased territorial overlap among adult male tigers and elevated levels of inter and intra-sex competition would be manifest in populations with male-biased adult sex ratios (ASR). We also assessed whether inter-sex variation in adult survival or degree of habitat connectivity resulted in skewed ASR. We evaluated these hypotheses using camera trap data from three tiger populations occupying habitat patches with varying levels of connectivity and ASRs. Data were analyzed using multi-state occupancy models, where states were defined as habitat use by one or more male tigers in sites with and without female use. As predicted, in populations with male-biased or even ASR we found evidence for increased spatial overlap between male tigers, particularly pronounced in areas adjacent to female territories. Given parity in adult survival, habitat fragmentation likely caused male-biased ASR. Our results suggest that the persistence of small tiger populations in habitat patches with male-biased ASR may be significantly compromised by behavior-mediated endogenous demographic processes that are often overlooked. In habitat fragments with pronounced male biased ASR, population recovery of territorial carnivores may require timely supplementation of individuals to compensate for population losses from intraspecific competition.

Increased male bias in eider ducks can be explained by sex-specific survival of prime-age breeders.

In contrast to theoretical predictions of even adult sex ratios, males are dominating in many bird populations. Such bias among adults may be critical to population growth and viability. Nevertheless, demographic mechanisms for biased adult sex ratios are still poorly understood. Here, we examined potential demographic mechanisms for the recent dramatic shift from a slight female bias among adult eider ducks (Somateria mollissima) to a male bias (about 65% males) in the Baltic Sea, where the species is currently declining. We analysed a nine-year dataset on offspring sex ratio at hatching based on molecularly sexed ducklings of individually known mothers. Moreover, using demographic data from long-term individual-based capture-recapture records, we investigated how sex-specific survival at different ages after fledgling can modify the adult sex ratio. More specifically, we constructed a stochastic two-sex matrix population model and simulated scenarios of different survival probabilities for males and females. We found that sex ratio at hatching was slightly female-biased (52.8%) and therefore unlikely to explain the observed male bias among adult birds. Our stochastic simulations with higher survival for males than for females revealed that despite a slight female bias at hatching, study populations shifted to a male-biased adult sex ratio (> 60% males) in a few decades. This shift was driven by prime reproductive-age individuals (≥5-year-old), with sex-specific survival of younger age classes playing a minor role. Hence, different age classes contributed disproportionally to population dynamics. We argue that an alternative explanation for the observed male dominance among adults–sex-biased dispersal–can be considered redundant and is unlikely, given the ecology of the species. The present study highlights the importance of considering population structure and age-specific vital rates when assessing population dynamics and management targets.

Adult sex ratio and social status predict mating and parenting strategies in Northern Ireland.

Evidence from animal species indicates that a male-biased adult sex ratio (ASR) can lead to higher levels of male parental investment and that there is heterogeneity in behavioural responses to mate scarcity depending on mate value. In humans, however, there is little consistent evidence of the effect of the ASR on pair-bond stability and parental investment and even less of how it varies by an individual's mate value. In this paper we use detailed census data from Northern Ireland to test the association between the ASR and pair-bond stability and parental investment by social status (education and social class) as a proxy for mate value. We find evidence that female, but not male, cohabitation is associated with the ASR. In female-biased areas women with low education are less likely to be in a stable pair-bond than highly educated women, but in male-biased areas women with the lowest education are as likely to be in a stable pair-bond as their most highly educated peers. For both sexes risk of separation is greater at female-biased sex ratios. Lastly, our data show a weak relationship between parental investment and the ASR that depends on social class. We discuss these results in the light of recent reformulations of parental investment theory.This article is part of the themed issue 'Adult sex ratios and reproductive decisions: a critical re-examination of sex differences in human and animal societies'.

High sex ratios in rural China: declining well-being with age in never-married men.

In parts of rural China male-biased sex ratios at birth, combined with out-migration of women, have led to highly male-biased adult sex ratios, resulting in large numbers of men being unable to marry, in a culture where marriage and reproduction are an expectation. The aim of this study was to test the hypotheses that older unmarried men are more predisposed to depression, low self-esteem and aggression than both those who are married, and those who are younger and unmarried. Self-completion questionnaires were administered among men aged 20–40 in 48 villages in rural Guizhou province, southwestern China. Tools used included the Beck Depression Inventory, the Rosenberg's Self-esteem Scale and the Bryant-Smith Aggression Questionnaire. Regression models assessed psychological wellbeing while adjusting for socio-demographic variables. Completed questionnaires were obtained from 957 never-married men, 535 married men aged 30–40, 394 partnered men and 382 unpartnered men aged 20–29. After adjusting for socio-demographic variables, never-married men were more predisposed to depression (p < 0.05), aggression (p < 0.01), low self-esteem (p < 0.05) and suicidal tendencies (p < 0.001). All the psychological measures deteriorated with age in never-married men. In contrast, married men remained stable on these dimensions with age. Never-married men are a psychologically highly vulnerable group in a society where marriage is an expectation. Since the highest birth sex–ratio cohorts have not yet reached reproductive age, the social tragedy of these men will last for at least another generation.This article is part of the themed issue ‘Adult sex ratios and reproductive decisions: a critical re-examination of sex differences in human and animal societies’.

Sex-specific early survival drives adult sex ratio bias in snowy plovers and impacts mating system and population growth

Adult sex ratio (ASR) is a central concept in population biology and a key factor in sexual selection, but why do most demographic models ignore sex biases? Vital rates often vary between the sexes and across life history, but their relative contributions to ASR variation remain poorly understood-an essential step to evaluate sex ratio theories in the wild and inform conservation. Here, we combine structured two-sex population models with individual-based mark-recapture data from an intensively monitored polygamous population of snowy plovers. We show that a strongly male-biased ASR (0.63) is primarily driven by sex-specific survival of juveniles rather than adults or dependent offspring. This finding provides empirical support for theories of unbiased sex allocation when sex differences in survival arise after the period of parental investment. Importantly, a conventional model ignoring sex biases significantly overestimated population viability. We suggest that sex-specific population models are essential to understand the population dynamics of sexual organisms: reproduction and population growth are most sensitive to perturbations in survival of the limiting sex. Overall, our study suggests that sex-biased early survival may contribute toward mating system evolution and population persistence, with implications for both sexual selection theory and biodiversity conservation.

Climate predicts which sex acts as helpers among cooperatively breeding bird species.

Among avian cooperative breeders, help in raising offspring is usually provided by males or by both sexes. Sex bias in helping should evolve in response to sex-specific ecological constraints on independent reproduction, with mate shortage for males and breeding vacancy shortage for each sex. Given that male-biased adult sex ratios are prevalent among birds, we predict that male-only helping mainly occurs in temperate species where fast population turnovers deriving from low adult annual survival allow all adult females to hold breeding vacancies, whereas some males overflow as helpers, and both-sex helping in tropical species where saturated habitats prevent not only males, but also females from breeding themselves. As expected, we found that across species, adult survival increased towards tropical zones and warmer climates, and higher adult survival tended to be associated with both-sex helping. Furthermore, sex bias in helping was predicted by latitude and ambient temperature. Our findings of demographic response of species to climate as a potential determinant of bias in helper sex uncover how ecological constraints operate to limit independent reproduction in sex-specific ways.

Simple settlement decisions explain common dispersal patterns in territorial species.

Dispersal is one of the least-understood aspects of animal behaviour. For example, little is known of the mechanisms that determine how individuals express different dispersal behaviours in response to different circumstances. Uncovering these mechanisms is important for our understanding of spatial population dynamics. Using agent-based simulations, we examine how simple decision rules generate individual-level dispersal plasticity, and how this can influence population-scale dispersal dynamics. We model a territorial, monogamous population inhabiting a completely homogeneous environment. Dispersal variability therefore emerges solely as a result of between-individual interactions (competition, settlement, reproduction), which are governed by simple decision-making algorithms. We show that complex dispersal dynamics, including sex biases and strong density dependence, emerge naturally from simple rule-based behaviours. Dispersal is particularly sensitive to the inclusion of mate availability as a criterion for settlement: if neither sex evaluates mate availability, dispersal distances tend to decline at low densities, leading to a strong Allee effect from reduced pairing success. If one sex evaluates mate availability (females), Allee effects are largely avoided, but female-biased dispersal generates increasingly male-biased adult sex ratios at low densities. Sex biases are eliminated if both sexes evaluate mate availability, but population growth rates tend to be reduced due to survival costs and reduced pairing success. Our models suggest that simple decision mechanisms can explain several dynamic patterns that are commonly observed among territorial species. Importantly, these patterns emerge in the absence of environmental heterogeneity or between-individual variation in dispersal phenotypes, two conditions that are often invoked to explain dispersal heterogeneity in nature. This has implications for studies seeking to examine the causes of dispersal variability in wild populations, suggesting that observed patterns could be largely driven by the social and demographic conditions experienced by sampled individuals. Further insights could be gained by examining how selection operates on decision rules in different life-history and environmental circumstances, and how this might interact with selection on other demographic traits. Uncovering the decision rules used during settlement should be a priority for those wishing to understand and predict dispersal patterns in nature.

The effects of adult sex ratio on mating competition in male and female guppies (Poecilia reticulata) in two wild populations

When competing for mates, males typically exhibit higher rates of intrasexual aggression and courtship than females. Operational sex ratio, represented here by adult sex ratio (ASR) as a proxy, is likely the best predictor of this competition, which typically increases between members of one sex as members of the opposite sex become rarer. Moreover, in populations subject to high predation, males often decrease mating competitive behaviour due to predation risk. We explored the combined effects of ASR and population of origin (low vs. high ambient predation risk) on mating competition in male and female wild-caught Trinidadian guppies. Both male and female aggression rates increased with ASR, but the increase for males was only significant in the low-predation population. In regard to male mating tactics, courtship propensity was unaffected by ASR, while the propensity to sneak increased at male-biased ASRs. Guppies from a high predation population had lower aggression rates than their low predation counterpart, but male courtship and sneaking attempts did not differ between populations. Surprisingly, females were just as aggressive as males when competing for mates. These results highlight the trade-offs between antipredator and agonistic behaviour, which may affect sexual selection pressures in wild populations.