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Work from social and biological sciences has shown that adult sex ratios are associated with relationship behaviours. When partners are abundant, opportunities for mate switching may increase and relationship stability decrease. To date, most of the human literature has used regional areas at various levels of aggregation to define partner markets. But, in developed countries, many individuals of reproductive age spend a considerable amount of time outside their residential areas, and other measures may better capture the opportunities to meet a (new) partner. Here, we use Danish register data to test whether the sex ratio of the occupational sector is linked to divorce. Our data cover individuals in Denmark who married during 1981-2002 and we control for age at and duration of marriage, education and parity. Results support the prediction that a higher proportion of opposite-sex individuals in one's occupational sector is associated with higher divorce risk. This holds for both men and women, but associations are somewhat stronger for men and vary by education. Our results highlight the need to study demographic behaviours of men and women simultaneously, and to consider partner markets beyond geographical areas so that differing strategies for males and females may be examined.

Sex‐biased mortality can lead to altered adult sex ratios (ASRs), which may in turn lead to harassment and lower fitness of the rarer sex and changes in the mating system. Female critically endangered swift parrots (Lathamus discolor) suffer high predation while nesting due to an introduced mammalian predator, the sugar glider (Petaurus breviceps). High predation on females is causing severe population decline alongside strongly biased adult sex ratios (≥73% male). Our 6‐year study showed that 50.5% of critically endangered swift parrot nests had shared paternity although the birds remained socially monogamous. Shared paternity increased significantly with the local rate of predation on breeding females, suggesting that rates of shared paternity increased when the ASR became more biased. Nests that were not predated produced fewer fledglings as the local ASR became more male‐biased possibly due to higher interference during nesting from unpaired males. Population viability analyses showed that part of the predicted decline in the swift parrot population is due to reduced reproductive success when paternity is shared. The models predicted that the population would decline by 89.4% over three generations if the birds maintained the lowest observed rate of shared paternity. This compares with predicted population reductions of 92.1–94.9% under higher rates of shared paternity. We conclude that biases in the ASR, in this case caused by sex‐specific predation from an introduced predator, can lead to changes in the mating system and negative impacts on both individual fitness and long‐term population viability.

Studies of mammals have often produced results inconsistent with theories predicting adaptive sex-ratio manipulation. Some apparently strong trends weaken or disappear over time, suggesting that multiple variables affect sex ratio and that their relative importance may change over time. Mountain goats are sexually dimorphic ungulates that satisfy all the assumptions of the Trivers-Willard hypothesis. Therefore, females able to provide high levels of maternal care are expected to produce an excess of sons. An earlier study found that older females, that are larger and dominant, produced more sons than did younger females, which are smaller and subordinate. Here, we show that, as sample size tripled, that trend disappeared. A large-scale climate index, reproductive status at conception, and population density had no consistent effects on offspring sex ratio. A composite measure of female condition at conception revealed that offspring sex ratio varied with adult sex ratio, as predicted by the homeostatic hypothesis, but only for mothers in good condition at conception. These females were dominant, heavier, and older. Their probability of producing a son decreased from about 80 to 20 % as the adult sex ratio became more male-biased. For mothers with a low condition index, however, adult sex ratio had no effect on offspring sex ratio. Our research suggests that offspring sex ratio is affected by complex interactions between individual condition and other variables, whose importance may vary over time and can only be elucidated by long-term studies.

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'.

Adult sex ratio (ASR, the proportion of males in the adult population) is a central concept in population and evolutionary biology, and is also emerging as a major factor influencing mate choice, pair bonding and parental cooperation in both human and non-human societies. However, estimating ASR is fraught with difficulties stemming from the effects of spatial and temporal variation in the numbers of males and females, and detection/capture probabilities that differ between the sexes. Here, we critically evaluate methods for estimating ASR in wild animal populations, reviewing how recent statistical advances can be applied to handle some of these challenges. We review methods that directly account for detection differences between the sexes using counts of unmarked individuals (observed, trapped or killed) and counts of marked individuals using mark-recapture models. We review a third class of methods that do not directly sample the number of males and females, but instead estimate the sex ratio indirectly using relationships that emerge from demographic measures, such as survival, age structure, reproduction and assumed dynamics. We recommend that detection-based methods be used for estimating ASR in most situations, and point out that studies are needed that compare different ASR estimation methods and control for sex differences in dispersal.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'.

Studies of mammals have often produced results inconsistent with theories predicting adaptive sex-ratio manipulation. Some apparently strong trends weaken or disappear over time, suggesting that multiple variables affect sex ratio and that their relative importance may change over time. Mountain goats are sexually dimorphic ungulates that satisfy all the assumptions of the Trivers-Willard hypothesis. Therefore, females able to provide high levels of maternal care are expected to produce an excess of sons. An earlier study found that older females, that are larger and dominant, produced more sons than did younger females, which are smaller and subordinate. Here, we show that, as sample size tripled, that trend disappeared. A large-scale climate index, reproductive status at conception, and population density had no consistent effects on offspring sex ratio. A composite measure of female condition at conception revealed that offspring sex ratio varied with adult sex ratio, as predicted by the homeostatic hypothesis, but only for mothers in good condition at conception. These females were dominant, heavier, and older. Their probability of producing a son decreased from about 80 to 20 % as the adult sex ratio became more male-biased. For mothers with a low condition index, however, adult sex ratio had no effect on offspring sex ratio. Our research suggests that offspring sex ratio is affected by complex interactions between individual condition and other variables, whose importance may vary over time and can only be elucidated by long-term studies.

The evolutionary history of sexual selection in the geologic past is poorly documented based on quantification, largely because of difficulty in sexing fossil specimens. Even such essential ecological parameters as adult sex ratio (ASR) and sexual size dimorphism (SSD) are rarely quantified, despite their implications for sexual selection. To enable their estimation, we propose a method for unbiased sex identification based on sexual shape dimorphism, using size-independent principal components of phenotypic data. We applied the method to test sexual selection in Keichousaurus hui, a Middle Triassic (about 237 Ma) sauropterygian with an unusually large sample size for a fossil reptile. Keichousaurus hui exhibited SSD biased towards males, as in the majority of extant reptiles, to a minor degree (sexual dimorphism index -0.087). The ASR is about 60% females, suggesting higher mortality of males over females. Both values support sexual selection of males in this species. The method may be applied to other fossil species. We also used the Gompertz allometric equation to study the sexual shape dimorphism of K. hui and found that two sexes had largely homogeneous phenotypes at birth except in the humeral width, contrary to previous suggestions derived from the standard allometric equation.