scholarly journals Sex roles and adult sex ratios: insights from mammalian biology and consequences for primate behaviour

2017 ◽  
Vol 372 (1729) ◽  
pp. 20160321 ◽  
Author(s):  
Peter M. Kappeler
Keyword(s):  
Life History ◽  
Sex Ratio ◽  
Social Organization ◽  
Sex Roles ◽  
Paternal Care ◽  
Sex Role ◽  
Sex Ratios ◽  
Theoretical Models ◽  
Operational Sex Ratio ◽  
Minimum Length

Theoretical models and empirical studies in various taxa have identified important links between variation in sex roles and the number of adult males and females (adult sex ratio (ASR)) in a population. In this review, I examine these relationships in non-human primates. Because most existing theoretical models of the evolution of sex roles focus on the evolutionary origins of sex-biased behaviour, they offer only a general scaffold for predicting variation in sex roles among and within species. I argue that studies examining sex role variation at these more specific levels need to take social organization into account to identify meaningful levels for the measurement of ASR and to account for the fact that ASR and sex roles mutually influence each other. Moreover, taxon-specific life-history traits can constrain sex role flexibility and impact the operational sex ratio (OSR) by specifying the minimum length of female time outs from reproduction. Using examples from the primate literature, I highlight practical problems in estimating ASR and OSR. I then argue that interspecific variation in the occurrence of indirect forms of paternal care might indeed be linked to variation in ASR. Some studies also indicate that female aggression and bonding, as well as components of inter-sexual relationships, are sensitive to variation in ASR. Thus, links between primate sex roles and sex ratios merit further study, and such studies could prompt the development of more specific theoretical models that make realistic assumptions about taxon-specific life history and social organization. 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’.

scholarly journals Not all sex ratios are equal: the Fisher condition, parental care and sexual selection

2017 ◽  
Vol 372 (1729) ◽  
pp. 20160312 ◽  
Author(s):  
Michael D. Jennions ◽  
Lutz Fromhage
Keyword(s):  
Sex Differences ◽  
Sex Ratio ◽  
Parental Care ◽  
Sex Roles ◽  
Sex Role ◽  
Sex Ratios ◽  
Theoretical Models ◽  
Evolution Of Sex ◽  
Offspring Production ◽  
Adult Sex Ratio

The term ‘sex roles’ encapsulates male–female differences in mate searching, competitive traits that increase mating/fertilization opportunities, choosiness about mates and parental care. Theoretical models suggest that biased sex ratios drive the evolution of sex roles. To model sex role evolution, it is essential to note that in most sexually reproducing species (haplodiploid insects are an exception), each offspring has one father and one mother. Consequently, the total number of offspring produced by each sex is identical, so the mean number of offspring produced by individuals of each sex depends on the sex ratio (Fisher condition). Similarly, the total number of heterosexual matings is identical for each sex. On average, neither sex can mate nor breed more often when the sex ratio is even. But equally common in which sex ratio? The Fisher condition only applies to some reproductive measures (e.g. lifetime offspring production or matings) for certain sex ratios (e.g. operational or adult sex ratio; OSR, ASR). Here, we review recent models that clarify whether a biased OSR, ASR or sex ratio at maturation (MSR) have a causal or correlational relationship with the evolution of sex differences in parental care and competitive traits—two key components of sex roles. We suggest that it is more fruitful to understand the combined effect of the MSR and mortality rates while caring and competing than that of the ASR itself. In short, we argue that the ASR does not have a causal role in the evolution of parental care. We point out, however, that the ASR can be a cue for adaptive phenotypic plasticity in how each sex invests in parental care. 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’.

scholarly journals Evolution of male strategies with sex-ratio–dependent pay-offs: connecting pair bonds with grandmothering

2017 ◽  
Vol 372 (1729) ◽  
pp. 20170041 ◽  
Author(s):  
Sara L. Loo ◽  
Kristen Hawkes ◽  
Peter S. Kim
Keyword(s):  
Life History ◽  
Sex Ratio ◽  
Human Evolution ◽  
Paternal Care ◽  
Multiple Mating ◽  
Sex Ratios ◽  
Mate Guarding ◽  
Equation Model ◽  
Pair Bonding ◽  
Pair Bonds

Men's provisioning of mates and offspring has been central to ideas about human evolution because paternal provisioning is absent in our closest evolutionary cousins, the great apes, and is widely assumed to result in pair bonding, which distinguishes us from them. Yet mathematical modelling has shown that paternal care does not readily spread in populations where competition for multiple mates is the common male strategy. Here we add to models that point to the mating sex ratio as an explanation for pairing as pay-offs to mate guarding rise with a male-biased sex ratio. This is of interest for human evolution because our grandmothering life history shifts the mating sex ratio from female- to male-biased. Using a difference equation model, we explore the relative pay-offs for three competing male strategies (dependant care, multiple mating, mate guarding) in response to changing adult sex ratios. When fertile females are abundant, multiple mating prevails. As they become scarce, mate guarding triumphs. The threshold for this shift depends on guarding efficiency. Combined with mating sex ratios of hunter–gatherer and chimpanzee populations, these results strengthen the hypothesis that the evolution of our grandmothering life history propelled the shift to pair bonding in the human lineage. 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’.

Seahorses Exhibit Conventional Sex Roles in Mating Competition, Despite Male Pregnancy

Behaviour ◽  
1994 ◽  
Vol 128 (1-2) ◽  
pp. 135-151 ◽  
Author(s):  
Amanda C.J. Vincent
Keyword(s):  
Sex Ratio ◽  
Sex Roles ◽  
Laboratory Experiments ◽  
Sex Role ◽  
Male Reproduction ◽  
Operational Sex Ratio ◽  
Mating Competition ◽  
Male Pregnancy ◽  
The Impact

AbstractIn seahorses, only males undergo a pregnancy. It had been tacitly and explicitly assumed that seahorses were sex role reversed (that females competed more intensely than males for access to mates), on the basis that male pregnancy so limited male reproduction as to produce a female-biased operational sex ratio (OSR). However, this supposition had never been investigated. The laboratory experiments in this paper demonstrate that, contrary to expectations, seahorses exhibit conventional sex roles: male seahorses compete more intensely than females for access to mates, on both the first and final days of courtship. Competing males are more active than competing females in those courtship and competitive behaviours common to both sexes, and only males exhibit uniquely competitive behaviours (wrestling and snapping). Males which succeeded in copulating are heavier than their rivals and copulating seahorses of both sexes generally are more active in courtship and competition than are their unsuccessful rivals. The finding that seahorses maintain conventional sex roles requires us to reconsider the impact of male pregnancy on the OSR.

scholarly journals Sex Chromosome Meiotic Drive in Stalk-Eyed Flies

Genetics ◽  
1997 ◽  
Vol 147 (3) ◽  
pp. 1169-1180 ◽  
Author(s):  
Daven C Presgraves ◽  
Emily Severance ◽  
Gerald S Willrinson
Keyword(s):  
Sex Ratio ◽  
Sex Chromosome ◽  
Sex Ratios ◽  
Natural Populations ◽  
Meiotic Drive ◽  
Operational Sex Ratio ◽  
Genetic Modifiers ◽  
Ratio Distortion ◽  
The Impact ◽  
Sex Ratio Distortion

Meiotically driven sex chromosomes can quickly spread to fixation and cause population extinction unless balanced by selection or suppressed by genetic modifiers. We report results of genetic analyses that demonstrate that extreme female-biased sex ratios in two sister species of stalk-eyed flies, Cyrtodiopsis dalmanni and C. whitei, are due to a meiotic drive element on the X chromosome (Xd). Relatively high frequencies of Xd in C. dalmanni and C. whitei (13–17% and 29%, respectively) cause female-biased sex ratios in natural populations of both species. Sex ratio distortion is associated with spermatid degeneration in male carriers of Xd. Variation in sex ratios is caused by Y-linked and autosomal factors that decrease the intensity of meiotic drive. Y-linked polymorphism for resistance to drive exists in C. dalmanni in which a resistant Y chromosome reduces the intensity and reverses the direction of meiotic drive. When paired with Xd, modifying Y chromosomes (Ym) cause the transmission of predominantly Y-bearing sperm, and on average, production of 63% male progeny. The absence of sex ratio distortion in closely related monomorphic outgroup species suggests that this meiotic drive system may predate the origin of C. whitei and C. dalmanni. We discuss factors likely to be involved in the persistence of these sex-linked polymorphisms and consider the impact of Xd on the operational sex ratio and the intensity of sexual selection in these extremely sexually dimorphic flies.

scholarly journals Low reproductive skew despite high male-biased operational sex ratio in a glass frog with paternal care

2015 ◽  
Vol 15 (1) ◽  
Author(s):  
Alexandra Mangold ◽  
Katharina Trenkwalder ◽  
Max Ringler ◽  
Walter Hödl ◽  
Eva Ringler
Keyword(s):  
Sex Ratio ◽  
Paternal Care ◽  
Reproductive Skew ◽  
Operational Sex Ratio

scholarly journals A shortage of males causes female reproductive failure in yellow ground squirrels

2015 ◽  
Vol 1 (9) ◽  
pp. e1500401 ◽  
Author(s):  
Nina Vasilieva ◽  
Andrey Tchabovsky
Keyword(s):  
Life History ◽  
Sex Ratio ◽  
Breeding Success ◽  
Local Density ◽  
Conflict Theory ◽  
Ground Squirrels ◽  
Operational Sex Ratio ◽  
Female Reproduction ◽  
Female Mating ◽  
Solitary Living

Sexual conflict theory suggests that female breeding success is strongly influenced by individual life history and environmental conditions and is much less affected by mate availability. Female mating failure due to a shortage of males remains poorly studied and understood. We present data on the effects of male availability on female breeding success in a wild colony of yellow ground squirrels (Spermophilus fulvus). A female’s probability of breeding increased with the local density of males and was higher with higher male-biased operational sex ratio (OSR) but was independent of local female density, female age, and body condition, which are factors commonly assumed to influence female reproduction. The positive effect of male availability (as measured by OSR) on female breeding success was consistent across the years, and we conclude that male limitation contributes to female mating failure. This pattern, which is not commonly recorded in species with conventional sex roles, can be explained by a combination of sociodemographic and life history traits (sex differences in age of maturation, female-skewed adult sex ratio and seasonally varying OSR, solitary living at low population density, and low mobility of females combined with mate-searching tactics of males) that are not confined toS. fulvus. Our findings indicate that the role of female mating failure (due to a shortage of males) in shaping mammalian life history may be underestimated.

The Behavioral Ecology of Sex Tourism: The Consequences of Skewed Sex Ratios

2020 ◽  
pp. 004728752094610
Author(s):  
Florian Kock
Keyword(s):  
Sex Ratio ◽  
Behavioral Ecology ◽  
Sex Ratios ◽  
Reproductive Age ◽  
Operational Sex Ratio ◽  
Future Research ◽  
Sex Tourism ◽  
Same Sex ◽  
Biological Perspective ◽  
Attitudes Toward Sex

The operational sex ratio (i.e., the ratio of reproductive-age females to males in a population) shapes both animal and human behavior in important ways. Drawing on research in evolution and ecology, the author proposes that a local male-skewed sex ratio (i.e., a surplus of males) influences local men’s attitudes toward sex tourism. Analyzing historical field (study 1) and experimental data (study 2), the author demonstrates that male-skewed sex ratios increase men’s sex tourism rationalization and intent, while women’s predispositions are not sensitive to sex ratios. Sex tourism is explained as a subconscious ecological plasticity in response to perceived increased intensities of same-sex competition for mates, signaled by male-skewed sex ratios. The findings demonstrate a link between mating ecologies and sex tourism, with the latter serving as a compensatory behavior of same-sex mating competition. This research contributes a novel, biological perspective on sex tourism with implications for future research and practice.

Operational Sex Ratio and Female Competition

2014 ◽  
Author(s):  
Haley M. Dillon ◽  
Lora E. Adair ◽  
Gary L. Brase
Keyword(s):  
Sex Ratio ◽  
Sex Ratios ◽  
Intrasexual Competition ◽  
Operational Sex Ratio ◽  
Female Competition ◽  
Future Directions ◽  
Adaptive Changes ◽  
Mating Market ◽  
Mating Behaviors ◽  
Theoretical Foundations

When there is a surplus of one sex in a population, members of that sex often compete against each other for access to the scarcer sex. This chapter reviews the theoretical foundations for this phenomenon, focusing on the concept of operational sex ratio (OSR; the ratio of viable and available males to females in a given mating market) versus overall sex ratio, as well as the phylogenetic evidence of sex ratios as an important factor influencing mating behaviors. Research on human OSR and its effects is a fairly recent development but has already led to findings that are generally coherent and consistent with the nonhuman evidence. The evidence to date indicates that people who find themselves in female-disadvantaged mating markets show systematic and adaptive changes in their behaviors, including increased female intrasexual competition. The chapter concludes with discussions of additional issues and future directions for research on OSR.

Host B chromosomes as potential sex ratio distorters of intestinal nematode infrapopulations in the yellow-necked mouse (Apodemus flavicollis)

2018 ◽  
Vol 93 (05) ◽  
pp. 552-558
Author(s):  
V.M. Jovanović ◽  
B. Čabrilo ◽  
I. Budinski ◽  
O. Bjelić-Čabrilo ◽  
T. Adnađević ◽  
...  
Keyword(s):  
Life History ◽  
Sex Ratio ◽  
Sex Ratios ◽  
Life History Strategy ◽  
B Chromosomes ◽  
Published Data ◽  
Apodemus Flavicollis ◽  
Egg Hatching ◽  
Trichuris Muris ◽  
Yellow Necked Mouse

AbstractThe yellow-necked mouse, Apodemus flavicollis, can be considered as a model for genetic polymorphism produced by the frequent presence of supernumerary or B chromosomes (Bs). Host genetic background is rarely taken into account in studies of parasite sex ratio. The main aim of this study was to investigate the range of infrapopulation sex ratios for nematode parasites of the yellow-necked mouse and to determine which factors most influence variation in parasite sex ratios. Six nematode species found in the collected yellow-necked mice were analysed. We confirmed the predominant pattern of female-biased sex ratios in vertebrate parasite infrapopulations. The presence of B chromosomes in host genomes played an important role in infrapopulations of Heligmosomoides polygyrus, Syphacia stroma and Trichuris muris, as hosts with B chromosomes carried a higher proportion of males. The relative increase of males in infrapopulations could result from a shift in parasite life history strategy, induced by adaptation to the specific host genotypes (Bs present). In a meta-analysis with previously published data, the sex determination system was demonstrated to play a significant role in nematode sex ratio variation, as well as specific life history patterns, such as the place of egg hatching.

Does perceived sex ratio influence intrasexual competitiveness in women?

2020 ◽  
Author(s):  
Thomas Richardson
Keyword(s):  
Sex Ratio ◽  
Statistical Power ◽  
Sex Ratios ◽  
Facial Attractiveness ◽  
Single Women ◽  
Operational Sex Ratio ◽  
Type I ◽  
Double Blind ◽  
Future Career ◽  
Biased Sex Ratio

The operational sex ratio has been shown to influence a variety of behaviours in humans and non-human animals, particularly relating to intrasexual competition. One way females compete for mates is by derogating other women’s attractiveness. Recent studies have shown that priming participants with different sex ratios can induce sex ratio effects on behaviour. In a pre-registered, double blind experiment, 71 single women came to the lab twice and were primed with either a favourable (many men) or unfavourable (few men) sex ratio. I assessed whether unfavourable sex ratios increased self reported intrasexual competitiveness, as well as competitor derogation in the form of decreased ratings of female facial attractiveness and kindness. I also assessed whether they expressed less choosiness by rating male faces as more attractive. I tested whether any sex ratio effects are weaker for more attractive women. Finally, I attempted to replicate previous work suggesting that a male biased sex ratio increases women's preference for their future career over their future family. Despite having higher statistical power than most previous studies of this type, I did not find evidence for any of the hypothesised effects and failed to replicate 2 previous findings. The data indicated that the sex ratio manipulation likely did not work.

Sign in / Sign up

Export Citation Format

Share Document