Sexual experience and testosterone during adolescence alter adult neuronal morphology and behavior

Steroid hormones released immediately before and after birth provoke sexual differentiation of neural circuits. Further, steroid hormones secreted during adolescence also exert long lasting effects on the nervous system. Hormones secreted during development may act through two distinct pathways: (1) hormones can directly affect neuron and synapse elimination and (2) endocrine changes in the nervous system may occur secondary to changes in social behaviors. Therefore, a critical period for organization of the nervous system by steroid hormones during adolescence may also be a sensitive period for the effects of social experience. The overall goal of this experiment was to determine whether the opportunity to mate with a sexually receptive female during this adolescent critical period would have enduring effects on behavior and neuronal morphology into adulthood. A second question was to determine the extent to which testosterone mediated the effects of these social interactions on adult outcomes. Compared to sexually inexperienced hamsters and those that experienced sex for the first time in adulthood, hamsters that experienced adolescent sexual experience displayed increased anxiety- and depressive-like behavioral responses. Adolescent sexual experiences decreased the complexity and length of dendrites on prefrontal cortical neurons and increased the expression of the pro-inflammatory cytokine interleukin 1β (IL-1β) in the PFC. In a second experiment, administration of testosterone during the adolescent period largely recapitulated the effects of adolescent sexual experience. These data support the overall hypothesis that a sensitive period extends into adolescence and that salient social stimuli during this time can significantly and persistently alter adult phenotype.     

Current and historical factors drive variation of reproductive traits in unisexual mosses in Europe: A case study

Unisexual bryophytes provide excellent models to study the mechanisms that regulate the frequency of sexual versusasexual reproduction in plants, and their ecological and evolutionary implications. Here, we determined sex expression, phenotypic sex ratio, and individual shoot traits in 242 populations of the cosmopolitan moss Pseudoscleropodium purum spanning its whole distributional range. We tested whether niche differentiation, sex-specific differences in shoot size, and biogeographical history explained the spatial variation of reproductive traits. We observed high levels of sex expression and predominantly female-biased populations, although both traits showed high intraspecific variation among populations. Sex expression and sex ratio were partly explained by current macroscale environmental variation, with male shoots being less frequent at the higher end of the environmental gradients defined by the current distribution of the species. Female bias in population sex ratio was significantly lower in areas recolonized after the last glacial maximum (recent populations) than in glacial refugia (long-term persistent populations). We demonstrated that reproductive trait variation in perennial unisexual mosses is partially driven by macroscale and historical environmental variation. Based on our results, we hypothesize that sexual dimorphism in environmental tolerance and vegetative growth contribute to sex ratio bias over time, constraining the chances of sexual reproduction, especially in long-term persistent populations. Further studies combining genetic analyses and population monitoring should improve our understanding of the implications of the intraspecific variation in the frequency of sexual versusasexual reproduction in bryophyte population fitness and eco-evolutionary dynamics.     

Application of organization-activation theory to alternative male reproductive strategies: a review

Many species have extreme within-sex morphological and behavioral polymorphisms, most commonly different male phenotypes that practice different reproductive strategies. Although much is known about the role of hormones in sexual differentiation, little is known about what role hormones might play in within-sex differentiation. The relative plasticity hypothesis is derived from the classical organization-activation model of hormone action. It distinguishes between two types of polymorphic systems: a fixed system in which individual males assume one phenotype for their adult lives and a plastic system in which individual males can change phenotypes at least once. By analogy to sexual differentiation, the relative plasticity hypothesis generally predicts that organizational influences of hormones will be more important in fixed systems and activational influences of hormones will be more important in plastic systems. A review of our knowledge of the role of hormones in differentiation of within-sex polymorphisms indicates that the relative plasticity hypothesis accounts for otherwise diverse and contradictory results. This further supports the hypothesis that the organizational-activational model of hormone action derived from sexual differentiation generalizes to within-sex polymorphisms. However, studies of the effects of hormone manipulations on within-sex differentiation are rare but are desperately needed to further our understanding of this problem. Further studies of discontinuous behavioral variation characteristic of polymorphic species may further our understanding of the physiological basis of within-sex behavior variation in all species.     

Sexual differentiation of motivation: A novel mechanism?

Sex differences in motivation are apparent for the motivation to engage in sexual behavior, the motivation to take drugs of abuse, and the motivation to engage in parental behavior. In both males and females there is an increase in NAcc DA associated with motivated behaviors. Here it proposed that sex differences in the regulation of DA activity in the ascending mesolimbic projections may underlie sex differences in motivation. In particular, sex differences in the neuroendocrine regulation of this brain system play a role in the expression of sex differences in motivated behaviors. Here it is proposed that sexual differentiation of motivation is mediated, at least in part, by a novel mechanism in which ovarian hormones secreted at puberty in the female actively feminize the DA system.     

Endocrine mechanisms,behavioral phenotypes and plasticity: known relationships and open questions

Behavior of wild vertebrate individuals can vary in response to environmental or social factors. Such within-individual behavioral variation is often mediated by hormonal mechanisms. Hormones also serve as a basis for among-individual variations in behavior including animal personalities and the degree of responsiveness to environmental and social stimuli. How do relationships between hormones and behavioral traits evolve to produce such behavioral diversity within and among individuals? Answering questions about evolutionary processes generating among-individual variation requires characterizing how specific hormones are related to variation in specific behavioral traits, whether observed hormonal variation is related to individual fitness and, whether hormonal traits are consistent (repeatable) aspects of an individual's phenotype. With respect to within-individual variation, we need to improve our insight into the nature of the quantitative relationships between hormones and the traits they regulate, which in turn will determine how they may mediate behavioral plasticity of individuals. To address these questions, we review the actions of two steroid hormones, corticosterone and testosterone, in mediating changes in vertebrate behavior, focusing primarily on birds. In the first part, we concentrate on among-individual variation and present examples for how variation in corticosterone concentrations can relate to behaviors such as exploration of novel environments and parental care. We then review studies on correlations between corticosterone variation and fitness, and on the repeatability over time of corticosterone concentrations. At the end of this section, we suggest that further progress in our understanding of evolutionary patterns in the hormonal regulation of behavior may require, as one major tool, reaction norm approaches to characterize hormonal phenotypes as well as their responses to environments.In the second part, we discuss types of quantitative relationships between hormones and behavioral traits within individuals, using testosterone as an example. We review conceptual models for testosterone-behavior relationships and discuss the relevance of these models for within-individual plasticity in behavior. Next, we discuss approaches for testing the nature of quantitative relationships between testosterone and behavior, concluding that again reaction norm approaches might be a fruitful way forward.We propose that an integration of new tools, especially of reaction norm approaches into the field of behavioral endocrinology will allow us to make significant progress in our understanding of the mechanisms, the functional implications and the evolution of hormone–behavior relationships that mediate variation both within and among individuals. This knowledge will be crucial in light of already ongoing habitat alterations due to global change, as it will allow us to evaluate the mechanisms as well as the capacity of wild populations to adjust hormonally-mediated behaviors to altered environmental conditions.     

Reproductive allocation between the sexes,across natural and novel habitats,and its impact on genetic diversity

Allocation to reproductive mode (sexual and asexual) can vary depending on environmental conditions but is often examined at the population level, whereas selection acts upon the individual. We examined individual variation in reproductive mode to identify how the interaction of sex and the environment affect population genetic diversity. Using the plant Marchantia inflexa, we tested whether reproductive allocation pattern varies consistently between males and females and among plants collected from different environments, and determined if morphological responses were the result of individual plasticity or genetic differences. We then quantified genetic variability between the different environments and between the sexes. Male and female plants were collected from two strikingly different habitats within the same region: along natural sites (rivers) and along novel human-modified sites (roadsides). Using a common garden approach, we found a strong sex by habitat interaction: male and female responses differed significantly by their source habitat. For females, relative to river-collected, road-collected plants had higher growth and asexual reproduction, while the pattern was reversed, although not significant, for males. Genetic differentiation was significant between the two habitats with no evidence of individual differences in plasticity for growth, but there was a genotype effect for asexual propagule production. Males and females did not differ genetically; but river-collected plants with lower sexual potential were more diverse than roadside-collected plants, possibly the result of founder events. These results show that individual variation in reproduction is controlled by the interaction of both the environment and genetics. Due to different selection pressures between natural and novel habitats, there are observable differences in life history traits with an associated evolutionary response to the novel habitat.     

Searching for Sex Differences in the Vomeronasal Pathway

The sexual differentiation of brain and behavior is reviewed from the findings of sex differences in the vomeronasal pathway. A motivational approach to sex differences in reproductive behavior is stressed by taking into account that sex differences are present in neural networks: from the receptor organ (the vomeronasal organ) to effector nuclei. Sex differences in the brain appear in two morphological patterns. In one, the male presents greater morphological measurements than the female; in the other, the opposite occurs. These two morphological patterns are actively differentiated by gonadal steroids. The functional significance of these two morphological patterns is addressed. Moreover, since the GABA_Areceptor is involved in the organization of sex differences in vomeronasal structures such as the accessory olfactory bulb and in maternal behavior, the role of membrane mechanisms, 5α reduced hormones, and neurosteroids in the sexual differentiation process is discussed.     

The expression of andromonoecy in Solanum hirtum (Solanaceae): phenotypic plasticity and ontogenetic contingency

Sex expression (the proportions of staminate and hermaphrodite flowers produced) in andromonoecious Solarium hirtum is phenotypically plastic, and there is genetic variation for sex expression plasticity. Changes in sex expression phenotype are inherently the result of altered development. However, the underlying developmental components of sex expression plasticity and of differences in plasticity among genotypes are unknown. This study takes an explicitly genetic and developmental approach to the study of phenotypic plasticity and examines changes in sex expression of ten clonally replicated genotypes at three levels of organization: among inflorescences, within inflorescences, and at the level of developing floral meristems. Changes in sex expression of individuals and differences among individuals are the result of a predictable interplay of resource, architectural, and floral level response within the hierarchical construction of the shoot system. Phenotypic plasticity of whole plant sex expression is ultimately due to sexual lability of individual developing flowers: floral sex is not determined until a primordium size of 9–10 mm. Until that time, sex expression remains labile and developing floral primordia can respond to changes in plant resource status. This flower level developmental lability, however, is expressed within the constraints set by the architecture and ontogenetic history of the organism. Only those floral primordia produced in distal portions of each inflorescence are labile, capable of developing into either a staminate or hermaphrodite flower, whereas those primordia in basal positions invariably develop as hermaphrodite flowers. The genotypes differ with respect to the architectural components of phenotypic plasticity and it is this architectural variation that results in differences in plasticity among genotypes. The phenomenon, in which the developmental fate of a primordium depends upon where and when it is produced within the architecture of an organism and what events have preceded it during ontogeny, can be termed “ontogenetic contingency.”     

Inferring the potentially complex genetic architectures of adaptation,sexual dimorphism and genotype by environment interactions by partitioning of mean phenotypes

Genetic architecture fundamentally affects the way that traits evolve. However, the mapping of genotype to phenotype includes complex interactions with the environment or even the sex of an organism that can modulate the expressed phenotype. Line‐cross analysis is a powerful quantitative genetics method to infer genetic architecture by analysing the mean phenotype value of two diverged strains and a series of subsequent crosses and backcrosses. However, it has been difficult to account for complex interactions with the environment or sex within this framework. We have developed extensions to line‐cross analysis that allow for gene by environment and gene by sex interactions. Using extensive simulation studies and reanalysis of empirical data, we show that our approach can account for both unintended environmental variation when crosses cannot be reared in a common garden and can be used to test for the presence of gene by environment or gene by sex interactions. In analyses that fail to account for environmental variation between crosses, we find that line‐cross analysis has low power and high false‐positive rates. However, we illustrate that accounting for environmental variation allows for the inference of adaptive divergence, and that accounting for sex differences in phenotypes allows practitioners to infer the genetic architecture of sexual dimorphism.     

Neural sex modifies the function of a C. elegans sensory circuit

Though sex differences in animal behavior are ubiquitous, their neural and genetic underpinnings remain poorly understood. In particular, the role of functional differences in the neural circuitry that is shared by both sexes has not been extensively investigated. We have addressed these issues with C. elegans olfaction, a simple innate behavior mediated by sexually isomorphic neurons. Though males respond to the same olfactory attractants as do hermaphrodites, we find that each sex has a characteristic repertoire of olfactory preferences. These are not secondary to other sex-specific behaviors and do not require signaling from the gonad. Sex-specific olfactory preferences are controlled by tra-1, the master regulator of C. elegans sexual differentiation. Moreover, the genetic masculinization of neurons in an otherwise wild-type hermaphrodite is sufficient to switch the sexual phenotype of olfactory preference behavior. These studies reveal novel and unexpected sex differences in a C. elegans sensory behavior that is exhibited by both sexes. Our results indicate that these differences are a function of the chromosomally determined sexual identity of shared neural circuitry.     

Hormonal and meta-hormonal determinants of sexual dimorphism

The role of hormones in the determination of sexual characteristics has been known for several decades. It has been shown, for example, that several products, including sex steroids, may influence the body development pattern, metabolic pathways, fat and muscle distribution and vocal cord anatomy, thus producing an overall outcome consistent with a masculine or feminine phenotypic pattern. These qualities are usually described as secondary sexual traits, so as to be distinguished from primary sex traits, usually referring to the gonads and external genitalia. However, it must be noted that hormonal regulation may not explain the full range of the sexual phenotype, since the central nervous system retains a significant role in the establishment of sexual identity, thus giving rise to a higher sex determination stage exclusively described in humans, namely behavioral or psychological sex. Recently, it has been suggested that differences among the sexes are not limited to brain function but they may also refer to anatomical differences and different biochemical profiles, including a distinct pattern of AR and ER distribution. This new aspect of sexual dimorphism suggests a whole system of meta-hormonal regulation, recently described as the sexual brain model. The role of local androgen and/or estrogen concentrations in the initial establishment of brain sexual dimorphism is still under evaluation, since the first results are relatively inconclusive and no direct cause and effect relationship has been proven so far. On the other hand, sex hormones have recently been found to participate in processes well beyond their initially suggested spectrum of action. For instance, ER interacts with EGFR in a number of ways, affecting development of a number of epithelial structures. Estrogen receptors have also been detected in a number of non-classic targets of steroids, such as the brain and the lungs. This observation may imply that sexual dimorphism goes a lot deeper than previously estimated, affecting virtually every organic system, suggesting, in essence, the existence of two different functional models for the whole human body, formulated and conserved throughout the evolutionary progress.     

Possible constraints on adaptive variation in sex ratio at birth in humans and other primates

There is general agreement that adaptive variation of sex ratio at birth has not been decisively demonstrated in primates (including human beings). So some workers have questioned whether it actually exists. Others have conjectured that it exists but is subject to as yet unidentified 'constraints' (factors opposing the modifying influences of selection in the phenotype). Meanwhile though most workers have called for research to reveal the proximate causes of sex ratio variation, few (if any) have directed studies toward that end. Here it is argued that hormonal action is responsible both for much adaptive and non-adaptive sex ratio variation, and for constraints on the adaptive variation. My hypothesis proposes that levels of steroid hormones (testosterone and oestrogen) of both parents around the time of conception are positively associated with offspring sex ratio (proportion male at birth) of mammals including man. Testosterone in men and oestrogen in women are also known to be positively associated with the health, attractiveness and fertility of individual human beings. However, high levels of testosterone in women are frequently associated with adverse medical conditions. It is suggested that for these reasons (and contrary to some adaptive theory) some classes of people (particularly women) in suboptimal health ("condition") produce excesses of sons. It seems that gonadal hormones are responsible for adaptive variation; and that maternal adrenal hormones are responsible for maladaptive variation. In evolutionary terms, gonadal hormones precede adrenal hormones.     

Quantitative genetics of plumage color: lifetime effects of early nest environment on a colorful sexual signal

Phenotypic differences among individuals are often linked to differential survival and mating success. Quantifying the relative influence of genetic and environmental variation on phenotype allows evolutionary biologists to make predictions about the potential for a given trait to respond to selection and various aspects of environmental variation. In particular, the environment individuals experience during early development can have lasting effects on phenotype later in life. Here, we used a natural full‐sib/half‐sib design as well as within‐individual longitudinal analyses to examine genetic and various environmental influences on plumage color. We find that variation in melanin‐based plumage color – a trait known to influence mating success in adult North American barn swallows (Hirundo rustica erythrogaster) – is influenced by both genetics and aspects of the developmental environment, including variation due to the maternal phenotype and the nest environment. Within individuals, nestling color is predictive of adult color. Accordingly, these early environmental influences are relevant to the sexually selected plumage color variation in adults. Early environmental conditions appear to have important lifelong implications for individual reproductive performance through sexual signal development in barn swallows. Our results indicate that feather color variation conveys information about developmental conditions and maternal care alleles to potential mates in North American barn swallows. Melanin‐based colors are used for sexual signaling in many organisms, and our study suggests that these signals may be more sensitive to environmental variation than previously thought.     

The end of gonad-centric sex determination in mammals

The 20th-century theory of mammalian sex determination states that the embryo is sexually indifferent until the differentiation of gonads, after which sex differences in phenotype are caused by the differential effects of gonadal hormones. However, this theory is inadequate because some sex differences precede differentiation of the gonads and/or are determined by non-gonadal effects of the sexual inequality in the number and type of sex chromosomes. In this article, I propose a general theory of sex determination, which recognizes multiple parallel primary sex-determining pathways initiated by genes or factors encoded by the sex chromosomes. The separate sex-specific pathways interact to synergize with or antagonize each other, enhancing or reducing sex differences in phenotype.     

Sexual segregation in ungulates: individual behaviour and the missing link

Previous "explanations" of sexual segregation in ungulates establish no more than a prerequisite for habitat segregation because they do not include a model of competitive habitat selection. Here we provide one based on the ideal free distributions of mutually competing, optimally foraging, individual deer. We parameterised our model using field data collected from a population of fallow deer (Dama dama) in a Mediterranean forest. The predictions of the inter-sex competition model were in full agreement with observational data, but those of single sex distributions (conventional theory) were not. The "conventional" hypothesis, that segregation arises simply from sex differences, predicted no more than moderate (20–40%) levels of segregation, even in optimal conditions. By introducing inter-sex resource competition, the predicted segregation can generally more than double and full segregation becomes possible in some circumstances. The modelling showed segregation to be density-dependent, varying in complicated ways with season and animal density. Sensitivity analysis showed the vulnerability of the "conventional" understanding of environmental variation and uncertainty. Using our competition model we show that as diet difference increases, direct competition between the sexes declines, so that as males increasingly differ from females, segregation declines and the two sexes are more likely to be found mixed (as long as the chosen food is available to both in the same area). Conversely, small differences among male and female deer are amplified by both food depletion and inter-sex competition to give substantial levels of segregation. The theoretical framework on which our model is built strongly suggests that sexual dimorphism in the context of scramble competition may be the fundamental cause of sexual habitat-segregation among ungulates.     

Seasonal covariation in progesterone and odorant emissions among breeding crested auklets (Aethia cristatella)

Crested auklets emit a citrus-like odorant that is seasonally modulated, suggesting that it is a secondary sexual trait. We hypothesized that expression of the chemical odorant is facilitated by steroid hormones, similar other secondary sexual traits in birds. Therefore we examined variation in concentrations of hormones in blood plasma and odor production during incubation and early chick rearing. A novel method was used to obtain and measure chemical emissions of crested auklets. Blood plasma samples were analyzed by radioimmunoassay. Progesterone was detected in all birds, and it varied during the breeding season. Octanal emissions covaried with progesterone levels in males but not in females. No seasonal patterns were detected in testosterone, estrogen or DHT, and these hormones were not detected in all breeding adults. Covariance of progesterone and octanal emissions in males suggests there could be at least an indirect relationship between odor emissions and steroid hormones in this species. Thus expression of the citrus-like odorant of crested auklets, like other secondary sexual traits in birds, could be regulated by steroid hormones.     

Genetic determination of plasma apolipoprotein AI in a population-based sample.

Apolipoprotein AI (apo AI) is the major protein of high-density lipoprotein (HDL). Using radioimmunoassay, we measured plasma apo AI levels in 1,880 individuals in 283 pedigrees randomly selected from the population with respect to disease status and risk factors for coronary artery disease. Apo AI levels were first adjusted for date of assay (6.8% of apo AI variation) and then adjusted for variability in age and body mass index (an additional 6.6%, 20.4%, and 23.0% of apo AI variations for males, females not using exogenous hormones, and females using exogenous hormones, respectively). A mixture of two normal distributions fit the adjusted data better than did a single normal distribution. Genetic and environmental models that could explain the mixture of normal distributions were investigated using complex segregation analysis. Heterogeneous etiologies for individual differences in adjusted apo AI levels were suggested by the data in the 283 pedigrees. In a subset of 126 pedigrees, there is evidence for the major effect of a nontransmitted environmental factor that explains the mixture of distributions as well as polygenic loci that influence apo AI levels within each distribution. The environmental factor and polygenic loci account for 32% and 65% of the adjusted variation, respectively. In the other 157 pedigrees there is strong support for a single locus with a major effect that accounts for 27% of the adjusted variation. The effect of the polygenic loci is not different from zero in these 157 pedigrees. This is the first study to present evidence for the segregation of a single unmeasured locus with a major effect on levels of apo AI in a population-based sample of pedigrees.     

Growth hormone and thyroid stimulating hormone concentrations in captive male orangutans: implications for understanding developmental arrest

There are two morphs of reproductive male in orangutans. Both morphs span the age range from adolescent to adult, but "subadult" males are smaller in size and lack secondary sexual features. In this study, urine samples were collected over a 2 year period from 23 captive male orangutans in order to define the endocrinology of this apparent arrest of secondary sexual development. Three males were juveniles, 3 to 5 years of age; seven males showed no secondary sexual trait development and were over 7 years of age; six males were in the process of developing secondary sexual features, with the youngest male being 6 years of age; and seven males were fully mature adults. Morning samples were analyzed by radioimmunoassay for levels of growth hormone (GH) and thyroid-stimulating hormone (TSH) and group hormone profiles were compared by analysis of variance. GH is the primary hormone of growth and development and its increase in teenage boys is associated with the adolescent growth spurt. TSH stimulates the thyroid to produce and secrete hormones that have metabolic effects and required for normal growth and development. Results show that arrested adolescent male orangutans have a GH level about 1/3 that of developing adolescents (P = .0006). TSH levels do not differ significantly between arrested and developing adolescents. These data complement other endocrine data showing significantly lower levels of sex steroids and luteinizing hormone (LH) in arrested males than developing males [Maggioncalda, 1995a,b; Maggioncalda et al., 1999]. Together with documented behavioral differences between reproductive males with and without secondary sexual features, these endocrine data support the hypothesis that in male orangutans there are alternative developmental pathways and corresponding alternative reproductive strategies.     

Leptin's Sexual Dimorphism Results from Genotype by Sex Interactions Mediated by Testosterone

Objective: Recent studies have reported the existence of marked sexual dimorphism in serum leptin levels in humans, with women having approximately three times the levels of men. As we have shown for other measures of adiposity, such sexual dimorphism can arise from a special case of genotype by environment interaction, that of genotype by sex interaction. Research Methods and Procedures: Using maximum likelihood-based variance decomposition techniques, we examined the genetic and environmental architecture of sexual dimorphism in serum leptin levels in 1147 Mexican Americans from the San Antonio Family Heart Study. Results: Both the genetic and environmental variances for this trait differed significantly between the sexes (p < 0.001 and p < 0.01, respectively), with women displaying larger values for both components. We found significant evidence that different genes influence variation in serum leptin levels between the two sexes (p = 0.05). Furthermore, this pattern of sexual dimorphism in serum leptin levels persisted even after accounting for the effects of either the percentage of body fat or total body fat. However, this pattern of sexual dimorphism was eliminated after accounting for the effects of testosterone. Discussion: These findings suggest that the sexual dimorphism seen in leptin levels is not simply explained as differences in total adiposity between the sexes. We conclude that the genes, which influence variation in serum leptin levels, are differentially expressed depending on sex, and that the sexes also show differences in response of the expression of this obesity-related trait to unmeasured residual effects.     

Sex in a material world: why the study of sexual reproduction and sex‐specific traits should become more nutritionally‐explicit

Recent advances in nutritional ecology, particularly arising from Ecological Stoichiometry and the Geometric Framework for nutrition, have resulted in greater theoretical coherence and increasingly incisive empirical methodologies that in combination allow for the consideration of nutrient‐related processes at many levels of biological complexity. However, these advances have not been consistently integrated into the study of sexual differences in reproductive investment, despite contemporary emphasis on the material costs associated with sexually selected traits (e.g. condition‐dependence of exaggerated ornaments). Nutritional ecology suggests that material costs related to sex‐specific reproductive traits should be linked to quantifiable underlying differences in the relationship between individuals of each sex and their foods. Here, we argue that applying nutritionally‐explicit thought to the study of sexual reproduction should both deepen current understanding of sex‐specific phenomena and broaden the tractable frontiers of sexual selection research. In support of this general argument, we examine the causes and consequences of sex‐specific nutritional differences, from food selection and nutrient processing to sex‐specific reproductive traits. At each level of biological organization, we highlight how a nutritionally‐explicit perspective may provide new insights and help to identify new directions. Based on predictions derived at the individual level, we then consider how sex‐specific nutrient limitation might influence population growth, and thus potentially broader patterns of life history evolution, using a simple population dynamics model. We conclude by highlighting new avenues of research that may be more accessible from this integrative perspective.