Increased sexual dimorphism is often inferred to be correlated with increased energy costs of the larger sex (Harvey & Bradbury 1991; Kappeler & van Schaik 2004). Sexual dimorphism is often associated with costly behaviours (e.g. large males might have a competitive advantage in fighting, which is energetically expensive). The energy cost of dimorphism is most likely to be the cost of maintenance of a large body size, and not the cost of behaviours performed when an individual is large.

A new study published in Science and led by Monash University biologists reveals that the energy cost of reproduction is far greater than previously believed. The study found that the energy invested by parents in reproduction includes not only the energy contained in the offspring themselves (direct costs), but also the energy expended to produce and carry them (indirect costs). The study also highlights the sensitivity of reproductive energy costs to global warming, particularly in ectotherms. Warmer temperatures can increase metabolic rates, potentially raising the indirect costs of reproduction. "This could lead to smaller offspring and have implications for population replenishment in a warming world."

Sex-specific responses to temperature change, particularly heat, potentially alter population dynamics because males and females may differ in thermal tolerance or acclimation capacity. Sex differences in thermal tolerance may significantly impact population responses to climate change, yet most studies examining geographic variation in phenotype or responses to temperature focus exclusively on females. Our study on the freshwater crustacean Daphnia magna demonstrates sex differences in both thermal acclimation and thermal limits, with males generally exhibiting lower heat tolerance than females.

"Sexual dimorphism is often associated with costly behaviours (e.g. large males might have a competitive advantage in fighting, which is energetically costly and would increase daily energy expenditure), and there is some evidence to support this hypothesis." The authors measured daily energy expenditure in sexually dimorphic mole-rats and found that "larger males had significantly higher daily energy expenditure than smaller females, even after controlling for body mass." They conclude that "sexual dimorphism in body size is associated with increased energy costs in mole-rats," linking dimorphism to maintenance costs in metabolism and behaviour.

In terms of the energetic cost of gamete biomass production, differences between males and females were substantial. Specifically, our analysis indicates that males and females devoted about 0.1 and 300% of the energy used for basal metabolism to the production of gamete biomass respectively. Thus, the cost of egg production was roughly 3.5 orders of magnitude higher than the cost of sperm production. Together, these results suggest that whole-organism energetics may act as a primary constraint on gamete production among species.

In hot environments, where many species seek shade to decrease their heat load at midday, sexual differences in heat sensitivity could lead to different patterns of shelter use in males and females, constituting a sexual segregation mechanism. In sexually size-dimorphic species living in these areas, the larger males have more difficulties in dissipating body heat and reducing evaporative water loss. Our conclusion of a higher thermal sensitivity in males based on summer migration is reinforced by the results of the present study. This represents one of the few examples of differential thermal tolerance between sexes in vertebrates, for which the most plausible explanation is the strong sexual size dimorphism.

Climate change is increasing average temperatures and the frequency and intensity of thermal extremes in coastal marine environments. Organisms can respond to warming by adjusting their physiology through plasticity, but plastic responses are often costly. Here, we examined variation in thermal tolerance plasticity and the costs of heat hardening in an intertidal gastropod. We found that individuals with greater capacity for plastic increases in heat tolerance incurred higher energetic costs, evidenced by reduced growth or lower energy reserves after heat hardening. These results demonstrate that coping with thermal stress through plasticity can be energetically costly, with implications for how populations respond to climate warming.

Heat stress reliably induced male sterility, revealing genetic variation in male fertility tolerance, albeit only at moderate temperatures. In response to receiving a heat-induced sterile or sub-fertile ejaculate, females facultatively remated, a behavioural shift that also exhibited line-specific genetic variation, and allowed females to mitigate some of the fitness costs associated with male infertility. Therefore, as global temperatures rise and heat-induced male sterility increases, selection for polyandry may be promoted in this typically monandrous system. Sexual selection and mating behaviour thus interact with thermal stress, with heat-imposed fertility costs on males potentially reshaping reproductive strategies under climate change.

This study investigated sex-specific metabolic signatures associated with stress exposure in the mouse hypothalamus and pituitary. The authors report that "sex shapes phenotype-linked metabolic signatures of stress exposure" and describe distinct male–female differences in metabolites and pathways under stress conditions. They show that stress-induced changes in central energy metabolism and signalling are sexually dimorphic, suggesting that the energetic and metabolic consequences of stress differ between the sexes.

In walleye (Sander vitreus), a sexually dimorphic freshwater piscivore, the authors examined how temperature, body size, and sex affect standard metabolic rate (SMR). They report "a significant interaction between sex and temperature, whereby males had lower SMR below 16 °C, but increased such that SMR was ~16% higher than females at 20 °C." The paper notes that these results "indicate sex-specific differences in metabolic demands that are modulated by temperature," implying that thermal conditions differentially alter energetic maintenance costs for males and females in a sexually dimorphic species.

Researchers suggest that climatic variation and the associated stress may be significant factors in sexual dimorphism’s etiology. A more ‘male’ morphotype and reduced dimorphism are appreciated in the pelves of Native Alaskans than the Terry sample. This research highlights a reduction in sexual dimorphism in populations under greater climatic stress and contributes to the production of more accurate skeletal assessments in future investigations.

Sexually selected traits, such as weapons and ornaments, are hypothesized to be costly to produce and maintain, but the nature of these costs remains poorly understood. Here, we provide the first direct measurements of the metabolic costs of maintaining a sexually selected weapon. We show that males with relatively larger weapons have higher mass-specific resting metabolic rates, indicating that the maintenance of the weapon is energetically expensive. These results demonstrate a hidden energetic cost of sexually selected traits that can influence their evolution.

First, heat stress affected reproductive performance of males and females. Second, mating systems altered heat stress responses in both sexes. We showed that sexual selection moderates heat stress response in males and females in a beetle system, with polygynous mating intensifying male reproductive costs under high temperature compared to monogamous mating. These findings suggest that the interaction between sexual selection and heat stress can impose sex-specific fitness costs, potentially influencing the evolution of mating systems as environments become warmer.

This long-term experiment on the western clawed frog (Xenopus tropicalis) examined the effects of warming from zygote to adult. The authors state that "warming attenuates sexual dimorphism" and that a 5 °C increase in ambient temperature "can establish a new metabolic state, resulting in elevated oxidative stress and a shift in energy allocation towards immune defense at the expense of sexual development." They argue that limited energy resources must be flexibly invested among growth, immunity and reproduction, leading to temperature-dependent trade-offs that can reduce investment into sexually dimorphic traits under thermal stress.

This review summarizes mechanisms for sex differences in energy homeostasis. It notes that "sex differences exist in the regulation of energy homeostasis" and that multiple organs (brain, liver, fat, muscle) contribute to the maintenance of energy balance in a sexually dimorphic fashion. The authors highlight that high circulating estrogens "prevent obesity in female animals" and that estrogen receptor alpha and androgens differentially regulate energy expenditure, feeding, and body weight in males and females. The paper thus frames sexual dimorphism in hormonal and neural control of metabolism as tightly linked to energetic balance and maintenance costs.

One of the most generalized conclusions arising from studies analyzing the ecological variation of energy metabolism in endotherms is the apparent negative correlation between ambient temperature and mass-independent basal metabolic rate (residual BMR). As a consequence, ambient temperature has been considered the most important external factor driving the evolution of residual BMR. We found a significant interaction between sex and mean annual temperature as predictors of residual BMR, such that the effect of temperature on residual BMR decreases as a function of sex…We present a novel energetics model demonstrating that it is also possible that females have been under stabilizing selection pressure for an intermediate basal energy expenditure to maximize energy available for reproduction.

Growing evidence suggests that increasing temperatures can have direct negative effects on fertility, negatively impacting population fitness. Heat stress caused near-complete male sterility – with no evidence of recovery – and reduced female mating success…In addition to disrupting fertility, elevated temperatures can affect an organism’s ability to mate. For example, exposure to heat stress during courtship can directly alter activity levels and metabolism, while developmental exposure can influence adult physiology and behaviour. Male mating signals, which demand substantial energy investment, are thought to reflect fertility and/or quality.

Bioenergetic differences in energy acquisition and metabolism help to explain sexual size dimorphism in percids. Females consistently had higher energy densities and growth efficiencies than males, and males had higher mass-specific standard metabolic rates than females. These differences suggest that the energetic costs of being male are higher than those of being female, and that sexual size dimorphism may be driven by sex-specific energy acquisition and allocation.

Sexual dimorphism and sexual selection are analyzed within a unified economic framework in which the only fundamentals in a life history model are the energetic costs of various control variables. The paper interprets most observations of sexual dimorphism by focusing on the energetic costs and benefits of sexually selected traits and behaviors. It emphasizes that the evolution of sexual dimorphism is constrained by the energetic budget of individuals, making sexually selected traits costly to produce and maintain.

Results from this study will allow for the comparison of physiological and developmental responses to temperature stress between species with different thermal niches. We examined thermal stress responses between Drosophila species, including differences between males and females, to understand how sex and species identity shape tolerance to heat and cold. Our data show that thermal stress impacts development time, survival and reproductive traits, with some sex-specific differences in resilience to high temperatures, highlighting that sexual dimorphism in thermal physiology can influence species’ vulnerability under climate change.

This article introduces the Research Topic "Sexual Dimorphisms on Central and Peripheral Control of Energy Balance." The editors note that "sex differences in energy balance, obesity, and related metabolic disorders are well recognized" and that sex hormones and sex chromosomes influence central and peripheral mechanisms regulating feeding, energy expenditure, and substrate use. The collection aims to explore "how sexual dimorphisms in energy balance arise and how they impact susceptibility to metabolic disease," highlighting that maintaining sexually dimorphic energy balance involves distinct metabolic and regulatory costs in males and females.

This study examined sexual dimorphism in the serum metabolome following acute maximal exercise. The authors report that "distinct metabolic response patterns between males and females" are observed when standardized exhaustion endpoints are reached, with sex-specific differences in amino acid, lipid, and energy metabolism pathways. They conclude that men and women show divergent metabolic costs and recovery profiles under acute physiological stress, indicating that sexually dimorphic physiological states have different energetic and metabolic consequences under stress conditions.

We developed a model of energy expenditure for reproduction that includes energy expenditure for maintenance, hunting, prey capture, lactation, climbing, and thermoregulation. Using radio telemetry, we followed the movements of male and female tamarins to estimate energy expenditure. Sexual dimorphism in body size and reproductive roles led to differences in energy budgets, with lactating females incurring substantially higher energetic costs than males due to reproductive activities and thermoregulation demands.

This paper shows that heat impacts emerge from the interaction of biological differences and gendered social roles. Physiological factors shape thermoregulation and vulnerability to heat, including sex-specific differences in body composition, cardiovascular function and hormonal profiles. More specifically, the paper examines the gender-differentiated biological impacts of heat (including effects on fertility), how gender roles and responsibilities shape exposure, and the implications of intensifying heat stress for equality and resilience in a warming world.

This study investigated how sexual dimorphism shapes renal metabolic adaptation to a high-fat diet and other metabolic challenges. The authors report that kidneys, which "exhibit sexual dimorphism," show sex-specific changes in metabolic pathways, substrate utilization, and oxidative stress responses under metabolic stress. They conclude that "sexual dimorphism shapes renal metabolic adaptation" and that male and female kidneys respond differently to energetic and metabolic stressors, implying sex-specific maintenance costs of renal metabolic homeostasis.

Researchers suggest that climatic variation and the associated stress may be significant factors in sexual dimorphism’s etiology. The results reinforce the complex and multifaceted relationship between climate and sexual dimorphism. This research highlights a reduction in sexual dimorphism in populations under greater climatic stress and contributes to the production of more accurate skeletal assessments in future investigations.

Females bear the energetic cost of producing eggs, which is much greater than the cost of making sperm by the males. Presumably, increased sexual dimorphism means males are brighter and more conspicuous, leading to increased predation. Moreover, the production of more exaggerated ornaments in males may come at the cost of suppressed immune function. Reproductive benefits arise in the form of a larger number of offspring, while natural selection imposes costs in the form of reduced survival.

Thermal stress was linked to lower lipid and protein content, and individuals from disturbed sites had reduced energy reserves. We also found a weak but significant negative relationship between thermal stress and condition index, suggesting that high heat loads compromise energy balance. Our results indicate that high thermal stress has physiological costs, decreasing energy stores available for growth, reproduction and maintenance. These energetic costs of being hot may interact with other ecological pressures to shape organismal performance under climate warming.

Sexual selection plays a profound role in the process of adaptation, giving rise to extraordinary sexually dimorphic traits observed in various species, such as elaborate male ornaments and weaponry. These traits are often costly to produce and maintain, and their expression can depend on condition and environmental context. Understanding how genomic architecture underlies these sexually dimorphic traits and sexual conflict can reveal how such costly traits evolve and are maintained.

Sexual dimorphism, the differences between males and females of the same species, such as in color, shape, size, and structure, often arises from sexual selection. Many sexually dimorphic traits, including bright coloration, enlarged body size, or elaborate ornaments, are associated with increased energetic costs, higher predation risk, or reduced thermal tolerance, and thus involve trade-offs with survival. Environmental conditions, such as climate and resource availability, can influence the degree and expression of sexual dimorphism.

By eliminating reproductive investment via surgical ovariectomy (OVX), we can dramatically increase survival, energy storage, maintenance physiology, and locomotor performance relative to intact (SHAM) females. The survival and growth costs of reproduction increase as average body condition (an index of environmental quality) increases across islands. Natural selection on body size also changes as a function of average body condition, indicating that environmental quality modulates the energetic and survival costs of reproduction.

More specifically, the paper examines the gender-differentiated biological impacts of heat (including effects on fertility), how gender roles and responsibilities shape exposure, and the implications of intensifying heat stress for equality and resilience in a warming world. It synthesizes evidence that male fertility can be reduced by high temperatures and that pregnant women and older women are disproportionately affected by heat-related health risks, illustrating sex- and gender-based differences in the costs of thermal stress.

In this symposium short talk, Lydia Lynch discusses a metabolic basis for sex differences in the immune system using mouse models. She reports that males are heavier and have increased metabolic rate at room temperature, but when mice are housed at thermal neutrality "we actually find that there is no fundamental difference in whole body metabolism between sexes" once body mass is accounted for. However, at thermoneutrality "much more sex differences were revealed" in substrate utilization, with females maintaining lipid burning and higher core body temperature compared to males. These findings suggest that environmental temperature modulates how sex differences in metabolism and immune function are expressed and what their energetic costs are.

The University of Toronto thesis on walleye (Sander vitreus) explores "effects of temperature, body size, and sex on the standard metabolic rates" in a sexually dimorphic freshwater piscivore. The document reports that male and female fish differ in SMR across temperature gradients, and that an interaction between sex and temperature influences metabolic demands. The work supports the idea that thermal conditions interact with sexual dimorphism to shape baseline energetic costs in ectotherms.

Sexual dimorphism refers to the situation where the two sexes of a species display distinct traits, especially those not directly linked to reproduction. Historically, women spent a much larger portion of their fertile years pregnant—pregnancy is both energetically costly and risky. This created a markedly different environment for women compared to men, so it is logical that they evolved distinct adaptations. Sexual dimorphism is beneficial overall because it lets each sex specialize for distinct ecological roles or life histories, avoiding a phenotypic compromise between them.

Most of the studies on sexual dimorphism and energetic costs do not explicitly test thermal stress, but it is well established that metabolic rate and the energetic cost of maintenance increase with temperature in ectotherms. Because sexually dimorphic traits (larger body size, ornaments, weapons, pregnancy) add to maintenance and reproductive costs, it is expected that these costs will increase under more thermally stressful environments where metabolic demands are higher.