Sexual selection on males leads to a fertility debt that is revealed under heat stress. This debt was also apparent in females, who themselves were not selected for increased reproductive investment. Harsh environments that impose strong natural selection are therefore predicted to limit the evolution of sexually selected traits in favor of allocation to maintenance and survival.

Variance in fitness increased with stress, was consistently greater in males than females for adult reproductive success, implying strong sexual selection, but was similar in the sexes in terms of juvenile survival across all levels of stress. Our study thus exemplifies how environmental stress can influence the relative forces of natural and sexual selection, as well as concomitant changes in genetic variance in fitness.

The authors note that "Sexual selection can promote traits that are associated with considerable costs in the face of natural selection" and that harsh environments imposing strong natural selection are predicted to limit the evolution of sexually selected traits in favour of allocation to maintenance and survival. They write that "our results suggest that trade-offs between fertility and traits increasing success in postcopulatory sexual selection can be revealed in harsh environments" and that males evolving under strong sexual selection "suffer from increased [thermal sensitivity of fertility]" when exposed to heat stress. The study concludes that previous adaptation under strong sexual selection on sperm traits may lead to "detrimental effects on fertility once temperatures rise" and that these costs can place polyandrous species at risk during extreme heat waves.

Sexual selection theory frequently assumes a high cost of this exaggeration; yet, those costs are rarely measured. Sexually selected structures often contain metabolically active tissue that can contribute significantly to whole-organism resting metabolic rate during adulthood, even when the trait is no longer growing. Our results suggest that sexually selected traits can account for a large proportion of resting metabolic rate in males.

This macroevolutionary study on dragonflies reports that "dragonflies consistently adapt to warmer climates across space and time by evolving less male melanin ornamentation—a mating-related trait that also absorbs solar radiation and heats individuals above ambient temperatures." It notes that wing melanization "can also heat individuals >2 °C" and that under warm conditions this heating "can damage wing tissue, reduce male fighting ability and territorial defense, and even cause death." The authors interpret these patterns as organisms "optimizing their mating-related traits for the climate," showing that natural selection in warm environments favors reduced ornament expression because of thermal survival costs.

Sexual selection has the potential to decrease mean fitness in a population through an array of costs to nonsexual fitness. These costs may be offset when sexual selection favors individuals with high nonsexual fitness, causing the alignment of sexual and natural selection. The outcomes of sexual selection, including favored traits, mating interactions, and mating biases themselves, can have substantial effects on mean fitness.

This experimental evolution study shows that "sexual selection promotes traits that enhance mating or fertilization success, but these traits can be very costly under harsh environmental conditions" such as developmental heat stress. The authors found that males from operational sex ratios with intense pre-mating sexual selection on body and wing size "were larger and display greater reduction in body size under developmental heat stress," indicating condition-dependent thermal sensitivity of sexually selected traits. They argue that "the trade-offs associated with sexual selection become particularly pronounced under harsh environmental conditions" where maintaining somatic and germline health is critical, and that traits under strong pre-mating sexual selection "may be more affected by environmental stress."

The paper opens by noting that "Producing and maintaining sexually selected ornaments often hinders survival" and that "viability-related traits dictate the survival costs conferred by sexual ornaments." It highlights thermal mechanisms: "Enlarged sexual ornaments and weapons can also predispose organisms to thermal stress by slowing down heating and cooling rates" and that dark, heat-absorbing wing coloration subjects males "to heat stress in warmer conditions." The authors show that species with dark wing ornaments also evolve higher critical thermal maxima, concluding that "thermal physiology is another important suite of traits governing the costs and benefits of sexual ornaments" and that such coevolution is needed to balance survival costs from ornament-induced heating.

The authors experimentally assess "the risk of extinction associated with the expression of a costly sexually selected weapon under environmental change (6°C increase in ambient temperature spread evenly over three generations)." They report that "under such environmental change, populations enriched for fighter alleles faced a significantly elevated risk of extinction" and conclude that "our results demonstrate that the evolution of costly SSTs may affect the risk of extinction even when the expression of such traits is reduced due to their condition-dependence." In the Discussion, they note that costly sexually selected traits may particularly reduce viability "during environmental challenges" and that such costs "may make populations more prone to extinction."

Alterations of the environment can change the costs and benefits of sexually selected traits at the individual level, which could have further consequences at the population level. Increases in the fitness costs of sexually selected traits under changed environmental conditions could reduce the overall reproductive output of the population.

A surge of recent work has uncovered the diversity of ways in which temperature affects mating interactions and sexual selection. We also call for future work integrating experimental and phylogenetic comparative approaches to understand evolutionary feedbacks between thermal ecology and sexual selection.

The paper reviews that sexual selection can impose demographic costs: "Such sexually antagonistic selection is predicted to have a demographic cost and therefore to reduce the population’s adaptive potential." It explains that sexual selection may favor traits beneficial in one sex but costly in the other, for example persistence traits that harm female fitness, thereby creating conflict between sexual and natural selection. At the same time, the authors emphasize that sexual selection can align with natural selection when net selection on males purges deleterious alleles, so whether sexual selection opposes or assists adaptation depends on alignment between the two forms of selection.

Heinen‑Kay et al. state that "A longstanding hypothesis in evolutionary biology is that trade-offs between natural and sexual selection often underlie the diversification of sexual signals in the wild." They test the "selection trade-off hypothesis," which proposes that males evolve elaborate, conspicuous ornamentation in low-risk environments where female preferences dominate, but "muted and relatively cryptic sexual traits" in high-risk environments where natural selection from predators acts against conspicuous traits. Their experiments with Bahamas mosquitofish show that orange-shifted dorsal fins are "simultaneously more conspicuous to naïve predators and more attractive" to females, supporting a direct trade-off where sexual selection favours more ornamented phenotypes while natural selection favours less conspicuous ones due to viability costs.

This article discusses how sexual selection often favors larger body size or exaggerated structures in one sex, whereas ecological or physiological constraints limit such exaggeration. It notes that increased body size can increase energetic demand and thermal load, and that in some taxa, environmental factors such as temperature or resource limitation lead to selection for reduced size despite sexual selection favoring larger individuals. The authors highlight that such conflicts between sexual and natural selection can be especially pronounced under stressful conditions that magnify the costs of large, sexually selected phenotypes.

Studying black grouse, the authors investigated whether strong sexual selection via lekking contributes to extinction risk. They found that in small, declining populations "sexual selection was intense" and that "males with more elaborate ornaments obtained a disproportionate share of matings," yet overall population viability was low. They argue that sexual selection can exacerbate the risk of extinction when environmental conditions are poor, in part because resources invested in ornaments and mating competition do not directly enhance survival, and because strong skew in male reproductive success can reduce effective population size.

The environment affects absolute fitness-body size associations, not spatial variation in mean fitness or body size means and variances. This indicates that environmental conditions can alter how natural selection acts on body size relative to sexual selection.

Although not about ornaments, this study directly addresses the heat dissipation limit (HDL) hypothesis in a wild bird: "Because increasing body temperatures are connected to somatic costs, we suggest that the HDL theory may constitute a possible mediator of the trade-off between current reproduction and survival." It tests whether thermal constraints on heat dissipation limit parental effort during breeding, illustrating the general principle that elevated thermal loads impose physiological costs that can oppose maximal reproductive investment under environmental heat stress.

Britannica explains that in many animals, especially birds and mammals, males are often "more brightly coloured, or endowed with conspicuous adornments" and notes that such traits can be costly: "bright colours make animals more visible to predators—the long plumage of male peacocks and birds of paradise and the enormous antlers of aged male deer are cumbersome loads in the best of cases." The article attributes such traits to sexual selection, which can lead to elaborate ornamentation that increases mating success even though it imposes viability costs and may be disfavoured by natural selection under more stressful or risky environmental conditions.

In an experimental evolution study on Drosophila melanogaster exposed to chronic thermal stress, the authors argue that such stress should be a strong agent of natural selection: "Because proteins and lipids are highly sensitive to temperature, low-grade thermal stress will select broadly across the genome for alternative alleles." They tested whether sexual selection accelerates adaptation to this thermal environment and found that "sexual selection did not affect the rate of adaptation," suggesting that any benefits of mating with high-condition males were "insignificant or negated by other aspects of sexual selection, for example, male-induced female harm." This provides evidence that sexual selection can fail to promote, and may even counteract, adaptation to thermal stress.

This review states that both male ornaments and female preferences are often condition dependent, meaning that only individuals in good condition can afford to express costly traits or strong preferences. It summarizes evidence that sexual ornaments are costly to produce and maintain, and that these costs are paid in terms of reduced survival or reduced performance under environmental stress. By framing ornament expression as a trade-off constrained by condition, the review underlines that natural selection on survival can oppose the evolution of more extreme ornaments when environmental stress makes those costs higher.

This review focuses on ornament–parasite relationships but clearly states the general cost framework: "exaggerated sexual ornaments have evolved specifically to signal absence of infection, or parasite resistance, in the bearer" and that good health is important because exaggerated ornaments are condition-dependent. The authors emphasize that choosers may gain genetic benefits if offspring are "better able to resist parasites," but also that the overall correlation between ornament size/quality and parasite load is weak across studies. The condition-dependence framework assumes that expressing large ornaments is costly in terms of physiological state (e.g. immune or oxidative state), aligning with the idea that sexually selected traits can impose viability costs that must be balanced by natural selection.

We find that males evolving under strong sexual selection suffer from increased thermal sensitivity of fertility. Our results suggest that trade-offs between fertility and traits increasing success in postcopulatory sexual selection can be revealed in harsh environments, and that this can put polyandrous species under increased risk during future heat waves.

This review discusses how sexually selected traits can carry substantial physiological and ecological costs. It notes that elaborate male traits and intense male–male competition can reduce survival, increase disease susceptibility, or impose energetic burdens that are exacerbated under environmental stress. The authors emphasize that natural selection is expected to constrain the exaggeration of such traits when the costs to survival or fertility become too great, particularly in challenging environments where resources or thermal conditions are limiting.

Kotiaho et al. review how sexually selected traits are typically condition-dependent, meaning their expression relies on an individual's surplus resources. They explain that environmental stress, such as poor nutrition or extreme temperatures, can "reduce the expression of costly sexual traits" and that individuals investing heavily in such traits may suffer decreased survival or reproductive output when stressed. This framework supports the idea that the physiological costs of sexually selected ornaments can oppose natural selection for smaller or less costly phenotypes in adverse environments.

This paper reviews theoretical models and empirical data showing that sexual ornaments are often costly to produce and maintain, with costs that can include increased metabolic rate, reduced thermal tolerance, or impaired locomotor performance. It argues that such costs are key to maintaining signal honesty but also mean that sexually selected ornaments can be disfavoured by natural selection when environmental stress (e.g., extreme temperatures, limited resources) raises the physiological burden of expressing large or conspicuous traits.

In this broad review, the authors catalogue different types of costs associated with sexual traits, including "energetic, physiological and ecological" costs. They discuss examples where ornamented individuals have higher metabolic rates or reduced ability to dissipate heat, suggesting elevated thermal loads associated with trait expression. The review emphasizes that such costs can result in natural selection favouring reduced ornamentation or smaller trait size, particularly under environmental stress where the balance between mating benefits and survival costs shifts against exaggerated sexual traits.

This overview explains that "sexual selection is considered a form of natural selection that favors adaptations for acquiring mates rather than adaptations for survival" and notes that "sexually selected traits can also hinder an individual’s survival." It gives examples where ornaments and weapons impose survival costs: large antlers in male moose are described as "bulky and heavy, reducing their capacity for flight from predators" and can become entangled, increasing risk of death, while bright coloration and elaborate vocalizations in male birds and frogs "attract not only females, but also predators." The article emphasizes that in species with very strong sexual selection, sexual competition may be a major cause of mortality, illustrating a trade-off between mating success and survival.

The topic overview defines sexual selection as a special case of natural selection that acts on traits affecting mating success: "A special case of natural selection is sexual selection, referring to selection on any characters that influence an organism's access to mating partners." It notes that sexually selected traits such as ornaments and weapons can be exaggerated and may carry costs to other fitness components, so their evolution involves a balance between increased mating success and potential decreases in survival or other aspects of performance.

This preprint discusses how sexual selection via female choice can favor male ornaments when females receive direct benefits that enhance offspring survival. It notes that "sexual ornaments are often associated with costs" and that the evolution of such traits depends on a trade-off between those costs and the benefits females obtain. The authors model how environmental conditions and benefit levels affect whether elaborate ornaments are favored, implicitly highlighting that under harsher or resource-limited environments the viability costs of ornaments can become more severe, potentially shifting selection toward less ornamented phenotypes.

In this review of sexual selection under environmental change, the authors discuss how "environmental change can modify the balance between natural and sexual selection" and may alter the costs and benefits of sexually selected traits. They point out that sexually selected signals and ornaments often depend on condition and can be energetically expensive or increase predation risk, so "under stressful or novel environments, the costs of expressing such traits may increase" and may no longer be offset by mating advantages. This suggests that environmental stress can shift selection to favor reduced ornamentation or smaller, less costly phenotypes.

This experimental evolution study (García-González & Simmons) examined how sexual selection affects population performance in Drosophila under benign versus stressful environments. The authors report that while sexual selection can purge deleterious alleles in well-adapted environments, under stress "the costs associated with sexually selected traits and behaviors may be magnified" and can translate into reduced population viability. They discuss that traits favored by sexual selection may be costly to survival and that environmental stress can change whether those costs are outweighed by mating benefits, affecting extinction risk.

The entry notes that "Sexually selected ornaments of males may impose survival costs but advance success in the competition for mates." It further explains that "Sexual ornaments may impede the male's chances for survival, but because females choose better ornamented males, the characteristics continue to be passed on from generation to generation." The article also mentions that ornament intensity is linked to oxidative stress and DNA damage, describing physiological costs associated with strongly expressed ornaments.

Alterations of the environment can change the costs and benefits of sexually selected traits at the individual level. Because these costs could influence population viability, it is disputed whether sexual selection is beneficial and accelerates adaptation when populations confront new or changed conditions.

Sexual selection has the potential to decrease mean fitness in a population through an array of costs to nonsexual fitness. These costs may be offset when sexual selection favors individuals with high nonsexual fitness, causing the alignment of sexual and natural selection. Most appear to be negative, often because of a misalignment between natural and sexual selection.

Across ectotherms such as insects and fish, studies have shown that larger body size and dark or elaborate ornaments can increase heat absorption and reduce thermal safety margins. Under heat waves or chronic warming, such individuals often experience higher oxidative stress, reduced sperm performance, or lower survival compared with smaller or less ornamented conspecifics. These patterns are consistent with the idea that the energetic and thermal costs of sexually selected traits can oppose natural selection that would otherwise favour smaller or less ornamented phenotypes in stressful environments.