Abstract Global temperatures are rising and inbreeding is increasingly common in wild populations as habitats become fragmented and population sizes shrink. There is extensive research on how inbreeding or elevated temperatures affect animal performance, but little is known about how these two factors interact to affect sexually selected behavior and reproductive traits. We ran a 2x2 experiment to investigate how developmental temperature (26°C or 30°C) and inbreeding (outbreeding or full-sibling mating) affect female guppies (Poecilia reticulata). We reciprocally cross-bred full-siblings to create inbred and outbred females and measured their sexual attractiveness (via male mate choice tests), fecundity, egg diameter and total egg mass (N = 120 females). Female attractiveness was affected by an interaction between breeding status and temperature. For females reared at 26°C, males preferred outbred females over inbred females and over both inbred and outbred females raised at 30°C. In contrast, for females reared at 30°C, males did not prefer outbred females. Inbreeding and elevated developmental temperature independently reduced egg diameter, indicating that both factors reduce offspring provisioning. Neither inbreeding nor developmental temperature affected fecundity or total egg mass. However, larger and older females produced more eggs, and larger females had heavier clutches. Our results suggest that the fitness effect of inbreeding for females depends on developmental temperature. Climate warming may intensify these genotype–environment interactions thereby increasing the costs of inbreeding. The efficacy of sexual selection in removing deleterious recessive alleles could be reduced if males are less likely to avoid inbred females as temperatures rise.

Global temperatures are rising and inbreeding is increasingly common in wild animals as populations decline. There is extensive research on inbreeding and temperature stress, but little is known about how they interact to affect sexually selected traits. We therefore investigated how developmental temperature (26°C or 30°C) and inbreeding affect male guppies (Poecilia reticulata). We reciprocally cross-bred full-siblings to create inbred and outbred fish, then measured traits under either pre-copulatory (i.e., coloration, gonopodium length, sexual attractiveness, and mating behavior) or postcopulatory (i.e., sperm number, and velocity) sexual selection for ∼120 adult males. There was no evidence that temperature and inbreeding interact to affect trait expression; instead, their effects were additive. Males reared at 30°C were significantly less attractive to females, and made fewer sigmoid mating displays, than males reared at 26°C. Inbred males were also less attractive, but their mating behavior did not differ from that of outbred males. Sperm number and velocity were unaffected by inbreeding or developmental temperature. Unexpectedly, males reared at 30°C and inbred males both had significantly more orange coloration. Our results show that inbreeding and developmental temperature independently influence some sexually selected male traits, suggesting that climate change and habitat fragmentation might alter evolution under sexual selection.

Ageing of adult males could be accelerated by both high mating/reproductive effort and fighting for mates. Testing the relative importance of these factors is challenging, however, because males that win fights also tend to have more mates. We used a 2 × 2 experimental design to test how a prolonged (9 week) period of either winning or losing fights, and either high or low reproductive effort (manipulating by varying access to females) interacts to affect male ageing and future reproduction allocation in the mosquitofish, Gambusia holbrooki. We measured telomere length and several life-history traits, including mating effort and ejaculates (sperm count and velocity). After 9 weeks, there were significant differences between winners and losers in their mating effort but not in their ejaculates. Males with a higher past reproductive effort (i.e., access to females) had significantly lower current mating effort and grew more slowly. Males with a higher past reproductive effort also had slower swimming sperm, but only if they were smaller than average in body size. Surprisingly, neither males with a higher past reproductive effort nor males that repeatedly lost fights had shorter telomeres. Our findings show that past social dynamics affect how males allocate resources to reproduction and somatic maintenance.

Climate change and human activities are elevating the level and variability of salinity in freshwater ecosystems. Consequently, many aquatic species now experience more extreme developmental environments. Resultant shifts in developmental trajectories could change key life-history traits that persist into adulthood. The 'silver spoon' hypothesis posits that favourable developmental conditions lead to faster growth, earlier maturation and greater reproductive success. In contrast, the 'predictable adaptive response' hypothesis suggests that faster growth and earlier reproduction should be selected for under stressful developmental conditions because stress provides cues about a higher risk of mortality in future environments. To understand life-history responses to salinity during development, we reared a global pest, mosquitofish (Gambusia holbrooki), from birth in either freshwater control (0‰), stable-saline (10‰), or fluctuating-saline environments (0‰-20‰; mean = 10‰) until maturation. We then monitored their performance in early and late adulthood in a common garden setting. Fish in fluctuating salinity grew more slowly and had a reduced reproductive output (lower sperm count, smaller eggs) than those in stable elevated salinity. These differences are consistent with a more stable environment providing a 'silver spoon' effect. Conversely, fish in stable elevated salinity grew faster and matured earlier than those in freshwater, supporting a 'predictive adaptive response' whereby salinity is a stressor triggering faster development and accelerates reproduction. In addition, fluctuations in salinity altered the effect of higher salinity on self-maintenance. Stable elevated salinity caused a decrease in male telomere length and female gut length, but fluctuating salinity caused an increase in female gut length. Our results suggest that fluctuating versus stable salinity during development leads to distinct fish life histories. The effect sizes for some traits differed significantly between males and females, suggesting sex-specific responses to climate fluctuations.

Human‐induced climate change is a driver of extinction, with extreme events occurring more frequently. It increases both the amount and fluctuations in environmental stress that organisms experience. In such environments, greater intra‐specific trait variation creates more potential for adaptation through natural selection. Many studies focus on changes in trait means between novel and historic environments but overlook changes in trait variation. Here we tested how salinisation – which currently affects 20–50% of freshwaters worldwide – alters trait variation in a freshwater fish, Gambusia holbrooki . We reared newborn fish in freshwater, stable‐salinity or fluctuating‐salinity water until maturation, and then compared variation in fitness‐related traits in each sex during early and late adulthood. Salinisation had stronger effects on young virgin adults than older mated adults, with sex‐specific impacts (female: gut development; male: age at maturity and body size). When we accounted for the mean trait value in each environment, salinisation also affected relative variation (i.e. the coefficient of variation) in female fecundity (egg size, offspring number). Notably, the fluctuating‐salinity treatment did not magnify the effect of the stable‐salinity treatment on trait variation but sometimes reversed its effect. Our findings suggest that researchers should pay closer attention to fluctuating environmental stressors, as they can differ from stable stressors in how they affect between‐individual variation in trait expression.

In many animal species, cognitive abilities are under strong natural selection because decisions about foraging, habitat choice and predator avoidance affect fecundity and survival. But how has sexual selection, which is usually stronger on males than females, shaped the evolution of cognitive abilities that influence success when competing for mates or fertilizations? We aimed to investigate potential links between individual differences in male cognitive performance to variation in paternity arising solely from sexual selection. We therefore ran four standard cognitive assays to quantify five measures of cognitive performance by male mosquitofish (Gambusia holbrooki). Males were then assigned to 11 outdoor ponds where they could compete for females. Females mate many times, which leads to intense sperm competition and broods with mixed paternity. We genotyped 2,430 offspring to identify their fathers. Males with greater inhibitory control and better spatial learning abilities sired significantly more offspring, while males with better initial impulse control sired significantly fewer offspring. Associative and reversal learning did not predict a male's share of paternity. In sum, there was sexual selection on several, but not all, aspects of male cognitive performance.

With climate change, animals face both rising temperatures and more variable food availability. Many species have evolved an adaptative response to historic variation in food availability: they grow faster after a period of diet restriction ("compensatory growth"). However, higher temperatures may reduce the capacity for compensatory growth in ectotherms because individuals require more resources to support their increased metabolism. We experimentally tested how higher temperature affects compensatory growth by raising guppies (Poecilia reticulata) at a high or control temperature, and on a normal or temporarily restricted diet during early development. At the control temperature guppies on the restricted diet grew faster once their diet returned to normal. Both sexes showed compensatory growth. At the high temperature, both sexes also increased their growth rates after dietary restriction ended, but the life history outcomes differed. Males at the high temperature matured earlier and were smaller than males reared at the control temperature. In contrast, females at the high temperature matured later and were bigger than females at the control temperature. Our study highlights that males and females can have different responses to the same environmental stressors.

Inbreeding plays a strong role in shaping life-history traits and behaviours. Supporting evidence for this role often comes from observational studies. Experiments that establish causality and formally test how environmental factors moderate any effects of inbreeding remain underutilized. We ran an experiment to test how developmental temperature and inbreeding influence key life-history traits (growth rate, size and age at maturity, survival, and sex ratio) and locomotor behaviours (boldness and habituation) in juvenile guppies (Poecilia reticulata). We used a controlled breeding design to generate inbred and outbred individuals that were then reared under a control (26 °C) or an elevated temperature (30 °C) until maturity. Developmental temperature strongly affected life-history traits; both sexes matured earlier at 30 °C, but only males exhibited slower early growth and reduced size at maturity. Female growth and size at maturity were unaffected. The higher developmental temperature reduced boldness in both sexes; however, only females at 26 °C habituated to the novel test environment. In contrast, inbreeding had no significant effects on any of the measured traits, nor did it significantly interact with temperature. In sum, under our experimental conditions, developmental temperature is the primary driver of phenotypic plasticity in guppies, generating sex-specific responses in both life-history traits and behaviour, while inbreeding between siblings had no detectable effects on any of the measured traits. Our findings highlight the key role of temperature in shaping developmental and behavioural trajectories, and reveal that a single generation of inbreeding may not always affect life-history traits, even under environmental stress.

Males often strategically adjust the number of available sperm based on the social context (i.e. sperm priming response), but it remains unclear how environmental and genetic factors shape this adjustment. In freshwater ecosystems, high ambient temperatures often lead to isolated pools of hotter water in which inbreeding occurs. Higher water temperatures and inbreeding can impair fish development, potentially disrupting sperm production. We used guppies (Poecilia reticulata) to investigate how developmental temperature (26 degrees C, 30 degrees C) and male inbreeding status (inbred, outbred) influence their sperm priming response. We also tested if sperm priming was affected by whether the female was a relative (sister) and whether she was inbred or outbred. There was no effect of rearing temperature; male inbreeding status alone determined the number of available sperm in response to female presence, her inbreeding status, and her relatedness. Inbred males produced significantly more sperm in the presence of an unrelated, outbred female than when no female was present. Conversely, outbred males did not alter the number of sperm available in response to female presence or relatedness. Moreover, inbred males produced marginally more sperm when exposed to an unrelated female that was outbred rather than inbred, but there was no difference when exposed to an inbred female that was unrelated versus related. Together, a sperm priming response was only observed in inbred males when exposed to an outbred female. Outbred females in our study were larger than inbred females, suggesting that inbred males strategically allocated ejaculate resources toward females in better condition. In freshwater ecosystems, climate warming leads to isolated pools of hotter water which can lower local female availability and also increase the risk of inbreeding. We studied how elevated temperatures, inbreeding, mate availability, and mate quality affect strategic sperm investment in guppies. Developmental temperature did not affect the number of available sperm. Inbred males produced more sperm when exposed to an outbred female, while outbred male did not exhibit strategic sperm investment.

Male-male contests for access to females or breeding resources are critical in determining male reproductive success. Larger males and those with more effective weaponry are more likely to win fights. However, even after controlling for such predictors of fighting ability, studies have reported a winner-loser effect: previous winners are more likely to win subsequent contests, while losers often suffer repeated defeats. While the effect of winning-losing is well-documented for the outcome of future fights, its effect on other behaviors (e.g. mating) remains poorly investigated. Here, we test whether a winning versus losing experience influenced subsequent behaviors of male mosquitofish (Gambusia holbrooki) toward rivals and potential mates. We housed focal males with either a smaller or larger opponent for 24 h to manipulate their fighting experience to become winners or losers, respectively. The focal males then underwent tests that required them to enter and swim through a narrow corridor to reach females, bypassing a cylinder that contained either a larger rival male (competitive scenario), a juvenile or was empty (non-competitive scenarios). The tests were repeated after 1 wk. Winners were more likely to leave the start area and to reach the females, but only when a larger rival was presented, indicating higher levels of risk-taking behavior in aggressive interactions. This winner-loser effect persisted for at least 1 wk. We suggest that male mosquitofish adjust their assessment of their own and/or their rival's fighting ability following contests in ways whose detection by researchers depends on the social context. The winner-loser effect occurs when previous winners of fights are more likely to repeat their fighting success. But can the winner-loser effect influence male behaviors in other social contexts? In male mosquitofish, winners increase their aggressiveness in the early stage of male-male competition for mating, and this effect endures for at least 1 wk. Our results indicate a long-lasting influence of a winning/losing experience on behaviors closely related to male mating success.

Across many taxa, males use elaborate ornaments or complex displays to attract potential mates. Such sexually selected traits are thought to signal important aspects of male 'quality'. Female mating preferences based on sexual traits are thought to have evolved because choosy females gain direct benefits that enhance their lifetime reproductive success (e.g. greater access to food) and/or indirect benefits because high-quality males contribute genes that increase offspring fitness. However, it is difficult to explain the persistence of female preferences when males only provide genetic benefits, because female preferences should erode the heritable genetic variation in fitness that sexually selected traits signal. This 'paradox of the lek' has puzzled evolutionary biologists for decades, and inspired many hypotheses to explain how heritable variation in sexually selected traits is maintained. Here, we discuss how factors that affect mitochondrial function can maintain variation in sexually selected traits despite strong female preferences. We discuss how mitochondrial function can influence the expression of sexually selected traits, and we describe empirical studies that link the expression of sexually selected traits to mitochondrial function. We explain how mothers can affect mitochondrial function in their offspring by (a) influencing their developmental environment through maternal effects and (b) choosing a mate to increase the compatibility of mitochondrial and nuclear genes (i.e. the 'mitonuclear compatibility model of sexual selection'). Finally, we discuss how incorporating mitochondrial function into models of sexual selection might help to resolve the paradox of the lek, and we suggest avenues for future research.

Mating with close relatives (‘inbreeding’) is common in small, fragmented populations. Inbreeding leads to a higher frequency of loci with homozygous recessive alleles, which can have serious consequences for offspring fitness (‘inbreeding depression’). In addition, females may differentially invest resources when they mate with a related or a nonrelated male, which might affect offspring fitness. A decline in the value of traits of inbred offspring, particularly traits displayed early in life, may therefore be caused by lower maternal investment when females mate with a relative (i.e. differential allocation) rather than solely being attributable to greater homozygosity of inbred offspring. In this study, we mated female guppies (Poecilia reticulata) to a brother or an unrelated male. We then measured the proportion of females breeding, their gestation time, offspring number, and offspring size at birth. We also tested if offspring traits are related to their mother’s size, and their father’s sexual coloration (‘attractiveness’). Mating with a brother did not lower the gestation time, or the number or the size of offspring at birth. However, smaller females gave birth to fewer, smaller babies; and females mated to males with more black coloration gave birth significantly sooner. In addition, females were more likely to give birth when mated to a male with more black colouration, but only when he was an unrelated male, rather than their brother. In sum, reproductive success did not differ when a female mated with a brother or unrelated male. There was no evidence for either inbreeding depression or differential maternal allocation on early life history traits when mating with a relative.

The Darwin-Bateman paradigm predicts that females enhance their fitness by being choosy and mating with high-quality males, while males should compete to mate with as many females as possible. In many species, males enhance their fitness by defending females and/or resources used by females. That is, males directly defend access to mating opportunities. However, paternity analyses have repeatedly shown that females in most species mate polyandrously, which contradicts traditional expectations that male defensive behaviours lead to monandry. Here, in an extensive meta-analysis, encompassing 109 species and 1026 effect sizes from across the animal kingdom, we tested if the occurrence of defensive behaviours modulates sexual selection on females and males. If so, we can illuminate the extent to which males really succeed in defending access to mating and fertilisation opportunities. We used four different indices of the opportunity for sexual selection that comprise pre-mating and/or post-mating episodes of selection. We found, for both sexes, that the occurrence of defensive behaviours does not modulate the potential strength of sexual selection. This implies that male defensive behaviours do not predict the true intensity of sexual selection. While the most extreme levels of sexual selection on males are in species with male defensive behaviours, which indicates that males do sometimes succeed in restricting females' re-mating ability (e.g. elephant seals, Mirounga leonina), estimates of the opportunity for sexual selection vary greatly across species, regardless of whether or not defensive behaviours occur. Indeed, widespread polyandry shows that females are usually not restricted by male defensive behaviours. In addition, our results indicate that post-mating episodes of selection, such as cryptic female choice and sperm competition, might be important factors modulating the opportunity for sexual selection. We discuss: (i) why male defensive behaviours fail to lower the opportunity for sexual selection among females or fail to elevate it for males; (ii) how post-mating events might influence sexual selection; and (iii) the role of females as active participants in sexual selection. We also highlight that inadequate data reporting in the literature prevented us from extracting effect sizes from many studies that had presumably collected the relevant data.

Inbreeding impairs the cognitive abilities of humans, but its impact on cognition in other animals is poorly studied. For example, environmental stress (e.g. food limitation and extreme temperatures) often amplifies inbreeding depression in morphological traits, but whether cognition is similarly affected is unclear. We, therefore, tested if a higher temperature (30°C versus 26°C) during development exacerbates any difference in inhibitory control between inbred ( f = 0.25) and outbred guppies ( Poecilia reticulata ). Inhibitory control is an aspect of cognition that is often measured in vertebrates using a detour test, in which animals have to navigate around a transparent barrier to reach a reward. We also tested if inbreeding and temperature affect ‘boldness’, which is a putative personality trait in guppies. Inbreeding lowered inhibitory control of guppies raised at the higher temperature but not those raised at the control temperature. Inbred fish were significantly less bold than outbred fish. In addition, males, but not females, raised at the higher temperature had significantly lower inhibitory control. There was no effect of temperature on the boldness of either sex. Our study is among the first to test if experimentally induced inbreeding impairs cognition in a non-domesticated vertebrate. We show that both inbreeding and higher temperatures during development can affect the behaviour and cognitive abilities of fish. These findings are noteworthy given the twin threats of rising global temperatures and more frequent inbreeding as habitat fragmentation reduces population sizes.

1. Global warming is reducing prey availability in many aquatic systems, raising questions about the combined effects of higher temperatures and lower food availability on fish life histories and reproductive output. 2. In ectotherms, higher temperatures accelerate growth and promote an earlier onset of reproduction. However, when fish have less food during development, resource depletion might constrain these temperature-driven processes. 3. We manipulated water temperature (24 or 28°C) and early-life food availability (control or restricted) for female guppies ( Poecilia reticulata ). We measured how both factors affected key life history traits (growth, reproduction, survival, self-maintenance). 4. Higher temperature significantly affected female life histories. Females at 28°C matured at a larger size, but then grew more slowly and produced fewer, smaller offspring than females at 24°C. The effect of temperature on reproduction persisted even after controlling for body size, suggesting there was a shift in the fecundity-size relationship. 5. Adult mortality was greater at 28°C. Higher temperature also resulted in a longer gut, potentially enhancing resource acquisition, but a higher temperature did not affect immunity or telomere length of the surviving females. 6. Early-life food shortage affected very few traits, except for a weak interaction with temperature that affected total fecundity. At 28°C, females that experienced early-life food restriction produced fewer offspring than females with continual food supply. No such diet effect occurred at 24°C. 7. Our results suggest that tropical fish may be severely impacted by increased temperatures (i.e., decreased reproduction with increased morality), but are likely to be resilient to brief periods of food limitations during early development. 8. Interestingly, early-life food shortage caused a reduction in total offspring number but only at 28°C, suggesting that global prey decline might exacerbate the negative effects of a warming climate on stock recruitment of tropical fish. ### Competing Interest Statement The authors have declared no competing interest.

A long-standing problem in evolutionary theory is to clarify in what sense (if any) natural selection cumulatively improves the design of organisms. Various concepts, such as fitness and inclusive fitness, have been proposed to resolve this problem. In addition, there have been attempts to replace the original problem with more tractable questions, such as whether a given gene or trait is favored by selection. Here, we ask what theoretical properties the concept fitness should possess to encapsulate the improvement criterion required to talk meaningfully about adaptive evolution. We argue that natural selection tends to shape phenotypes based on the causal properties of individuals and that this tendency is, therefore, best captured by a fitness concept that focuses on these properties. We highlight a fitness concept that meets this role under broad conditions but requires adjustments in our conceptual understanding of adaptive evolution. These adjustments combine elements of Dawkinsian gene selectionism and Egbert Leigh's "parliament of genes."

ABSTRACT Climate change is elevating salinity levels in many freshwater systems, and more erratic rainfall is increasing variation in salinity. Consequently, many species now experience more extreme developmental environments. Resultant shifts in developmental trajectories could change key life history traits that persist into adulthood. To date, however, how variation in salinity affects the life histories of freshwater species has been neglected despite its implications for fisheries. We ran a large-scale experiment with a global pest, the mosquitofish ( Gambusia holbrooki ), and manipulated the salinity experienced by juveniles: freshwater (0‰), stable salinity (10‰) or fluctuating salinity (0-20‰; mean = 10 ‰). Fish developing in stable, high salinity grew faster and matured earlier, albeit with a decline in male telomeres and female gut development. Stable high salinity resulted in larger adult body size in females, but not males, which increased female fecundity. Conversely, fluctuations in salinity induced fish to grow more slowly and lowered female fecundity. Crucially, several of the long-term effects of salinity fluctuations were sex-specific, more adversely affecting females than males. We highlight that environmental variability alters an organism’s vulnerability to stressors, with implications that should be considered if we wish to understand the impact of climate change on population dynamics.

Past reproductive effort allows males to assess their ability to acquire mates, but it also consumes resources that can reduce their future competitive ability. Few studies have examined how a male's reproductive history affects his subsequent mate choice, and, to date, no study has determined the relative contribution of past mating behavior and past ejaculate production because these two forms of investment are naturally highly correlated. Here, we disentangled the relative effects of past mating behavior and past ejaculate production in mosquitofish (Gambusia holbrooki) by experimentally preventing some males from ejaculating when trying to mate. We assessed the effect of mating behavior on mate choice by comparing males that had previously been with or without access to females and male rivals for 8 and 16 weeks and assessed the effect of ejaculation on mate choice by comparing males that either could or could not ejaculate when they had access to females for 16 weeks. Reproductive treatment did not affect male attractiveness, but it did affect male mate choice. Somewhat surprisingly, in five of the six treatment-by-age at testing combinations, males preferred a female in the vicinity of a male rival over a solitary female. This preference was marginally stronger for males that had previously engaged in mating behavior but were unaffected by past ejaculate production. We discuss the potential benefits to males of associating with another male when seeking mates. This is the first study to quantify the relative influence of pre- and post-copulatory reproductive investment on male mate choice.

When males compete, sexual selection favors reproductive traits that increase their mating or fertilization success (pre- and postcopulatory sexual selection). It is assumed that males face a trade-off between these 2 types of sexual traits because they both draw from the same pool of resources. Consequently, allocation into mate acquisition or ejaculation should create similar trade-offs with other key life history traits. Tests of these assumptions are exceedingly rare. Males only ejaculate after they mate, and the costs of ejaculation are therefore highly confounded with those of mating effort. Consequently, little is known about how each component of reproductive allocation affects a male’s future performance. Here, we ran an experiment using a novel technique to distinguish the life history costs of mating effort and ejaculation for mosquitofish (Gambusia holbrooki). We compared manipulated males (mate without ejaculation), control males (mate and ejaculate), and naïve males (neither mate nor ejaculate) continuously housed with a female and 2 rival males. We assessed their growth, somatic maintenance, mating and fighting behavior, and sperm traits after 8 and 16 weeks. Past mating effort significantly lowered a male’s future mating effort and growth, but not his sperm production, while past sperm release significantly lowered a male’s future ejaculate quantity, but not his mating effort. Immune response was the only trait impacted by both past mating effort and past ejaculation. These findings challenge the assumption that male reproductive allocation draws from a common pool of resources to generate similar life history costs later in life. Instead, we provide clear evidence that allocation into traits under pre- and postcopulatory sexual selection have different trait-specific effects on subsequent male reproductive performance.