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Cuticular hydrocarbons (CHCs) play an essential role in mate recognition in insects but the form and intensity of sexual selection on CHCs has only been evaluated in a handful of studies, and never in a natural population.We quantified sexual selection operating on CHCs in a wild population of sagebrush crickets, a species in which nuptial feeding by females imposes an unambiguous phenotypic marker on males. Multivariate selection analysis revealed a saddle-shaped fitness surface, suggesting a complex interplay between the total abundance of CHCs and specific CHC combinations in their influence on female choice. The fitness surface resulting from two axes of disruptive selection reflected a trade-off between short- and long-chained CHCs, suggesting that males may be sacrificing some level of desiccation resistance in favour of increased attractiveness. There was a significant correlation between male body size and total CHC abundance, suggesting that male CHCs provide females with a reliable cue for maximizing benefits obtained from males. Notwithstanding the conspicuousness of males� acoustic signals, our results suggest that selection imposed on males via female mating preferences may be far more complex than previously appreciated and operating in multiple sensory modalities. [ABSTRACT FROM AUTHOR]

The reproductive costs associated with the upregulation of immunity have been well-documented and constitute a fundamental trade-off between reproduction and self-maintenance. However, recent experimental work suggests that parents may increase their reproductive effort following immunostimulation as a form of terminal parental investment as prospects for future reproduction decline. We tested the trade-off and terminal investment hypotheses in a wild population of house wrens (Troglodytes aedon) by challenging the immune system of breeding females with lipopolysaccharide, a potent but non-lethal antigen. Immunized females showed no evidence of reproductive costs; instead, they produced offspring of higher phenotypic quality, but in a sex-specific manner. Relative to control offspring, sons of immunized females had increased body mass and their sisters exhibited higher cutaneous immune responsiveness to phytohaemagglutinin injection, constituting an adaptive strategy of sex-biased allocation by immune-challenged females to enhance the reproductive value of their offspring. Thus, our results are consistent with the terminal investment hypothesis, and suggest that maternal immunization can induce pronounced transgenerational effects on offspring phenotypes. [ABSTRACT FROM AUTHOR]

[Display omitted] • Hatching order interacts with sex in its influence on nestling corticosterone. • First-laid eggs had significantly higher corticosterone than last-laid eggs. • Developmental asymmetries created by cross-fostering had no effect on CORT. • Pre-natal effects contribute more to developmental differences than post-natal. Variation in nestling growth and survival is often influenced by hatching order, with first-hatched offspring having an advantage over later-hatched younger siblings. In house wrens (Troglodytes aedon), this effect of hatching order is especially evident in asynchronously hatched broods and can lead to sex-specific differences in the size and condition of nestlings. Females appear to allocate the sex of their offspring across the laying order to capitalize on these differences. We hypothesized that levels of circulating corticosterone, the primary metabolic hormone in birds, mediates these sex-specific effects in nestlings. We predicted that: i) baseline levels of corticosterone in nestlings should vary along the hatching order, ii) effects of hatching order on baseline corticosterone should be sex specific, and iii) any sex-specificity of hatching order on baseline corticosterone could be contingent on the degree of hatching synchrony. We tested these predictions in a study in which we measured baseline corticosterone in first- and last-hatched nestlings in synchronously and asynchronously hatching broods. To assess whether any differences in nestling baseline corticosterone levels could be attributed to pre-natal maternal effects, the post-natal environment, or both, we conducted two additional studies in which we measured i) yolk corticosterone in first- and last-laid eggs and ii) baseline corticosterone in nestlings that were cross-fostered to create simulated 'asynchronously' hatched broods. There was a significant interaction between sex and relative hatching order in their effects on nestling baseline corticosterone, but no effect of hatching synchrony. Corticosterone levels remained relatively constant across the hatching order in males but decreased in females. There was a significant effect of laying order on yolk corticosterone, with first-laid eggs containing significantly higher levels of yolk corticosterone than last-laid eggs. Cross-fostering of nestlings at different points of development had no significant effect on nestling corticosterone levels. These results indicate that sex-dependent differences in corticosterone levels across the hatching order may arise, at least in part, from embryonic exposure to maternally derived corticosterone, whereas the post-natal rearing environment plays, at best, a minimal role in determining nestling baseline corticosterone levels. [ABSTRACT FROM AUTHOR]

In recent years, temperatures have increased globally, and nestlings of many bird species are likely regularly exposed to increased temperatures both pre- and postnatally. Even small increases in nest temperature during incubation affect offspring growth and survival in a variety of species, one cause of which is thought to be increased production of prooxidants in embryos and nestlings. Defences marshalled in response to this oxidative stress could, in turn, result in trade-offs that lead to reduced survival or growth. If so, any downstream negative effects on nestlings of increased ambient temperatures during incubation could be counteracted by increasing their antioxidant intake. We predicted, therefore, that dietary supplements of an antioxidant would reduce or eliminate any detrimental effects on nestling growth and survival of experimentally increased nest temperature during the incubation period. We employed a split-brood design in which we increased nest temperature of entire clutches and, after hatching, provided dietary supplements of the antioxidant vitamin E to half of the nestlings within broods. We also recorded female incubation and provisioning behaviour to control for the possibility that heating nests might also influence maternal behaviour. There was a significant interaction between nestbox heating treatment and vitamin E treatment in their effect on nestling mass, a trait that is positively correlated with survival and future reproductive success in the study population. Vitamin E supplementation promoted increased nestling mass in heated nests, whereas it had the opposite effect in control nests, but these effects were weak. Heating significantly affected female incubation behaviour, with females in heated nestboxes investing less in incubation than those in unheated boxes. These results suggest that within at least some range of expected increased ambient temperatures during the 21 st century, effects of climate change on nestling bird development can be mitigated by adjustments in female incubation behaviour., Competing Interests: CONFLICT OF INTEREST The authors declare no conflict of interest.

Hosts of obligate avian brood parasites often evolve defense mechanisms to avoid rearing unrelated young. One common defense is egg rejection, for which hosts often rely on eggshell color. Most research has assumed that hosts respond to perceived color differences between their own eggs and parasite eggs regardless of the particular color; however, recent experiments have found that many hosts respond more strongly to brown foreign eggs than to equally dissimilar blue eggs. Yet, none of these prior studies tested a brown-egg-laying species and, with only one exception, all were conducted in open nests where light levels are considered sufficient for effective color-based egg discrimination. Here, we explored how two cavity-nesting hosts of the parasitic brown-headed cowbird (Molothrus ater) - the blue-egg-laying eastern bluebird (Sialia sialis) and the brown-egg-laying house wren (Troglodytes aedon) - respond to experimental eggs painted six distinct colors ranging from blue to brown. Rejection responses of both hosts were best predicted by perceived differences in color between the model egg and their own eggs. Specifically, we found that house wrens preferentially rejected eggs bluer than their own eggs. However, although we found that bluebirds relied on perceived differences in color for their egg rejection decisions, further tests are needed to determine whether they preferentially rejected brown eggs or simply responded to absolute perceived differences in color. These findings demonstrate that these cavity-nesting birds treat perceived color differences in distinct ways, which has important implications on the coevolutionary arms races and the interpretation of avian-perceived color differences., Competing Interests: Competing interests The authors declare no competing or financial interests., (© 2023. Published by The Company of Biologists Ltd.)

Sexually antagonistic coevolution is predicted to lead to the divergence of male and female genotypes related to the effects of substances transferred by males at mating on female physiology. The outcome of mating should thus depend on the specific combination of mating genotypes. Although mating has been shown to influence female immunity in diverse insect taxa, a male-female genotype-by-genotype effect on female immunity post mating remains largely unexplored. Here, we investigate the effects of mating on female decorated cricket baseline immunity and the potential for a male-genotype-by-female-genotype interaction affecting this response. Females from three distinct genotypic backgrounds were left unmated or singly mated in a fully reciprocal design to males from the same three genotypic backgrounds. Hemocytes and hemocyte microaggregations were quantified for female cellular immunity, and phenoloxidase, involved in melanization, and antibacterial activity for humoral immunity. In this system, female cellular immunity was more reactive to mating, and mating effects were genotype-dependent. Specifically, for hemocytes, a genotype-by-mating status interaction mediated the effect of mating per se, and a significant male-female genotype-by-genotype interaction determined hemocyte depletion post mating. Microaggregations were influenced by the female's genotype or that of her mate. Female humoral immune measures were unaffected, indicating that the propensity for post-mating effects on females is dependent on the component of baseline immunity. The genotype-by-genotype effect on hemocytes supports a role of sexual conflict in post-mating immune suppression, suggesting divergence of male genotypes with respect to modification of female post-mating immunity, and divergence of female genotypes in resistance to these effects.

Fitness costs of incubation ensue whenever the trade-off between incubation and foraging leads to suboptimal incubation or decreased parental body condition. We examined the costs of incubation in a wild population of house wrens, Troglodytes aedon , by experimentally extending or decreasing the incubation period by cross-fostering eggs between nests at different stages of incubation (eggs from control nests were cross-fostered at the same stage of incubation). We determined whether parents or offspring bear the costs of incubation by measuring effects on females and offspring within the same breeding season during which the manipulation occurred, but also by evaluating potential trade-offs between current and future reproduction by monitoring return rates of experimental females and recruitment rates of offspring in subsequent breeding seasons. There was no difference in hatching or fledging success across treatments. There was also no effect of incubation duration on female size-corrected mass, and females from different treatments were equally likely to produce a second brood. Nestlings produced by females did not differ in body mass, tarsus length or residual mass. Neither return rates of females, nor the number of offspring recruited, differed across treatments. We conclude, therefore, that although prolonged incubation entails increased energy expenditures, females are able to offset these losses while foraging, thereby mitigating the costs of incubation. This resiliency is more likely to be seen in income breeders, such as house wrens, that retain some ability to recoup energy expended in incubation, than in capital breeders that are constrained by stored energy reserves.

Although reproductive strategies can be influenced by a variety of intrinsic and extrinsic factors, life history theory provides a rigorous framework for explaining variation in reproductive effort. The terminal investment hypothesis proposes that a decreased expectation of future reproduction (as might arise from a mortality threat) should precipitate increased investment in current reproduction. Terminal investment has been widely studied, and a variety of intrinsic and extrinsic cues that elicit such a response have been identified across an array of taxa. Although terminal investment is often treated as a static strategy, the level at which a cue of decreased future reproduction is sufficient to trigger increased current reproductive effort (i.e., the terminal investment threshold) may depend on context, including the internal state of the organism or its current external environment, independent of the cue that triggers a shift in reproductive investment. Here, we review empirical studies that address the terminal investment hypothesis, exploring both the intrinsic and extrinsic factors that mediate its expression. Based on these studies, we propose a novel framework within which to view the strategy of terminal investment, incorporating factors that influence an individual's residual reproductive value beyond a terminal investment trigger - the dynamic terminal investment threshold., Competing Interests: Conflict of Interest: The authors declare that they have no conflict of interest.

Predation is a significant cause of nest failure in passerine birds, and, thus, natural selection is expected to favor behavioral plasticity to allow birds to respond to perceived changes in predation risk. However, behavioral plasticity in response to perceived predation risk, and its potential fitness-related costs, are understudied. In a wild population of breeding house wrens ( Troglodytes aedon ), we tested the hypotheses that (1) birds show behavioral plasticity in response to perceived nest-predation risk to reduce self-risk or risk to offspring, but (2) this plasticity incurs fitness-related costs. We experimentally increased the perceived risk of nest predation by enlarging the diameter of the nestbox entrance from the standard 3.2 cm to 5.0 cm once incubation began. Unexpectedly, large-hole females spent significantly less time being vigilant than small-hole (control) females during late incubation. Both males and females also exhibited plasticity in their provisioning behavior. Large-hole males increased and large-hole females decreased provisioning visits with increasing brood size, whereas small-hole males and females behaved similarly and were unaffected by brood size. Females did not show plasticity in their incubation or brooding behavior. Notwithstanding this behavioral plasticity in response to increased perceived predation risk, treatment had no effect on hatching success or early hatchling survival, nor did it affect nestling body condition or fledging success. We conclude, therefore, that house wrens show behavioral plasticity in response to perceived nest-predation risk, but that any short-term fitness-related costs associated with this flexibility appear negligible., Competing Interests: Compliance with Ethical Standards Conflict of Interest The authors declare that they have no conflict of interest.

In altricial birds, parental provisioning is plastic and can respond to a variety of environmental stimuli. In this study, we manipulated the size of entrances into artificial nest cavities (i.e., nestboxes) in a population of house wrens ( Troglodytes aedon ) as a means of manipulating a male's sexual attractiveness, and examined changes in parental provisioning. Nest cavities with large entrances are less desirable as nesting sites, and the males at these sites are less attractive to females. Therefore, we predicted that males at boxes that had large entrances would invest more in parental care (i.e., those that succeeded in finding a mate would provision their offspring at a higher rate) than males at nestboxes with small entrances. As predicted, males provisioned their offspring with food at the highest rates at nestboxes with enlarged entrances, and male provisioning effort positively predicted the number of fledglings they produced per egg. Males at these boxes provisioned more than their mates and more than females and males at nestboxes with small entrances. At nestboxes with small entrances, males provisioned at the same rate as females, and female provisioning did not differ significantly between treatments, on average. Male and female provisioning rates were negatively correlated, such that the increase in provisioning by males at nestboxes with enlarged entrances did not enhance nestling condition, likely because food delivery by females declined with increased provisioning by males. However, the amount of time females spent providing warmth for their ectothermic young increased with increases in male provisioning, suggesting that levels of male parental care altered the mode, not necessarily the extent, of care provided by females. These findings suggest that male provisioning is related to sexual attractiveness, and that sexual conflict over biparental care may not be as simple as the assessment of food provisioning might otherwise suggest.

Females in a variety of taxa adjust offspring sex ratios to prevailing ecological conditions. However, little is known about whether conditions experienced during a female's early ontogeny influence the sex ratio of her offspring. We tested for past and present ecological predictors of offspring sex ratios among known-age females that were produced as offspring and bred as adults in a population of house wrens. The body condition of offspring that a female produced and the proportion of her offspring that were male were negatively correlated with the size of the brood in which she herself was reared. The proportion of sons within broods was negatively correlated with maternal hatching date, and varied positively with the quality of a female's current breeding territory as predicted. However, females producing relatively more sons than daughters were less likely to return to breed in the population the following year. Although correlative, our results suggest that the rearing environment can have enduring effects on later maternal investment and sex allocation. Moreover, the overproduction of sons relative to daughters may increase costs to a female's residual reproductive value, constraining the extent to which sons might be produced in high-quality breeding conditions. Sex allocation in birds remains a contentious subject, largely because effects on offspring sex ratios are small. Our results suggest that offspring sex ratios are shaped by various processes and trade-offs that act throughout the female life history and ultimately reduce the extent of sex-ratio adjustment relative to classic theoretical predictions.

Mass or body-size measures of 'condition' are of central importance to the study of ecology and evolution, and it is often assumed that differences in condition measures are positively and linearly related to fitness. Using examples drawn from ecological studies, we show that indices of condition frequently are unlikely to be related to fitness in a linear fashion. Researchers need to be more explicit in acknowledging the limitations of mass-based condition measures and accept that, under some circumstances, they may not relate to fitness as traditionally assumed. Any relationship between a particular condition measure and fitness should first be empirically validated before condition is used as a proxy for fitness. In the absence of such evidence, researchers should explicitly acknowledge that assuming such a relationship may be unrealistic.

In altricial birds, siblings raised within a nest usually leave the nest within hours of each other, despite often differing considerably in age. The youngest members of the brood are typically underdeveloped at this time and less likely than their older siblings to survive outside the nest, yet they risk abandonment if they do not fledge with their older siblings. Nest leaving is usually initiated by the older offspring, which may delay this process to provide more time for their younger siblings to mature, increasing the younger siblings' postfledging survival and their own inclusive fitness. We tested this hypothesis in a population of house wrens Troglodytes aedon and found that broods with broad age spans among siblings had longer nestling periods than broods with narrow age spans and that delayed fledging improves the survival and reproductive prospects of younger siblings, although at a potential cost to future siblings. We also manipulated age spans through cross-fostering and found that older foster nestlings postponed fledging when raised with younger broodmates, as predicted if the age of younger nestlings determines the time of fledging. Our results support kin-selection theory and demonstrate that the exact time of fledging is attributable, in part, to sib-sib interactions.

The Free Radical Theory of Ageing (FRTA) predicts that oxidative stress, induced when levels of reactive oxygen species exceed the capacity of antioxidant defenses, causes ageing. Recently, it has also been argued that oxidative damage may mediate important life-history trade-offs. Here, we use inbred lines of the decorated cricket, Gryllodes sigillatus, to estimate the genetic (co)variance between age-dependent reproductive effort, life span, ageing, oxidative damage, and total antioxidant capacity within and between the sexes. The FRTA predicts that oxidative damage should accumulate with age and negatively correlate with life span. We find that protein oxidation is greater in the shorter lived sex (females) and negatively genetically correlated with life span in both sexes. However, oxidative damage did not accumulate with age in either sex. Previously we have shown antagonistic pleiotropy between the genes for early-life reproductive effort and ageing rate in both sexes, although this was stronger in females. In females, we find that elevated fecundity early in life is associated with greater protein oxidation later in life, which is in turn positively correlated with the rate of ageing. Our results provide mixed support for the FRTA but suggest that oxidative stress may mediate sex-specific life-history strategies in G. sigillatus., (© 2012 The Author(s). Evolution© 2012 The Society for the Study of Evolution.)

Nuptial food gifts function to enhance male fertilization success, but their consumption is not always beneficial to females. In decorated crickets, the spermatophore transferred at mating includes a gelatinous mass, the spermatophylax, which is consumed by females after mating. However, females often discard spermatophylaxes shortly after mating, whereupon they terminate sperm transfer. We hypothesized that females discard gifts based on their assessment of the gift itself, and specifically the composition of free amino acids. We tested this hypothesis by comparing spermatophylaxes discarded by females after mating with those that were destined to be fully consumed, and employed multivariate selection analysis to quantify the strength and form of multivariate sexual selection operating on the free amino acid composition of gifts. The analysis yielded a saddle-shaped fitness surface with two local peaks. Different amino acid profiles appear to elicit continued feeding on the spermatophylax either because they offer the same level of gustatory appeal, or because they differentially affect both the gustatory appeal and texture of the spermatophylax. We conclude that the gustatory response of females to males' nuptial food gifts represents an important avenue of post-copulatory mate choice, imposing significant sexual selection on the free amino acid composition of the spermatophylax.

Although female multiple mating is ubiquitous in insects, its adaptive significance remains poorly understood. Benefits to multiple mating can accrue via direct material benefits, indirect genetic benefits, or both. We investigated the effects of multiple mating in the decorated cricket, Gryllodes sigillatus, by simultaneously varying the number of times that females mated and the number of different males with which they mated, measuring aspects of female fecundity and elements of offspring performance and viability. Multiple matings resulted in enhanced female fitness relative to single matings when females mated with different partners, but not when females mated repeatedly with the same male. Specifically, polyandrous females produced significantly more offspring surviving to reproductive maturity than did monogamous females mating once or mating repeatedly with the same male. These results suggest that the benefit females gain from multiple mating is influenced primarily by genetic and not material benefits.

Females, by mating with more than one male in their lifetime, may reduce their risk of receiving sperm from genetically incompatible sires or increase their prospects of obtaining sperm from genetically superior sires. Although there is evidence of both kinds of genetic benefits in crickets, their relative importance remains unclear, and the extent to which experimentally manipulated levels of polyandry in the laboratory correspond to those that occur in nature remain unknown. We measured lifetime polyandry of free-living female decorated crickets, Gryllodes sigillatus, and conducted an experiment to determine whether polyandry leads to an increase in offspring viability. We experimentally manipulated both the levels of polyandry and opportunities for females to select among males, randomly allocating the offspring of experimental females to high-food-stress or low-food-stress regimes to complete their development. Females exhibited a high degree of polyandry, mating on average with more than seven different males during their lifetime and up to as many as 15. Polyandry had no effect on either the developmental time or survival of offspring. However, polyandrous females produced significantly heavier sons than those of monandrous females, although there was no difference in the adult mass of daughters. There was no significant interaction between mating treatment and offspring nutritional regimen in their effects on offspring mass, suggesting that benefits accruing to female polyandry are independent of the environment in which offspring develop. The sex difference in the extent to which male and female offspring benefit via their mother's polyandry may reflect possible differences in the fitness returns from sons and daughters. The larger mass gain shown by sons of polyandrous females probably leads to their increased reproductive success, either because of their increased success in sperm competition or because of their increased life span.