Sexual Conflict Hypothesis Research Articles

An experimental increase in female mass during the fertile phase leads to higher levels of extra-pair paternity in pied flycatchers Ficedula hypoleuca

Female mass in most altricial birds reaches its maximum during breeding at egg laying, which coincides temporally with the fertile phase when extra-pair paternity (EPP) is determined. Higher mass at laying may have two different effects on EPP intensity. On the one hand, it would lead to increased wing loading (body mass/wing area), which may impair flight efficiency and thereby reduce female’s capacity to resist unwanted extra-pair male approaches (sexual conflict hypothesis). On the other hand, it would enhance female condition, favouring her capacity to evade mate guarding and to search for extra-pair mates (female choice hypothesis). In both cases, higher female mass at laying may lead to enhanced EPP. To test this prediction, we reduced nest building effort by adding a completely constructed nest in an experimental group of female pied flycatchers (Ficedula hypoleuca). Our treatment caused an increase in mass and thereby wing loading and this was translated into a significantly higher EPP in the manipulated group compared with the control group as expected. There was also a significant negative relationship between EPP and laying date and the extent of the white wing patch, an index of female dominance. More body reserves at laying mean not only a higher potential fecundity but a higher level of EPP as well. This interaction had not previously received due attention but should be considered in future studies of avian breeding strategies. While most research has been focused on determining possible criteria for extra-pair mate choice by females, less effort has been made on establishing if female traits are related to EPP and its intensity. One such trait is mass at laying which attains its highest level for breeding females of altricial birds. Our study indicates that a higher mass during the fertile phase not only has implications for female fecundity and predation risk but also for EPP in the resulting brood as more mass means a higher EPP.

Experimentally flight-impaired females show higher levels of extra-pair paternity in the pied flycatcher Ficedula hypoleuca.

There is no consensus yet on the reasons why females engage in extra-pair copulations (EPCs). In some species, females have been shown to accrue some indirect benefits, but these effects are not consistent across species and studies. The sexual conflict hypothesis posits that extra-pair paternity (EPP) is the result of strong selection for male pursuit of EPC without real benefits for females. In order to test this hypothesis, we experimentally reduced wing area (reversibly tying together some primary feathers), in a group of pied flycatcher females (Ficedula hypoleuca). The manipulation increases wing loading (body mass/wing area), which is negatively associated with flying ability, and thus with the capacity to escape from unwanted copulations. We compared the levels of EPP in this experimental group with those of a group of un-manipulated females. Experimental females almost doubled the proportion of extra-pair young (EPY) with respect to control females. In addition, more males sired EPY in experimental than in control broods containing EPY. These results suggest that in our study population, EPP could be partially a product of female capacity to avoid EPCs. We also discuss the alternative hypothesis that results might be due to an eventual reduction of female attractiveness.

Female secondary sexual traits in spiders: Adaptive interpretations of the sternum projection in the pholcid Holocnemus pluchei

Sexually dimorphic traits are the outcome of a network of selective pressures acting upon each sex. While male secondary sexual traits are frequently restricted to intrasexual competition, female ornaments or weapons are mostly associated to social interactions for the access to limited resources. Here we investigate a sexually dimorphic trait expressed within the female's sternum of the spider species Holocnemus pluchei (Pholcidae), and its adaptive maintenance under the three leading hypotheses: sexual receptivity signaling, mate choice and sexual conflict hypothesis. We provide fine-scaled behavioral descriptions of the copulatory behavior of Holocnemus pluchei, corroborating that mating interactions are mediated by the female's sternum projection. The single moment of contact between the female projection and male body occurs during sperm transfer, always followed by a reduction on the intensity of the male genital movement. Additionally, biometrical properties of the female sternum projection corroborate our functional interpretation for the trait as restricting the intensity of males' copulatory movements. We claim that female sternum projection is a sexually selected trait mediating sexual conflict, with several adaptive consequences upon the sexes.

Asynchronous hatching provides females with a means for increasing male care but incurs a cost by reducing offspring fitness.

In species with biparental care, sexual conflict occurs because the benefit of care depends on the total amount of care provided by the two parents while the cost of care depends on each parent's own contribution. Asynchronous hatching may play a role in mediating the resolution of this conflict over parental care. The sexual conflict hypothesis for the evolution of asynchronous hatching suggests that females adjust hatching patterns in order to increase male parental effort relative to female effort. We tested this hypothesis in the burying beetle Nicrophorus vespilloides by setting up experimental broods with three different hatching patterns: synchronous, asynchronous and highly asynchronous broods. As predicted, we found that males provided care for longer in asynchronous broods whereas the opposite was true of females. However, we did not find any benefit to females of reducing their duration of care in terms of increased lifespan or reduced mass loss during breeding. We found substantial negative effects of hatching asynchrony on offspring fitness as larval mass was lower and fewer larvae survived to dispersal in highly asynchronous broods compared to synchronous or asynchronous broods. Our results suggest that, even though females can increase male parental effort by hatching their broods more asynchronously, females pay a substantial cost from doing so in terms of reducing offspring growth and survival. Thus, females should be under selection to produce a hatching pattern that provides the best possible trade-off between the benefits of increased male parental effort and the costs due to reduced offspring fitness.

Origins of female genital diversity: Predation risk and lock-and-key explain rapid divergence during an adaptive radiation.

The study of male genital diversity has long overshadowed evolutionary inquiry of female genitalia, despite its nontrivial diversity. Here, we identify four nonmutually exclusive mechanisms that could lead to genital divergence in females, and potentially generate patterns of correlated male-female genital evolution: (1) ecological variation alters the context of sexual selection ("ecology hypothesis"), (2) sexually antagonistic selection ("sexual-conflict hypothesis"), (3) female preferences for male genitalia mediated by female genital traits ("female-choice hypothesis"), and (4) selection against inter-population mating ("lock-and-key hypothesis"). We performed an empirical investigation of all four hypotheses using the model system of Bahamas mosquitofish inhabiting blue holes that vary in predation risk. We found unequivocal support for the ecology hypothesis, with females exhibiting a smaller genital opening in blue holes containing piscivorous fish. This is consistent with stronger postmating female choice/conflict when predators are present, but greater premating female choice in their absence. Our results additionally supported the lock-and-key hypothesis, uncovering a pattern of reproductive character displacement for genital shape. We found no support for the sexual conflict or female choice hypotheses. Our results demonstrate a strong role for ecology in generating female genital diversity, and suggest that lock-and-key may provide a viable cause of female genital diversification.

The role of sexual conflict in the evolution of facultative parthenogenesis: a study on the spiny leaf stick insect

Parthenogenesis is an asexual mode of reproduction that is rare in nature compared to sex. Parthenogenetic animals are derived from sexual progenitors, but little is known about how transitions to parthenogenesis evolve and persist. One hypothesis is that selection promotes transitions to parthenogenesis when males are rare or when mating is impossible (the mate scarcity hypothesis). However, we hypothesized that selection might also favour sex-to-parthenogenesis transitions when sexual interactions are costly (the sexual conflict hypothesis). These hypotheses lead to contrasting predictions about the nature of male–female interactions and their effects on female fitness. To test these hypotheses, we conducted a series of life history and behavioural experiments on the facultatively parthenogenetic spiny leaf stick insect, Extatosoma tiaratum. As predicted by sexual conflict theory, we found that females appeared to neutralize the costs of sex by utilizing counterevolved resistance traits: male-paired females resisted matings by curling their abdomens and kicking their legs during copulation attempts, prereproductive virgin females produced an antiaphrodisiac that repelled males, and parthenogenetic females made themselves inconspicuous to males by altering their pheromonal signals. Although measures of offspring viability point to possible advantages of sexual reproduction, females that were experimentally switched from a parthenogenetic to a sexual reproductive mode suffered elevated mortality and egg production costs that were not observed in exclusively parthenogenetic or exclusively sexual females. Our results suggest that females can benefit by avoiding mating in at least some circumstances, therefore supporting the hypothesis that sexual conflict mediated by female resistance could contribute to the evolution of facultative parthenogenesis.

Allometry of Male Grasping Apparatus in Odonates Does Not Suggest Physical Coercion of Females

Male abdominal grasping apparatus that are used to secure a female prior, during and after mating, are widespread in arthropods. The scarce evidence regarding its selective regime suggests that they are male adaptations to circumvent female mating decisions, as predicted by the sexual conflict hypothesis. A recent discussion regarding this way of selection suggests that, similar to weapons and traits that have to do with physical endurance, grasping apparatus should show hyperallometry (proportionally larger compared to body size) as an indication of selection towards increased size. We have tested this idea by measuring the length, width and area of the grasping apparatus of five dragonfly species (Anax junius, Rhionaeschna multicolor, Dythemis nigrescens, D. sterilis and Phyllogomphoides pacificus). We used two proxies of body size (wing and body length). Our measures did not indicate any pattern of hyperallometry. Thus, the grasping apparatus in these animals does not seem to be positively selected for increased size as would be expected if they were forcing females to mate. Given this, we discuss three other explanations for the maintenance of the grasping apparatus in odonates: 1) a firm grip that secures the tandem and mating position; 2) courtship devices subject to female choice; and, 3) isolation structures that mechanically prevent interspecific matings. The first hypothesis, however, could not explain the highly elaborated and species specific morphology of grasping apparatus in these animals. Support for the second hypothesis comes from the fact that odonate females have mechanoreceptor sensilla embedded in their mesostigmal plates (the place grabbed by the grasping apparatus). For the third hypothesis, coevolutionary patterns in morphology in the grasping apparatus and mesostigmal plates in some Zygoptera can also be used as support.

Large testes and brain sizes in odontocetes (order Cetacea, suborder Odontoceti): the influence of mating system on encephalization

Little is known about their mating systems, but odontocetes may utilize the same types of mating systems as terrestrial mammals. Species with relatively large testes are likely to be polygynandrous, while species with smaller testes and greater sexual size dimorphism (SSD) are predicted to be polygynous. The “Machiavellian intelligence or sexual conflict” hypothesis predicts that polygynadrous species also evolved larger brains both to coerce conspecifics to mate and to resist mating attempts by undesirable mates. The “costly tissue” hypothesis predicts that species investing heavily in testes invest less in brain tissue and vice versa to conserve energy. Residual testes and brain mass measurements were used to test the sexual conflict and costly tissue hypotheses in 40 species of odontocetes. Correlations were performed on both raw data and independent contrasts to control for phylogeny. There was a significant positive correlation between residual testes mass and SSD in both data sets, and between residual testes mass and residual brain mass in the non-phylogenetically controlled data set. Results indicate a negative relationship between increased testes masses and SSD in odontocetes. There was no support for the costly tissue hypothesis. Support for Machiavellian intelligence or sexual conflict hypothesis was found only when phylogenetic effects were not considered.

Costly infidelity: low lifetime fitness of extra-pair offspring in a passerine bird.

Extra-pair copulation (EPC) is widespread in socially monogamous species, but its evolutionary benefits remain controversial. Indirect genetic benefit hypotheses postulate that females engage in EPC to produce higher quality extra-pair offspring (EPO) than within-pair offspring (WPO). In contrast, the sexual conflict hypothesis posits that EPC is beneficial to males but not to females. Thus, under the sexual conflict hypothesis, EPO are predicted to be no fitter than WPO. We tested these two hypotheses in a 12-year dataset with complete life-history and pedigree information from an isolated island population of house sparrows (Passer domesticus). We compared fitness components of EPO and two types of WPO: (1) WPO from genetically polyandrous “unfaithful” mothers, and (2) WPO from genetically monogamous mothers. We found that all three groups of offspring had similar probabilities of hatching and nestling survival. Unexpectedly, EPO had the lowest probability of recruiting into the breeding population and the lowest lifetime reproductive output. Our results indicate that EPO incurred indirect genetic costs, rather than benefits, which is contrary to indirect benefit models. Importantly, the indirect costs we observed are also underappreciated in current sexual conflict models. Our results call for improved theoretical frameworks that incorporate indirect costs by extending current sexual conflict models.

Mito-nuclear interactions as drivers of gene movement on and off the X-chromosome.

BackgroundMito-nuclear gene interactions regulate energy conversion, and are fundamental to eukaryotes. Generally, mito-nuclear coadaptation would be most efficient if the interacting nuclear genes were X-linked, because this maximizes the probability of favorable mito-nuclear allelic combinations co-transmitting across generations. Thus, under a coadaptation (CA) hypothesis, nuclear genes essential for mitochondrial function might be under selection to relocate to the X-chromosome. However, maternal inheritance predisposes the mitochondrial DNA (mtDNA) to accumulate variation that, while male-harming, is benign to females. Numerous nuclear genes were recently reported in Drosophila melanogaster, which exhibit male-specific patterns of differential expression when placed alongside different mtDNA haplotypes, suggesting that nuclear genes are sensitive to an underlying male-specific mitochondrial mutation load. These genes are thus candidates for involvement in mito-nuclear interactions driven by sexual conflict (SC), and selection might have moved them off the X-chromosome to facilitate an optimal evolutionary counter-response, through males, to the presence of male-harming mtDNA mutations. Furthermore, the presence of male-harming mtDNA mutations could exert selection for modifiers on the Y-chromosome, thus placing these mito-sensitive nuclear genes at the center of an evolutionary tug-of-war between mitochondrion and Y-chromosome.We test these hypotheses by examining the chromosomal distributions of three distinct sets of mitochondrial-interacting nuclear genes in D. melanogaster; the first is a list of genes with mitochondrial annotations by Gene Ontologies, the second is a list comprising the core evolutionary-conserved mitochondrial proteome, and the third is a list of genes involved in male-specific responses to maternally-inherited mitochondrial variation and which might be putative targets of Y-chromosomal regulation.ResultsGenes with mitochondrial annotations and genes representing the mitochondrial proteome do not exhibit statistically-significant biases in chromosomal representation. However, genes exhibiting sex-specific sensitivity to mtDNA are under-represented on the X-chromosome, over-represented among genes known to be sensitive to Y-chromosomal variation, and among genes previously associated with male fitness, but under-represented among genes associated with direct sexual antagonism.ConclusionsOur results are consistent with the SC hypothesis, suggesting that mitochondrial mutational pressure selects for gene movement off-the-X, hence enabling mito-nuclear coadaptation to proceed along trajectories that result in optimized fitness in both sexes.

Multiple origins of sex chromosome fusions correlated with chiasma localization in Habronattus jumping spiders (Araneae: Salticidae).

Entelegyne spiders rarely show fusions yielding neo‐Y chromosomes, which M. J. D. White attributed to a constraint in spiders, namely their proximal chiasma localization acting to upset meiotic segregation in males with fusions. Of the 75 taxa of Habronattus and outgroups studied, 47 have X1X20 sex chromosomes in males, 10 have X1X2Y, 15 have X1X2X3Y, 2 have X0, and one has both X1X20 and X1X2X3Y. Chromosome numbers and behavior suggest neo‐Ys formed by an autosome‐X fusion to make X1X2Y, with a second fusion to an autosome to make X1X2X3Y. Phylogeny shows at least 8–15 gains (or possibly some losses) of neo‐Y (i.e., X‐autosome fusions), a remarkable number for such a small clade. In contrast to the many X‐autosome fusions, at most one autosome–autosome fusion is indicated. Origins of neo‐Y are correlated significantly with distal localization of chiasmata, supporting White's hypothesis that evolution of neo‐Y systems is facilitated by looser pairing (distal chiasmata) at meiosis. However, an alternative (or contributing) explanation for the correlation is that X‐autosome fusions were selected to permit isolation of male‐favored alleles to the neo‐Y chromosome, aided by distal chiasmata limiting recombination. This intralocus sexual conflict hypothesis could explain both the many X‐autosome fusions, and the stunning complexity of male Habronattus courtship displays.

Costly interactions between the sexes: combined effects of male sexual harassment and female choice?

Male and female interests can differ profoundly regarding quality and quantity of mates. Especially in promiscuous mating systems, males often inflict costs on females that may precipitate in negative fitness consequences. In reality, however, discerning between female costs arising from a sexual conflict and costs arising from female mate choice is not trivial. In livebearing fishes, for example, costs of male sexual harassment are often quantified as female feeding time reductions, but female feeding times may also be affected by competitive interactions and the willingness of females to interact with a particular mate (i.e., mate choice). Using the tropical live-bearer Poecilia mexicana as a model, we manipulated male and female nutritional state and measured focal females' feeding times in presence of another female as well as in presence of males of varying quality. Consistent with other studies quantifying effects of male harassment, female feeding times were negatively affected by sexually active males. Also, females were feeding more in presence of starved males than in presence of well-fed males. In subsequent dichotomous choice tests, females preferred to associate with well-fed females. These results are not consistent with the sexual conflict hypothesis: If the reduced female feeding time in presence of a well-fed male were solely a cost imposed by the male, females would be expected to avoid such males in a choice situation. Consequently, our study suggests that both male sexual harassment and active female choice simultaneously affect female feeding times in livebearing fishes. Copyright 2011, Oxford University Press.

Mating asymmetry resulting from sexual conflict in the brachypterous grasshopper Podisma sapporensis

We explored why interpopulation crosses often yield mating asymmetry in the grasshopper Podisma sapporensis. Previous studies show that when local populations are crossed, mating frequency differs significantly between the 2 types of heterotypic mating. Mating asymmetry has been explained by 3 hypotheses: female choice, sexual conflict, or the consequences of bottlenecking events (Kaneshiro's hypothesis). The present study assessed which hypothesis best explained the observed mating patterns. Each test population was crossed with populations used in the previous studies. Of the 10 combinations of interpopulation crossing, 6 exhibited significant asymmetry in the frequency of heterotypic mating, 2 had marginal P values, and 2 exhibited symmetry. The mating frequency of one sex of a test population was mainly determined by the mating propensities of 2 crossed populations, but no interactions were detected between the populations. Conspicuous mating asymmetry arose when the 2 populations had greatly different mating propensities (i.e., the combination of vigorous males in 1 population and receptive females in the other population). In contrast, when 2 populations with similar female receptivity were crossed, the mating tended to be assortative. The results of crossing of ancestral and derived chromosomal races do not support Kaneshiro's hypothesis. Furthermore, the finding that mating propensities for both sexes vary greatly among populations does not agree with the female choice hypothesis. In conclusion, our results are consistent with the sexual conflict hypothesis, which postulates that mating vigor/receptivity varies geographically due to antagonistic coevolution between the sexes. Copyright 2011, Oxford University Press.

Individual differences in protandry, sexual selection, and fitness

Protandry is the difference in arrival date between males and females, with competition among males for access to preferred territories (the rank advantage hypothesis) or mating success (the mate opportunity hypothesis) supposedly driving the evolution of protandry. The fitness costs and benefits of protandry accruing to individuals differing in degree of protandry (arrival date of a male relative to the arrival date of his partner) have never been quantified. We analyzed the fitness consequences of sex differences in arrival date in the barn swallow Hirundo rustica, in which arrival date can be precisely estimated and the fitness of pairs differing in degree of individual protandry assessed. Early arriving males had greater mating success than late arriving males. The number of extrapair offspring in own nests decreased with increasing degree of individual protandry, whereas the number of offspring fathered by a focal male was unrelated to individual protandry. There was directional selection on individual protandry as shown by pairs with a larger than average degree of protandry reproducing early and, hence, supposedly producing more recruits. There was also stabilizing selection on individual protandry as shown by pairs with an intermediate degree of protandry reproducing early. Annual production of fledglings increased with early arrival of males, but not with early arrival of females, once the effect of laying date had been considered, with no additional effect of individual protandry. Neither male nor female survival was significantly related to degree of individual protandry. These findings are consistent not only with the mate opportunity hypothesis but also with a sexual conflict hypothesis, suggesting that males and females differ in their optimal timing of arrival due to sex-specific fitness costs and benefits. Key words: barn swallow, extrapair paternity, Hirundo rustica, optimal arrival time, sex ratio. [Behav Ecol]

EVOLUTIONARY PATHWAYS IN SHOREBIRD BREEDING SYSTEMS: SEXUAL CONFLICT, PARENTAL CARE, AND CHICK DEVELOPMENT

Sexual selection, mating opportunities, and parental behavior are interrelated, although the specific nature of these relationships is controversial. Two major hypotheses have been suggested. The parental investment hypothesis states that the relative parental investment of the sexes drives the operation of sexual selection. Thus, the sex that invests less in offspring care competes more intensely and monopolizes access to mates. The sexual conflict hypothesis proposes that sexual selection (the competition among both males and females for mates), mating opportunities, and parental behavior are interrelated and predicts a feedback loop between mating systems and parental care. Here we test both hypotheses using a comprehensive dataset of shorebirds, a maximum-likelihood statistical technique, and a recent supertree of extant shorebirds and allies. Shorebirds are an excellent group for these analyses because they display unique variation in parental care and social mating system. First, we show that chick development constrains the evolution of both parental care and mate competition, because transitions toward more precocial offspring preceded transitions toward reduced parental care and social polygamy. Second, changes in care and mating systems respond to one another, most likely because both influenced and are influenced by mating opportunities. Taken together, our results are more consistent with the sexual conflict hypothesis than the parental investment hypothesis.

Natural selection and genetic variation for female resistance to harm from males

The sexual conflict hypothesis predicts that males evolve traits that exploit the higher parental investment of females, which generates selection for females to counter-evolve resistance. In Drosophila melanogaster it is now established that males harm females and that there is genetic variation among males for the degree of this harm. Genetic variation among females for resistance to harm from males, and the corresponding strength of selection on this variation, however, have not been quantified previously. Here we carryout a genome-wide screen for female resistance to harm from males. We estimate that the cost of interactions with males depresses lifetime fecundity of females by 15% (95% CI: 8.2-22.0), that genetic variation for female resistance constitutes 17% of total genetic variation for female adult fitness, and that propensity to remate in response to persistent male courtship is a major factor contributing to genetic variation for female resistance.

Ejaculate feeding and female fitness in the sexually dimorphic fly Prochyliza xanthostoma (Diptera: Piophilidae)

Recent theory and empirical evidence suggest a role for sexual conflict in the evolution of male ejaculates, including ‘nuptial gifts’. According to the sexual conflict hypothesis, the greater the probability of female remating, the stronger the selection on males to induce an elevated reproductive rate in their mates, even if this reduces some components of female fitness, such as survivorship. Piophilid flies show an unusual form of nuptial gift: following copulation, females expel and ingest much of the sperm and accessory fluids received from the male. We investigated the effects of ejaculate ingestion on female fitness in the piophilid carrion fly Prochyliza xanthostoma by manipulating females' opportunity to ingest the ejaculate under two background food levels (fed ad libitum versus starved). We also estimated females' probability of remating. Few P. xanthostoma females mated more than once in repeated pairings with males. Females permitted to ingest the ejaculate oviposited sooner and laid more eggs than females prevented from ingesting the ejaculate, but ejaculate ingestion did not affect female survival. Hence, ejaculate feeding appeared to increase females' net fitness. However, these effects were evident only in fed females, suggesting that ejaculate meals provide mainly stimulation rather than nutrition. The benign effects of ejaculate feeding on female fitness, and the lack of evidence of sexual conflict, are consistent with the low female remating rate in this species. None the less, our results suggest that sexual conflict has played a role in the evolution of these traits.

Hatching Asynchrony and Feeding Rates in Yellow Warblers: A Test of the Sexual Conflict Hypothesis

T. Slagsvold and J. T. Lifjeld have proposed that hatching asynchrony in bird species in which only the female incubates is a strategy used by females to extract more parental investment from their mates. We examined this hypothesis by comparing feeding rates of female and male yellow warblers (Dendroica petechia) tending asynchronously and synchronously hatched broods. Regardless of brood size, feeding rates during the early, middle, and late nestling periods were similar for females tending asynchronously and synchronously hatched broods. Nevertheless, there was a general trend for females tending asynchronous broods to have lower feeding rates, which suggests that females may prefer hatching asynchrony. This is consistent with the sexual conflict hypothesis. Male feeding rates exhibited trends similar to those of females. That is, although the result is not statistically significant, males tending asynchronous broods, regardless of brood size, generally made fewer feeds than males tending synchronously hatched broods. However, males tending asynchronous broods of five did make significantly fewer feeds during the early nestling period than males tending similar-sized synchronous broods. This was also true when the data were combined across brood sizes. This suggests, contrary to the sexual conflict hypothesis, that males, especially those tending larger broods, should favor hatching asynchrony. Consequently, the sexes do not appear to be in conflict as to when incubation should begin, but rather females and males prefer, or at the very least are neutral with respect to, the initiation of incubation prior to clutch completion.

Hatching Asynchrony in Birds: The Hypothesis of Sexual Conflict Over Parental Investment

For animals with parental care, a sexual conflict may exist over the degree of parental investment involved. This may explain why, in birds, the eggs in a clutch do not hatch together but over a period of one or more days (asynchronous hatching). By adjusting the onset of incubation during the egg-laying period, the female is able to regulate the hatching times of the young and thereby the parental-investment pattern of the male. Some of the field data from the pied flycatcher support this sexual-conflict hypothesis. Females rearing asynchronous broods had higher body weights at the end of the nestling period than those rearing synchronous broods. However, it remains to be shown whether the post-fledging survival of the young is improved if the females are in good condition, and thus whether asynchronous hatching also benefits males.