<p>Harmony and cooperation was for long believed to dominate sexual interactions. This view slowly started to change 25 years ago and is today replaced with a view where males and females act based on what is best from a costs-benefits perspective. When sex specific costs and benefits differ, concerning reproductive decision influenced by both sexes, sexual conflict will occur. The basis for discordant reproductive interests between the sexes is that males produce many small gametes, while females’ produce few and large gametes. One result of this difference is that the optimal mating rate differs between the sexes. Males, with their many small sperm, maximize their reproductive output by mating with many females, while females often do best by not mating more frequently than to fertilize their eggs, since mating often entails a cost. Sexual conflict over mating is thus an important factor shaping the interactions between the sexes. In this thesis I study this and related conflicts between the sexes, using mathematical models, fruit flies and comparative methods. Mathematical modelling was used to explore how males and females may coevolve under sexual conflict over mating. This model shows that sexual conflict over mating results in the evolution of costly female mate choice, in terms high resistance to matings, and costly exaggerated male sexual traits, aimed to manipulate females into mating. A key assumption in this model is that males which females find attractive also are more harmful to females. This assumption was tested by housing fruit fly females with either attractive or unattractive males. Females kept with attractive males were courted and mated more, and suffered a 16 percent reduction in lifetime offspring production. In another study I measured genetic variation in two antagonistic male traits used to compete over females; offence - a male’s ability to acquire new mates and supplant stored sperm, and defence - a male’s ability to induce fidelity in his mates and prevent sperm displacement when remating occurs. Independent additive genetic variation and positive selection gradients were found for both these traits, indicating an ongoing arms race between these male antagonistic traits. This arms race also had a negative impact on females, since high values of offence compromised female fitness. Genetic variation in female ability to withstand male harm was also tested for and found, indicating that females evolve counter adaptations to reduce the effect of harmful male traits. Finally, the proposed link between sexual conflict and speciation was tested. Theory suggests that perpetual sexual arms races will cause allopatric populations to evolve along different evolutionary trajectories, resulting in speciation. This theory was tested using comparative methods by contrasting the number of extant species in taxa with high and low opportunity for sexual conflict. The study showed that taxa with high opportunity for sexual conflict, on average, has four times as many species as those with low opportunity, supporting that sexual conflict is a key process in speciation.</p>

<p>This thesis examines parental condition, how it is traded off against reproduction and how it is displayed in a secondary sexual trait. The studies were performed on nest-box breeding collared flycatchers Ficedula albicollis on the island of Gotland, in the Baltic Sea. Early breeding and high fitness were found to be associated with high levels of glycosylated haemoglobin possibly governed by migratory exertion and infectious disease. In order to test if immune function is expressed in secondary sexual traits and how it is traded off against reproductive effort a series of experiments were performed, in which birds were challenged with an antigen, via a vaccine containing neutralised paramyxovirus. The forehead patch of the male collared flycatcher serves as a badge of status and is under sexual selection. Good condition, as reflected in strong immune response and low levels of blood parasites was found to be associated with bigger patch size. Patch size was also found to vary in size within the same breeding season in a pattern predictable from immune response data. Immune response, in itself, was found to be costly in terms of reduced survival, confirming that trade-offs involving suppression of immune response may increase fitness. Mating effort was found to be traded off against immune function and moult. Experimental brood size manipulations revealed a trade-off females between number of offspring and immune function. Thus I suggest a set of parameters useful for condition estimation. I also show that immune response is costly and, second, that pathogen resistance probably plays an important role in the shaping of secondary sexual traits and life-history decisions.</p>

<p>The importance of cryptic female choice, i.e. female post-copulatory influence over male reproductive success, in driving the evolution of male traits remains controversial. The main aim of this thesis was to understand the post-copulatory consequences of sexual interactions and the importance of cryptic female choice in two species of beetle.</p><p>Males of the red flour beetle Tribolium castaneum use their legs to rub the lateral edges of the female elytra during mating. When manipulating female perception of this behaviour, I found that females preferentially use the sperm of males with vigorous leg rubbing when they mate with more than one male. Leg rubbing also appeared to increase female rate of oviposition. Females do not seem to gain any indirect benefits by preferring males with an intense leg rubbing behaviour since this behaviour was found to have very low narrow sense heritability and did not appear to be condition dependent in its expression.</p><p>Males of the bruchid beetle Callosobruchus maculatus have spiny genitalia that harm their mates. Females kick males during copulation and when prevented from kicking, suffered reduced lifetime offspring production as a consequence of more extensive injuries. Males were not able to delay female remating, increase rate of oviposition or increase sperm precedence by inflicting relatively severe injuries to non-kicking females. Hence, the injuries appear to be side effects of male efforts to remain in copula. When copulation duration was manipulated, ejaculate size and female lifetime offspring production increased with the length of copulation. Females reduced their mating rate when they had access to water, suggesting that they obtain water from the large ejaculates and trade-off their need for additional water against the costs of mating. Males may then reduce the benefits of remating by providing their mates with a large amount of water. Females did not increase their remating propensity to avoid inbreeding when they had mated to brothers. Together, these studies reveal the complexity of sexual interactions and the importance of post-copulatory processes for the fitness of both males and females.</p>

This thesis examines parental condition, how it is traded off against reproduction and how it is displayed in a secondary sexual trait. The studies were performed on nest-box breeding collared flycatchers Ficedula albicollis on the island of Gotland, in the Baltic Sea. Early breeding and high fitness were found to be associated with high levels of glycosylated haemoglobin possibly governed by migratory exertion and infectious disease. In order to test if immune function is expressed in secondary sexual traits and how it is traded off against reproductive effort a series of experiments were performed, in which birds were challenged with an antigen, via a vaccine containing neutralised paramyxovirus. The forehead patch of the male collared flycatcher serves as a badge of status and is under sexual selection. Good condition, as reflected in strong immune response and low levels of blood parasites was found to be associated with bigger patch size. Patch size was also found to vary in size within the same breeding season in a pattern predictable from immune response data. Immune response, in itself, was found to be costly in terms of reduced survival, confirming that trade-offs involving suppression of immune response may increase fitness. Mating effort was found to be traded off against immune function and moult. Experimental brood size manipulations revealed a trade-off females between number of offspring and immune function. Thus I suggest a set of parameters useful for condition estimation. I also show that immune response is costly and, second, that pathogen resistance probably plays an important role in the shaping of secondary sexual traits and life-history decisions.

Multiple mating by females is common in nature. Yet, the evolution and maintenance of polyandry remains a bit of an evolutionary puzzle. It was my aim in this thesis to reach a greater understanding of this phenomenon as well as to investigate the consequences of polyandry on the evolution of reproductive divergence in insects. In an extensive meta analysis addressing the direct effects of multiple mating on female fitness in insects, I found that insects gain from multiple matings in terms of increased lifetime offspring production. In species without nuptial feeding, increased mating rate leads to decreased female lifespan and my results strongly support the existence of an intermediate optimal female mating rate. However, results from an experimental study where I examined the relationship between female fitness and mating rate in the bean weevil (Callosobruchus maculatus) showed that female fitness was maximized at two alternative mating rates, indicating that some species may exhibit a more complex relationship between the costs and benefits of mating. In the meta analysis on species with nuptial feeding, I found only positive effects of increased mating rate and the puzzle is rather what constrains the actual mating rates of females in these groups. Sexual selection is a very potent driver of rapid evolutionary change in reproductive characters. Most research has focussed on precopulatory sexual selection, but in promiscuous species sexual selection continues after copulation and variance in male fertilization success gives rise to postcopulatory sexual selection. In this thesis I found that three allopatric populations of the red flour beetle (Tribolium castaneum) have diverged in traits related to reproduction. Male genotype affected all aspects of female reproduction, but more interestingly, males and females interacted in their effect on offspring production and reproductive rate, showing that the divergence was due to the evolution of both male and female reproductive traits. When studying postcopulatory sexual selection, sperm competition has been put forward as the main source of variance in fertilization success. The results from a set of double-mating experiments, using the same populations of flour beetles, provided strong evidence that cryptic female choice is also important in generating variance in male fertilization success. I found not only main effects of female genotype on male fertilization success but also male-female interactions which provide more unambiguous evidence for cryptic female choice. Finally, I attempted to uncover which male signals-female receptors are involved in the reproductive divergence observed in the Tribolium populations. In a double-mating experiment I manipulated female perception of two male reproductive signals, copulatory courtship and cuticular hydrocarbons, and the results indicate that, within populations, both signals are sexually selected. However, only male cuticular hydrocarbons seem to be involved in the reproductive divergence between the populations. In conclusion, multiple mating by female insects can be understood solely in terms of direct fitness benefits resulting from increased offspring production. I have shown that postcopulatory sexual selection can lead to rapid divergence in reproductive traits related to mating and that cryptic female choice plays an important role in this divergence.

Sexual selection arises through variation in reproductive success. This thesis investigates different aspects important in sexual selection, namely nest building, sperm competition, paternity and paternal care, and their mutual interrelationships. In the studied species, the sand goby (Pomatoschistus minutus) and the common goby (Pomatoschistus microps), sperm competition did arise when small males, so called sneakers, sneaked into other males nests and released sperm. They seemed to use female behaviour as their prime cue for a sneaking opportunity. However, also nest-holders, both with and without eggs, were found to fertilize eggs in the nests of other males. Clearly, nest-holding males tried to prevent other males from spreading their sperm in their nests, since they showed aggression towards such males. A nest building experiment indicated that the small nest-openings found in the sneaker male treatment were sexually selected through protection against sneaking or by female choice. Yet, no behavioural or genetical support for the hypothesis that the nest functions as a physical or visual defence, or that sneaker males prefer to sneak upon nests with wide nest-openings, were found in the other studies. Still, individual nest-holding males showed a higher mucus preparation effort inside the nest in the presence of a sneaker male than when alone. In close relatives, such mucus contains sperm, suggesting an importance in sperm competition. However, the mucus may also have pheromone and anti-bacterial functions and may constitute a mating effort, as found in other gobies. Both a behavioural and a mate choice experiment suggested that the males were not less eager to spawn in the presence of a sneaker male. Sneak intrusion did not affect nest defence, fanning or filial cannibalism, nor had paternity an effect on filial cannibalism. This and various life history aspects, together with the fact that the parasitic male only fertilized a fraction of the clutches, would predict females to ignore sneaker males. This was also the case, as the presence of sneaker males was found not to affect female spawning decision. Still, several females spawned in two nests, which coincided with parasitic spawnings, suggesting a cost of disturbance for the females and thus a substantial cost to the nest-holding males in terms of lost mating success. However, females paid attention to other traits in their choice of mate since spawning was associated with sand volume of the nest, but not with nest-opening width. Also, female (but not male) courtship was correlated with partial clutch filial cannibalism, indicating that females are able to anticipate future male cannibalism. In a partial correlation of nest opening, sand volume, male courtship display, displacement fanning and male size, a large number of traits were correlated both positively and negatively with regard to how we may expect them to be appreciated by females. For instance, males which fan well also build large nests or display intensely (but not both). Together with all the other results of this thesis, this shows the entangled selection pressures working on breeding animals, as well as the different male and female tactics employed to maximize their reproduction.

The importance of cryptic female choice, i.e. female post-copulatory influence over male reproductive success, in driving the evolution of male traits remains controversial. The main aim of this thesis was to understand the post-copulatory consequences of sexual interactions and the importance of cryptic female choice in two species of beetle. Males of the red flour beetle Tribolium castaneum use their legs to rub the lateral edges of the female elytra during mating. When manipulating female perception of this behaviour, I found that females preferentially use the sperm of males with vigorous leg rubbing when they mate with more than one male. Leg rubbing also appeared to increase female rate of oviposition. Females do not seem to gain any indirect benefits by preferring males with an intense leg rubbing behaviour since this behaviour was found to have very low narrow sense heritability and did not appear to be condition dependent in its expression. Males of the bruchid beetle Callosobruchus maculatus have spiny genitalia that harm their mates. Females kick males during copulation and when prevented from kicking, suffered reduced lifetime offspring production as a consequence of more extensive injuries. Males were not able to delay female remating, increase rate of oviposition or increase sperm precedence by inflicting relatively severe injuries to non-kicking females. Hence, the injuries appear to be side effects of male efforts to remain in copula. When copulation duration was manipulated, ejaculate size and female lifetime offspring production increased with the length of copulation. Females reduced their mating rate when they had access to water, suggesting that they obtain water from the large ejaculates and trade-off their need for additional water against the costs of mating. Males may then reduce the benefits of remating by providing their mates with a large amount of water. Females did not increase their remating propensity to avoid inbreeding when they had mated to brothers. Together, these studies reveal the complexity of sexual interactions and the importance of post-copulatory processes for the fitness of both males and females.

Harmony and cooperation was for long believed to dominate sexual interactions. This view slowly started to change 25 years ago and is today replaced with a view where males and females act based on what is best from a costs-benefits perspective. When sex specific costs and benefits differ, concerning reproductive decision influenced by both sexes, sexual conflict will occur. The basis for discordant reproductive interests between the sexes is that males produce many small gametes, while females’ produce few and large gametes. One result of this difference is that the optimal mating rate differs between the sexes. Males, with their many small sperm, maximize their reproductive output by mating with many females, while females often do best by not mating more frequently than to fertilize their eggs, since mating often entails a cost. Sexual conflict over mating is thus an important factor shaping the interactions between the sexes. In this thesis I study this and related conflicts between the sexes, using mathematical models, fruit flies and comparative methods. Mathematical modelling was used to explore how males and females may coevolve under sexual conflict over mating. This model shows that sexual conflict over mating results in the evolution of costly female mate choice, in terms high resistance to matings, and costly exaggerated male sexual traits, aimed to manipulate females into mating. A key assumption in this model is that males which females find attractive also are more harmful to females. This assumption was tested by housing fruit fly females with either attractive or unattractive males. Females kept with attractive males were courted and mated more, and suffered a 16 percent reduction in lifetime offspring production. In another study I measured genetic variation in two antagonistic male traits used to compete over females; offence - a male’s ability to acquire new mates and supplant stored sperm, and defence - a male’s ability to induce fidelity in his mates and prevent sperm displacement when remating occurs. Independent additive genetic variation and positive selection gradients were found for both these traits, indicating an ongoing arms race between these male antagonistic traits. This arms race also had a negative impact on females, since high values of offence compromised female fitness. Genetic variation in female ability to withstand male harm was also tested for and found, indicating that females evolve counter adaptations to reduce the effect of harmful male traits. Finally, the proposed link between sexual conflict and speciation was tested. Theory suggests that perpetual sexual arms races will cause allopatric populations to evolve along different evolutionary trajectories, resulting in speciation. This theory was tested using comparative methods by contrasting the number of extant species in taxa with high and low opportunity for sexual conflict. The study showed that taxa with high opportunity for sexual conflict, on average, has four times as many species as those with low opportunity, supporting that sexual conflict is a key process in speciation.

The ultimate goal of my thesis is to develop a better understanding of the contribution of indirect benefits (i.e. good genes) to the evolution of female mate preferences. It is genetic variance in, and genetic correlations (covariances) among, male sexual displays, female preferences for them, and fitness that in part determine the degree to which females preferring certain male displays over others will gain an indirect benefit by having higher fitness offspring. Recent advances in quantitative genetic theory provide the mathematical means for quantifying the strength of indirect selection for female mate preferences (Kirkpatrick and Hall 2004), at least under certain conditions, but there are few empirical systems for which such data exist (Brooks and Endler 2001; Qvarnström et al. 2006). I have undertaken a classic half-sibling breeding design with the ultimate goal of estimating the specific parameters of this model in a population of the Australian fruit fly Drosophila serrata. The breeding design was performed across two environments - one to which the population was well adapted and a novel environment to which it was not - thereby also providing insight into genotype-by-environment interactions for this suite of traits and their effects on good genes indirect benefits in a novel environment. General insight is also gained into the genetic covariance of male and female fitness and the prevalence of intralocus sexual conflict, the quantitative genetic basis of female mate preferences for multiple male traits, the condition-dependence of these traits, and the genetic association between sexual displays and fitness when mutation-selection balance is inferred. My results advocate caution in the application of existing theory to quantify the strength of indirect selection, suggesting that a good genes process may be fundamentally different when the exaggeration of sexual displays is eventually halted and an equilibrium is reached between opposing selection.

The ultimate goal of my thesis is to develop a better understanding of the contribution of indirect benefits (i.e. good genes) to the evolution of female mate preferences. It is genetic variance in, and genetic correlations (covariances) among, male sexual displays, female preferences for them, and fitness that in part determine the degree to which females preferring certain male displays over others will gain an indirect benefit by having higher fitness offspring. Recent advances in quantitative genetic theory provide the mathematical means for quantifying the strength of indirect selection for female mate preferences (Kirkpatrick and Hall 2004), at least under certain conditions, but there are few empirical systems for which such data exist (Brooks and Endler 2001; Qvarnström et al. 2006). I have undertaken a classic half-sibling breeding design with the ultimate goal of estimating the specific parameters of this model in a population of the Australian fruit fly Drosophila serrata. The breeding design was performed across two environments - one to which the population was well adapted and a novel environment to which it was not - thereby also providing insight into genotype-by-environment interactions for this suite of traits and their effects on good genes indirect benefits in a novel environment. General insight is also gained into the genetic covariance of male and female fitness and the prevalence of intralocus sexual conflict, the quantitative genetic basis of female mate preferences for multiple male traits, the condition-dependence of these traits, and the genetic association between sexual displays and fitness when mutation-selection balance is inferred. My results advocate caution in the application of existing theory to quantify the strength of indirect selection, suggesting that a good genes process may be fundamentally different when the exaggeration of sexual displays is eventually halted and an equilibrium is reached between opposing selection.