Daphnia females adjust sex allocation in response to current sex ratio and density

Cyclical parthenogenesis presents an interesting challenge for the study of sex allocation, as individuals' allocation decisions involve both the choice between sexual and asexual reproduction, and the choice between sons and daughters. Male production is therefore expected to depend on ecological and evolutionary drivers of overall investment in sex, and those influencing male reproductive value during sexual periods. We manipulated experimental populations, and made repeated observations of natural populations over their growing season, to disentangle effects of population density and the timing of sex from effects of adult sex ratio on sex allocation in cyclically parthenogenetic Daphnia magna. Male production increased with population density, the major ecological driver of sexual reproduction; however, this response was dampened when the population sex ratio was more male-biased. Thus, in line with sex ratio theory, we show that D.magna adjust offspring sex allocation in response to the current population sex ratio.Peer reviewe

Daphnia invest in sexual reproduction when its relative costs are reduced

The timing of sex in facultatively sexual organisms is critical to fitness, due to the differing demographic consequences of sexual versus asexual reproduction. In addition to the costs of sex itself, an association of sex with the production of dormant life stages also influences the optimal use of sex, especially in environments where resting eggs are essential to survive unfavourable conditions. Here we document population dynamics and the occurrence of sexual reproduction in natural populations of Daphnia magna across their growing season. The frequency of sexually reproducing females and males increased with population density and with decreasing asexual clutch sizes. The frequency of sexually reproducing females additionally increased as population growth rates decreased. Consistent with population dynamic models showing that the opportunity cost of sexual reproduction (foregoing contribution to current population growth) diminishes as populations approach carrying capacity, we found that investment in sexual reproduction was highest when asexual population growth was low or negative. Our results support the idea that the timing of sex is linked with periods when the relative cost of sex is reduced due to low potential asexual growth at high population densities. Thus, a combination of ecological and demographic factors affect the optimal timing of sexual reproduction, allowing D. magna to balance the necessity of sex against its costs.Peer reviewe

Artificial selection on male genitalia length alters female brain size

Male harassment is a classic example of how sexual conflict over mating leads to sex-specific behavioural adaptations. Females often suffer significant costs from males attempting forced copulations, and the sexes can be in an arms race over male coercion. Yet, despite recent recognition that divergent sex-specific interests in reproduction can affect brain evolution, sexual conflict has not been addressed in this context. Here, we investigate whether artificial selection on a correlate of male success at coercion, genital length, affects brain anatomy in males and females. We analysed the brains of eastern mosquitofish (Gambusia holbrooki), which had been artificially selected for long or short gonopodium, thereby mimicking selection arising from differing levels of male harassment. By analogy to how prey species often have relatively larger brains than their predators, we found that female, but not male, brain size was greater following selection for a longer gonopodium. Brain subregion volumes remained unchanged. These results suggest that there is a positive genetic correlation between male gonopodium length and female brain size, which is possibly linked to increased female cognitive ability to avoid male coercion. We propose that sexual conflict is an important factor in the evolution of brain anatomy and cognitive ability

Sex ratio bias leads to the evolution of sex role reversal in honey locust beetles

The reversal of conventional sex roles was enigmatic to Darwin, who suggested that it may evolve when sex ratios are female biased [1]. Here we present direct evidence confirming Darwin’s hypothesis. We investigated mating system evolution in a sex-role-reversed beetle (Megabruchidius dorsalis) using experimental evolution under manipulated sex ratios and food regimes. In female-biased populations, where reproductive competition among females was intensified, females evolved to be more attractive and the sex roles became more reversed. Interestingly, female-specific mating behavior evolved more rapidly than male-specific mating behavior. We show that sexual selection due to reproductive competition can be strong in females and can target much the same traits as in males of species with conventional mating systems. Our study highlights two central points: the role of ecology in directing sexual selection and the role that females play in mating system evolution

Competition, conflict and cooperation : social interactions within and between species

This thesis addresses several aspects of cooperation and conflict in social interactions. Male fiddler crabs use their enlarged claw as a weapon in territorial interactions. Clawless males are severely disadvantaged in fighting. Chapter 2 investigates whether clawless males use their resemblance to females to avoid male aggression during burrow acquisition and defence. Clawless and intact males differed in burrow acquisition methods and fighting behaviour, but were equally successful at acquiring and retaining burrows. Although treated as female by courting males, clawless males did not mimic female behaviour, but altered their territorial strategies to avoid conflicts. Chapter 3 examines the 'dear enemy' effect, where residents are more aggressive towards strangers than neighbours. The greater threat posed by strangers explains why residents are differentially aggressive, but not how they distinguish neighbours from strangers. I manipulated the residency status of fiddler crab neighbours to determine whether familiarity facilitates reduced aggression. Fights between neighbours were less escalated than fights between residents and both familiar and unfamiliar 'strangers', suggesting that residents use behavioural cues to determine the threat posed by an intruder. Cooperative territory defence is expected when helping a neighbour is less costly than establishing new boundaries with the usurper of a neighbouring territory. In fiddler crabs cooperation depends on the relative sizes of participants - large residents help smaller neighbours repel intermediate-sized intruders. Chapter 4 tests whether familiarity or the relative size of the smaller neighbour affect the likelihood that a resident helps, by replacing neighbours and simulating intrusions onto their territories. The likelihood of helping familiar and unfamiliar neighbours of the same size did not differ, but decreased when the replacement neighbour differed in size from the original neighbour. This suggests that residents do not recognise their neighbours individually, but use size as a cue to neighbour identity. Chapter 5 demonstrates that cooperative defence occurs between heterospecific neighbours. Residents were equally likely to help a smaller conspecific or heterospecific neighbour during simulated intrusions by intermediate-sized conspecifics. Helping was significantly less likely when the intruder was heterospecific. Chapter 6 investigates the effect of simultaneous vs. sequential mate encounter on male mate discrimination between conspecific and heterospecific females. Under sequential encounter, males were equally likely to court conspecific and heterospecific females. However, when simultaneously viewing a conspecific and a heterospecific female, males courted and attempted to mate conspecifics more often, indicating that males can discriminate between conspecific and heterospecific females and prefer conspecifics when given a choice. Chapter 7 explores the effects of social context on male reproductive success, testing whether the mate choice of male mosquitofish depends on the size of competitors, and quantifying the effect of competitor size on male mating success. In two-choice trials, males preferred large over small males. However, when females were present males associated equally with large and small males. The mating success of competing males increased with male size, and with decreasing competitor size. The benefits of associating with small competitors in a mating context appear to be balanced by other benefits of associating with larger males

Male fiddler crabs prefer conspecific females during simultaneous, but not sequential, mate choice

Mate choice is potentially beneficial whenever prospective mates vary in quality, but when mates are encountered sequentially the cost of rejecting a current mating opportunity affects the net benefit of choosiness by lowering the mating rate. There is, however, no reduction in mating rate when choosing among potential mates that are encountered simultaneously. In general, mating with a heterospecific is costly as the resultant offspring are of low fitness. It is often argued that males, unlike females, will court and even mate with heterospecifics because the lost opportunity cost is minimal if they rarely encounter potential mates. In the fiddler crab Uca mjoebergi, we show that, in a natural situation, where females arrived sequentially males were equally likely to court conspecifics and heterospecifics. Females were released individually into the population, and nearly every male they passed performed a courtship waving display whether the female was conspecific or heterospecific. Taken alone, this result implies that males exhibit no species discrimination. However, in an experimental setting where males simultaneously viewed a conspecific and a heterospecific female, males waved faster and for longer at conspecific females, and attempted to mate more often with conspecifics. This indicates that U. mjoebergi males can discriminate between conspecific and heterospecific females and prefer to court conspecifics when given a choice. We used mate choice among rather than within species (to maximize variation in mate quality) to illustrate the need to distinguish between simultaneous and sequential mate choice when quantifying mating preferences

The complex interplay of sex allocation and sexual selection

It is well recognized that sex allocation strategies can be influenced by sexual selection, when females adjust offspring sex ratios in response to their mates' attractiveness. Yet the reciprocal influence of strategic sex allocation on processes of sexual selection has only recently been revealed. Recent theoretical work demonstrates that sex allocation weakens selection for female preferences, leading to the decline of male traits. However, these results have been derived assuming that females have perfect knowledge of mate attractiveness and precise control over cost-free allocation. Relaxing these assumptions highlights the importance of another feedback: that adaptive sex allocation must become difficult to maintain as traits and preferences decline. When sex allocation strategies erode not only traits and preferences but also their own selective advantage, predictions can no longer be expressed as a simple linear correlation between ornament exaggeration and adaptive sex allocation. Instead, strongest sex ratio biases may be found at intermediate trait levels

Sexual Selection: The Weevils of Inbreeding

A recent study has used inbreeding depression to gain insight into the maintenance of additive genetic variation in populations, with intriguing implications for good genes models of sexual selection

Competitor size, male mating success and mate choice in eastern mosquitofish, Gambusia holbrooki

Males usually compete for mates but, by choosing a favourable social environment (e.g. avoiding stronger competitors), males might increase their reproductive success. We first tested whether the mate choice of male eastern mosquitofish depended on the size of potential competitors. In two-choice trials, focal males preferred to associate with a group of large males over a group of small males. However, when both stimulus groups also contained females, focal males associated equally often with the group with large males and the group with small males. We then quantified the effect of competitor size on the relative mating success (proportion of all mating attempts) of males competing for access to a female. In mating trials, the relative mating success of focal males increased with focal male size. In addition, focal males had higher mating success when competing in a group of small males than a group of large males. We suggest that the benefits of associating with small male competitors in a mating context (greater mating success) are balanced by other benefits that have led to a general preference for associating with larger males