Costs of breeding and their effects on the direction of sexual selection

A recent life-history model has challenged the importance of the operational sex ratio and the potential reproductive rates of males and females as the factors most important for the control of sexual selection, arguing that the cost of breeding, interpreted as the probability of dying as a consequence of the current breeding attempt, is the single most important factor that best predicts a mating system. In one species of bushcricket, the mating system can be reversed by resource manipulation. Here, we examine the costs of breeding in this system. Consistent with the model, increased costs of breeding can explain female competition and increased male choosiness under resource limitation. However, this is due to differences in the time required for a breeding attempt, rather than differences in breeding mortality which did not differ between the sexes. In general, males lived longer than females and we discuss the possible reasons behind this pattern of sex-biased non-breeding mortality

Larger ejaculate volumes are associated with a lower degree of polyandry across bushcricket taxa

In numerous insects, including bushcrickets (Tettigoniidae), males are known to transfer substances in the ejaculate that inhibit the receptivity of females to further matings, but it has not yet been established whether these substances reduce the lifetime degree of polyandry of the female. The aim of this study was to test the hypothesis that larger ejaculate volumes should be associated with a lower degree of polyandry across tettigoniid taxa, controlling for male body mass and phylogeny. Data on ejaculate mass, sperm number, nuptial gift mass and male mass were taken primarily from the literature. The degree of polyandry for 14 species of European bushcrickets was estimated by counting the number of spermatodoses within the spermathecae of field-caught females towards the end of their adult lifespans. Data for four further species were obtained from the literature. Data were analysed by using both species regression and independent contrasts to control for phylogeny. Multiple regression analysis revealed that, as predicted, there was a significant negative association between the degree of polyandry and ejaculate mass, relative to male body mass, across bushcricket taxa. Nuptial gift size and sperm number, however, did not contribute further to interspecific variation in the degree of polyandry. A positive relationship was found, across bushcricket taxa, between relative nuptial gift size and relative ejaculate mass, indicating that larger nuptial gifts allow the male to overcome female resistance to accepting large ejaculates. This appears to be the first comparative evidence that males can manipulate the lifetime degree of polyandry of their mates through the transfer of large ejaculates

The ultrasonic mating signal of the male lesser wax moth

Abstract. Male lesser wax moths, Achroia grisella (Fabricius) (Lepidoptera: Pyralidae: Galleriinae), produce both a pheromone and an ultrasonic acoustic signal that function in mate attraction. We describe the structure of the acoustic signal, in particular the interpulse intervals and the spectral properties of the pulses. The song consists of a train of ultrasonic pulses. The interpulse interval is usually bimodally distributed, but can sometimes be unimodal. This reflects variation in the duration of the up and down wing strokes. The pulses are also usually paired which can produce multimodality of the interpulse intervals. These paired pulses probably reflect wingbeat asynchrony because they are not found in males in which the signalling capability of one wing's sound producing structure is abolished. The song's frequency spectrum has peaks at around 80 and 100 kHz. The first peak varies significantly with male size, with larger males producing a lower frequency peak. The second peak is associated with male age, with 1‐day‐old males producing songs with a lower frequency second peak. Thus the ultrasonic song of lesser wax moths is more complex in structure than previously reported and could provide potentially important cues to females. However, the ability of females to discriminate such detail is not known.</p

Lonely hearts or sex in the city? Density-dependent effects in mating systems

Two very basic ideas in sexual selection are heavily influenced by numbers of potential mates: the evolution of anisogamy, leading to sex role differentiation, and the frequency dependence of reproductive success that tends to equalize primary sex ratios. However, being explicit about the numbers of potential mates is not typical to most evolutionary theory of sexual selection. Here, we argue that this may prevent us from finding the appropriate ecological equilibria that determine the evolutionary endpoints of selection. We review both theoretical and empirical advances on how population density may influence aspects of mating systems such as intrasexual competition, female choice or resistance, and parental care. Density can have strong effects on selective pressures, whether or not there is phenotypic plasticity in individual strategies with respect to density. Mating skew may either increase or decrease with density, which may be aided or counteracted by changes in female behaviour. Switchpoints between alternative mating strategies can be density dependent, and mate encounter rates may influence mate choice (including mutual mate choice), multiple mating, female resistance to male mating attempts, mate searching, mate guarding, parental care, and the probability of divorce. Considering density-dependent selection may be essential for understanding how populations can persist at all despite sexual conflict, but simple models seem to fail to predict the diversity of observed responses in nature. This highlights the importance of considering the interaction between mating systems and population dynamics, and we strongly encourage further work in this area