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Dwarf spiders (Linyphiidae, Erigoninae) are especially suitable for sexual selection research as many of them exhibit sexual dimorphism, with males possessing modified prosomata. In those species that have been investigated in detail the modified structures are equipped with a glandular tissue that produces secretions, which the females contact and take up during courtship/copulation. The time of secretion release, and refilling of the reservoirs was analysed on an ultrastructural level in male Oedothorax retusus. The results suggest that the main function of the secretions is gustatorial courtship and not the emission of volatile pheromones for mate attraction. Mating decisions and reproductive success are influenced by secondary sexual traits that evolved under sexual selection. However, an individual´s nutritional status is also important for mate choice. Since spiders are regularly exposed to limited prey availability, adult feeding status can be considered an important component of spider mating behaviour. In order to test for the effects of dietary restriction, females of the closely related species O. retusus and O. apicatus were subject to a short period of food shortage. The effects of low- (LD) vs. high-diet (HD) treatment on courtship, mating probability and behaviour, and reproduction were analysed. We found that short phases of diet restriction as adults have a high impact on copulation and reproduction in the two dwarf spider species. Whenever females mate with more than one male, and sperm is stored prior to fertilization, males may suffer from sperm competition. Mating plugs that block the female genital openings after mating are a male strategy to avoid sperm competition. Although mating plugs occur in many species, their function and origin has hardly been investigated. O. retusus males transfer amorphous material onto the female genitalia during mating. We investigated the location of plug production using x-ray microtomography (μCT) as well as light and transmission electron microscopy (TEM). Furthermore, we asked whether males are limited in the production of the amorphous plug material in successive matings. The plug material is produced in a gland inside the male pedipalp and stored close to the blind end of the sperm storage compartment. The size of the first plug a male produced significantly influenced the size of subsequent plugs. Obviously, males do not possess unlimited amounts of plug material in a certain period, which may severely limit their ability to secure paternity through subsequent mating plugs. Even though mating plugs seem to be an obvious means to secure paternity, their potential in securing paternity depends on their mechanical efficacy and persistence. Consequently, the influence of the size of the plug material (mating duration as a proxy) and the age of the mating plug (time interval between successive copulations) on its efficacy was investigated. Small and fresh plugs were least effective, whereas large plugs were highly effective. We were able to show that mating plugs in O. retusus are a powerful mechanical safeguard whose efficacy varies with plug size and age. Genitalia in animals with internal fertilization are complex, species-specific, and underlie rapid evolution. In spiders, male and female genitalia are paired, and have to interact during mating, which results in an even higher complexity. Pedipalps (transformed pair of legs) in male spiders are used as secondary sperm transfer organs that are not directly connected to the gonads. Due to the high complexity of male pedipalps, it has been taken for granted that pedipalps are side specific and cannot be used flexibly into either female copulatory opening. We investigated potential flexible pedipalp use in O. retusus. Our findings demonstrate a flexible insertion mode in a dwarf spider with complex pedipalps but relatively simple female genitalia. Our findings corroborate sexual selection as the selective regime for the evolution of complex and diverse genitalia. The results of this thesis show how complex sexual selection acts in the dwarf spiders O. retusus and O. apicatus. It shapes the evolution of male and female genitalia, affects mate choice (pre- and postcopulatory), mating behaviour, and influences mating success and reproduction. All these factors and traits affect an individual´s evolutionary fitness, and their interactions help to understand how sexual selection acts.
In holometabolic insects, senescence starts at sexual maturation and condition diminishes with age. Young virgin males should gain the highest mating success. Although sperm quality and quantity typically decreases with age, older males have been shown to have a higher mating success in a variety of insect taxa. Life-history theory predicts an increased aggressive and persistent courtship behaviour, due to a decrease of the residual reproductive value and thus, the opportunity for future reproduction. In the butterfly Bicyclus anynana, older males gain up to a 4x higher mating success despite a poorer condition, compared to younger males. As older males were found to court more often and for longer time bouts,suggesting a higher intrinsic motivation, there is evidence for the residual reproductive value hypothesis. On the other hand, an age-specific variation of male sex pheromones may provide females with information, being helpful to chose specific males. The latter could be males which have proven a high fitness in terms of survival, thus essentially reflecting a “good genes" hypothesis. This doctoral thesis investigated the reasons underlying old male mating advantage in B. anynana. The first study tested whether old male mating advantage prevails, even if females were unable to distinguish between older and younger males. The results were backed up by examining female rejection rates, male courtship frequency, courtship duration, time to copulation and mating duration. Older males had a significantly higher mating success compared to younger ones regardless of differences in pheromone blends or the females ability to smell. Older males courted more often and longer. The results support the residual reproductive value hypothesis. Study 2 investigated post-copulatory sexual selection in B. anynana. Females were double-mated, allowing for sperm competition and cryptic mate choice. Older males had a higher paternity success than younger ones, when the former were the last mating partner. The paternity success was balanced across age classes when older males were the first mating partner. Older males transferred larger spermatophores with higher numbers of eupyrene sperm than younger males. B. anynana does not exhibit cryptic mate choice and last-male precedence is the most probable fertilization pattern. The higher proportion of offspring sired by older males is due to significantly increased numbers of fertile sperm compared to younger males. The latter are clearly disadvantaged, as the higher sperm numbers provided by older males counteract the benefits of last-male precedence. Study 3 explored determinants of mating success in B. anynana. Successful males had longer wings, a heavier thorax, a lighter abdomen, a higher fat content, and higher phenoloxidase expression levels. Mating success seemed to be directly linked to proxies of flight performance. Successful males showed a better flight performance, in turn increasing mating success. As energy reserves are of crucial importance for flight manoeuvres, they may play a significant role in male mating success. Study 4 tested the effects of male age and mating number on spermatophore mass, sperm number, male oxidative status, and reproductive success by manipulating age and mating frequency in male B. anynana. Spermatophore mass and sperm numbers increased significantly with age, while antioxidant defences and oxidative damage declined. Female fecundity and egg-hatching success was highest when being mated with young virgin males. Antioxidant defences decreased with age, being a possible reason for the negatively affected reproductive success in females. Ejaculate quality diminished with age and mating number, despite larger spermatophores and higher sperm numbers being found in older males. Therefore, spermatophore size and sperm numbers can not be considered as reliable proxies of male condition. The final study proofed whether male sex pheromone comprise honest signals and which traits might be associated with increased pheromone titres. Pheromone titres were analysed among successful and unsuccessful males when being mated with either a control or a scent-blocked female. Both groups did not differ in pheromone levels. Successful males had significantly higher numbers of eupyrene sperm. No correlation between male pheromones and any investigated trait was found, suggesting that pheromone titres do not provide reliable information on male quality. Nothing indicates that male sex pheromones in B. anynana play a decisive role in female mate choice. Successful males generally have a better body condition, resulting in a more vigorous courtship behaviour and higher sperm numbers. Hence, variation in body condition rather than pheromone titres is more likely to determine male mating success. Mating decisions are primarily driven by male behaviour. The results suggest that old male mating advantage arises from sexual conflict.