Testing sexual and other selection pressures influencing male size in sexually size dimorphic spidersQuinones Lebron, Shakira Guani (Avtor) Kuntner, Matjaž (Mentor) Kralj Fišer, Simona (Komentor) Sexual size dimorphismdevelopmental plasticitysexual selectionnatural selectionNephilidaeSexual size dimorphism (SSD) refers to the phenotypic difference in body size between males and females and it has puzzled scientists since Darwin first described it in 1871. The processes underlying the evolution of SSD have been extensively studied in vertebrates, where male-male competition commonly favours larger males. In many arthropod groups, however, the females of many species are much larger than males. Several hypotheses have been proposed to explain the evolution of SSD in spiders, including natural and sexual selection pressures. The hypotheses related to natural selection pressures include the fecundity hypothesis, the differential mortality hypothesis, and the gravity hypothesis. Here, we investigate the interplay of several natural and sexual selection pressures influencing male size in SSD spiders, specifically in Nephilingis cruentata. We found no directional selection towards smaller male sizes in most of the tested hypotheses. Small male sizes do not confer an advantage in either climbing or bridging ability. In fact, larger males are faster climbers. Similarly, we found no correlation between size and time of maturation. In other words, earlier maturation does not pose a size constraint in males and we therefore reject the scramble competition hypothesis as a possible explanation for the maintenance of small male sizes. Surprisingly, male size heritability is low, and depended mostly on food supply and maternal effects. Furthermore, we found evidence for a dynamic trade-off between size and fitness, mediated by sexual cannibalism. Larger males are more likely to get cannibalized, truncating their future mating opportunities. However, cannibalized males transferred more sperm. The fitness of smaller males almost doubles when cannibalized, but they are less likely than larger males to be attacked by females. Both the rate of sexual cannibalism and the occurrence of male-male contests were relatively low. Generally, larger males have an advantage when fighting off rivals, however fights are rare. Also, the presence of a rival, regardless of his size, did not affect male mating success. In fact, most of the time, males ignored the presence of a competitor. Lastly, we found that the co-occurrence of Trichonephila species suggests reproductive interference. Larger males might be outcompeting both heterospecific and conspecific rivals, but this hypothesis remains to be tested. In conclusion, male size is a highly variable trait that is maintained by both sexual selection and developmental plasticity.20202020-04-22 07:15:04Doktorsko delo/naloga115280sl