Emasculation in orb web spiders has been a poorly understood form of sexual mutilation that has been assumed to be maladaptive. Through a review of all evidence and original phylogenetic reconstructions we showed that the so called eunuch phenomenon is adaptive. Males engage in genital mutilation in order to become more agile and aggressive in combating their rivals. Through emasculation, the eunuchs even secure remote copulation. These behaviors have co-evolved with extreme sexual size dimorphism as a response to sexual conflict.
COBISS.SI-ID: 37090861
Sexual dimorphism describes substantial differences between male and female phenotypes. In spiders, sexual dimorphism research almost exclusively focuses on size, and recent studies have recovered steady evolutionary size increases in females, and independent evolutionary size changes in males. Their discordance is due to negative allometric size patterns caused by different selection pressures on male and female size (converse Rensch’s rule). Here, we investigated macroevolutionary patterns of sexual size dimorphism (SSD) in Argiopinae, a global lineage of orb weaving spiders with varying degrees of SSD. We devised a Bayesian and maximum likelihood molecular species level phylogeny, then used it to reconstruct sex specific size evolution, to examine general hypotheses and different models of size evolution, to test for sexual size coevolution, and to examine allometric patterns of SSD. Our data revealed ancestral moderate sizes and SSD, and by failing to reject the Brownian motion model, these results suggest a nondirectional size evolution. Contrary to predictions, male and female sizes were phylogenetically correlated, and SSD evolution was isometric. We interpreted these results to question the classical explanations of female biased SSD via fecundity, gravity, and differential mortality.
COBISS.SI-ID: 37591341
We present the first species-level molecular phylogeny that includes the enigmatic orb weavers ‘Zygiellidae’ and Caerostris. Both groups belong among basal lineages of Araneidae. Caerostris represents the most striking case of web gigantism. We also find behavioral exclusivity that distinguish "basal" araneids from "classical" ones.
COBISS.SI-ID: 38559789
Integrating the insights derived from both phylogenetic and experimental approaches offers a more complete understanding of evolutionary patterns and processes, yet it is rarely a feature of investigations of the evolutionary significance of trait variation. The study combines these approaches to reinterpret the patterns and processes in the evolution of female biased sexual size dimorphism in Nephilidae, a spider lineage characterized by the most extreme sexual size dimorphism among terrestrial animals. A reconstruction of size evolution for each sex reveals a case of “sexually dimorphic gigantism”: both sexes steadily outgrow their ancestral sizes, but the female and male slopes differ, and hence sexual size dimorphism steadily increases. A review of the experimental evidence reveals a predominant net selection for large size in both sexes, consistent with the phylogenetic pattern for females but not for males. Thus, while sexual size dimorphism in spiders most likely originates and is maintained by fecundity selection on females, it is unclear what selection pressures prevent males from becoming as large as females. This integrated approach highlights the dangers of inferring evolutionary significance from experimental studies that isolate the effects of single selection pressures.
COBISS.SI-ID: 37217325
Several clades of spiders whose females evolved giant sizes are known for extreme sexual behaviors, but these behaviors have only been tested in a handful of size dimorphic spiders. Here, we bring another lineage into the picture by reporting on sexual behavior of Caerostris darwini. We uncover a rich sexual repertoire that predictably involves sexual cannibalism, genital mutilation, male preference for teneral females, and emasculation. Surprisingly, C. darwini males also engage in oral sexual encounters, a rarely reported phenomenon outside mammals.
COBISS.SI-ID: 39794477