Intraspecific differences in the growth rates and adult body sizes of male and female offspring are frequently observed in both vertebrate and invertebrate taxa. In some species, males are the larger sex, whereas in others females are larger, a process described as ‘sexual size dimorphism’ (SSD). Several hypotheses have been postulated to explain SSD, such as intra-sexual competition for access to mates, differences in the mass-scaling of gamete production costs (eggs, sperm), and sexual variation in mortality risks during early development. Among spiders, differences in the body masses of males and females at maturity are among the largest in the animal kingdom, with females sometimes attaining a body mass a 100-fold greater than that of males. Here, we tracked growth trajectories of brown widow spiders (Latrodectus geometricus) from neonate to adult. To assess whether prey availability affects sexual variation in adult body mass, we either fed spiderlings weekly (‘continuous-feeding treatment’) or once every three weeks (‘intermittent-feeding treatment’) on fruit flies and then crickets or blow flies. Approximately 60% of continuously-fed spiderlings survived to adulthood, compared to only 2 spiderlings in the intermittent-feeding treatment. Males molted on average 4 times, and reached maximum virgin body masses after around 100 days, whereas females molted 6-7 times, and reached maximum virgin body masses after around 170 days. Growth rate of males was consistent across different instars whereas it was highest in later female instars. Mature males were less than 1% as large as mature females, revealing extreme (SSD) in this species. We discuss different factors that may account for extreme sexual size dimorphism (ESSD) in widows and other spiders.
Harvey, Jeffrey and Gerosa, Francesco and Gols, Rieta and Verberk, Wilco C.E.P., Development, Adult Body Mass and Extreme Sexual Size-Dimorphism in the Brown Widow Spider, Latrodectus Geometricus.
Available at SSRN: https://ssrn.com/abstract=4741608 or http://dx.doi.org/10.2139/ssrn.4741608