Grant: Discovery Grants Program - Individual

In many animals males and females differ in characteristics such as size, weaponry, or colouration. In some groups, notably the orb-web spiders, females can weigh more than one hundred times as much as males. The reasons for such extreme size differences are not well understood. Identifying the processes that underlie the evolution of extreme body size differences between the sexes are important to understand how natural selection and the genetic architecture of organisms interact to result in observed patterns of morphological and behavioural diversity. This research project examines factors that contribute to the evolution of extreme sexual size dimorphism in the orb-web spider Argiope aurantia, a large and conspicuous spider common to Eastern North America. I address the consequences of the adult size differences for the early juvenile stages. Juvenile males and females have to pursue different foraging strategies: females have to catch more prey, but may as a result engage in risky behaviours, exposing themselves to predators. Males of all web-building spiders have to leave their webs to search for females. I examine how habitat structure effects male movement through the vegetation and if males of different sizes are more efficient at moving through particular habitat types. Large females are commonly hypothesized to have the advantage to be able to lay more eggs and to have outgrown relevant predators. Large females may however have the disadvantage of taking more time to grow to maturity so that less time to mate and to lay clutches is available. I investigate reproductive success of females in detail to evaluate the various hypotheses. Furthermore, I estimate the variability in male mating success and compare this to the obtained estimate for female reproductive success, which is the number of offspring produced. I test the hypothesis that, unlike in the majority of species, the variability is greater in females than in males, leading to stronger selection on females and thus enabling the evolution of giant size in the female sex.