The evolutionary significance of the extreme sexual size dimorphism in orb-web spiders

在许多动物中，雄性和雌性在大小、武器或颜色等特征上有所不同。在某些群体中，特别是球网蜘蛛，雌性蜘蛛的体重可能是雄性的一百多倍。造成这种极端规模差异的原因还不是很清楚。对于理解自然选择和生物体的遗传结构如何相互作用，从而导致观察到的形态和行为多样性的模式，确定两性之间极端体型差异的演化过程是重要的。这项研究项目考察了导致圆网蜘蛛Argiope aurantia极端性别二态进化的因素，Argiope aurantia是一种常见于北美东部的大型且引人注目的蜘蛛。我讨论了成年个体大小差异对幼年早期的影响。幼年的雄性和雌性必须采取不同的觅食策略：雌性必须捕捉更多的猎物，但结果可能会从事危险的行为，使自己暴露在捕食者面前。所有建网蜘蛛的雄性都必须离开它们的网去寻找雌性蜘蛛。我研究了栖息地结构如何影响雄性在植被中的移动，以及不同大小的雄性在通过特定栖息地类型时是否更有效率。体型较大的雌性通常被认为具有优势，能够产下更多的卵，并比相关的捕食者发育得更好。然而，体型较大的雌性可能有一个缺点，即需要更多的时间才能发育到成熟，因此交配和产卵的时间较少。我对女性的生殖成功进行了详细的调查，以评估各种假设。此外，我估计了雄性交配成功的变异性，并将其与获得的雌性生殖成功的估计进行比较，后者是产生后代的数量。我测试的假设是，与大多数物种不同的是，雌性的变异比雄性的更大，导致对雌性的更强选择，从而使雌性进化出巨大的体型。

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.