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

在许多动物中，雄性和雌性在体型、武器或颜色等特征上有所不同。在一些群体中，尤其是圆网蜘蛛，雌性的体重可以是雄性的100多倍。造成如此极端大小差异的原因尚不清楚。确定性别之间巨大体型差异的进化过程，对于理解自然选择和生物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模式，具有重要意义。本研究项目研究了导致北美东部常见的一种巨大而引人注目的蜘蛛Argiope aurantia球形网蜘蛛的极端性别大小二型性进化的因素。我解决的后果成年尺寸差异的早期少年阶段。年轻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，但可能会因此做出危险的行为，将自己暴露在捕食者面前。所有织网的雄性蜘蛛都必须离开自己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，以及不同体型的雄性是否在特定的栖息地类型中更有效地移动。体型较大的雌性通常被认为有优势，能够产下更多的卵，并且能够战胜相关的捕食者。然而，体型较大的雌性可能需要更长的时间才能发育成熟，因此交配和产卵的时间就更少。我详细调查了雌性的繁殖成功率，以评估各种假设。此外，我估计了雄性交配成功的可变性，并将其与雌性繁殖成功的估计值进行比较，雌性繁殖成功是指产生的后代数量。我测试了这样一个假设，即与大多数物种不同，雌性的可变性比雄性更大，这导致了对雌性的更强选择，从而使雌性进化为巨大的体型。

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.