臺灣博碩士論文加值系統

人面蜘蛛具有極端的雌雄大小二型性，因此人面蜘蛛向來為探討此主題的熱門物種。學者普遍認為人面蜘蛛雌雄大小二型性的演化成因是由於天擇對於雌性子代生產量的性擇壓力而造成雌性體型在演化上有增加的趨勢。最近的親緣分析確實發現人面蜘蛛雌性的體型隨著演化過程有增加的趨勢，而雄性體型則維持在小體型的祖先型特徵。許多前人研究支持在人面蜘蛛物種中，會因為雄性競爭而偏好體型較大的雄性。然而，雄性蜘蛛體型並未隨著演化過程而有增加的趨勢，這意味著必定有反向的作用力偏好體型較小的雄性。大人面蜘蛛具有極端的雌雄大小二型性，因此為合適的物種去了解何種演化壓力使得人面蜘蛛雄性維持著小體型。我進行野外調查及室內操控實驗去了解此物種在交配前後面臨的性擇壓力是否有所不同。野外調查結果顯示，較大雄性雖能佔據雌性網上的優勢位置，但卻不一定能獲得交配成功率，因為佔有優勢位置的雄性無法長時間守衛雌性因此容易錯失適當的時機與雌性交配，也就是在雌性蛻皮時。且大部分的交配事件皆發生在雌性蛻皮且網子上只有一隻雄性的狀態下，這可能是因為大人面蜘蛛雄性則必須移動長距離去搜尋雌性，此過程具有高死亡風險，因此降低了網子上雄性競爭的可能性。由上述結果可看出，對於大體型的選擇壓力相對來說較不重要，反之如何找到適合交配的即將蛻皮雌性則是最重要的。從實驗室的操控實驗發現，交配後的精子競爭似乎與交配順序較無關聯，第一雄性並無法使較多的卵授精。這些結果顯示交配前的性擇壓力可能是造成大人面蜘蛛極端雌雄大小二型性的重要原因之一。

Extreme female-biased sexual size dimorphism is generally assumed to result from female gigantism as a result of selection for fecundity in spiders. However, the mechanisms that maintain male’s smaller size are still not clear. The giant wood spiders Nephila pilipes is a species with extreme sexual size dimorphism, making it a suitable organism for studying which evolutionary pressures keep males in small body size. In this study, we performed field census and laboratory manipulations to realize potential sexual selection pressures on males of this species both before and after mating. Results of an intensive field census suggested that lager males could occupied a favorable position on a female’s web; however they might not obtain mating opportunities since males occupied the favorable position might not able to guard female for a long time and mate with the female at the right time which when female were molting. Also, most of mating events happened while there were only one male present on the web which means no male-male competition. N. pilipes have to travel for long distance to locate females. Since mate searching suffers high mortality risk, this might relax male-male competition in N. pilipes males and larger body size seems to be relatively unimportant under this circumstance. Results of field study thus indicate that how to find an appropriate female at the right time represents a strong selection pressure which favors protandry and small size in males. Results of laboratory manipulation showed that sperm competition pattern in N. pilipes followed a mixed rather than first male sperm priority pattern, suggesting a relatively weak post-copulatory sexual selection pressutr. Our field and laboratory studies thus show that selection pressures from pre-copulatory stage seem to be one important determinant of extreme sexual size dimorphism in N. pilipes.

Abstract (in Chinese) ----------------------------------------------------------4
Abstract --------------------------------------------------------------------------5
Introduction --------------------------------------------------------------------7
Materials and Methods -------------------------------------------------------12
Study organisms ------------------------------------------------------12
Study sites and field investigations ------------------------------- 13
Sperm priority pattern -----------------------------------------------16
Results --------------------------------------------------------------------------20
Population characteristics -------------------------------------------21
Female-male interactions -------------------------------------------22
Male-male interaction -----------------------------------------------23
Factors determining male mating success ------------------------25
Sperm release and observed paternity pattern ----------------------26
Discussion ---------------------------------------------------------------------27
References ---------------------------------------------------------------------32
Tables ---------------------------------------------------------------------------46
Figures --------------------------------------------------------------------------47






Appendix (Nephila clavata)
Materials and Methods -------------------------------------------------------50
Study organisms ------------------------------------------------------50
Study sites and field investigations --------------------------------50
Statistical analysis ---------------------------------------------------51
Results --------------------------------------------------------------------------52
Female-male interactions -------------------------------------------52
Male-male interaction -----------------------------------------------52
Factors determining male mating success ------------------------53
Discussion ---------------------------------------------------------------------53
References ---------------------------------------------------------------------56
Tables ---------------------------------------------------------------------------57
Figures --------------------------------------------------------------------------58

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