翡翠樹蛙之雌雄生殖策略
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2023
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性擇(sexual selection)透過雄性競爭(male-male competition)以及雌性選擇(female choice)來影響個體的生殖成功。性擇理論預期雄性和雌性通常採取不同的生殖策略來增加自己的生殖成功：雄性會尋求較多的交配機會，而雌性則會挑選較高品質的配偶。群聚展示(lek-breeding system)的物種，雄性會聚集到特定地點進行行為或者鳴叫聲的展示來吸引雌性，雌性會在展示場(lek)中透過比較雄性的展示來挑選牠們的配偶。持久力競賽(endurance rivalry)（一種雄性之間不直接互動的競爭機制）和雌性選擇(female choice)在展示場生殖系統中是兩個重要的的性擇機制，這兩種機制造成雄性間之間的交配成功有所不同。持久力競賽指的是雄性透過提高自己在展示場的出席率來增加自己的交配機會。持久力競賽假說(endurance rivalry hypothesis)認為因為較高品質的雄性才有能力較多次數的出席展示場進行展示，雌性在隨機交配的情況下，仍然有機會跟高品質的雄性交配，因此雌性不需要有選擇性。而優良基因假說(good gene hypothesis)則認為，雌性可以透過選擇展示誇張或精緻的形態或行為特徵的雄性作為配偶來把優良的基因傳給自己的子代。因為製造身上的裝飾形態或者展示行為是耗能的，所以這些特徵能夠反映出雄性本身的品質。但在同一個物種上，雄性的持久力競賽以及雌性選擇這兩個機制對於雄性交配成功的影響很少被同時探討。此外，當雄性與雌性各自的生殖策略不同時，為了最佳化自己的利益，可能就會造成性別之間的衝突(sexual conflict)。在群聚展示的物種中，雌性會在展示場出選擇最佳的雄性來把優良的基因傳給子代。雌性如果是因為雄性的強迫，而被迫接受不偏好的配偶而造成一雌多雄交配(polyandrous mating)，對雌性來說，會承受到間接遺傳上的代價(indirect genetic costs)，因為牠一部分的子代將會被品質較低的雄性所受精。在這樣的狀況下，一雌多雄交配的雌性所產下的子代會有較低的適存度(fitness)。然而，一雌多雄交配可能可以提供雌性利益。其中一個適應性的假說認為，參與一雌多雄交配可以讓雌性獲得較高受精率的利益（受精保險假說）(fertilization insurance hypothesis)。雖然在一些群聚展示的物種已經發現有一雌多雄交配的行為，但針對一雌多雄交配對於雌性的利益或代價的研究還很少。因此，我以群聚合唱生殖(lek-chorusing)而且被觀察到有一雌多雄交配的行為的翡翠樹蛙(Zhangixalus prasinatus)為研究對象，探討持久力競賽以及雌性選擇對於雄蛙的交配成功的重要性，亦探討雌蛙參與一雌多雄交配是否會承受代價或者獲得利益。本研究分為四個章。在第一章，我介紹雄性以及雌性的生殖策略，當中包含雄性競爭以及雌性選擇如何影響雄性交配成功的假說，以及一雌多雄交配如何對雌性造成利益及代價的假說。在這一章，我也敘述翡翠樹蛙的生殖行為，以及本論文的研究目的。第二章，我研究持久力競賽以及雌性選擇如何共同的影響雄蛙交配成功，以及如何影響雄蛙的形態。主要的結果顯示：(1)雄蛙的展示場出席和其本身的體長大小和身體狀況有正相關，且展示場出席叫高的雄蛙有較高的交配成功，(2)雄蛙宣告叫聲的主音頻(dominant frequency)和其本身的體長大有有負相關，且叫聲主音頻較低的雄蛙有較高的交配成功，(3)雄蛙的體長大小和交配成功有不顯著的正相關，但身體狀況跟交配成功無關，(4) 雌蛙在二選一聲音回播實驗中顯示，對於較低音頻的雄性叫聲有顯著的偏好。整合來說，持久力競賽以及雌性選擇都在雄蛙的交配成功上扮演重要的角色，且這兩個機制都與雄蛙的形態特徵有關連。第二章，我研究一雌多雄交配是否對於雌蛙造成代價或者提供利益。我檢測一雌多雄交配是否造成子代的適存度降低(代價)以及一雌多雄交配是否增加窩卵的受精（利益）。主要結果顯示：(1)一雌一雄交配的窩卵和一雌多雄交配之間的窩卵之間的胚胎存活率、子代的存活、生長與發育(跟適存度相關的表現)沒有顯著差異，(2) 一雌一雄交配的窩卵和一雌多雄交配之間的窩卵之間的受精率沒有顯著差異，(3)一雌多雄交配後的窩卵具有多重父子關係(multiple paternity)，且主要雄蛙（被雌性選擇的雄蛙）的子代較周圍雄蛙(未被雌性選擇的雄蛙)多。整合這些結果顯示，參與一雌多雄交配的雌蛙沒有遭受到間接遺傳上的代價，也沒有獲得受精保險的好處。而這些結果也顯示了雄蛙如果沒有被選擇成為主要雄蛙，加入已配對的交配組合成為周圍雄蛙，是一個替代性的生殖策略(alternative reproductive tactic)來獲得生殖成功。周圍雄蛙可以從一雌多雄交配中獲得利益，而雌蛙沒有從一雌多雄交配中獲得任何利益。因此，我推測翡翠樹蛙的一雌多雄交配，是由雄蛙為了獲得生殖成功而造成的。第四章，我總結本研究第二、三章的結果，並提出未來進一步對於翡翠樹蛙的性擇研究方向的建議。
Sexual selection operates through male-male competition and female choice to influence organisms’ reproductive success. Sexual selection theory predicts that males and females should adopt different mating strategies to increase their reproductive success: males seek more mating opportunities, whereas females choose to mate with high-quality mates. In the lek-breeding system, males aggregate in areas where they perform physical or acoustic displays to attract females, allowing females to compare and choose their mates. Endurance rivalry (interaction-independent male-male competition) and female choice are two important mechanisms of sexual selection in lek-breeding systems that contribute to the variation in male mating success. Endurance rivalry is when males compete for opportunities to mate by spending more time in leks than others. The endurance rivalry suggests that, because high-quality males can afford to have high lek attendance, females have a higher chance of mating with good-quality males even when they mate randomly. The good gene hypothesis proposes that females can pass good genes on to their offspring by choosing males that display elaborate morphological and/or behavioral traits. Because ornaments and advertisement displays are costly to produce, these traits reflect the males’ genetic quality. However, the relative importance of endurance rivalry and female choice to males’ mating success in the same species is rarely evaluated. Moreover, sexual conflict can occur when the two sexes have divergent and conflicting reproductive strategies to optimize their own fitness. One sex, to maximize reproductive success, may impose costs on the other sex. In lek-breeding species, females may choose the best males at the lek that can pass good genes on to their offspring. Suppose females are forced to accept superfluous mates against their preference (for high-quality males), leading to polyandrous mating. In that case, a proportion of their offspring will be sired by lower-quality (non-preferred) males. Therefore, females are expected to have offspring with lower fitness (genetic costs). However, polyandrous mating may provide female benefits. One direct benefit to females is that polyandrous mating can ensure egg fertilization. Although polyandrous mating has been observed in some lek-breeding species, the studies on the costs and benefits of polyandrous mating to females in lek-breeding species are few. The emerald treefrog (Zhangixalus prasinatus), a lek-chorusing treefrog that has been observed with polyandrous mating, was used as the study organism to investigate the importance of endurance rivalry or female choice to males’ mating success and the effects (costs and benefits) of polyandrous mating on females’ reproductive success in this dissertation. This dissertation has four chapters. Chapter 1 introduces the reproductive strategies of males and females, including the hypotheses of how male-male competition and female choice can influence male mating success and how polyandrous mating provides female benefits and costs. In the same chapter, I also described the breeding behavior of Z. prasinatus and the aims of this dissertation. Chapter 2 investigates how endurance rivalry and female choice might jointly influence male mating success and shape males’ morphological traits in Z. prasinatus. The main results of this chapter showed that (1) male lek attendance was positively correlated with body size and condition, and males with higher lek attendance had higher mating success, (2) the dominant frequency of males’ advertisement calls was negatively correlated with body size and males producing lower frequency calls have higher mating success, (3) male body size, but not body condition, had a non-significant positive relationship with mating success and (4) females show a preference for calls with lower dominant frequencies in two-choice playback. Overall, both endurance rivalry and female choice play an important role in the mating success of male emerald treefrogs in the field, and both are influenced by male body size/condition. Chapter 3 investigated the costs and benefits of polyandrous mating to female Z. prasinatus. I examined whether polyandrous mating reduced offspring fitness (costs) and whether polyandrous mating increased clutch fertilization (benefits). The main results of this chapter showed that (1) polyandrous mating did not reduce embryo viability and offspring’s survival, growth, and development (which links to fitness), and (2) polyandrous mating did not increase the fertilization of the clutch. In polyandrous clutches. I (3) detected multiple paternity in polyandrous clutches and further found that primary males (chosen by female) sired more offspring than peripheral males (non-chosen by female). These results showed that females that mated with multiple males did not suffer the indirect genetic cost and obtained no benefits from increasing fertilization. The results suggest that when a male is not chosen by a female, joining a mating group as a peripheral male could serve as an alternative reproductive tactic to obtain reproductive success. Therefore, I suggest that polyandrous mating in Z. prasinatus is driven by the males so that it only benefits peripheral males but not females. In Chapter 4, I summarized the results of Chapters 2 and 3, and proposed future work on investigating sexual selection in Z. prasinatus.
Sexual selection operates through male-male competition and female choice to influence organisms’ reproductive success. Sexual selection theory predicts that males and females should adopt different mating strategies to increase their reproductive success: males seek more mating opportunities, whereas females choose to mate with high-quality mates. In the lek-breeding system, males aggregate in areas where they perform physical or acoustic displays to attract females, allowing females to compare and choose their mates. Endurance rivalry (interaction-independent male-male competition) and female choice are two important mechanisms of sexual selection in lek-breeding systems that contribute to the variation in male mating success. Endurance rivalry is when males compete for opportunities to mate by spending more time in leks than others. The endurance rivalry suggests that, because high-quality males can afford to have high lek attendance, females have a higher chance of mating with good-quality males even when they mate randomly. The good gene hypothesis proposes that females can pass good genes on to their offspring by choosing males that display elaborate morphological and/or behavioral traits. Because ornaments and advertisement displays are costly to produce, these traits reflect the males’ genetic quality. However, the relative importance of endurance rivalry and female choice to males’ mating success in the same species is rarely evaluated. Moreover, sexual conflict can occur when the two sexes have divergent and conflicting reproductive strategies to optimize their own fitness. One sex, to maximize reproductive success, may impose costs on the other sex. In lek-breeding species, females may choose the best males at the lek that can pass good genes on to their offspring. Suppose females are forced to accept superfluous mates against their preference (for high-quality males), leading to polyandrous mating. In that case, a proportion of their offspring will be sired by lower-quality (non-preferred) males. Therefore, females are expected to have offspring with lower fitness (genetic costs). However, polyandrous mating may provide female benefits. One direct benefit to females is that polyandrous mating can ensure egg fertilization. Although polyandrous mating has been observed in some lek-breeding species, the studies on the costs and benefits of polyandrous mating to females in lek-breeding species are few. The emerald treefrog (Zhangixalus prasinatus), a lek-chorusing treefrog that has been observed with polyandrous mating, was used as the study organism to investigate the importance of endurance rivalry or female choice to males’ mating success and the effects (costs and benefits) of polyandrous mating on females’ reproductive success in this dissertation. This dissertation has four chapters. Chapter 1 introduces the reproductive strategies of males and females, including the hypotheses of how male-male competition and female choice can influence male mating success and how polyandrous mating provides female benefits and costs. In the same chapter, I also described the breeding behavior of Z. prasinatus and the aims of this dissertation. Chapter 2 investigates how endurance rivalry and female choice might jointly influence male mating success and shape males’ morphological traits in Z. prasinatus. The main results of this chapter showed that (1) male lek attendance was positively correlated with body size and condition, and males with higher lek attendance had higher mating success, (2) the dominant frequency of males’ advertisement calls was negatively correlated with body size and males producing lower frequency calls have higher mating success, (3) male body size, but not body condition, had a non-significant positive relationship with mating success and (4) females show a preference for calls with lower dominant frequencies in two-choice playback. Overall, both endurance rivalry and female choice play an important role in the mating success of male emerald treefrogs in the field, and both are influenced by male body size/condition. Chapter 3 investigated the costs and benefits of polyandrous mating to female Z. prasinatus. I examined whether polyandrous mating reduced offspring fitness (costs) and whether polyandrous mating increased clutch fertilization (benefits). The main results of this chapter showed that (1) polyandrous mating did not reduce embryo viability and offspring’s survival, growth, and development (which links to fitness), and (2) polyandrous mating did not increase the fertilization of the clutch. In polyandrous clutches. I (3) detected multiple paternity in polyandrous clutches and further found that primary males (chosen by female) sired more offspring than peripheral males (non-chosen by female). These results showed that females that mated with multiple males did not suffer the indirect genetic cost and obtained no benefits from increasing fertilization. The results suggest that when a male is not chosen by a female, joining a mating group as a peripheral male could serve as an alternative reproductive tactic to obtain reproductive success. Therefore, I suggest that polyandrous mating in Z. prasinatus is driven by the males so that it only benefits peripheral males but not females. In Chapter 4, I summarized the results of Chapters 2 and 3, and proposed future work on investigating sexual selection in Z. prasinatus.
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雄性競爭, 持久力競賽, 雌性選擇, 展示場出席, 交配成功, 一雌多雄交配, 多重父子關係, 間接遺傳代價, 受精保險, 樹蛙, male-male competition, endurance rivalry, female choice, lek attendance, mating success, polyandrous mating, multiple paternity, indirect genetic costs, fertilization insurance, rhacophorid treefrog