赤尾青竹絲之族群生態學研究

Abstract

中文摘要

生物個體在生殖、成長與棲息地選擇方面的差異是導致它們擁有不同適合度（fitness）的重要原因，同時也是天擇作用於物種或是族群的原動力之一。赤尾青竹絲（Trimeresurus s. stejnegeri）是台灣常見的樹棲性蝮蛇，夜行性，採定點埋伏捕食策略；具有明顯的性別體型二型性（sexual-size dimorphism，SSD），雌蛇體型較大。本論文以赤尾青竹絲為對象，研究其族群生態、生活史特性與微棲地選擇，內容共分為三章。 第一章與第二章是我藉由對苗栗縣造橋鄉平興村的野外赤尾青竹絲族群，以重複捕取（mark-recapture）的方法獲得長達9年以上的群體研究（demography）資料，據以估算赤尾青竹絲的存活率，並探討此一族群的雄蛇明顯多於雌蛇的原因；此外我也研究赤尾青竹絲的生長，並探討其性別體型二型性的形成原因。第三章則是研究植被遮蔽（vegetation cover）、獵物可及性（prey availability）與環境溫度對於赤尾青竹絲選擇日間停棲地點(diurnal retreat site)的影響性，為了避免野外眾多環境因子交互影響又常難以釐清，此部分研究在戶外網室中進行。 在第一章中，我從1996年5月至2005年8月期間共計標放577隻赤尾青竹絲，所有的捕取資料共計2095隻次。以Jolly-Seber模式估算的結果，整體族群的平均存活率為0.72；尚未性成熟的個體（immature）其平均存活率為0.62，顯著較性成熟後的成蛇存活率（0.73）為低，但年輕成蛇與年長成蛇的存活率則無顯著差異。族群內雄蛇與雌蛇隻數的平均比值為2.51（雄蛇/雌蛇），而且雄蛇顯著多於雌蛇的現象持續存在於研究中的每個半年期間（以雌蛇的生殖期前、後為區分）。惟在不同的生活史階段，新生幼蛇（newborn）與尚未性成熟的亞成蛇（juvenile），其性別比均未偏離1：1；但在性成熟後，雄性成蛇顯著多於雌性成蛇，而且隨著體長或年齡的增長，雄蛇多於雌蛇的趨勢更加顯著。比較不同性別之存活率，性成熟前，雄蛇與雌蛇之平均存活率並無顯著差異（雄蛇為0.62，雌蛇為0.63）；惟性成熟後，雌蛇的平均存活率（0.65）顯著低於雄蛇（0.75），而兩性間的捕取率並無顯著差異（雄蛇為0.65，雌蛇為0.61）。從研究結果推測，雌性赤尾青竹絲在性成熟後，因為較高的生殖成本（cost of reproduction）而導致其存活率較雄蛇低，同時也導致此一族群的雄性成蛇數量顯著多於雌性。 在第二章中，我分析造橋赤尾青竹絲的生長模式、估測其年齡結構，並比較性別間的差異。性成熟前，赤尾青竹絲的平均吻肛長（SVL）與體重並未有性別間的差異；性成熟後，雌蛇的體重顯著大於雄蛇。不同生活史階段的群體間，新生幼蛇的吻肛長（SVL）與體重並未有性別間的差異；但在亞成蛇階段，雌蛇的吻肛長與體重均顯著大於雄蛇，且此差異性同樣出現於年輕成蛇與年長成蛇階段，顯示赤尾青竹絲的性別體型二型性是從亞成蛇階段開始發生。此外，性成熟前雌蛇的生長速率（0.28 mm/day）明顯高於雄蛇（0.19 mm/day），性成熟後二者則無顯著差異。雌蛇較雄蛇更早性成熟，雄蛇達到性成熟體長所需的平均時間為2.6年，雌蛇僅需1.9年；而年齡結構也有顯著的性別間差異，雌性赤尾青竹絲的平均年齡（4.0）顯著低於雄蛇（5.2），5歲以及5歲以上的雄蛇佔所有雄性個體的36.8 %，而同樣年齡層的雌蛇僅佔11.9 %。赤尾青竹絲的性別體型二型性肇因於雌蛇在性成熟前的快速生長，以及在性成熟之後以與雄蛇相近的速率持續生長；這可能是因為赤尾青竹絲面臨雌性成蛇較低的存活率與較短的壽命，而演化出快速生長與快速生殖以獲得較高生殖成功（reproductive success）的生存策略；而雌、雄蛇間的存活率差異導致年長雌蛇的數量較少，也使得所有雌性成蛇的平均吻肛長並未大於雄性成蛇，致使性別間實際存在的體長二型性未能顯現。 第三章，我藉由操控植栽排列的疏密程度與距離食物資源遠近，以及記錄環境溫度變化的情況下，研究植被遮蔽、獵物可及性以及溫度對赤尾青竹絲選擇日間停棲地點的影響性。結果發現，溫度是影響赤尾青竹絲選擇日間停棲地點的最重要因子，赤尾青竹絲於日間停棲在植栽上的高度明顯受到環境溫度的影響，當環境溫度較高時，牠們趨向停棲於溫度較低的植被下層。赤尾青竹絲對不同植被遮蔽程度的選擇也同樣受到溫度的影響，當環境溫度高於25C時，赤尾青竹絲趨向停棲於溫度較低的植被較密處；當環境溫度低於25C時，除了完全不選擇去除所有葉片而最為稀疏的植被結構外，赤尾青竹絲對於植被的疏密程度並未有明顯偏好。其日間停棲地點在不同時間與不同季節的變異，應該是為了滿足體溫調節需求所作的調整。Abstract

Reproduction, growth and habitat selection of an organism can make a strong impact on its long-term fitness. Variation in fitness among individuals plays a primary force in natural selection. The Chinese green tree viper (Trimeresurus s. stejnegeri), a nocturnal, arboreal sit-and-wait predator, is one of the most common snakes in Taiwan. This serpent exhibits an obviously female-biased sexual-size dimorphism (SSD). It often retreats into vegetation during daytime and moves to ambush sites at dusk. Furthermore, a pattern of male-biased sex ratio was found in a population of Trimeresurus s. stejnegeri at Tsaochiao in the northern Taiwan. In this dissertation, I used demographic analysis from an over nine-year (1996-2005) mark-recapture data set of the population of Trimeresurus s. stejnegeri at Tasochiao, to explore the causes of observed male-biased sex ratio and female-biased SSD. Additionally, I applied an experimental approach to investigate the role of temperature, prey availability and vegetation density with respect to diurnal retreat site selection of the Chinese green tree vipers in outdoor enclosures. In Chapter I, the sex ratio and survivorship of the Chinese green tree viper Trimeresurus s. stejnegeri in Tsaochiao, Taiwan were investigated monthly by mark-recapture method from 1996 to 2005. A total of 577 individuals were captured and 2095 observations were made. They showed significant male-biased sex ratios with an average of 2.51 (male/females) for the half-year reproductive and postpartum periods. The male-biased sex ratio was not observed at the immature (newborn + juvenile) stage but at the adult stage. The average estimates of half-year survival rates and catch probability were 0.72 and 0.65, respectively. The survival rates were lower at the immature stage than adult stage, but they were not significantly different between immature males (mean survival rate, 0.62) and immature females (0.63). In contrast, the survival rate of adult females (0.65) was lower than that of adult males (0.75), but there was no significant difference in average capture probabilities between the two sexes (0.65 for males and 0.61 for female). Costs of reproduction in females, such as physical burden during gestation and emaciated condition after parturition are suggested as the primary causes for lower survivorship of adult females than adult males. In Chapter II, using the data of 577 individuals collected from the nine-year mark-recapture study, sexual differences in growth and body sizes were examined for the different life stages of the Chinese green tree viper Trimeresurus s. stejnegeri in Tsaiochio. The results showed that there were no significant sexual differences in SVL and body weight at the newborn stage but the differences were significant at the juvenile stage, young adult stage, and old adult stage. These indicated that the SSD of the Chinese green tree viper occurs at the post-newborn life stages after the first year of life. Growth rates were significantly faster for females (0.28 mm/day) than males (0.19 mm/day) at the immature stage but not at the adult stage. The Chinese green tree viper became reproductive maturity at the ages of 2.6 years for males and 1.9 years for females. Male-biased age distributions were observed for the old adult snakes; the average age of adult males was 5.2 years whereas females was 4.0 years, indicating that the females had lower survivorship and shorter life span than the males at the adult stage. The proximate mechanisms for SSD of the Chinese green tree viper are that females grow faster and larger than males at the immature stage. By doing so, the females reach sexual maturity faster and increase their reproductive fecundity. This may be evolved as a surviving strategy to compensate the lower survivorship and shorter life span at the adult stage of female Chinese green tree vipers. In Chapter III, I manipulated vegetation structure, distance to prey source, and monitoring the microhabitat temperature within the test enclosures. The results indicated that the height of daytime perch sites was influenced by ambient temperature. Snakes perched on lower layers of vegetation seeking cooler conditions when ambient enclosure temperature was high. In addition, when the ambient temperature rose above 25C, Trimeresurus s. stejnegeri retreated into denser vegetation, which provided significantly lower temperatures. In contrast, Trimeresurus s. stejnegeri in low temperature environments did not exhibit any preference for vegetation structures in terms of density, except for an apparent avoidance of vegetation with bare branches and no leaves. The distance to prey source did not appear to have any significant influence on the green tree viper’s selection of retreat site, this could be attributed to the fact that these snakes do not only hunt terrestrial preys, but also prey upon arboreal species. This study demonstrated that temperature was the most important factor influencing retreat site selection of Trimeresurus s. stejnegeri. The temporal variations and seasonal differences in the space utilization patterns may have been for the purpose of satisfying their thermoregulation needs.