Population ecology of chinese tree viper, Trimeresurus stejnegeri stejnegeri

• Main Authors: Hua-Ching Lin, 林華慶
• Other Authors: Ming-Chung Tu
• Format: Others
• Language: en_US
• Published: 2006
• Online Access: http://ndltd.ncl.edu.tw/handle/07265107498101357900

博士 === 國立臺灣師範大學 === 生命科學研究所 === 94 === 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.