Skewed sex ratio of the Chinese green tree viper, Trimeresurus stejnegeri stejnegeri, at Tsaochiao, Taiwan

Preliminary collecting of adult Chinese green tree vipers, Trimeresurus stejnegeri stejnegeri, in Tsaochiao, northwestern Taiwan, yielded a male-biased sample. According to the sex ratio theory, a skewed sex ratio is unlikely at birth, and the above results may thus reflect sampling bias or differential mortality after birth. We collected litters of neonates of this viviparous snake from a broader area over a longer period to examine causes of such numerical dominance of males in the adult sample. We collected 23 gravid females from the Tsaochiao area in 3 consecutive reproductive seasons and obtained 50 male and 40 female neonates. The sex ratio at birth was not significantly biased. We marked and released a total of 169 male and 79 female T. s. stejnegeri. Marked snakes were recaptured 940 times. Regardless of season, time of day, transect, or sample area, we always encountered more males than females. Thus, sampling bias did not likely account for this skewed sex ratio. In the adult sample, the ratio of males was greatest in the largest size class, and this suggests that females are subjected to higher mortality. We thus hypothesize that the skewed sex ratio observed in overall adult samples of T. s. stejnegeri reflects such differential mortalities between sexes that are probably derived from a higher cost of reproduction in females.