Sexual dimorphism and annual reproductive cycle of common asiatic softshell turtle Amyda cartilaginea

Abstract

Sexual dimorphism and annual reproductive cycles of mature male and female common asiatic softshell turtles were studied in a captive population of Amyda cartilaginea maintaind under semi-natural conditions at a temple pond at Prayurawongsawas temple, Bangkok, Thailand. The study was carried out from October 1996 to September 1997. Sexual dimorphism was determined from mean comparison of parameters of morphological characters and regression analysis of plots of morphological characters. It was found that this softshell turtle species exhibited significant differences (p<0.05) in various parameters of morphological characters. Several of these sexually dimorphic traits are related to reproductive performance, including tail width, tail length, position of cloacal opening and size. The other traits might play important roles in decreasing intersexual competition for resources, including head size. These data indicated that Amyda cartilaginea in sexually dimorphic. The annual reproductive cycle was investigated from changes in levels of plasma testosterone, estradiol and progesterone. These sex steroids were detected by radioimmunoassay according to the WHO matched reagent programme. It was found that male and female softshell turtles exhibited a seasonal reproductive cycle. The males displayed high levels of testosterone in prenesting period, while the females showed a prenesting peak of estradiol and high levels of progesterone during the prenesting and perinesting period. Detectable levels of heterologous sex steroids i.e. androgens in females and estrogens in males were apparent in both sexes indicating evidence for functions of heterologous sex steroids in this softshell turtle. Plasma estradiol levels in both sexes and plasma progesterone levels in the males were well correlated (p<0.05) with temperature of the Bangkok Metropolis area. The results suggest that softshell turtles exhibit a prenuptial reproductive cycle which results in rising plasma sex steroid levels prior to the mating season and decreasing to basal level at other periods of the year.