Male Biased Sex Ratio in Captive Bred Siamese Narrow-headed Softshell Turtles, Chitra chitra: Does the Incubation Temperature Influence Hatchling Sex in the Family Trionychidae?

A captive breeding program for the critically endangered Chitra chitra (Testudines: Trionychidae) at Kanchanaburi Inland Fisheries Development Center (KIFDC), Thailand, produced 710 hatchlings between 2000 and 2004, with eggs being incubated in shaded areas only. Thirty-one captive bred C. chitra that died during rearing were dissected and their gonads examined histologically and microscopically to determined if they were ovaries or testes. Of the 29 samples that could be sexed, 23 were males and 6 were females (79% males). The male biased sex ratio in captive-bred C. chitra indicates that the incubation conditions at KIFDC, which are likely to be cooler than in natural nests on sun exposed river sand banks, may have had a masculinising effect on hatchlings. This is potentially contrary to the generally accepted wisdom that softshell turtles have genotypic sex determination and that incubation temperature has no effect on hatchling sex ratios. Although we cannot formally rule out differential developmental mortality between males and females, it is possible that sex determination in the family Trionychidae is more variable and complex than currently recognised. Our finding that the incubation environment in a captive breeding facility can produce skewed sex ratios in C. chitra has important implications for the recovery projects of other critically endangered softshell turtles.