Incubation, Pivotal Temperature of Sex Determination, and Endoscopic Sexing of the Burmese Black Giant Tortoise (Manouria emys phayrei)
Paul M. Gibbons1,2, DVM, MS, DABVP (Reptile and Amphibian Practice)
The Burmese black giant tortoise (Manouria emys phayrei) is critically endangered and native to Southeast Asia. Manouria is the most primitive genus of extant tortoises, inhabiting wet, cool, montane forests; feeding on bamboo and mushrooms; and exhibiting nest-building and nest-guarding behaviors. Clutch size can exceed 50 eggs. Temperature-dependent sex determination has been previously reported. This study aims to specify incubation temperatures that produce each sex and to describe the endoscopic appearance of juvenile gonads. Eggs (n=56) were collected from the nest of a single dam in a breeding group in a private collection, positioned on a bed of vermiculite:water 2:1 w/w, covered with moistened and loose sphagnum moss, and divided into three groups incubated at different temperatures. Hatchlings were raised communally for 10 months before endoscopic sexing. Tortoises were anesthetized, and gonads were photographed during endoscopic examination via the prefemoral fossa. Standard error for parameters, pivotal temperature, and transitional range of temperatures were estimated using a Bayesian method with Monte-Carlo with a Markov chain and Metropolis-Hastings algorithm using weighted average temperature during the thermosensitive period. Thirteen eggs hatched from each group; higher temperatures produced more females; and incubation duration decreased with increasing temperature. The pivotal temperature (95% CI) was 28.87°C (27.58–30.06°C), with a transitional range of temperatures (95% CI) from 27.98°C (25.33–29.36°C) to 29.29°C (28.90–31.54°C). Testes were elongated, solid yellow, smooth-surfaced, with a uniform reticulated internal vascular pattern. Ovaries were elongated, colorless to pale yellow, with irregularly distributed regions of superficial granularity characterized by refractile ring structures.