LOTT,D.B.; ACKERMAN,R.A.*: Thermal dose and embryonic sex determination in Chelydra serpentina.

The thermal dose (degree hours) necessary to determine the sex of Chelydra serpentina embryos was assessed over two nesting seasons (1996,1997) using programmable incubators. A total of 48 clutches from Iowa, Illinois, Minnesota and Louisiana were divided among treatments and among experimental boxes within treatments where they were incubated fully buried in sand at -7 kPa. All treatment incubators were programmed to produce an average temperature of 25 C, which is close to the average summer soil temperature at nest depth in Iowa. One incubator produced a constant 25 C incubation treatment (control). The other four incubators were programmed to produce quasi square wave thermal pulses in the boxes (up 6 C and down 6 C for 1, 2, 4, and 6 hours in each daily cycle). The 6 C amplitude represented the average for Iowa calculated from an extensive data set of soil temperatures at nest depth. Dose treatment strongly influenced hatchling sex with 0% females in the control, >90% females in the 6 hr treatment and progressively more females at each of the intermediate treatments (1,2,4 hrs). There appeared to be an influence attributable to geographic origin. Dose response analysis indicates that about 160 degree hours over the incubation period (I) were needed to produce 50% female hatchlings. I was a positive function of increasing dose and was influenced by geographic origin. Hatchling mass was not influenced by treatment.