Influence of temperature on the life history of turtles : an exploration of the embryonic and maternal adaptations to incubation temperature

Abstract

This study explores the role of temperature on the early life history traits of Murray River short-necked turtles, Emydura macquarii. Firstly, I investigate the role of incubation temperature in inducing adaptive behavioural and physiological strategies within the nest. Secondly, I explore the interaction between genes, developmental conditions and the post-hatching environment to determine long-term effects on growth and survival of turtles. Lastly, I review the impact of climate change and habitat modifications on Murray River turtles. Investigation of heart rates of embryonic turtles incubated at constant temperatures (26oC and 30oC) revealed that heart rates experience circadian rhythms independent to time of day and the peak heart rate varies throughout the day between individuals. Heart rate patterns were consistent throughout the individuals monitored in that the peak heart rate and the minimal heart rate were observed 12 h apart, and intermediate heart rates occurred 6 h before and after these extremes. Heart beats per minute varied between individuals, but embryos showed 15-20% difference in heart rate throughout at 24 h period. This provides evidence of flexibility in metabolic activity independent of temperature. Examination of the differences in heart rates of embryonic turtles incubated in either a group environment or individually, at a constant 30oC, and in darkness, showed that mortality rates were not affected but embryos incubated in the group environment had significantly shorter incubation duration. Heart rates were significantly higher in the embryos incubated individually during the middle of incubation, but were similar for the final two weeks of incubation. The mean heart rate variability was significantly higher in embryos incubated individually during the final two weeks of incubation. There was no significant difference between the two treatments in either neuromuscular ability or morphology. Changes in heart rates within a nest environment may be important for coordinating development and hatching between individuals in a nest. Analysing the order of oviposition in relation to incubation temperature in embryonic turtles incubated in fluctuating regimes indicated that eggs may be differentially suited for a particular temperature environment. Control eggs had shorter incubation duration when incubated with respect to oviposition order under two fluctuating temperature regimes (both equivalent to a constant 26 oC) reflecting temperatures experiences at the top and at the bottom of a nest. Bottom nest incubation temperatures fluctuated between 23oC and 29 oC and top of nest incubation temperatures fluctuated between 19 oC and 33 oC over a 24 h period. That is, if an embryo was oviposited early, it would incubate at the bottom of the nest and be exposed to slight fluctuations in temperature, and if an embryo was oviposited late it would be exposed to high fluctuation in temperature at the top of a nest. Order of oviposition also affected egg mass, with eggs oviposited early being heavier than those oviposited last, but this was not reflected in hatchling mass. There was no significant difference in neuromuscular ability between the treatment groups. Embryonic heart rates in the high fluctuating treatment group maintained significantly higher heart rates at the intermediate temperature of 26oC in the second last week of incubation, but in the final week of incubation the embryos in the control group had significantly higher heart rates at both an intermediate temperature (26oC) and a low temperature (19oC). In the slight fluctuating temperature regime, the embryos in the control group had significantly higher heart rates in the final week of incubation during the hotter temperature period (29oC). This study indicates turtle embryos are optimised for an incubation environment that their oviposition order, and subsequent position within a nest, would dictate. Studying the interaction between genotype, incubation temperature, and post-hatching environment (population density or temperature) on growth in hatchling turtles indicated effects of genotype and incubation temperature was still evident 11 months after hatching. Hotter post-hatching environments encouraged higher feeding rates in turtles and had significant effects on growth 11 months after hatching. The effect of higher population densities on growth was evident one month after hatching but not at 5 or 11 months after hatching. Oviposition order also affected growth, whereby late oviposited hatchings incubated at a hotter temperature (30oC) and reared in a warmer post-hatching environment, grew faster than turtles from the same clutch oviposited earlier. Long-term effects of genotype, oviposition order, incubation temperature, and post-hatching environment are evident in turtles. The results from this thesis identify areas of plasticity in embryonic and hatchling turtles at different life stages. Long-lasting environmental effects exist and a narrow range of thermal optima has been identified as being advantageous to hatchling turtles when they are at their most vulnerable.

Date of Award	2014
Original language	English