In both freshwater turtles Emydura macquarii. signata and Chelodina expansa clutch size varied more than egg size, female body size was related to clutch mass but did not affect egg or clutch size and pelvic size did not constrain egg size. Thus egg size variation in these species loosely supported some predictions of optimal egg size theory, however such theories need refining, particularly the amount of egg size variation
allowable within a population. Both species exhibited a large amount of variation in egg and clutch sizes between individuals and across years. An egg-size-clutch-size trade-off was not evident for these species however. The extent of variation in egg and clutch size in this study suggests that such variation in reproductive characteristics may support theories of plasticity or of random adaptive coin-flipping with egg and clutch sizes highly influenced by different combinations of both genetic and environmental factors, rather than the evolution of an optimal egg size. Further longer term studies are needed to assess parameters of reproductive ecology in both these species of freshwater turtle taking into account differences in ecology, latitude, behaviour, and physiology to determine whether adaptive plasticity or adaptive coin-flipping occurs and whether some predictions of optimal egg size theory are relevant.
Incubation temperature played a significant role in determining the morphology and growth of both E. m. signata and C. expansa as well as swim speed in E. m. signata. For E. m. signata eggs incubated at 30°C yielded hatchlings that, although significantly smaller at hatching, grew faster than those incubated at either of the cooler temperatures (24°C and 27 °C) over their first year of life. Despite growing faster, hatchlings from 30°C were significantly slower swimmers compared to hatchlings incubated at 27°C and 24°C. Higher incubation temperatures offer 3 potential advantages; shorter incubation period, a longer growth period before winter and inherently faster growth, which may increase survival by reducing the risk of predation by gape limited predators. Swim speed was not repeatable between hatchling and 1 year of age suggesting care
should be taken in extrapolating results in early ontogeny to those later in life. C.expansa hatchlings from eggs incubated at 25°C were larger, grew significantly faster and suffered much lower mortality than those from 30°C. Clutch of origin significantly affected hatchling size and growth rate in both E. m. signata and C. expansa with some clutches producing much larger hatchlings and some clutches exhibiting healthier individuals with faster growth rates than other clutches regardless of size. It was also shown that the effects of incubation temperature on hatching growth affected different clutches in different ways which acts as a further source of variation in hatchling characteristics and success. More research is required on many aspects of reproductive ecology in these species in relation to hatchling characteristics, growth and survivorship further along in ontogeny to truly assess the long term
effects of factors such as clutch and incubation temperature on success.
In the prey handling time study hatchling mass and head width of E. m. signata had a significant effect on prey handling time. There were also significant clutch effects on prey handling time after the effects of two measures of hatchling size (mass and HW) were removed. This suggests that not only size but also genetic factors influence prey handling time with some clutches showing better foraging ability than others. Handling time was faster for small worms than large worms across all sizes of hatchling tested. An interesting morphological finding in this study was that the ratio of head size to body size was not uniform across all sized hatchlings with smaller hatchlings having proportionately larger heads than large hatchlings. The larger head to body size ratio found in smaller turtles may maximise gape size to ensure that
foraging of prey with adequate energy content can occur. Another finding was that small and large hatchlings differed in their times to consume small worms but were more similar in their processing times of large worms and this may be related to differences in behaviour associated with consuming large worms. All correlations between body size and prey handling time were more significant for handling small worms than large worms. Profitability, or the mass of prey consumed per unit of handling time, was similar for small and large worms. This suggests that although large size offers advantages in terms of prey handling time, prey profitability does not necessarily increase on a size specific basis as large worms took longer for hatchlings of all sizes to handle. Size differences of this magnitude between individuals therefore, may not influence optimal prey size and further studies on the biological significance of such differences in head size as well as prey handling behaviour in
the wild need to be quantified as other factors are known to influence handling success of prey such as amount of feeding experience and level of satiation.