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Spencer, R-J., M.B. Thompson & I.D. Hume. 1998. The diet and digestive energetics of the Murray short-necked turtle, Emydura macquarii. Comp. Biochem. Physiol. 121A: 341-349.
We described the diet of Emydura macquarii, an omnivorous turtle from south-eastern Australia, compared its digestive performance on diets of fish or plants at two temperatures, and related how both diet and temperature affect its food selection in nature. Filamentous algae constituted 61% of the stomach content of E. macquarii. The turtles rarely fed on motile prey, but selected carrion from the lagoon bottom and terrestrial insects (Diptera, Hymenoptera and Coleoptera) trapped on the surface of the water. Digestive efficiency of E. macquarii was affected little by body temperature, in contrast to consumption rates and rates of passage which were strongly influenced by both temperature and diet. In combination, these responses resulted in a slower rate of digestion at 20C than at 30C. Digestive efficiency of E. macquarii on a herbivorous diet at 30C (49%) was about half that of turtles on a carnivorous diet (91%), but they had longer transit times (118 h on the plant diet versus 70 h). Lower consumption rates and longer mean retention times in turtles fed plants compared those fed fish relate to slower digestive processing of the plant. Rapid processing and higher consumption rates of fish by E. macquarii resulted in higher energy gains compared to turtles consuming plants (almost 100 times more energy at 30C). The laboratory results suggest that fish carrion and aquatic and terrestrial invertebrates are probably essential dietary items of E. macquarii in the wild, because its metabolic requirements cannot be met from aquatic macrophytes alone.
Spencer R-J, Thompson M.B. and Banks P.B. 2001. Hatch or wait: a dilemma in reptilian incubation. Oikos 91:401-406.
Animals often form groups to reduce the risk of predation through the per capita dilution of their individual predation risk. The advantages of grouping also influence the timing of reproduction in many species. In particular, synchrony in the timing of births may have evolved as a predator-avoidance strategy as it dilutes the risk of predation upon vulnerable newborn and naive young. Eggs of an Australian fresh-waterturtle, Emydura macquarii, can hatch synchronously despite developmentalasynchrony among eggs of a clutch and hatchlings have a reduced predation risk by emerging from the nest as a group. Developmental asynchrony within clutches was induced to reflect natural nests by dividing clutches and incubating them at either 25C or 30C. Some eggs were then reunited with their clutch-mates and hatching occurred synchronously in some of these groups. In groups where synchronous hatching did not occur, less advanced eggs still hatched earlier than the normal incubation period. Synchrony occurred because the less advanced eggs hatched up to five days earlier than the control embryos. We conclude that the less advanced embryos within a clutch either accelerate their development or hatch prematurely to ensure synchrony of hatching and hatchling group formation may facilitate emer-gence from the nest and dilute predation risk.
Spencer R-J. 2002. Experimentally testing nest site selection: Fitness trade-offs and predation risk in turtles. Ecology. 83: 2136-2144.
The nesting habitat of most freshwater turtle species has been described, but factors influencing maternal nest site selection have rarely been tested experimentally. Offspring fitness is assumed to be the major factor influencing nest site selection because habitat characteristics and nest microenvironments affect offspring survival. However, two opposing factors drive maternal nest site selection: minimizing female mortality and max-imizing offspring fitness. In Australia, introduced red foxes are the major predator of turtle nests, and they also destroy nesting females. Thus, females may trade off maximizing nest survival or offspring fitness to avoid predators. In this paper, I show that the risk of predation affects maternal nest site selection and has negative effects on reproductive success in a freshwater turtle. I also show that the mechanisms behind predator detection vary between native and introduced species. From 1996 to 2000, I observed female freshwater turtles, Emydura macquarii, nesting around four lagoons in southeastern Australia to determine nesting habitat characteristics. During 1997 and 1998, foxes were removed from two sites, and nest predation rates declined by .50%, but remained .85% in nonremoval sites. Foxes destroyed ;3% of the female population only in high-risk areas. Female turtles nest away from shore to maximize offspring fitness when foxes are removed from an area. The dilemma in high-risk areas is that predation risk limits females from nesting in preferred areas away from shore, where nest predation is reduced. However, females may sacrifice some offspring by nesting in inappropriate substrate, where incubation conditions are not optimal, but nest predation is significantly reduced. Nesting turtles do not detect foxes by chemical recog-nition, but they have an innate avoidance response to the odor of a native predator. Nesting habitat affects offspring fitness, but factors affecting female survival may ultimately drive maternal nest site selection in turtles.
Growth patterns of two widely distributed freshwater turtles and a comparison of common methods used to estimate age. Australian Journal of Zoology 50: 477-490.
Spencer R-J. 2002
Turtles are long lived and demographic models requiring estimates of age, growth, fecundity, and survival are central for management. Most studies estimating age and growth of freshwater turtles use annuli as an index of age without estimating its error and very few studies using growth models include many juveniles, where growth is often large and variable. In this paper, I compare the reliability of growth annuli and common models in determining age and growth of two widely distributed turtles in Australia. Most turtles are carnivorous during the juvenile stage but many species shift to a lower quality omnivorous diet prior to maturing. Patterns of growth are often characterised by this dietary shift and I compared the growth of a common omnivorous turtle (Emydura macquarii) and a vulnerable sympatric species that is an obligate carnivore (Chelodina expansa). Mark-recapture programs were established in three lagoons on the Murray River. 1218 hatchling E. macquarii were released into two of the lagoons and growth annuli were unreliable in estimating their age by year two. The von Bertalanffy and logistic growth models can reliably estimate age of both male and female E. macquarii and C. expansa respecitively. Growth is extremely rapid during the juvenile stage of E. macquarii, but is highly variable in C. expansa, with rapid growth only occurring beyond age three. Hence growth models fail to predict age when juveniles are excluded from the analyses. Female E. macquarii delay maturity until 9-12 years of age because clutch size is positively related to body size and they can only produce one large clutch per year. Female C. expansa mature later (~14 years) than E. macquarii and both species are sexually dimorphic, as males mature earlier at smaller sizes than females. Common growth models describe the growth of two widely distributed freshwater turtles, but different patterns of growth and age at maturity relate to quality of diet and reproduction.
The significance of predation in nest site selection of turtles: An experimental consideration of macro- and microhabitat preferences. Oikos 103:592-600.
Spencer R-J. and Thompson M.B.
Nest selection has important consequences for fitness in freshwater turtles, as nest predation is extremely high in most populations. However, nest predation has rarely been considered a major factor affecting nest site selection. Thermal qualities of a nest are often suggested as a factor influencing maternal choice of nest site because incubation temperature affects offspring performance and growth, as well as the sex ratio of the clutch in species with temperature dependent sex determination (TSD). Yet experimental manipulations of the nesting environment (habitat and threat) to test factors influencing maternal nest site selection in turtles are uncommon. In this paper, we show that nest site selection in an Australian turtle (Emydura macquarii ) with genetically determined sex determination (GSD) exists at two spatial scales, with predation as the major factor affecting the location of a nest. Females prefer to nest in areas where nest predation is minimal but when the risk of direct predation is increased, they trade-off minimising nest predation with survival by locating their nests closer to shore. However, experimentally reducing cover demonstrated that females prefer to nest in open areas, which are more common closer to shore. Hence females are forgoing preferred microhabitat to locate their nests away from shore to minimise nest predation. Despite a preference for open nesting areas, females exhibit similar repeatability of microenvironment selection to a North American species with TSD. Repeatability of nesting overstory vegetation in the North American turtle, Chrysemys picta, suggests that females may exert some control over the thermal qualities of the nests; a key assumption in theoretical models of the microevolution and adaptive significance of TSD in reptiles. Incubation temperature is one factor that may affect microhabitat preferences in both turtles but other factors (e.g. predation and soil moisture) affected by microhabitat may be critical in the evolutionof repeatability of overstory cover.
Experimental analysis of the impact of foxes on freshwater turtle populations using large-scale field and modelling techniques: Implications for management. Conservation Biology (In Press).
Spencer R-J and Thompson M.B. 2004
Advances in freshwater turtle population dynamics have lagged well behind the understanding of the population ecology of other taxa (eg. mammals), primarily because studies are rarely manipulative or comparative. Common demographic parameters, such as survival and fecundity, are regularly estimated in freshwater turtles, but to determine the dynamics of a population requires large-scale manipulative experiments in several populations. The resilience of a population to stage specific predation is primarily dependent on density dependent inversity or compensation. Some species are also more susceptible to predation because of their life history strategies. Nest predation by introduced red foxes on populations of Australian freshwater turtles is a major source of mortality, but its full impact has yet to be evaluated. Freshwater turtles are long-lived and a population decline due to predation by foxes may take a long time to become evident. We evaluated the impact of foxes on the population growth and patterns of survival of two widely distributed Australian freshwater turtles on the Murray River, where nest predation rates have previously been reported at over 95%. We estimated densities and survivorship of Emydura macquarii and Chelodina expansa from the nest to adult stage from three lagoons in the upper Murray River. A BACI designed fox removal program determined the impact of foxes on these parameters. A type III survivorship curve describes the life history of Emydura macquarii, because nest predation and adult survival rates are greater than 0.9. Nest predation rates of C. expansa, which are considered rare in Victoria, were half that of E. macquarii and juvenile survival rates were over 0.8 to compensate for delayed sexual maturity. Nests of C. expansa were in low densities and situated away from shore. Emydura macquarii occur at densities more than 5 times that of C. expansa, but projection matrices predict that E. macquarii populations may be declining slowly, but respond to changes in adult survival, which has the greatest elasticity value. Foxes destroy a proportion of nesting adult females each year; however there appears to be no density dependent compensation in adult survival and foxes potentially regulate these populations. Emydura macquarii populations have low productivity and rely on a standing crop of turtles for population maintenance, whereas C. expansa has relatively more juveniles in the population and is less reliant on adult survival.