Environmental Influences on Painted Turtles
Ecological & environmental influences on nest-site selection in the Painted Turtle (Chrysemys picta)
Nicole Davidson ‘13, Nick Schwab ’14, and Boback, S.M.
Student Faculty Research Project
Funded by: Center for Sustainability Education
Funds from: NASA Cooling the Curriculum, Cool Climate Grant Fund
In some animals, sex is not determined by genetics but rather by temperature experienced during development. This phenomenon, referred to as temperature-dependent sex determination or TSD, is found in all crocodylians, many turtles and lizards, and some fish. Painted turtles (Chrysemys picta) are one such species with TSD in which warm temperatures produce females and cool temperatures produce males. As a result, these species are highly susceptible to global temperature changes. In fact a recent study has shown that an average increase in temperature of just 1.1 degrees C would be sufficient to induce all female clutches and the eventual extinction of the species (Telemeco et al., 2013). However, previous research has demonstrated that female Painted turtles can induce cooler nest temperatures by laying eggs earlier in the season and/or by placing their nests under a thick canopy of vegetation. We know from previous studies that female Painted turtles do not place their nests randomly within the environment but seem to select certain sites for nesting (Janzen, 1994a). Therefore an understanding of where and when a female creates a nest is of paramount importance and although we know females are choosing particular locations for nesting, exactly how a female chooses her nest site remains less well understood.
The proposed study aims to evaluate the environmental and ecological parameters influencing the sex and survival of Painted turtle hatchlings. Specifically we focus our efforts on nesting ecology in which the female turtles choose a nest site, lay their eggs, and the embryos develop into hatchlings before finally emerging from the nest. Painted turtles exhibit temperature-dependent sex determination (TSD) whereby nest temperatures above about 28OC will produce mostly females, temperatures below about 24OC will produce mostly males, and temperatures around 26OC will produce an even sex ratio. Our objectives are to record both static (depth, width, and distance to water) and dynamic (temperature, soil moisture) variables of the nest site to characterize where females have chosen to lay their eggs. Second, we recorded the identity and basic morphology of each female so we can document nest site locations within and among individuals. Thirdly, we measured clutch characteristics (clutch size, egg size, and egg length). Lastly, we monitor the emergence of hatchlings from the nest; some emerge in the fall while others don’t emerge until spring the following year. By comparing these parameters among nests at our site and the sites from other populations, we hope to discover how this species has evolved to maximize hatchling success and how it might respond to increases in average daily temperature predicted to occur with global climate changes.
Our study site is an anthropogenic pond at the Hunstsdale Fish Hatchery property in Huntsdale, PA. Twice daily we conducted visual encounter surveys to intercept females during their spring nesting forays. Our surveys occurred around the entire pond however our past work has identified that nesting activity is concentrated towards the northern edge of the pond adjacent to a railroad right-of-way. In Pennsylvania, female Painted Turtles begin nesting in late May and the majority of nests are dug in the afternoon and evening. When a female is encountered, she was observed from a distance and allowed to complete her nest. After nesting, the female was captured, measured, and uniquely marked using a sterile passive integrated transponder (PIT) tag; this provides a unique combination of 15 numbers that can be read using a reader. The PIT tag was be injected beneath the skin of the left hind limb; the wound was then sealed with cyanoacrylate (superglue). Additionally, animals were also marked by filing a unique sequence of notches in the small scales (scutes) that surround the margin of the turtles’ shell. This allowed unambiguous confirmation of an animal’s identity if a PIT tag were to fail. After processing, the females were released at their point of capture.
After releasing the female, we returned to the nests where we carefully excavated the nests, marking each egg’s position in the nest. Eggs were measured for length using a millimeter ruler and mass using a digital scale. Eggs were replaced in the nest in the same position and orientation in which they were found. Additionally we deployed three temperature recording devices (ibuttons, Embedded Data Systems) within the nest: bottom, middle, and top (closest to surface). The ibuttons recorded temperature to the nearest 0.1OC every 4 hours for 365 days. After launching the ibuttons the nest was refilled and was covered with a wire mesh cage (1 cm x 1 cm spacing) to minimize the threat of nest predation. Positions of nest locations were recorded using hand-held GPS units (Juno SB, Trimble) and we obtained the following characteristics for each nest site: canopy coverage (measured with a spherical densitometer – basically a convex mirror with a grid system etched on it that, when placed on the ground above the nest, reflects an image of the trees and plants above it), soil composition, slope, aspect, and vegetation within a 1 meter buffer around the nest.
For the past five years my students and I have been conducting a long-term population study of the turtle community at the Hunstdale Fish Hatchery. To date my students and I have marked over 1000 Painted turtles in a 7 acre pond at this site. These marked turtles form the basis for understanding nesting ecology because the identity of most of the adult females in this population is known.
Over the past three years, we have made significant progress in understanding turtle nesting behavior at this site. This summer we located and processed 15 nests bringing the total nests processed over the last three years to 33. Clutch size has averaged 5.6 eggs and this measure is similar to previous studies. However, the depth of the nests was shallower at our site relative to those from other studies (Table 1).
Temperature profiles from this summer’s nests will be retrieved this spring when all hatchlings have emerged. From past years we have a total of 17 nests with temperature profiles. Preliminary analysis of these temperature profiles has revealed that some nests at Huntsdale Fish Hatchery are achieving temperatures likely too high for hatchling survival (47OC; Figure 1). This result may be due to an extremely hot summer (2011), the notably shallow nest depths, or a combination of the two. In addition to these variables, overstory vegetation or canopy cover can significantly affect nest temperature with cooler temperatures predicted in areas with high canopy coverage. We found that canopy coverage above nests averaged 72% at this site and this is greater than has been documented at other sites (Janzen, 1994b). Therefore despite high levels of canopy coverage, Painted turtle nests at our site became extremely warm and some nests at our site may fail due to these extremely high incubation temperatures. While this finding is provoking, we need one additional year of data to make any robust conclusions. Additionally, we have not found any pattern in regards to the amount of canopy cover (shade) and nest temperature (Figure 2). This second finding is in contrast to most studies on nesting ecology where researchers have shown a predictable negative correlation between nest temperature and percent canopy cover – nests under shade result in cooler temperatures (Janzen, 1994b).
Janzen, F. J. (1994a). Climate change and temperature-dependent sex determination in reptiles. Proceedings of the National Academy of Sciences 91, 7487-7490.
Janzen, F. J. (1994b). Vegetational cover predicts the sex ratio of hatchling turtles in natural nests. Ecology 75, 1593-1599.
Telemeco, R. S., Abbott, K. C. and Janzen, F. J. (2013). Modeling the effects of climate change-induced shifts in reproductive phenology on temperature-dependent traits. The American Naturalist 181, 637-648.