In several major groups of vertebrates, environmental temperatures experienced during embryonic development determine offspring sex. Temperature-dependent sex determination (TSD) will be addressed with a multi-year field study of a wild population of the painted turtle, Chrysemys picta. The project will investigate several aspects of this unusual phenomenon: (1) the impact of long-term climatic variation on population structure, particularly the ratio of males to females, (2) the effect of lifetime patterns of nesting behavior on offspring sex ratio, and (3) climatic effects on variation in predator numbers and differential predation on nests, and hence shifts in population sex ratio. A team of student researchers will record nesting date and vegetation cover over nests, follow females across years to evaluate nesting behavior, monitor nests to investigate the influence of predators on offspring recruitment, and relate these data to information on climate.
The findings of this research will have important implications for the impacts of climate change and human habitat use on the biology and conservation of species with TSD. This long-term research program will also continue to provide extensive educational opportunities for numerous students (including underrepresented minorities and women) and the public.
Whether an individual is male or female is generally decided by genetic factors at conception. Yet, remarkably, environmental temperatures that embryos experience during development permanently determine the sex of individuals in several major groups of animals. This unusual system (known as TSD), where genetically-identical individuals can become fundamentally different genders, can serve as a model for understanding environmental effects on essential biological traits, but how TSD works in nature is unclear. Addressing this question also assumes great urgency with the rapidity of contemporary environmental change and the fact that most species with TSD are already imperiled globally. This ongoing, long-term research project is investigating significant aspects of TSD by focusing on a wild population of painted turtles. This project particularly targets three issues: (1) the impacts of long-term climatic variation on population structure, especially the numbers of males and females (=sex ratio), (2) the influence of lifetime patterns of nesting behavior on offspring sex ratio, and (3) climatic effects on variation in nest predation and subsequent shifts in population sex ratio. These issues are being examined by teams of students based on the principle that how populations respond to changing environmental conditions is crucial to long-term persistence. In the first instance, this project is establishing that populations with TSD can be exquisitely sensitive to temperature shifts from the long-term average that are as small as 3 degrees Fahrenheit (F). For example, a decline in the average July air temperature of 3 F in the study population would alter offspring sex ratio from the typical one male for every one female to a situation where only male offspring generally would be produced. If such a dramatic shift in sex ratio occurred swiftly, it almost certainly would exert an unfavorable impact on a population. While populations with TSD may respond to temperature shifts in several ways, changes in nesting behavior may be the most likely avenue for acclimation, at least in the short term. This project is finding that where and when mothers make nests affects offspring sex ratio. Nests located in shadier sites earlier in the season are cooler and more likely to produce male offspring than are nests located in sunnier sites laid later in the season. Thus, by altering nesting behavior, mothers can potentially reduce biases in sex ratio caused by changing environments. Consistent with that possibility, these two traits seem to shift at the population level in response to changing winter temperatures: after warmer winters, turtles generally nest earlier and in shadier sites. Even so, these behavioral shifts alone turn out to be insufficient to fully reduce biases in sex ratio caused by a change in average July air temperature of as little as 3 F. This project is further revealing that changing environments also affect sex ratios of populations with TSD indirectly. Raccoons vary in intensity of predation, ranging from 20-99% of turtle nests destroyed annually, which is a primary predictor of surviving turtle offspring. When nest predation is modest in a year that also experiences typical July air temperatures, the turtle population remarkably receives a major influx of new adult females 6-7 years in the future. Thus, predation intensity and environmental severity interact to strongly shape the structure of populations with TSD. The results of this project continue to be disseminated widely in talks, publications, and data archives. Among other practical outcomes, these findings are providing guidance for more effective conservation strategies for long-term viability of populations with TSD. The massive amounts of diverse information collected are further being incorporated into a computerized database that will allow Internet users to explore the data to address new scientific questions as they arise. This project is generating additional broader impacts. In the past 6 years, 87 students (many involved for multiple years), from high school to undergraduate to graduate and beyond, contributed to the extensive research, education, and outreach activities. Nearly every participant has obtained (or is obtaining) higher education in science, and many have proceeded (or are proceeding) to advanced degree programs in health, law, education, and science where they continue to help solve substantive challenges with a scientific emphasis. Moreover, 77 of these students were women, underrepresented minorities, and/or persons with disabilities, thereby contributing importantly to diversifying the scientifically-literate workforce. With few exceptions, people like turtles. Thus, this project is serving as a highly effective gateway for agreeably conveying basic scientific principles to engaged audiences. The participants in this project productively interact with members of the public daily at the field site as well as through planned outreach events with K-12 teachers, Boy Scout troops, children at a local library, and even 5th-grade, 7th-grade, and high school classrooms via cutting-edge, high-tech curricular innovations. This ongoing long-term project will continue to provide such educational opportunities for numerous students and the public.
