Chances are good that you have seen a turtle cross a road. Perhaps you have even stopped to assist one on its journey out of concern that a car would hit it. The body of a dead turtle, its shell crushed by an automobile, is clear evidence that road crossing can be fatal for an individual turtle. A more complex question is if roads and other aspects of urbanization have population-level consequences for turtles. For example, if nesting adult female turtles are at greater risk than males of being hit by a car or a lawnmower, then you’d expect fewer females in populations at higher levels of urbanization. The difficulty of testing this hypothesis is obtaining enough data at enough sites of varying degrees of urbanization. We solved this problem by creating a collaborative research project named TurtlePop through the Ecological Research as Education Network (www.erenweb.org). Over 1000 undergraduates worked with faculty at 24 institutions across central and eastern United States to learn about, measure, and share data on freshwater turtles.

We found, contrary to expectation, the proportion of adult female painted turtles in a population increased with urbanization. While we do not definitively know the reason for this surprising result, we suspect more urbanized areas produce more female hatchling turtles. The sex of most turtle species is determined during incubation, with higher temperatures producing females. If more urbanized areas have warmer soil, as would be expected due to the urban heat island effect, then more females than males may be produced, and this biased hatchling sex ratio may continue into adulthood. We need to conduct a lot more research to understand how human-induced changes in the landscape influence populations of turtles. In the meantime, continue to keep an eye out for our shelled friends trying to cross a road!

Further Reading:
Bowne, D.R., B.J. Cosentino, L.J. Anderson, C. P. Bloch, S. Cooke, P. W. Crumrine, J. Dallas, A. Doran, J.J. Dosch, D. L. Druckenbrod, R. D. Durtsche, D. Garneau, K. S. Genet, T. S. Fredericksen, P.A. Kish, M.B. Kolozsvary, F.T. Kuserk, E.S. Lindquist, C. Mankiewicz, J.G. March, T.J. Muir, K.G. Murray, M.N. Santulli, F.J. Sicignano, P.D. Smallwood, R.A. Urban, K. Winnett-Murray, C.R. Zimmermann.2018. Effects of urbanization on population structure of freshwater turtles across the United States. Conservation Biology. https://doi.org/10.1111/cobi.13136.