Sex in the City: Female turtles may be more common in urban areas

Cover photo source: Dr. David Bowne

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

Beyond Weird Pond

In a lot of ways, TurtlePop is just like any other research group. They’ve got protocols, data sheets, and equipment galore. They’ve got an interesting research question, and now they have the data to start answering it. But this particular team is special. TurtlePop is made up of over a thousand researchers. Even more revolutionary, most of those researchers aren’t professional scientists (yet). They’re students!

Students and a professor wade through a pond surveying turtles
Class at Weird Pond (Source: Dr. David Bowne)

The TurtlePop project started in the fall of 2009 when the General Ecology class at Elizabethtown College (Elizabethtown, PA, USA) went on a class project to survey turtles in their local pond, Weird Pond. Students asked interesting questions about the population of turtles in the area, but it quickly became clear that this research was destined to be part of something greater. Thinking well beyond their small pond, researchers developed an interesting question: could cities around the country be affecting the ratio of female turtles to male turtles?

On first glance, you might say Weird Pond inspired a really weird question. But the idea didn’t come out of nowhere; it was based on previous studies that have found fewer female turtles than male turtles in a few American cities.

According to those studies, if you hit a turtle with your car there’s a good chance it’s a female. Female turtles have to cross roads more than males because they are in charge of scouting out a good place to set up their nest. Because females are more likely to be hit by cars, there end up being fewer females where there are more roads. But those results had never been tested on a national scale.

Enter TurtlePop.

Not long after that first project, the class’s professor, Dr. David Bowne enlisted help from a network of similar classes around the country. They put together what would ultimately become the largest study on turtle population structure that has ever conducted.

The network they joined is called the Ecological Research as Education Network (EREN; erenweb.org), and it aims to get students exposed to the process of conducting scientific research while also answering important ecological questions on a national scale. Funded by the National Science Foundation, EREN provides curriculum materials, video protocols, and an active network of like-minded collaborators. TurtlePop is one of several projects supported by EREN.

Weird Results

Ultimately the folks from Elizabethtown College enlisted the help of over a thousand researchers in 11 states across the central and eastern United States. The turtles they sampled ranged from city slickers with access to over 3000 meters of road to country folks with a mere 4 meters of road. And after pooling all of these data, what did the researchers find?

Just about the opposite of what they expected. The proportion of female turtles actually increased near cities!

To understand this result, let’s zoom out and take a look at what’s actually happening in cities. Don’t worry, we’ll come back to the turtles.

A Hot Topic

Imagine walking around a big city on a hot summer day. The sun is beating down from all sides and reflecting off of the buildings around you. The pavement feels like it could melt the soles of your shoes. You find yourself pausing every time you walk past a tree just so you can take advantage of every last second of shade.

Now imagine walking into a nearby park to eat your lunch in the shade by the river. How do you feel?

Chances are if you have ever spent time in a major city you are already pretty familiar with the urban heat island effect. The pavement and lack of shade in cities makes them great at trapping heat, while nearby parks and rural areas stay slightly cooler. This difference between urban areas and the surrounding rural land can average up to 6ºF (3ºC) warmer, and even more at night.

Graph showing that heat increases in urban areas
Temperatures are higher in urban areas (Source: NASA, 1999)

So how does that affect turtles? Well, it turns out most freshwater turtles have “type 1a temperature-dependent sex ratios.” What that really means is that when mother turtles lay eggs in warm soil they get more female babies, and when they lay eggs in cold soil they end up with more boys. This may seem crazy, but it is actually fairly common for reptiles and some fish.

Graph showing that warmer temperatures produce more female turtle offspring
Turtles produce more female offspring at high temperatures (Source: Mrosovsky & Godfrey, 2010)

Putting it All Together

This study found is that urban heat islands across the country are causing the soil to heat up, which in turn leads to warmer eggs and more female turtles in cities! By collaborating in a national project, researchers were able to pick up this cool result that has never been observed in smaller-scale experiments.

That said, Dr. Bowne is most excited about another aspect of the project. “My favorite part of the project was knowing that we motivated hundreds of students and many faculty to get outside, get dirty, and learn about turtles” says Bowne. Those students and faculty have gone on to develop more turtle-related projects and present their work at regional and national scientific meetings.

As cities continue to grow, it’s important to know how we are affecting other organisms and how we can control those effects. Fortunately, this project has helped to prepare over one thousand students to deal with those important questions.

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Abigail Lewis

I am a fourth-year undergraduate at Pomona College with interests in limnology, microbial ecology, and nutrient cycling. In between rehearsals, hikes, and long dinners I am working to build a career that will address environmental issues and build a more inclusive scientific community.

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