How painted turtles end up male or female isn’t so predictable

A painted turtle. (Credit: Fredric Janzen)

Some painted turtle nests within a single population may be significantly more sensitive to temperature than others, research finds.

The new study could flip the established framework for how scientists believe geography influences sex determination in painted turtles on its shell.

The study, which appears in Functional Ecology, analyzes decades of data on painted turtles, a species that undergoes temperature-dependent sex determination. That means the temperatures an incubating painted turtle egg experiences influence whether the embryo develops the physical characteristics biologists describe as male or female. Warmer temperatures tend to produce female turtles, and cooler temperatures tend to produce male turtles.

The study’s findings defied theoretical expectations for how painted turtle populations respond to environmental variation, which could lead scientists to rethink how they look at the topic, says lead author Anna Carter, a postdoctoral research associate in ecology, evolution, and organismal biology at Iowa State University.

Painted turtles cover a vast geographical range, from New Mexico to Canada. That means populations experience wide variation in temperatures and environmental conditions. For years, scientists emphasized “pivotal temperature,” or the temperature that produces an equal number of male and female turtles in a given population, when studying how the turtles respond to environmental variation. This framework would expect populations that live in warmer regions to have a higher pivotal temperature as well.

Previous studies found patterns related to latitude, Carter says. The closer a population was to the equator, the higher its pivotal temperature. But using a massive dataset on painted turtle populations allowed the scientists to take an unprecedented look at the relationship between latitude and pivotal temperature, and the new analysis didn’t find a convincing pattern.

Instead, the researchers found wide variation in pivotal temperature within local populations, as much as 5 degrees Celsius (9 degrees Fahrenheit).

“The implication of our study is that our understanding of local adaptation in this species isn’t as good as we thought it was,” Carter says. “It might be useful to move away from pivotal temperature as a model.”

The study, however, did find patterns connecting geography to the transitional range, or the range of temperatures that produce a mix of males and females. Transitional ranges tended to be wider at lower latitudes, Carter says.

The unexpectedly wide variation in pivotal temperature within populations could suggest painted turtles are more resilient to changes in temperature than previously thought. It’s possible female painted turtles can nest successfully in a multitude of environments, they say.

The study drew on a huge dataset that Fred Janzen, a professor of ecology, evolution, and organismal biology, and his colleagues collected. Janzen says his lab has collected data on painted turtle populations on the Mississippi River near Clinton for 32 years. The data include nesting and temperature measurements.

For the study, the researchers modeled temperature dependent sex determination in 12 geographically distinct painted turtle populations using both field data and lab incubation experiments.

Source: Iowa State University