Droughts threaten the delicate balance of painted turtle populations

In an unprecedented research project from the University of Nebraska-Lincoln, researchers have investigated how increasingly frequent drought conditions may affect the survival and gender distribution of painted turtles, a native species found in the freshwater bodies of Nebraska. The study was based on a vast amount of data collected over 11 years by more than 50 undergraduates.

The analysis, published in the journal Climate Change Ecology, revealed that drought conditions significantly reduce the survival chances of painted turtles and slow down their growth rates. Drought also disrupts the female-to-male ratio of these aquatic reptiles. Interestingly, these effects were apparent even when the water levels in the studied southwestern Nebraska pond remained relatively constant throughout the drought periods.

“I have to admit: When the students launched into it, I wasn’t sure that we were going to find drought impacts. I think it’s interesting that we found the drought impacts that we did, just given the fact that this is not a pond that completely dried up,” said Professor Larkin Powell, who supervised the studies.

Drought and sex determination 

Painted turtles, like many other reptile and fish species, have temperature-dependent sex determination. Cooler incubation temperatures typically yield male turtles, while higher temperatures tend to produce females. As drought is often associated with higher temperatures, more frequent and intense droughts could potentially distort the female-to-male ratio in painted turtles.

In an intriguing turn of events, researchers Ellen Dolph and Charrissa Neil found that male painted turtles made up 60-85 percent of the sample in the absence of drought. However, the proportion significantly shifted during periods of drought with nearly 60 percent of the sample being female. This shift was apparent when temperature, groundwater conditions, and other factors indicated a strong influence of drought.

Survival rates

Past research has indicated that a drastic shift towards a female-dominated ecosystem could pose a significant threat to turtles and other reptile species. This scenario becomes increasingly probable under the warmer, drier conditions expected due to climate change. 

Although the painted turtle, which is widely distributed across North America, faces no immediate risk of extinction, Powell warns that other more vulnerable species could bear the brunt of these changes.

“There’s nothing crazily unique about a painted turtle compared to the other turtles – Blanding’s turtles, mud turtles, map turtles – that live in rivers and lakes,” said Powell.

The study also provided the first evidence that drought can decrease survival rates in an intact pond, with a decrease of approximately seven percent in female painted turtles and 10 percent in males. One model suggested that each 10 percent increase in the probability of drought could result in a drop in the annual survival rate.

Slowed growth rates

Furthermore, the research showed that painted turtles, particularly females, grew slower during years of drought. Dolph and Neil established this by noting changes in shell length. The slowed growth rate is a concern because a turtle’s size can limit its reproductive capacity, explained Powell.

“Females grow faster than males, and one of the reasons that they are selected to grow faster is that they can’t reproduce until they have enough space inside their shell to hold the eggs.”

Although the specific drought-related factors contributing to the decreased survival rate remain uncertain, Powell suggests that the concurrent slowing of growth may be indicative of increased competition for limited food resources, primarily insects.

The perfect study animals

Powell, an ecologist primarily known for his work with avian species, found himself delving into the realm of herpetology when he took over a wildlife management techniques course at Cedar Point Biological Station in 2005. There, instead of capturing birds for population studies, he turned to the slow-moving, easy-to-capture painted turtles.

Thanks to their slow pace, painted turtles were deemed “perfect study animals.” With the assistance of a local landowner, Powell discovered a pond near Keystone, Nebraska, teeming with these shelled creatures.

Ever since that fateful encounter, Powell has been delving into the world of herpetology, a stark departure from his previous ornithology-focused career. His studies with painted turtles have not only brought attention to the potential effects of climate change on these creatures, but have also provided invaluable experience to numerous students over the years. 

More about painted turtles 

Painted turtles (Chrysemys picta) are a species of turtle native to North America. They are known for their strikingly colorful markings and are the most widespread native turtle of North America. Painted turtles are part of the family Emydidae, which also includes other well-known species like the box turtle and the diamondback terrapin.

Interesting facts:

Coloration and appearance

They are called ‘painted’ because of their vividly colored markings. The skin of the painted turtle is dark and smooth, with red, orange, or yellow stripes on the neck, legs, and tail. The bottom shell, or plastron, is usually yellow with some red or black.

Size

Depending on the subspecies, adult painted turtles can range from 4 to 10 inches in shell length. Females tend to be larger than males.

Subspecies

There are four subspecies of painted turtles, each with slightly different coloration and size. These include the eastern, midland, southern, and western painted turtles.

Habitat

Painted turtles live in a variety of aquatic environments, including ponds, lakes, marshes, and slow-moving rivers. They prefer areas with soft bottoms, aquatic vegetation, and plenty of places to bask in the sun.

Diet

They are omnivorous, feeding on a mix of aquatic vegetation, small fish, insects, and even carrion. The younger turtles tend to be more carnivorous, transitioning to a more herbivorous diet as they mature.

Behavior

Painted turtles are known for their basking behavior. They can often be seen lined up on logs or rocks, soaking up the sun. This behavior is crucial for their thermoregulation and metabolism.

Reproduction

Mating usually occurs in spring. The female digs a hole in a sunny location and lays between 2 to 20 eggs. The eggs are left to incubate, and the hatchlings emerge after 72-80 days. The sex of the hatchlings is temperature-dependent: higher temperatures result in females, and lower temperatures result in males.

In many areas, painted turtles can often be found in close proximity to human activity, making them a familiar sight to many people. Despite their widespread presence, they play an important role in their ecosystems and contribute to the biodiversity of their habitats.

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