Winter wetland adventures – how do researchers study reptiles in the winter?

By Tharusha Wijewardena

My name is Tharusha Wijewardena and I am a PhD Candidate at Laurentian University. I am collaborating with the Toronto Zoo Adopt-A-Pond team to study the winter ecology of freshwater turtles in Rouge National Urban Park. This blog post summarizes  some of the exciting things that happen in the wetland during winter! 

We know that summer is a busy time for amphibians and reptiles in our wetlands. It is a time to forage, mate, and lay eggs. But as the fall arrives, amphibians and reptiles reduce their activity levels and start looking for a good overwintering location (hibernaculum) to survive the winter. For turtles, finding the right spot can be a little tricky. They have to find a location with enough oxygen to last the winter, but also avoid freezing. Most turtles choose to overwinter underwater or buried in the mud below the frostline.1 Some hatchlings even spend their first winter in their nest cavity.2 The picture below is an example of a juvenile Blanding’s turtles hibernaculum. If you look closely, you can see that the turtle is sitting just below the ice in a floating vegetation mat.  

Over the years, researchers have studied the overwintering habitat preferences of adult turtles. For example, the three turtle species in our wetlands, Blanding’s, painted, and snapping turtles, are known to be anoxia tolerant. That means they can survive in low oxygen conditions for a prolonged period of time. For example, the painted turtle can survive in anoxic waters for more than 150 days!3 But we still do not know a lot about the overwintering habitat preferences of juvenile Blanding’s turtles, especially in an urbanized area.  

As part of my PhD, I am investigating the water chemistry parameters that allow juvenile Blanding’s turtles to survive the winter. For example, I am measuring dissolved oxygen level, water temperature, pH, ice depth, and snow depth to understand what kind of overwintering locations are preferred by juvenile Blanding’s turtles. Let’s have a look at how it is done.  

First, we have to find where the turtles are overwintering. To do this, we continue to radio-track our Blanding’s turtles in the winter using radio telemetry. If you want to know more about radiotelemetry, watch one of our videos or check out our website.

As you can see in the pictures above, the ponds are frozen during winter fieldwork and there is usually a very thick layer of ice. To access the water, we first drill a hole in the ice using a hand auger. We are very careful to not disturb the turtle, so we drill the hole about 0.5m away.  

Once we drill a hole, we insert our water chemistry probes (YSI ProDSS), which records different water chemistry parameters. Once we have taken all the water chemistry measurements, we close the hole we created by covering it with ice. This way, we minimize any changes to the water column where we drilled.  

 Although our survey effort may not be as rigorous as in the summer, the wetland is still full of exciting events! For example, on a cool November morning, we spotted this sluggish painted turtle moving ever so slowly under the ice. Although turtles may not move as much as in the summer, they do move under the ice column and periodically come up to breathe when the ice thaws.  

Winter is a very challenging time for turtles, and despite the thick layer of ice, they are still vulnerable to predation. Another part of my study is to determine the presence of turtle predators and how often predation occurs. Some of the most common predators belong to the Mustelidae family. This includes otters, minks and martens. Our wetlands  are not large enough to regularly support otters but there have been sightings of minks. Luckily, we have not seen any winter turtle predation in our wetlands so far.  

Predation is not the only cause of death in winter for wetland species. For example, we saw a green frog trapped and frozen under ice. Sometimes, amphibians and reptiles take a gamble trying to bask for as long as possible so that they can thermoregulate their bodies to stay alive. However, this time the gamble cost them their life.  

Although the wetlands may be covered in ice in the winter and be void of beautiful frog calls and other reptile and amphibian sightings, the wetlands are still full of action; sometimes exciting and sometimes gruesome. There is a lot to learn about how reptiles and amphibians behave and survive the harsh winters and I hope this blog post shed some light into how researchers study these interesting behaviours!   


  1. Ultsch, G. R. (2006). The ecology of overwintering among turtles: Where turtles overwinter and its consequences. Biological Reviews of the Cambridge Philosophical Society81(3), 339–367.  
  1. Ultsch, G. R. (1985). The viability of nearctic freshwater turtles submerged in anoxia and normoxia at 3 and 10 degrees C. Comparative Biochemistry and Physiology Part : Physiology, 81(3), 607-611. 
  1. Riley, J. L., Tattersall, G. J., & Litzgus, J. D. (2014). Potential sources of intra-population variation in the overwintering strategy of painted turtle (Chrysemys picta) hatchlings. Journal of Experimental Biology, 217(23), 4174-4183.