Trapped Under The Ice: How Wood Turtles Survive the Winter.



It’s hard to imagine a reptile holding its breath for more than 130 days, but that is exactly what Wood Turtles do in Northern New England many winters. And it’s not just air, they don’t eat in the winter either. During brumation (similar to hibernation in mammals), when water temperatures are barely above freezing, a Wood Turtle’s metabolism shuts down to roughly 5% of what it would be mid-summer, so from a metabolic standpoint, 130 days is only about a week by warm-season standards. At such low metabolisms, Wood Turtles don’t need to breathe air at all, but that doesn’t mean they go completely without oxygen. Wood Turtles are actually very dependent on having constant access to oxygen in the water all winter, which means water quality is very important to their survival. But how do they manage without filling their lungs with air?

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While Wood Turtles do sometimes overwinter in rivers with lower water quality, hot spots for the species usually have crystal clear water that fades to an ocean-like teal color beneath the surface.

People may assume Wood Turtles bury themselves in the muck at the bottom of a river in the winter, similar to the strategies sometimes used by Snapping and Painted Turtles, but that would cut them off from oxygen and could suffocate a Wood Turtle. A major reason why Wood Turtles overwinter in streams instead of ponds and other bodies of sanding water is that the bottoms of standing water bodies are prone to oxygen depletion mid-winter, especially when it ices over. Likewise, burying into in the muck would mean overwintering in anoxic conditions, so instead, Wood Turtles just rest in pools at the bottom of the river, inside log jams or undercut banks, or in leaf packs that provide some shelter from flood surges and predators. That still leaves the matter of how they survive without breathing unresolved. 

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Resting at the bottom of river pools, Wood Turtles usually overwinter in the open water rather than burying themselves in the muck so they can maintain access to oxygenated water through the winter, sometimes wedging themselves into branches and roots for shelter.

Pretty much any aquatic turtle is capable of absorbing oxygen from the water through capillary beds on the roof of their mouth and lining of their cloacas (aka: butt breathing). They can do this at any time, but at warmer temperatures the ‘butt breathing’ technique isn’t enough to keep a turtle alive for very long, so they need to breathe air most of the year. When water temperatures are near freezing and turtle metabolisms are low, Wood Turtles can get all the oxygen they need right from the water, but only if there is plenty of oxygen in the water to begin with.

Painted and Snapping Turtles take it a step farther and, when oxygen levels crash, they metabolize glycogen from their muscle tissue instead of oxygen. This causes an acid buildup in their blood that would be fatal to most other reptiles, but they buffer the acidity by leaching calcium and other minerals from their shells (in laboratory experiments, Painted Turtles have survived over four months with zero access to oxygen). Even when oxygen is available, Painted and Snapping Turtles sometimes bury themselves in the muck and cut themselves off from oxygen deliberately, better protecting themselves from semi-aquatic predators such as mink and otter, which are known to collect and eat turtles they find under the ice. 

Several days after a reservoir draw down, this Snapping Turtle emerged from a thick layer of sediment it had buried into for the winter. The turtle remained in place for at least 48 hours after emerging because the high levels of acidity in its blood caused by a long period of anoxic respiration made it very lethargic.

Wood Turtles, however, can’t use the same shell buffering technique as Painted and Snapping Turtles, so they cannot overwinter in anoxic environments. Clean flowing water with high levels of oxygen all winter is critical to the survival of Wood Turtles, which is why restoring healthy buffers along rivers is an important conservation strategy for this species. Riparian buffers, which are strips of land separating human land use from water and where building, farming, and mowing are not allowed, help to maintain and restore the clean water Wood Turtles need, while also providing the turtles with terrestrial foraging habitat (Wood Turtles forage extensively on land in the summer, especially in river valleys). 

Riparian buffers are strips of land that separate human land use from water and are a hands off zone where plants are allowed to grow without human disturbance. When improving water quality is the purpose of a buffer, as in the case pictured above, approximately 50 feet is usually the target, but at sites with Wood Turtles , a 300-foot buffer is ideal. A 300-foot wide buffer can be challenging to accomplish on working lands, but any buffer is better than nothing and the wider it is the better.

Not everybody cares about biodiversity and the importance of restoring wildlife habitat, but I think most people do appreciate clean water, and in the case of Wood Turtles, the two things go hand in hand. Riparian buffers certainly benefit Wood Turtles and other wildlife, but buffers also reduce sediment and nutrient runoff into the water, which are linked to declining fisheries and toxic algal blooms that are a risk to human safety. Furthermore, ecologically in-tact floodplains also protect agricultural fields, infrastructure, and homes from damage caused by intense flooding. 

While our mission is to conserve and restore critical habitat for imperiled reptiles and amphibians, if you take a step back and look at the bigger picture, it’s not just about our flagship species such as the Wood Turtle. It’s about clean water, flood resiliency, and agricultural sustainability, to name just a few things that all people should care about. As conservation biologists, we usually think of things through the lens of our species and ecosystems of interest, but in order to bring more people on board with habitat restoration, we need to highlight the connections between wildlife conservation, ecosystem services, and how it all ties back to us humans. Wood Turtles absolutely depend on clean rivers, something we all benefit from.

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Perhaps not everybody cares about turtles, but we should all appreciate the value of clean water.