It’s hard to imagine an air-breathing vertebrate holding their breath for 130 days, but that is exactly what all fifteen of the Wood Turtles I tracked last year using radio telemetry have done since late November. And it’s not just air, they likely haven’t eaten in that time either. During brumation (akin to, but not quite the same as hibernation in mammals), when water temperatures are just 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. However, despite not needing access to surface air during their winter dormancy, Wood Turtles are actually very sensitive to oxygen levels in the water.
People often assume that Wood Turtles bury themselves in the muck at the bottom of a river, similar to the overwintering strategies often employed by both Snapping and Painted Turtles, but that would cut them off from oxygen and cause the turtles to suffocate in short order. The entire reason Wood Turtles overwinter in streams instead of ponds, wetlands, or other water bodies lacking current is that standing water is prone to oxygen level crashes mid-winter, especially when it ices over. Likewise, burying themselves in the muck would mean overwintering in anoxic conditions, so instead the turtles just rest in pools at the bottom of the river that provide some shelter from flood surges that could wash the turtles downstream. Undercut banks or wedged inside root masses and log jams are also the sorts of places a Wood Turtle might hunker down for the winter.
But not all turtles are so sensitive to oxygen during the cold season. 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) when water temperatures are near freezing and turtle metabolisms are low, but that trick requires there to actually be oxygen in the water to begin with. Painted and Snapping Turtles can take it a step farther and, when oxygen levels crash, metabolize glycogen from their muscle tissue instead. This causes an acid buildup in their blood that would be fatal to most other reptiles, but to buffer the acidity and maintain a somewhat neutral pH in their blood they leach 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 in the water remains high, Painted and Snapping Turtles sometimes bury themselves in the muck and cut themselves off from oxygen deliberately, better protecting themselves from semi-aquatic mammalian predators such as mink and otter, which are known to collect and eat turtles they find under the ice.
Because Wood Turtles don’t have some of the same tricks up their shells as Painted and Snapping Turtles, they are greatly limited in the sorts of places they can overwinter. Clean flowing water with high levels of oxygen all winter is absolutely critical to the survival of this species, which is why restoring healthy buffers along rivers to the greatest extent possible is an important conservation strategy for this species. River buffers, which are strips of land separating human land use from water where building, farming, and mowing are not allowed, not only provide critical foraging habitat for Wood Turtles, which might wander over 1000 feet from streams in the summer, but also improve water quality in the process.
Well known is the fact that 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. Establishing and enhancing river buffers certainly benefits Wood Turtles and any other wildlife they share the landscape with, but also results in lower 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. So 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. 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, it is important 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.
