One of the most common questions I am asked during the winter is “where did all the frogs and turtles go?” Frequent visitors know of the variety of turtles and number of bullfrogs that are found in Quarry Pond. In the summer, you hear the croaks and splashes of frogs as you traverse the trail. You see turtles sitting upon a floating log in the sun. The pond is a busy place! Naturally, when the temperatures drop and ice forms on the pond, one wonders where the animals living there go.
Frogs and turtles both hibernate in the winter. Hibernation is typically thought of as a long nap in the winter—images of a bear curled up in its den come to mind. This method of winter survival, however, is more complicated than a long sleep. Hibernation is an extended period during which an animal’s metabolism slows significantly—their heart rate and breathing slow down to conserve energy when temperatures are cold and food is scarce. Frogs and turtles both exhibit amazing feats during hibernation.
Let’s start with frogs. All the frogs you see hopping about the pond during the summer are American bullfrogs. They get through the freezing winter temperatures by hibernating at the bottom of the pond where the temperature remains above freezing. In order to survive a winter below the surface, frogs engage what seems like a superpower to humans. Frogs can breathe through their skin. As they lay on the pond’s bottom, water runs over their skin and provides them with oxygen. Frogs will stay just above the mud or slightly buried, occasionally swimming slowly about the pond bottom. They cannot bury themselves under the mud because they would suffocate without that contact between their skin and oxygen-rich water. They remain in this state until the spring. They require so little energy during this period that the carnivorous frog can survive without eating until the spring.
The frog with what might be the most surprising “superpower” is the wood frog. Being ectotherms (or “cold-blooded”) animals, frogs do not regulate their body temperature and rely on environmental factors to heat or cool their bodies. When temperatures drop below freezing, so does the frog’s body temperature. This puts the frog at risk of freezing. While bullfrogs move below the water’s surface where the temperature remains above freezing, wood frogs get through the winter on land— burrowing themselves into small crevices in trees, rocks or logs. Despite being exposed to below-freezing temperatures and turning into frozen frogsicles, wood frogs are able to survive. Wood frogs survive freezing because they protect their vital organs with their own version of antifreeze. Wood frogs use extra sugars to lower the freezing temperature of water in their organs’ cells enough that they are not damaged by a freeze. Thus, when the spring comes with warmer temperatures, the frog can simply thaw and continue its life cycle.
Aquatic turtles, like bullfrogs, spend the winter at the bottom of the pond where temperatures are stable. During the summer, when turtles are active, they breathe in air to their lungs at the pond’s surface. When temperatures fall and ice forms on the pond, though, the turtles cannot come up to breathe. To survive this period, aquatic turtles reduce their metabolic rate almost completely, keeping their need for energy and oxygen low. They are able to take in the small amount of oxygen needed through their skin, mouth and cloaca. At the end of the winter, the pond’s oxygen may be depleted or reaching that point. If this occurs, some species of turtle are actually able to hibernate without any uptake of oxygen. To do this, turtles go through a process that produces lactic acid, which can be harmful to their bodies. To survive this, the turtles actually neutralize the acidic waste using the calcium from their shell and bones.
Turtles and frogs are beloved all summer long, their presence in the pond is appreciated by visitors that walk by and see these animals enjoying sunny days. At this time of year these animals are deep in the pond, waiting for the ice to melt so they can head to the surface again.