By Aaron Weinstein, Naturalist
Winter has come quickly this year and the freezing temperatures are only further justification to finally start planning that trip to the tropics. In fact, the harshness of winter brings about a frantic sense of needing to be extra prepared. We find ourselves bundling on more layers than we ever thought possible, running to start our cars before work, and refusing to leave the house past 7 p.m. Unsurprisingly, the changing seasons illicit a similar reaction from the animal kingdom as nature begins its own frenzied preparations.
Scientists use terms like “hibernation, migration and adaptation” to label these preparations, but we really know them through examples: bears sleep through the winter, geese fly south to escape the cold, and foxes grow winter coats. The resiliency of nature is remarkable, but this topic brings up far more complicated questions of survival. What about turtles, frogs and fish that appear far less prepared to take on freezing conditions than even you and I? In a world of frozen creeks and barren landscapes, how can anything without fur survive?
Before we can delve into the particularities of surviving winter, it is important to refresh ourselves on what terms like hibernation, migration and adaptation actually mean to the animal kingdom. Hibernation, unlike a daily slumber, drastically lowers body temperature and energy consumption, allowing an animal to go months without food or needing to move at all. There are different degrees to hibernation as well. Black bears are considered “light hibernators,” due to their ability to wake up multiple times throughout the winter to feed. Alternatively, groundhogs and bats are classified as “true hibernators,” because they stay asleep for the whole winter. Regardless of the label, most of these animals spend late summer and fall gorging themselves with food in order to build up enough body fat to last them through the winter. Once spring arrives, their body senses the warming temperatures and longer days, and wakes them from their slumber.
Migration occurs when an animal changes geographical location based upon the season and changing temperature, like the monarch butterfly’s preference for spending summers in Canada and winters in Mexico. Butterflies and birds are common examples of migratory animals that rely on finding warmer temperatures and more abundant food to make it through the winter. If an animal does not hibernate or migrate, what else can they do? Mammals like beavers and squirrels feel the cold coming and begin to store food away for the tough months ahead, while white-tailed deer, rabbits and foxes grow a different coat with each season. These behaviors are considered adaptations that help animals that do not migrate or hibernate to survive through the bleak weather.
Even knowing that animals can hibernate, migrate and adapt to outlast the winter, we still have more questions than answers. How do turtles spend summers basking on logs and then go through the whole winter without breathing air? How do frogs and toads, who need to have constant moisture to protect their sensitive skin, resist freezing to death at first frost? And where do fish go to survive once ponds and lakes freeze over? As it turns out, the answers to these questions are not as far off as we thought.
As fall turns to winter and the nights dip below freezing, most aquatic dwellers are at the bottom of the water column. The denser warm water sinks to the bottom of the pond and, despite its presentation, even an iced-over pond has habitable water just a few feet below the surface. To maximize this effect, aquatic turtles dig into the mud to hibernate at the bottom. In this state, their metabolism is so low that they barely require oxygen or food and hardly have to move all winter. Even more mind-boggling is their ability to absorb oxygen through skin tissue by their throat and tail, which further reduces their need to spend energy rising to the surface.
Aquatic frogs also take advantage of the tolerable temperatures found deep within a body of water but must seek out oxygen rich areas in which they can rest. While turtles can bury themselves into the leaves and mud at the bottom, the low oxygen levels in this area are not enough for a frog to survive. Therefore, it is less surprising to see frogs moving around during the wintertime than turtles, as they seek oxygen-rich zones around the pond or lake. Finding oxygen may be a concern, but temperature is, surprisingly, not. Terrestrial frogs, like the wood frog or spring peeper, take cover in between rocks and leaf litter, where they risk being frozen. Even with ice crystals forming within the frog, its body protects its necessary organs until the environment warms and the frog thaws out, no worse for wear.
Much like the two species from before, fish seek out the deepest areas of a lake where the water will be the warmest. In a similar fashion to aquatic frogs, and even more so than aquatic turtles, fish are in a constant battle of finding proper oxygen levels while maintaining a livable temperature. While the cold water is better than warm water for holding oxygen, layers of ice and dying vegetation reduce the amount of oxygen dissolved over the winter, leaving larger fish on the hunt for habitable water. To their benefit, the metabolism of a fish, or the conversion of food to energy, slows down with cooler water. As they use less energy, their heart rates slow down and their need for oxygen lowers.
It really is incredible how nature finds ways to persist even through the most extreme conditions. The ability of aquatic creatures such as turtles, frogs and fish to survive through winter is due in large part to their adaptations, like growing a winter coat or storing food away. As each animal fills a niche to survive through hardship, one can’t help but wonder what environment their ancestors grew up in and how that may have shaped their behavior. What environment did turtles face that drove them to switch each season between breathing through lungs and absorbing oxygen through the water? What did prehistoric frogs experience that allowed them to be frozen and thawed without harm? The answers to these questions are beyond this article but show how even the most unassuming creatures are prepared for the worst.