Most of our readers probably already know we have American alligators in Texas. However, many residents of North Central Texas seem to be unaware of this fact and generally are surprised to learn that a healthy breeding population of alligators resides at the Nature Center. One of the most common questions I am asked about our alligator population is, “How do they survive our cold winters?” The short answer: They retreat to underwater dens, utilize deep-water refugia, or resort to “icing.” To better understand these thermoregulatory behaviors, it helps to have a basic understanding of alligator physiology.
Alligators are ectotherms, meaning they derive most of their body heat from their external environment. The disadvantage of having this life history strategy is that ectotherms become lethargic during periods of cold weather; contrarily, the advantage is a decreased metabolic rate, which reduces caloric requirements and the need to feed during cold spells. This behavioral and physiological response to cold weather is known as brumation in reptiles. Most reptiles can exhibit some degree of brumation, but alligators can take this behavior to extremes during periods of ice formation.
So how do alligators survive freezing weather? The simplest and best option to avoid freezing weather is to brumate in a den. Although alligators can dig, they prefer to occupy dens created by mammals. In my experience, they also prefer dens located in the sides of steep banks. Den entrances are typically submerged, but many biologists hypothesize that the main chamber is positioned higher than water level, giving them access to an air pocket. As ectotherms, alligators don’t need to feed when it’s cold. They can remain in their dens for weeks at a time, allowing them to avoid ice formation and endothermic predators (raccoons, bobcats, herons, etc.). Next time you are hiking along a waterway during winter in an area known to have alligators, pay attention to cloudy, murky areas adjacent to the shore. The entrance to an active den is almost always cloudier than the surrounding water. Exercise caution if you happen to discover a den. Mothers will protect the entrance from perceived threats, so please keep your distance and notify Nature Center staff.
Seeking deep-water refugia is another effective method that alligators occasionally utilize to avoid freezing temps and ice. During the winter, the temperature of the water column is inversely stratified. This means that the temperature of water increases with increasing depth. By retreating to deeper depths, alligators, especially smaller individuals, can avoid ice that has formed on the surface. Younger alligators that are cold and lethargic are also susceptible to being snatched from the shallow littoral areas by avian and mammalian predators. Alligators can remain submerged for many hours during the winter because the cold water lowers their metabolism, reducing their oxygen demands and need to breathe.
The most unique and interesting behavior alligators exhibit during periods of extreme cold is “icing.” This behavior occurs when alligators must endure the formation of a layer of ice on the surface of the water. When this happens, they will become dormant, while keeping their snouts above water. As the ice freezes around their snouts, they will slowly rise up and down in the water column to prevent becoming frozen in the ice as they maintain their air pocket. This behavior increases their odds of survival during the most extreme winter weather conditions because they can continue breathing while keeping their vital organs submerged in deeper, warmer water. Although this survival strategy works for older, larger alligators, younger, smaller alligators are vulnerable to becoming frozen in the ice if the layer grows too thick for them to maintain an air pocket. Hatchling alligators, particularly, depend on their mothers during their first winter to maintain a large air pocket in the ice. Hatchlings are simply too small to maintain their own air pocket, and their heads are too short to generate enough of a thermal gradient between the ice, their snouts, and their brains to prevent mortality.
Despite these incredible survival mechanisms that alligators use to survive extreme cold, our winters remain a primary limiting factor for alligators existing on the periphery of their range. Areas such as Southeastern Oklahoma and North Central Texas will never have the abundance of alligators that our warmer, neighboring states have, but I think these alligators — alligators that are enduring the harshest winters occurring across their species’ range — are more fascinating and special. Even after 15 years of observing and studying the wintering ecology of alligators in Oklahoma and North Central Texas, my respect for these animals continues to grow each time I see them make it through another blizzard event. Therefore, I challenge you to leave the comforting warmth of your home and explore the Refuge the next time we have a freeze. Doing so will give you the opportunity to observe animals displaying adaptations that you may never see otherwise, and, if you’re lucky, you might also see the tip of an alligator snout emerging through the ice.
