As I watched my bird feeders on a chilly December 30th several years ago, the temperature around the feeders was 40°F. If I had sat outside to watch, I would have felt very cold. How do animals get through this time of year? The birds at my feeder seemed to be doing OK, but they paid a high price to keep themselves warm. Their bodies use energy to generate heat, and they need to eat a lot of food to provide that energy. Birds are “warm-blooded,” as are mammals like raccoons, deer, and people. The more technical term for warm-blooded is “endothermic” (endo=inside, and thermic=temperature). They generate warmth inside their bodies.
All the other animals – reptiles, amphibians, fish, invertebrates – are “cold-blooded.” The better term for this is “ectothermic” (ecto = outside, thermic = temperature), meaning that their sources of body heat come from outside their bodies. There are a few minor exceptions, like the pythons that coil around their eggs and use shivering to generate just a little bit of heat. A few larger reptile species can retain some metabolic heat, such as the leatherback sea turtle. But for the most part, ectotherms keep from getting too hot or too cold through strategies like basking in sunlight or seeking shelter from extreme temperatures.
Shifting from one place to another to try to maintain a certain temperature means that the best an ectotherm can do is to keep themselves within a workable range of temperatures. Too low, and most of them cannot stay active, and remaining below freezing will kill ectotherms because ice crystals form within their cells (there are a few “sort of” exceptions to this – see below). Too hot, and their bodies cannot work right, and above a certain temperature, they will die. Some reptile and amphibian (“herp”) species can tolerate more heat than others. For example, lizards in the racerunner family are active generally between 81 to 113°F (Vitt & Caldwell, 2014). Members of many other reptile families choose more moderate temperatures. And amphibians often do best within cooler temperatures; for example, species of mole salamanders that live in temperate areas generally prefer between 34 and 80°F (again based on a table in Vitt & Caldwell).
So what do ectotherms do on winter days like today when it’s 40°F or cooler? In the Dallas-Fort Worth area of north Texas, wintertime highs average in the 50s or 60s, and the average minimum temperatures are in the thirties. Most of our local reptiles and amphibians cannot function well in those temperatures, although on a warmer winter day you may see turtles basking at a pond. Where do they go in winter?
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The first image you may have is of underground dens with dozens of snakes escaping freezing in deep crevices below boulders and slabs of rock. Some snakes may use dens in our area, but many do not. Different species may use different refuges to escape the coldest temperatures. Their basic need is for:
One of the best ways to be protected from freezing is under a layer of soil or rock, which is a good insulator. At Fort Worth Nature Center & Refuge they monitor soil temperatures a couple of feet below the surface. Their records in 2014 showed that minimum temperatures averaged 12 to 17 degrees warmer in the soil than in the air. In January, for example, when the minimum air temperature averaged 29.0°F, the minimum temperature under the soil was 41.1°F. Staying underground smooths out the highs and lows, so that on a very cold day, the soil temperature will be warmer. On a particularly warm winter day, the soil does not warm quickly and so it may be cooler than the air. For this reason, a reptile sheltering deep within a burrow may not come out on a sunny and warm winter day.
However, other reptiles may not choose such deep shelters. A cottonmouth is more cold-tolerant and may shelter in a hollow log, and may emerge on some of those warm winter days to bask in the sunshine.
Some of our pond turtles may go beneath the water of a pond or river and even dig in to the mud at the bottom. Their heart rates drop quickly during such underwater dives, and because their metabolism becomes very slow in the cold, some of them may be able to get by for a while on anaerobic respiration, which involves getting the energy needed to keep life going from chemical reactions without oxygen. Some semiaquatic turtles are very cold tolerant, and have sometimes been seen moving around under the ice of a frozen pond!
Some of our salamanders stay in burrows or below ground for much of the year, and so maybe winter does not seem like such a big deal to them. They make their way through leaf litter and soil, under logs and rocks where they are protected from extremes of heat and cold. And most of them breed in late fall, winter, or early spring after rainfall.
At least a few frogs dig in for the winter in fairly shallow places and may become active in early spring for breeding. In early spring, things begin to warm up and then may freeze with a sudden cold front. This immediately places the frogs in danger of freezing, and at least a few species use a sort of “anti-freeze” to keep themselves alive. When ice crystals begin to form, their bodies produce a lot of glucose (some produce glycerol) in a hurry, and get it to the cells of the body (Stebbins & Cohen, 1995). Just as the green-yellow gunk we put in the radiator of the car will not freeze at 32°F, glucose does not freeze at that temperature. As a result, the water between the cells may freeze into ice crystals, and the frog may appear frozen, but inside the cells where it counts, there is no ice. If the freezing is only brief, the frog will thaw and resume normal life! Here in north Texas, some chorus frogs and the gray treefrog are known to be able to do this. Maybe we will discover that some others can, too.
The term we most often use to describe an animal that has become dormant during the winter cold is “hibernation.” The term is used for reptiles and amphibians, whose temperatures fall when their surroundings are cold, and so their metabolism and activity level decrease accordingly. The term is also used for some mammals whose bodies seem to program a winter period of dormancy. This difference, between animals that simply become dormant when it’s cold vs. animals whose bodies take an active role in decreasing their metabolism and becoming dormant, has led to some people using the term “brumation” to describe winter dormancy in herps. You don’t usually find the term “brumation” in herpetology textbooks, and I don’t think you’re committing some sort of terrible mistake if you use “hibernation” to refer to what herps do. If you would like to read further about this issue, there is a good discussion in the Obligate Scientist blog.
Here are a few photos of places where herps might hibernate … or brumate, or whatever!
Stebbins, R.C., & N.W. Cohen. 1995. A Natural History of Amphibians. Princeton: Princeton University Press.
Vitt, L.J., & J.P. Caldwell. 2014. Herpetology: An Introductory Biology of Amphibians and Reptiles (Fourth Edition). London: Academic Press