by Whit Gibbons

January 17, 2010

The severe and persistent cold weather that visited most of the country in December prompted many questions about how different kinds of animals manage to survive in freezing weather. The following are a few of those questions.

Q. How do animals survive the extreme, record-low winter temperatures that most eastern states experienced, even as far south as Florida, without going extinct?

A. The simple answer is that many individuals of many species did not or will not survive the cold temperatures, and some of the introduced tropical species will be hardest hit. But populations of native species at higher latitudes have experienced such temperatures in the past and evolved to withstand them. Those that survived carried the genetic material to persist and passed it on to their offspring. Populations of many species may be temporarily reduced in size, but no native species will go extinct because of the extreme cold.

Q. It is sunny and 26 degrees in East Texas today. A small lizard has been hanging out on a window sill at my home. He is pretty skinny and clearly hibernating, because he moved around very little. I was wondering if there was anything I could do to help this little guy? Would he be better off outside on his own where he could access a food source? Should I just leave him alone on the window sill? Should I try and move him to a warmer place or offer him any type of food? He was brown and turned greenish when he warmed up.

A. The lizard is probably a green anole, presumably hibernating. It may be an individual that is nearing the end of what is typically a short life--only a year or two in the wild. They would not typically eat at this time of year. If it is outside and finds a sheltered spot (under tree bark, on a log, or on your window sill) on the south-facing side so it gets direct sunlight, it will have a good chance of making it through the winter. However, we do find many anoles that have been killed during unusually cold winters, so there are no guarantees for what it may experience naturally.

Q. How do frogs survive the winter? I saw tadpoles beneath the ice in a lake on the golf course.

A. Most frogs in the world live in tropical or subtropical regions, where there is effectively no winter. However, many live in the temperate zones, some as far north as the Arctic Circle; for them, protection from winter cold is essential for survival. Overwintering or winter dormancy, also referred to as hibernation or brumation, is accomplished in various ways by frogs. Adults of many species avoid freezing temperatures during the winter by selecting hibernation sites that do not freeze. Some overwinter on land, buried beneath vegetation or in burrows, and some stay under water and bury themselves in the mud at the bottom of ponds. A frog's metabolism is very low at cold temperatures and its oxygen requirements are minimal. Because the oxygen available in cold water is relatively high, many frogs are able to survive under water during the winter with only the dissolved oxygen that diffuses across their skin. Some species, including bullfrogs and green frogs, overwinter as tadpoles in the muddy bottom of lakes or ponds. Even when ice is on the surface, water temperatures at the bottom remain above freezing.

Another mechanism used by some species of frogs to deal with cold temperatures is to produce antifreeze in the body; this allows them to survive temperatures several degrees below freezing. These species still seek out areas that protect them from severe cold, but they live in cold climate regions where winters can be especially harsh so that antifreeze measures are highly adaptive. Most produce glucose in their blood and other tissues to provide protection from freezing at temperatures in the 20s. Many of these species actually do partially freeze, and wood frogs can withstand freezing of up to 70 percent of their total body water. One native species, the gray treefrog, avoids freezing by producing glycerol in its tissues instead of glucose.

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