Means of overwintering span from antifreeze to staying the night under snow

Winter frosts are dangerous to animals. If cells freeze, they break and the animal dies. The winter for animals is a full-time battle against freezing, and they have many ways to do it.

Cold-blooded animals, such as reptiles, amphibians, and the invertebrate, have no way to prevent cooling with their own heat production. They solve the problem with antifreeze substances, or with supercooling, which involves controlled freezing. The latter occurs, for example, in amphibians, who can appear to be thoroughly frozen.

”Freezing requires a seed, a particle where the freezing starts. A complex mechanism directs the freezing seeds outside the cells”, explains the University of Oulu professor emeritus of biology, Esa Hohtola. “Then their temperature is below freezing without the cell freezing, or in other words, it has supercooled.”

Many cold-blooded species, such as certain arctic fish, supercool in a different way. “The so-called anti-freeze proteins insulate the ice crystal inside the cell so that it cannot grow.”

Anti-freeze substances reduce the freezing point of a cell. Insects especially use them. Substances such as ethylene glycol decrease the freezing point all the way below minus 30 celsius; the temperature of a butterfly in hibernation is the same as air temperature, but if temperature goes down even a little below the critical limit, cells freeze and break.

Insects especially use anti-freeze substances, which decrease the freezing point of their cells. The Chionea in the picture can move about on sub-zero snow all the way down to −5 celsius (picture: Esa Hohtola).

The freezing point of cell fluid will also decrease when it is concentrated by removing water. That way a viper can stay the winter in a location where its temperature decreases below zero.

Occasionally, concentration is connected with controlled freezing, and sometimes combinations of anti-freeze and supercooling occur.

Small mammals stay active under snow

The hibernation of cold-blooded animals, who follow the temperature of their environment, is called brumation. When warm-blooded animals such as mammals and birds hibernate, their metabolism and body temperature do not reduce as dramatically.

Hibernation is more controlled. If body temperature gets close to going below freezing point, the body starts reactions to prevent this.

”The animal will not usually react to external stimuli. It does not move, eat or defecate, but it warms itself to normal temperature every few weeks. One theory is that hibernation accumulates sleep debt, making the animal ‘wake up to sleep’. There are, however, only a few hibernating mammals in Finland: the hedgehog, garden dormouse, birchmouse, and the bats.”

Many mammals are active in winter time. “The stoat, for example, dwells in hollows, and there are small mammals living under snow who do not get exposed to hard frosts. Even though it has not been studied yet, I bet that voles and shrew mice go into hibernation for short periods”, says Hohtola.

Winter hibernation takes many forms. It varies in duration and can last up to half a year. The arctic ground squirrel reduces its temperature to slightly below zero celsius. The hibernation of the bear is in the other extreme.

”The bear’s body temperature reduces by only a few temperatures, but its metabolism and heart rate decrease relatively more. The bear keeps turning in its nest, and the mother also gives birth there. The bear also reacts to its surroundings, as witnessed by hunters who get a darn angry bear attacking them at the nest. The badger and the raccoon dog hibernate the same way as the bear.”

Birds are miniature heating plants

Conserving energy and reducing body temperature are needed because temperature production requires energy ie. food, which is scarce in wintertime.

Fatty tissue does not burn energy but stores it. ”A large animal with a similar percentage of fat is in a better position than a small one, because the big one has a smaller surface and expends less energy in relation to its weight”, says Hohtola.

Therefore, it is surprising that only one bird species, the North American common poorwill, is in hibernation for weeks on end. Most birds are only in short hibernation, which can be deep or shallow, lasting over one night or day.

”Birds have a tremendous ability to produce heat in their muscles. They have no problems with minus 40 degrees celsius”, says Hohtola. “Temperatures between minus 20 and 30 degrees raise their energy requirement to maybe three times higher than during the level of inactiveness, and flying to ten times as high.”

Of the 250 bird species in Finland, some two hundred will fly south during winter, but a few individuals will always stay behind to test their cold tolerance. Partially moving species have a larger number of ones remaining.

Those birds who stay for the winter take care of their energy balance by eating more and flying less.

”First they fly from the night shelter to the feeding place, then they rest, eat, and get back to the shelter”, says Hohtola. “Little birds cannot stay without eating fo long because of their small size, as they expend all the fat they accumulated during the night. The great tit, for example, uses most of the winter day for eating and can get up even at dark to eat.”

Tits are individualists, as most birds are, and they do not warm each other by huddling together. Many of them freeze to death alone in their hole.

Birds that sleep together include the Grey partridge, long-tailed tit, and Eurasian wren. During the day, partridges also feed and rest side by side, whereas treecreepers can form a ring around a tree.

”We know annoyingly little about how birds stay the night. The Bohemian wax simply disappears out of sight for the night. Of the small birds, the redpoll, snow bunting, and bullfinch go under soft snow, and of the bigger ones, the black grouse and hazel grouse can go under harder snow.”

The latest research information on the overwintering of birds says that they conserve energy with periferal cooling. The University of Oulu has participated in the research by shooting pigeons with an infrared camera.

”Pigeons can restrict heating to their vital core tissue. Right under their plumage their tissues are cooler. Even our human fingers are cooler than 37 degrees celsius, but birds take this even further.”

Text: Jarno Mällinen

Main picture: Erkki Alasaarela

Last updated: 20.11.2017