Survival Below the Freezing Point
BY ALPER ÖZKAN (MSN/PhD I)
d_ozkan@ug.bilkent.edu.tr
Welcome to the first biology column of the new year! We've finally seen substantial amounts of snow as a bonus present for January, and this is a good time to write on animals particularly adapted to the cold. I am aware that this so-called history-and-biology column is quickly shifting to full-on biology, but let's say that I'll have a winter-themed mythology draft ready for a fortnight later.
Providing resistance to adverse weather does not require too large an investment for larger beasts such as seals, deer and bears; and those animals often show general patterns that allow them to survive in the cold with relative ease, including shorter body extremities and smaller nasal passages (a tendency called Allen's rule) in addition to larger bodies in general (Bergman's rule). But while thick fur, layers of blubber and the like may protect larger animals from low temperatures, their smaller kin are mostly unable to take advantage of such protective measures, for small body masses equal greater surface area to volume ratios, and heat is lost from the surface. This means that the smaller an animal gets, the harder it is to conserve its body temperature (which is also the reason that small, warm-blooded animals such as shrews and mice need to consume extremely large amounts of food for their size, as their energy expenditure is far higher than ours on a per-weight basis), and microscopic fauna in colder regions have to deal with the ever-present possibility of being frozen in their entirety. Bacteria, nematodes, rotifers, tardigrades and many other tiny creatures therefore have to come up with other methods to either prevent being frozen or to control the process in a way that minimizes tissue damage.
Nature offers a number of ways to pull off just that. The first is to prevent an animal from freezing much in the same way that one prevents a car engine from freezing, by judicious application of antifreeze. Indeed, compounds used in commercial antifreeze products, such as glycerol, occur in very high concentrations as a component of blood in animals frequently exposed to low temperatures, and equally commonplace are proteins that prevent the growth of ice crystals instead of merely dropping the freezing point. Both strategies focus on preventing the formation of ice, and they tend to fall short when the animal does freeze - this generally involves the rupture of any and all membranes, leading to a swift death. As such, species employing such tactics are called freeze-avoiding, since ideally the animal never freezes even when encased in solid ice. Defense against freezing itself is also possible, and often seen in the form of structures that divert the ice formation to non-vital compartments. This is accomplished with the help of nucleating proteins, which induce the formation of ice in a controlled manner and allow the animal to freeze and thaw repeatedly with little damage. Animals making use of this method are called freeze-tolerant, since they can freeze completely in winter, endure the damage, and once again go about their business when summer comes around. A third way to avoid freezing is simply not containing any water to freeze - by desiccating themselves, many small critters can survive harsh winters and revive with impunity when the stars are right again.
Such measures confer varying degrees of protection. Insects are rather complex creatures and can only withstand relatively higher temperatures. (Weta, massive Australian insects often called "demon crickets," are famous for their freeze tolerance but cannot survive below -15°C. Their ill repute is hardly warranted, though - dragonhead grasshoppers are much more terrifying.) There are smaller animals, however, that can survive in much colder environments, up to and including space. Tardigrades, the unsurmountable paragons of endurance, can survive brief exposures to temperatures nearing absolute zero (about -273.15°C, the surface temperature of Cocytus) and have been demonstrated to endure both solar radiation and the vacuum of space for at least 10 days. (On a side note, vacuum exposure is not as harmful as movies would have you believe and certainly does not involve all your blood exploding out of your body in an act of rebellion. You suffocate, freeze and/or dehydrate quite quickly, though.) As a brief venture into Wikipedia will tell you, tardigrades can also survive temperatures up to about 150°C, several thousand times the radiation that would kill a human and pressures higher than those experienced in the deepest parts of the ocean, making them one of the most persistent creatures imaginable.
Indeed, I greatly regret the fact that I was born as a human and not one of the proud and noble phylum of tardigrades. Sapience, superb cognitive capacity, sentience and all that nonsense mean nothing when you can't survive unaided in space.