The last time we had a really cold snap like last week I dug up something unexpected in my backyard. It was not as exciting as a human skeleton, but nonetheless interesting.
I was digging frozen turf with a sharp spade to prepare a flower bed. It was like cutting ice cream cake into slices. My hands blanched without gloves, but was better working in the cold than waiting for the thaw to turn the ground to mud.
I noticed a strange object after turning over a slice of turf. It was the size of a hen’s egg, but as hard as a stone. Brushing off ice, it looked buffish with a yellowish stripe, and a long “process” extended from the “body.” When more ice and caked clay were removed I found another long process and two short ones attached to the corners of the body. A French scientist would have guessed sooner they were frog’s legs.
It was a frozen wood frog, which is well-named Lithobates sylvaticus (meaning a stone that walks in the woods). Tidewater Virginia is as far south as you are likely to encounter this species, whose range extends above the frigid Arctic Circle. How this cold-blooded creature survives freezing is still an enigma, and most animals can’t.
My students in Edinburgh learned about the effects of freezing by some simple experiments with red blood cells. They diluted blood in a test-tube of isotonic salt solution, which turned pink and turbid. After freezing and thawing, the solution became so transparent that print could be read through the tube. Before freezing, blood cells are still packed with hemoglobin which scatters light and makes the tubes look cloudy. After freezing their membranes have been perforated, which allows hemoglobin to escape and leaves the cells like transparent bags. To confirm the theory, the tubes were spun in a centrifuge. The unfrozen cells formed a red pellet at the bottom overlaid with colorless liquid showing the cells were still intact, whereas the frozen tube was uniformly pink from dissolved hemoglobin.
Lastly, the students added glycerol at the start of the experiment, which completely prevented freezing damage.
The protective effects of glycerol were discovered by accident in Audrey Smith’s lab over
sixty years ago. It was one of those beautiful errors to warm a scientist’s heart. Colleagues who remembered working with her told me that Audrey was a delightfully dotty Englishwoman who took her beloved Labradors everywhere, even opening her lab doors so they could join her at work.
Glycerol was the first of a bunch of substances that now preserve a long list of cells and tissues—blood, semen, eggs, embryos, stem cells, heart valves, skin, et cetera. They work in several ways—not simply by lowering the freezing point like car radiator fluid. When they bind water molecules or substitute for water the number and size of ice crystals that do harm inside cells are reduced. Audrey’s research team also showed they reduce the risks of cells pickling in their own brine, because salt reaches harmful concentrations when water starts to freeze.
Unlike amphibians, mammals and birds tightly regulate their body temperature, and those adapted to cold climates are protected with extra insulation and a counter-current blood supply reduces heat loss through cold feet, like penguins. Some species can turn down their internal thermostat—squirrels hibernate and bears become torpid—but none can survive whole body freezing. Hibernators wake up when temperatures fall dangerously low and start to shiver and stoke their metabolic fires.
Nakedness is a great adaptation for hot climates but no good in the Arctic. Humans generate heat by shivering and conserve it by reducing blood circulation in the skin, but for migration to the Far North more clothing and changes in behavior were needed. People with Raynaud’s phenomenon have an exaggerated vasoconstriction to chilling of their fingers and toes (penguin feet). I do. Their digits blanch and become painful at temperatures that don’t affect others, and turning blue is a frostbite warning.
Raynaud’s can be ameliorated by drugs that reduce vasoconstriction, but in one of my more eccentric projects I tested if a herbal remedy could keep my digits warm. It was a fire water of concentrated ginger and peppers, and barely drinkable. But somehow I managed to gulp it down every day for six weeks followed by six weeks off before repeating the cycles throughout the Scottish winter. I weighed my fingernail clippings at the end of each week to the nearest milligram. If the herbal was improving my cold sensitivity I theorized that my nails would grow faster on treatment from a better blood supply to the nail bed. There was absolutely no difference, but warming pads for hands and feet worked!
Frozen storage brings to mind episodes of Doctor Who traveling through time and space and coming back from the Planet of the Dead. And that reminds me about Cryonics in which whole bodies of legally-dead people (sometimes just their heads) are frozen in the hope that in the distant future biomedical science will restore them to consciousness. It’s a good business while customers are stiff in liquid nitrogen containers and cannot complain about the service, but I think it’s better to invest in the living.
Frozen frogs are not dead, but almost. In fact they are not completely frozen, even if three-quarters of their body water is iced-up. There is glycerol in the body, but it is not responsible for cold survival. Before hibernating, stores of liver glycogen are converted into glucose, which is another molecule that helps in cryopreservation. More importantly, ice forms in their body cavities rather than inside organs and cells, both of which shrink from partial dehydration and become supercooled. Their hearts slow and finally stop without fibrillating as ours do by only a few degrees of hypothermia.
I can’t be sure if the frog in our backyard really was alive or had frozen to death. Warming it would have given me an answer, but it might not have survived the rest of the winter without a food supply or possibly had insufficient energy reserves for successful hibernation. Besides, there is a You Tube video of frogs waking up to satisfy my curiosity. So I quickly returned the stone to frozen ground.
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