Surviving winter in the great outdoors is difficult especially when body temperature is unregulated and subjected to subfreezing temperatures. Insects and amphibians have some remarkable adaptations that enable them to successfully over-winter.
The dangers of ice formation in insects and amphibians are similar to those of freezing in plants. Ice must be prevented from growing within cells, instead growing in a controlled fashion in spaces between cells.
Insects either allow themselves to freeze or they avoid freezing altogether. Each strategy is not without risks.
Insects that avoid freezing, like the mountain pine bark beetles, actively lower their super-cooling point, and prevent ice formation by lowering their freezing point enabling them to tolerate temperatures as low as minus 40 degrees.
In order to achieve this level of tolerance they select a dry hibernating site. A waxy outer layer is impermeable to water and it serves as an excellent mechanical barrier against external ice seeding. One organ within bugs that is particularly susceptible to freezing is their gut. So insects empty it, completely, before the onset of hibernation.
In order to super-cool their blood (correctly called hemolymph) they manufacture anti-freeze proteins, alcohols, sugars and even ethylene glycol (the same compound we use in our radiators). The correct mix of anti-freeze compounds blocks ice formation and prevents its growth.
Supper-cooling is a gamble. At any temperature below 32 degrees the only stable state in which water can exist is solid. Any kind of disturbance for a super-cooled bug results in spontaneous flash freezing — an instant death.
The only way to avoid this risk is to promote early and gradual freezing of fluids between cells more specifically in the cell walls. And it’s a strategy used by a variety of insects, some frogs, garter snakes and hatchlings of the painted turtle.
There are four requirements in order to successfully freeze: ice growth is promoted at temperatures just beneath 32 degrees; it must be restricted to spaces between cells; the total amount of body ice is limited; and cell membranes must be protected from structural damage.
The ability to create early ice formation is unique and it is undertaken by special ice-attracting proteins in fluids between cells in their walls. These proteins are manufactured in the autumn and disappear in the spring. They essentially reduce energy barriers needed to make ice and promote a lattice (just like a fence) for ice formation.
Here’s the tricky part: If ice grows unchecked (i.e. larger and larger) it will dehydrate the inside of cells. The lethal limit of ice, in a living organism, seems to be about 65 percent of total body weight. In order to control ice, once its formation is induced, freeze-tolerant organisms must also make anti-freeze proteins. Their job is to keep ice masses small.
Some bugs in the far north are so well adapted that they posses the ability to switch yearly between freezing or super-cooling!
How do insects know when to get ready for winter? Just like plants in the Northern Hemisphere, insects rely on low, above freezing, air temperatures and reduced daylight hours as their cue to get ready to freeze, or barf and get on the super-cooling rollercoaster.
Land hibernating frogs, with water permeable skin, must withstand freezing. They over-winter beneath a scant cover of leaf litter under snow where they may experience subfreezing temperatures and are not able to control their body temperatures. In one of Mother Nature’s most awesome winter feats spring peepers, chorus frogs, gray tree frogs and wood fogs freeze solid and resemble a hard baseball.
They do this by initiating freezing at 28 degrees with high amounts of the sugar glucose (rather than alcohols that bugs use). Glucose is extremely important in protecting membranes like skin.
Very suddenly glycogen (stored glucose) in the liver is rapidly converted to glucose (sugar) and dumped quickly into the blood stream. In fact the glucose level increases 200-fold in about eight hours, until the frog becomes severely diabetic.
Not only is this risky but it becomes a race to deliver the sugar to the body tissues while freezing is progressing. A strong heart is mandatory to pump and within one minute of the first ice formation a frog’s heart beat doubles.
Within 20 hours after initial freezing 60 to 65 percent of its body water is frozen. Their heart stops, breathing ceases — the frog teeters on the edge of life. The thin litter layer and a good insulative snow cover are of paramount importance. For no frog has yet been found to survive below 19 degrees.
Once temperatures rise in the spring, within an hour of thawing, the heart resumes beating, six hours later at 41 degrees the heart rate is back to normal.
To freeze or not to freeze? It’s a question that Mother Nature has at least two exceptional answers for.
Dr. Reese Halter is a public speaker, conservation biologist and founder of the international conservation institute Global Forest Science. His most recent book is “The Incomparable Honeybee and the Economics of Pollination,” Rocky Mountain Books. Contact him through www.DrReese.com.