Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [115]
We had our first letup of the bitter cold at the end of the first week in January. The weatherman talked of “the January thaw.” It was warm enough so that the kids could get out and play in the snow without running back inside within two minutes crying from the cold. Kids were not the only warm-blooded creatures who ventured out, at least briefly. Some of my bees did so as well, and bees are decidedly more vulnerable to cold than kids, given their huge size-disadvantage for heat retention.
The bees had by then already been confined in the hive for about two months. Admittedly, they had been fueled with honey, a relatively clean-burning fuel. But the honey also contains small amounts of impurities (such as amino acids) that result in bulk uric acid wastes accumulating in the gut. The thaw came just in time, I thought, for some cleansing flights.
At the first hint of warmer weather, my honeybees flew out quickly and some of them left yellow stains on the snow. They also left corpses; not all bees survived the hazardous outdoor ventures to void their bowels: The snow in front of my three hives under the roof against the side of the chicken pen was pocked with fifty-three seemingly dead bees.
Unlike other insects here in the north, honeybees cannot survive freezing. They must keep body temperature above about 15°C at all times to be able to stay active (or crawl), and they need a muscle temperature of at least 30°C to operate their wing muscles to generateenough power to achieve lift for level flight. Within the hive in winter, many bees tolerate their body temperature dropping to 12° to 15°C. These temperatures are their lower tolerable limit for walking, and it is also the low body temperature set-point that they “defend” when on the swarm or bee cluster mantle, because at still lower body temperatures they become immobile. At body temperatures below about 12°C they start losing physiological control. They cannot shiver to warm themselves back up by their own metabolism, and they are then also unable to crawl back into the social cluster to be warmed by the collective metabolism of the colony. Thus, if a bee should get separated from the winter colony cluster, it would die as soon as the temperature surrounding them (such as on snow) is about 3° to 4°C lower than 0°C since once outside the hive, and at a body temperature of -2°C, internal ice-crystal formation kills them. When honeybees fly out to void themselves at near 0°C, they can therefore not drop their body temperature appreciably before they must return to the warmth of the bee cluster in the hive.
Honeybees are only capable of maintaining a modest difference (of about 15°C) between body and air temperature by shivering and/or flight metabolism; thus if they leave the hive into 0°C air, they are in mortal danger. The bees’ corpses littering the snow in front of my hives were of those individuals whose body temperatures had first declined to below 30°C, and then plummeted to lethal temperatures because they lost the ability to control their body temperature. That is, they were those that lost power, fell to the snow, and then cooled to -2°C or less, to freeze solid.
Do the bees experience a reluctance similar to my own to go to the outhouse at my Maine cabin on subzero nights? And when they do fly out do they try to be as brief as possible, since to tarry even a minute means to court death? Do they take off as hot as possible, to thus increase flight speed and delay the inevitable cooling to lethal low temperatures?
Given the bees’ small body size, cooling occurs in seconds. Caution in leaving is likely an important variable for natural selection and caution differs among different populations. I found previously that African honeybees (the so-called “killer” bees), for example, are at least as able as our native (European) honeybees at regulating their body temperatures