Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [113]
Once the cool bees on the swarm mantle are warmed up and capable of flight, another signal, called “buzz running,” made also by the scouts, initiates takeoff of the cloud of thousands of bees. It launches them into flight and on their way to their new home. The more synchronized the bees are, the fewer members of the colony will be left behind. The next problem is of bees potentially getting lost from the huge crowd of thousands flying to the new nest site, which only a few of the bees in the swarm have seen.
Only some of the bees in the swarm actually know where they are heading. But the swarm crowd is guided by what scientists have dubbed “streakers” (scouts presumably) who zip through the swarm drawing attention to themselves and leading it in the proper direction. The others, and the queen, follow. However, the bees continue to fly on only if the queen is with them; they detect her presence by scent.
After the swarm is ensconced in its new nest site, which in the wild is generally a hollow tree, its next main task is foraging for nectar and pollen and building receptacles (out of wax produced from special glands in the abdomen) for the pollen and honey that will be hoarded. Honey is the bee’s energy fuel for heat production, and pollen or “bee bread” is the primary protein food fed to the young for growth. Honey is an almost pure sugar concentrate, and it is collected as nectar in the bee worker’s stomachs and then regurgitated into the wax storage cells.
A colony of north temperate honeybees with ample fields surrounding them can produce nearly two hundred pounds of honey in a single summer. This is more than enough fuel to keep the hive warm all winter, so that we can in good conscience collect and eat the hive’s surplus without destroying the colony. Two or threee hives are sufficient for providing all of my family’s sugar needs for a year.
Honey, as such, of course gives off no heat until it is combusted (burned), or metabolized. In both cases the retrieval of the energy from the carbon-carbon bonds of the sugar molecules requires oxygen and yields carbon dioxide and water. Honeybees metabolize honey (i.e., the sugar in it) during flight while collecting pollen and more honey, and exclusively to get heat during shivering. Shivering involves the same muscles used for flight, only the wings don’t move because the upstroke and downstroke wing muscles are each in a slow tetanus contraction, one pulling against the other until both are pulled taut.
Shivering, since it uses up valuable honey stores, is minimized by the bees if it can be. Instead, their first response in the winter cluster, which is much like a swarm cluster, is energy conservation. As temperatures outside and then within the hive get lower, the bees begin to draw closer toward one another to form an ever-smaller and -tighter cluster.
The overall effect of cluster temperature regulation can be explained by distinguishing the bees on the outside of the bee cluster—the mantle bees—from those in the center of the cluster—the core bees. The lower the external temperatures, the more the mantle bees try to crawl into the cluster. When the cluster has shrunk to near minimum size, then the outermost mantle bees can finally only force their front ends inside. They plug every hole, and heat produced by bees’ metabolism within the cluster becomes trapped. Of course, some heat still leaks out by convection and conduction, and for a while the mantle bees are still sufficiently warmed by it. But eventually the mantle may become cold enough for the bees in it to finally have to shiver, to rely on heat produced on their own. In contrast, when external temperatures rise, then there is a lesser temperature gradient from the inside to the outside of the cluster. Less heat then leaves, so the bees