National Geographic : 1966 Aug
Half-drugged yearling chases John Craighead around his tripod. Eyesight still affected, she charges toward sounds and finally "trees" a now-smiling John on the hood So we let well enough alone, content that our primary mission had been accomplished. We retreated silently and cautiously, with the thrill of having confirmed our suspicion that grizzlies move directly to their hibernation dens during snowstorms. Thus they leave no telltale paths. Radiotracking had given us this informa tion. Our success with No. 164 proved that biotelemetry can be an invaluable tool in wildlife behavioral research. Winter Dens Dug Dry and Warm In the fall of 1964 and again in 1965, we instrumented a dozen grizzlies all told, seek ing more information on prehibernation be havior, den construction, and home ranges. We tracked to their dens a young mother with her cub, and an old ill-tempered grizzly with three large yearlings. We followed a pregnant sow to her winter hideout, as well as a weaned male yearling. All had dug dens. None used natural shelters. All these dens were lined with evergreen boughs for warmth. They had been dug into 264 slopes, minimizing accumulation of water during winter thaws, and all faced north, assuring a deep, insulating snow blanket. The grizzlies had hollowed their dens at the bases of large trees, with the entrances lo cated between thick, steeply descending roots. One we found empty was a split-level den: The bedchamber lay higher than the entrance. Air warmed by the body heat of the bear was thus trapped, as in an Eskimo igloo. Was this construction accidental or by design? Since immobilizing our first bears in 1959, we have determined the age structure of the Yellowstone population. We now know that at a given time 19 percent will be cubs, 12 percent yearlings, 10 percent two-year-olds, 14 percent three- and four-year-olds, and adults will make up the remaining 45 percent. Only about a third of the adult sows are candidates for motherhood each year, because females taking care of cubs or yearlings do not mate. Not until the youngsters are weaned, some as yearlings and some as two-year-olds, will sows accept the attentions of the boars. The average annual crop of 33 cubs is sired by a few large, aggressive boars. Male parents play no other role in family life.