Late in July, a cub-exchange occurred between two female grizzly bears in Grand Teton National Park. This spring, fifteen-year-old grizzly bear #399 (a research number assigned to her in 2001) emerged from her winter den with three cubs, while 399’s five-year-old daughter, #610, emerged with two cubs. 399 and 610 have overlapping territories in the northern portions of Grand Teton National Park, and have treated our Wildlife Expeditions participants to marvelous grizzly behavior all summer.This cub swapping behavior is not unheard of, but it is certainly considered rare. We are not sure why this exchange occurred, or whether it is temporary or permanent. Biologists speculate that cub adoption in bears can increase cub survival when cubs get separated from their mothers, the mother dies, or some other stressful event occurs. Perhaps 610 is producing more milk. We so often hypothesize that there must be some survival advantage to animal behaviors. Maybe 399’s third cub just prefers the company of 610’s twins rather than his/her own siblings.Our Wildlife Expeditions biologists were out interpreting this memorable ethological event immediately after it occurred. Our biologists continue to facilitate unforgettable educational experiences in the Greater Yellowstone Ecosystem.