数年前,美国丹佛动物园遇到了一个棘手的问题。动物园来了一头美丽、健壮的北极熊,但这里没有适于北极熊生活的居所,只好将它安置在一个狭小的、临时性的笼子里,北极熊的活动受到了严重的限制,以致它无论往哪个方向行走,每走三步,就不得不转弯或者回头。 A few years ago, Denver Zoo in the United States encountered a thorny problem. The zoo came with a beautiful and robust polar bear, but there was no shelter for polar bears and had to be housed in a small, temporary cage where the activities of the polar bear were severely limited so that no matter which direction Walking, walking three steps, you have to turn or look back.