Wolves belong to the animal family Canidae, which also includes such animals as the jackal, coyote, fox, and dog. Some of these canids, such as the fox, are solitary hunters and raise their young alone; wolves, ___51____, are regarded as social animals, living in well-organized packs of 8 to 20 or more animals. A pack generally includes only one breeding pair and their offspring, but occasionally it may include one or two ____52___ wolves. Occasionally, different packs join together during difficult times, such as harsh winters, to find and bring down large game(猎物). The ___53____ nature of these packs is remarkable. Raising the pups, for instance, is a collective responsibility. Adult wolves in the pack may bring ___54___ to the mother while she is nursing; later they will feed the growing pups on partly ___55____ food they have regurgitated(反刍). Various adult wolves watch over the young ones, ___56_____ them, playing with them, and teaching them to hunt. Wolves, which usually live in northern forests, often search for food with all other wolves in the pack, working collectively to bring down large animals such as caribou and elk. Each pack usually has its own ____57___ for hunting. Wolves are protective of these territories, and they sometimes kill other wolves that trespass(擅自进入). When members of a pack are separated, ____58____--including howling, yelping and growling—help them communicate across long distances, stay in touch during hunting forays, and ____59____ their territory. Each wolf occupies a particular place in the hierarchy(等级) of the pack, in which high-ranking, powerful wolves—called dominant wolves--___60____ the weaker, subordinate wolves. The leading male wolf is often designated(指定) the alpha male, and the leading female is the alpha female. When a dominant member of the pack encounters a subordinate one, the dominant wolf stands erect, holding its tail high and its ears forward; it many also growl or bare its teeth. The subordinate wolf, ___61____, crouches down, lowers its tail between its legs, and flattens its ears; it may whine as well. It is possible that this submissive(温顺的) posture may play an important role in creating ___62____ bonds between wolves. According to current studies, the alpha pair in wolf pack is the only ___63____ pair. Alpha status in a wolf pack is ___64____ permanent, however. New research suggests that as younger wolves pair off, they assume the alpha roles, and animals no longer breeding ___65___ their alpha status. 51. A. otherwise B. thus C. plus D. however 52. A. dominating B. unrelated C. respected D. rebelling 53. A. artificial B. basic C. cooperative D. distinctive 54. A. shelter B. food C. comfort D. safety 55. A. left B. rotten C. digested D. cooked 56. A. driving B. catching C. beating D. protecting 57. A. methods B. techniques C. territories D. tools 58. A. traps B. pups C. hunters D. sounds 59. A. defend B. expand C. ruin D. conquer 60. A. train B. govern C. inspire D. challenge 61. A. on the other side B. in contrast C. in addition D. for example 62. A. common B. natural C. cultural D. emotional 63. A. controlling B. leading C. breeding D. feeding 64. A. rarely B. relatively C. occasionally D. virtually 65. A. keep B. abandon C. seize D. enjoy