Elephants don’t forget—at least,female elephants don’t.Elephant families are matriarchal. And the social knowledge gained by the oldest females is the key to a family group’s survival, according to a study published in April by Karen McComb,a biologist at Sussex University in England. Elephants announce their presence by making a deep,long sound,a practice referred to as contact calling.An unfamiliar call may mean that an elephant from outside the family group is nearby.A stranger can cause trouble,interrupting feeding or disturbing the young.So an elephant matriarch signals the family to gather around her;then they all lift their trunks in the air to smell the unfamiliar caller.False alarms can disturb the group and take time and energy away from feeding,so survival may depend in part on getting it right.   Working with Cynthia Moss,who founded the Amboseli Elephant Research Project in Kenya 30 years ago,McComb tested the social knowledge of 21 Amboseli elephant families with matriarchs 27 to 67 years old.She played recordings of contact calls to each family and found that the oldest matriarchs were much better at picking out unfamiliar calls.In fact,a group with a matriarch in her fifties was several thousand times more likely to form into a group upon hearing an unfamiliar contact call than when hearing a familiar call.However,families with younger matriarchs were less than twice as likely to gather together upon hearing an unfamiliar contact call as compared with a familiar call.And they gathered together a lot.Moreover,the social knowledge of older matriarchs translated into favourable results:Families with older matriarchs produced more baby elephants in each female-reproductive year. This finding shows how difficult it is to protect the oldest members of elephant families. As elephants age,they continue to grow larger,as do their much wanted tusks .So the older—and wiser—a matriarch is,the greater the chance she will be killed.About 800 000 elephants have been killed by people in the past 20 years. 53.What does the underlined word “matriarch” mean? A.An old member of an elephant family. B.A female head of an elephant family. C.A wise elephant. D.A large elephant. 54.The research with recordings of contact calls shows _________. A.how fast elephants form into groups B.how important the age of a leading elephant is C.how frightened elephants are when hearing a strange call D.how frequently old elephants call other members of the family 55.The older a female elephant is,_________. A.the stronger she will be B.the poorer memory she will have C.the more useless her tusks will be D.the more likely she will be killed 56.We can infer from the passage that elephants may _________. A.run into other elephant families B.give wrong warnings to their mothers C.run away upon hearing a strange sound D.produce more babies by gathering together more often