英语听力:自然百科 北极熊 冰上间谍 Spy_Bear—7(在线收听

 She leaves little behind. 

 
Males are still on the lookout for females. For her cub's safety, she heads back to the hills. But as spring continues, lengthening days bring sudden and dramatic change. The highest tides of the year combine with the rising temperatures to trigger a vast of breakup of the ice that surrounds the islands.  In just one night, it can crack apart and drift 20 miles or more. Svalbard is a series of islands once more. 
 
The mother and single cub are still on the snowy hills, but signs of change are all around. 
 
At the shoreline, things are not as expected. The sea ice has drifted northwards and with it the bears' food supply. It's a huge swim for an adult bear. For the cub, it's out of the question. Their ship has sailed and left them behind. They are marooned on the island for the summer. 
 
The two-cub family managed to board the sea ice before it drifted too far.
  原文地址:http://www.tingroom.com/lesson/zrbaike/2010/259375.html