By Victoria Gill, BBC News

媒体英语会带大家一起学习BBC撰稿人在报道世界大事时常用到的单词和短语。

Background: 虎鲸，俗称杀人鲸，本是非常善于交流的物种。不过最近，英国圣安德鲁斯大学的研究人员发现，虎鲸在捕食过程中不发出任何声音，以确保不被猎物发现。

Killer whales whistle to talk to each other and click to find their way around. It's the echoes from these clicks that help the animals to map their surroundings and to pinpoint the location of their next meal.

But while one type of killer whale eats fish, the other hunts marine mammals, including seals and porpoises. A shoal of salmon can't hear the clicking of an approaching killer whale, but the mammals, with their highly sensitive underwater hearing, can.

The researchers used underwater microphones to listen to killer whales hunting seals off the coast of Alaska. They found that the animals fell completely silent when they were hunting. But, somehow, they still managed to organise themselves into groups - often spreading out up to a mile apart, before coming back together and calling loudly to each other while they shared their catch.

The scientists now hope to attach satellite trackers to individual killer whales, to find out more about this stealthy behaviour.