By Victoria Gill, BBC News

媒體英語會帶大家一起學習BBC撰稿人在報道世界大事時常用到的單詞和短語。

Background: 虎鯨，俗稱殺人鯨，本是非常善于交流的物種。不過最近，英國圣安德魯斯大學的研究人員發現，虎鯨在捕食過程中不發出任何聲音，以確保不被獵物發現。

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.