听力原文：P Youve probably all seen Hollywood movies or performances at a marine park where humans seem to be talking to dolphins. And youve seen them nod their heads and shake their tails when the trainers speak to them. But are they really communicating? Well, the simple answer is YES. They are capable of giving and receiving information by using a variety of sounds and gestures. Thats the focus of todays discussion. Right, so lets begin by examining the concept of nonverbal communication. Its important because its the key to understanding communication in the animal kingdom. Now the first question is this: how do dolphins communicate with each other? Is there any one who knows about that? S1 Yeah, maybe by producing peculiar sounds as dogs or cats do. I remember watching a documentary film about sea life. P Yes, youre right. Well, they rely on sound or acoustics. In particular, dolphins have a very complex communication system because they live together in a large social group called a pod. So the very first thing they learn is how to introduce themselves to the pod. For example, when the young are born, the mother will continually whistle a specific sound to her young for many days. This whistle teaches them how to identify their mother. So, if the young get separated from the pod, their mother simply keeps making this sound until they get reunited. S2 But how can the young recognize whether the sound is their mothers or not? P Oh, thats the point. Just like your signature that identifies who you are, each dolphin has a unique sound called a signature whistle. S3 Like human voices? P Yes ... something like that, but not exactly the same. And the second pattern of communication is used for hunting. You see, dolphins hunt in groups, so they make different clicking and squeaking sounds to convey the action theyre going to take. Let me give you an example. When theyre hunting together, they take turns herding the prey into a narrow area. Theyre like cowboys, who surround the cattle and force them to move in one direction. And while one group herds the fish, the other group swims right into the middle of the herd and begins to eat. Then they switch roles so that everyone can get a chance to eat. Its a clever way to save energy and time, isnt it? Whales also use different clicking sounds to woo potential mates. So its obvious that marine mammals do have a very complex language system. S2 Wow, they must be very intelligent.., especially for their role sharing. P Yes, but there remains one more important thing. The third pattern of communication is, in my opinion, the most fascinating. Its called echolocation ... from the word echo. Marine mammals use this technique to navigate the oceans. For instance, when a dolphin makes a clicking sound, the sound travels through the water until it hits an object and then bounces back to the mammal. They can identify the size, shape and speed of an object and distance ... just by listening to the different echoes that each object makes. So, its like seeing with your ears! In fact, humans can also use this technique in real life. Theres a young blind boy in the U.S. who taught himself how to use echolocation to navigate his neighborhood alone. He just makes clicking sounds with his tongue, listens for echoes to measure distances, and recognizes different objects. Isnt that incredible? S1 Oh, wonderful! They are like a submarine. P Scientists have also adopted the principle of echolocation. They created sonar technology to explore the ocean depths. And the military has adopted this technology for underwater surveillance. In fact, they recently developed a new sonar system to detect super-silent submarines. The problem is ... the ping it sends out is so loud that its just like standing beside a twin-engine jet plane at takeoff. And these sonar speakers are designed to cover 80% of the Earths oceans. So, you can imagine what effect this has on marine mammals. Some biologists believe that the sonar interferes with the whales echolocation and disorients them. So theyre concerned that there is a link between the use of sonar and the increasing number of dolphins and whales that run aground on beaches. Take for instance the case of the sixteen whales on islands in the Bahamas and the twelve whales in Greece that beached themselves within hours after the super sonar was used. In fact, scientists have found bleeding around the brain and ear bones of eight dead whales. These injuries are consistent with hearing damage caused by exposure to loud sounds. So there is increasirig evidence that commercial and military activities using sonar are negatively affecting the lives of marine mammals. P In fact, they recently developed a new sonar system to detect super-silent submarines. The problem is ... the ping it sends out is so loud that its just like standing beside a twin-engine jet plane at takeoff. And these sonar speakers are designed to cover 80% of the Earths oceans. So, you can imagine what effect this has on marine mammals. What does the professor imply when she says this: P So, you can imagine what effect this has on marine mammals. What is the main topic of this lecture?