四、篇章結構：      Dolphins use sound to detect the size, shape, and speed of objects hundreds of yards away. Fascinating and complex, the dolphin's natural sonar, called echolocation, is so precise it can determine the difference between a golf ball and a ping-pong ball based solely on density. Although humans have researched these intelligent marine mammals for decades, much of their acoustical world remains a mystery.      26 Sound waves travel 4.5 times faster in water than they do in the air. Dolphins use this totheir advantage, in ways that would make a superhero envious. Using nasal sacs in their heads,dolphins send out rapid clicks that pass through their bulbous forehead, or “melon.” 27 The soundwave speeds through the water, bounces off the object under investigation, and is reflected back tothe dolphin. Fat-filled cavities in the dolphin's lower jaw receive this information and auditorynerves conduct it to the middle ear and brain, where an acoustic picture is created.      Scientists say that dolphins may also use clicking to communicate with one another. 28 Amother dolphin may whistle to her newborn for days, apparently to imprint a signature whistleupon her baby that will enable it to recognize her. It is believed that dolphins use whistles toidentify one another and possibly for other functions, such as communicating strategic alerts whilehunting in a group, but scientists have yet to crack the code. 29      Since the 1960s, American military scientists have studied dolphins, and have trained them toperform such tasks as attaching explosives and eavesdropping devices to enemy ships or submarines. 30 In 2003, dolphins were deployed for the first time in a real war situation to probe the seafloor for mines near the Iraqi port of Umm Qasr.      Efforts to replicate dolphin echolocation continue to fall short, as humans have yet to achievethe complexity and precision that 50 million years of evolution has bestowed upon dolphins.Perhaps if scientists could understand dolphin-speak, they’d have more luck, but for now the truenature of dolphin communication remains mysterious.(AB) The sound is focused, and then beamed out in front of the dolphin.(AC) Sound production is also influenced by group types, habitats, and behaviour.(AD) One of the keys to dolphin echolocation is water's superb conduction of sound.(AE) Many doubt, however, that dolphins have a formal language akin to that of humans.(BC) In the mid-1980s, the U.S. Navy began training dolphins to search for mines using theirecholocation.(BD) Dolphins detect sonar to identify obstacles and map their surroundings when navigatingthrough the ocean.(BE) Experiments conducted in the mid-1990s with a bottlenose dolphin named BJ demonstratedthis sensitive ability.(CD) Although dolphins do not possess vocal cords, they still“speak”using sounds such as whistles,squeaks, and trills25.