TOEFL Listening: ETS-TOEFL听力机经 - 38V3ZT6KW824463DQ$

Lecture: Manatees: Narrator: Listen to part of a lecture in a marine biology class. Professor: I want to elaborate a little bit on something we talked about last week when we talked about how fish navigate. We had briefly discussed that fish are able to sense changes in water pressure and water currents because of their lateral line. The lateral line, if you'll remember, is a line of small holes that runs horizontally along a fish's body. Each of these holes is connected to a canal that sits just under the fish's skin and within the canal there are these small sensory hairs that move back and forth when they're stimulated by changes in water pressure from water entering the canal. This sensitivity to water pressure allows the fish to detect changes in currents, which helps them to navigate, to make their way through the environment and I bring this up because you need to understand it for today's discussion, which is actually about how the manatee navigates. You should be familiar with manatees. They're large marine mammals that are found in warm water, coastal and river ecosystems; places where salt water and fresh water meet. In North America, they can be found in Florida and the Caribbean. The manatee's habitat is often characterized by dark, murky water. As a result, it's very difficult for manatee's to see in this environment. It's difficult to see any distance. It's difficult to see any objects. Manatees don't have very good vision anyway and unlike other marine mammals, such as whales and dolphins, manatees don't use echolocation. Echolocation's when animals produce really high-frequency clicks and then detect the echoes of those clicks that bounce off objects in the water. It allows animals like whales and dolphins to sense how far away objects are and to navigate. So, manatees cannot echolocate and their habitat makes visual orientation less than ideal. So, how do they navigate? Well, manatees have an unusual characteristic. They have these very tough hairs that are spaced more or less evenly all over their bodies and about a hundred years ago, one scientist, well his instinct was and don't ask me how he came up with this, was that this might have something to do with manatee navigation. His opinion was that these were tactile hairs. Tactile has to do with the sense of touch and tactile hairs are so called because they allow animals to feel what's going on in their environment. They're different from other hairs because of the hair follicle sinus they sit in. The hair follicle sinus is just like a small sack within which a single hair sits in the skin of the animal and importantly, the sinus is attached to lots of nerves; more nerves than other kinds of hair follicles. So when the hairs touch something or move, the nerves sense the movement and send a signal to the brain. Now, here's what was a little peculiar about the scientist's proposal. Other mammals have tactile hairs, but they're usually restricted to the facial region. An example of this is the whiskers on cats and dogs, so what this scientist was proposing, just based on an instinct, was kind of unusual, but if his instinct was correct, that these were tactile hairs, then it's possible these hairs do constitute a system that enables navigation, something similar in function to what fish have. Anyway, that was a hundred years ago, but all of this came to light again recently when a researcher analyzed some magnified images of manatee hair follicles. He found out that indeed these are sinus follicles and the hairs in the mare indeed tactile hairs, so together these hairs form a sensory system for manatees. What he also found is that on manatees, these hairs are spaced far enough apart that they don't touch each other, so that they don't overlap and interfere with each other. In other words, so the manatee doesn't sense itself. So this research is all very interesting, but the big question is how similar manatee tactile hairs and fish lateral lines are. What information is the manatee receiving from the tactile hairs? Like, can tactile hairs allow manatees to detect stationary objects such as rock formations or riverbanks? See, there's patterns of flow in the water and stationary objects disrupt that flow. Fish perceive these kinds of disruptions because of their lateral lines and the research is ongoing, but preliminary findings seem to say that yes, manatees have this ability. Lets take a look now at one particular experiment.