In this question, you will listen to a lecture. Then you will be asked to su

Everyone knows that beavers build dams. And they have a good reason to. Dams are the source of their protection and food. By increasing the volume of water in their habitat, they increase the area in which they can move around safely and increase the amount of aquatic plant life on which they feed. Scientists wondered about the mechanism by which beavers build their dams. How do they choose the location? Do they consider the environment from an engineer’s perspective, or is their behavior strictly instinctual?
    In one experiment, researchers set up loudspeakers around a beaver habitat and played the sound of running water. Consistently, they found the speakers buried under mounds of sticks and mud, which muted the sound. In some cases, beavers built dams around speakers even when there was no water in the area. These results led biologists to conclude that it was the sound of rushing water that stimulated the dam-building response. However, subsequent experimentation showed that some beaver communities did not respond right away to the sound of rushing water. Only after considerable time passed did these beavers get around to piling up material around the speakers. This hesitation suggests that beavers have other considerations for building.
    In other experiments, ah, let’s see, there were drainpipes installed, bypassing the dams. Even when the pipes were set low in the water, taking in and releasing water far from the dams, the beavers found them and clogged them up with debris. This means that sound was not a factor in these cases. Water intake and release occurred underwater. Here’s another interesting behavior. During flooding, beavers allow water to run and do not build up their dams. These behaviors indicate that beavers do have some kind of understanding of other factors--perhaps water level, perhaps threat to food supply--and that they are not instinctively responding to just a sound stimulus.