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Listen to part of a lecture in a biology class. The professor is discussing animal communication. Professor: Today we're going to talk about a fascinating aspect of animal behavior: communication. While many animals use visual signals, such as the bright colors of a peacock's tail, or chemical signals, like pheromones, one of the most efficient methods is acoustic communication. Now, acoustic signals have a major advantage—they can travel around obstacles and work in the dark. For example, frogs in a noisy stream face a challenge: their calls might be drowned out by the sound of rushing water. So some frog species have adapted by shifting the frequency of their calls to a higher pitch, which is less masked by the low-frequency noise of the stream. This is a clear example of how environmental pressure can shape communication systems. However, this adaptation comes at a cost. Higher-frequency calls attenuate more quickly over distance, meaning the signal doesn't travel as far. So frogs must balance the need to be heard by potential mates against the energy cost and range limitations. This trade-off is a common theme in the study of animal communication.