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Listen to part of a lecture in a biology class. The professor is discussing animal communication. Professor: Now, we often think of animal communication as being about sound, like birdsong or whale calls. But visual signals are equally important, especially in species that live in open habitats. Take the fiddler crab, for example. The male has one oversized claw that he waves in a rhythmic pattern to attract females. This claw is also used in territorial disputes with other males. Interestingly, the waving speed and the size of the claw can indicate the male's fitness. Females tend to prefer males with larger claws and faster waving. However, there's a trade-off. A larger claw is heavier and requires more energy to move, making the crab more vulnerable to predators. So, the signal is honest—it reflects the male's overall health and ability to survive despite the handicap. This concept is known as the handicap principle, first proposed by Amotz Zahavi. It suggests that costly signals are reliable because only high-quality individuals can afford them. So, in summary, the fiddler crab's claw is not just a weapon or a tool; it's a communication device that conveys important information about the sender's quality.