Male plainfin midshipman fish (Porichthys notatus) serenade for sex. By contracting a large sonic muscle around their air-filled swim bladders to generate low-frequency humming sounds, they woo females into their staked-out nest sites. But in about 10 percent of males, the sonic muscle is some 20 times smaller, allowing the production of only little buzzing sounds called grunts. Unable to sing, the smaller males, known as sneakers or type Ⅱ males (photograph at right, bottom), swim about stealing fertilizations in the nests of the guarding, or type Ⅰ, males (right, top).
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