Have you heard of humming fish?

Well, it's less of a hum really, more like a motor-boat drone. You see, during mating season, male midshipman fish excavate nests and then start humming by rapidly contracting the muscles on their swim bladder, hoping to lure the silent midshipman females to lay eggs in their nests.

After the female deposits all her eggs, she swims away, leaving the male to fertilize and guard them until they hatch and mature. In the meanwhile, the male starts humming again, hoping to lure additional females and add more eggs to his nest.

Not all midshipman males can hum, just those known as Type I males. The smaller Type II males, then, can't attract females on their own. Instead, they linger around the Type I males' nests, darting in and out and fertilizing eggs on the sly.

What they lack in attractiveness, the Type II males make up in their capacity for reproduction. their gonads can comprise up to 15% of their body weight, compared to 1% for the Type I males. The equivalent would be 22 1/2 pound testes on a 150 pound human male.

Anyway, scientists are interested in humming fish because of what they can teach about hearing. For example, the way female fish zero in on a particular hum may eventually help us understand how humans focus on a particular sound. Plus, by studying the way hormones affect the midshipmen fish's ability to sing, scientists hope to learn about the general mechanisms by which the same hormones influence behavior in other animals, including humans.