I saw an interesting nature show last night about crocs and it showed this male sending out a booty call -- he would have most of his body just below the water surface and then he would produce an enormous vibration that made the water surrounding his body shake. Apparently this produces subsonic sounds, which due to their tremendous wavelength travel great distances through the water at great speed. And sure enough, they showed a female respond to his call. Interesting that she would come at his call. I've heard before that Elephants keep in touch with subsonic sounds as well. I don't know why, but this always strikes me as cool. I expect Whales do this as well. Maybe this is the hope of all men who buy giant subwoofers, that the ladies will come running?