Propeller-free robots may soon be swimming with style. A new model of how fish and other aquatic species are able to propel themselves forward without expending much energy may help create energy-efficient underwater robots that swim just like the real thing. By closely studying and monitoring how fish, dolphins and other sea creatures swim, Mehdi Saadat at Harvard University and his colleagues found that many of them have remarkably similar styles that can be described with a simple model, depending on how fast and far the tail whips back and forth and the length of the animal. Scientists previously homed in on just one parameter that relates the beat of an oscillating fish's tail and how far it oscillates to the animal's forward motion. But it turned out to be slightly more complicated.