The movements of swimming fish and flapping birds are remarkably similar despite great differences in their bodies and physical environments, in a surprise finding which could be used to design futuristic flying machines.

Animals have a narrow range of efficient motion, perfected over millions of years, regardless of whether they are swooping or swimming, report Dr Graham Taylor and colleagues from the University of Oxford in Nature.

The researchers have discovered that the way most animals move while travelling at cruising speed is similar, whether they're flapping their wings or flipping their fins. They suggest this indicates that an 'optimal physiology' has evolved for both flying and swimming motion.

Research has previously shown that cruising dolphins and fish show a remarkable similarity in the ratio of frequency and speed of tail flapping to forward motion.

The researchers wondered if the same was true for flapping wings. To find out they looked at 16 studies on animal locomotion that had recorded wingbeat frequency, stroke amplitude and flight speed.

Amazingly, it turned out that the relationship of wingbeat speed and amplitude to forward speed, known as the Strouhal number, was the same, regardless of whether the animal was a flyer or a swimmer.

The researchers suggest that evolutionary selection pressure has resulted in animals which cruise in the most efficient manner possible.

"We were surprised to find that the rule worked for both flying and swimming organisms. Animals tune their wing or tail flap frequency to be most efficient when cruising - just like a well-designed car," says co-researcher Dr Robert Nudds from the University of Leeds.

"It applies to animals moving through air and through water. It broadly applies whether the propulsion is driven by wings or tail ... and to animals ranging in size from moths to dolphins."

Future flight
But the finding isn't just a lesson in evolution - it may also help design the flying machines of the future, say the researchers.

The research is particularly relevant to developing micro-air vehicles (MAVs), finger-sized flying machines that can be equipped with sensors or cameras and used in emergency situations or defence.

MAV designer and PhD student John Young from the School of Aerospace, Civil and Mechanical Engineering at the Australian Defence Force Academy says the results are very interesting.

"It's very relevant in that they've looked at a whole range of animals and whether they fall in this range of Strouhal number, which is a good number for categorising how animals flap their wings and what sort of thrust and efficiency they get out of them.

"At the moment the sort of MAVs people are building are powered by batteries which are still quite heavy and you can only use them for so long, so you want the highest possible efficiency," says Young.

"The efficiency of insect and bird flight is interesting since they've had millions of years of evolution to refine their techniques. This paper is one of the few that show it is a good idea to look at these techniques in the design of MAVs."

MAVs have applications in surveillance, reconnaissance, and could be an extremely useful counter-terrorism tool. They could be used to spot and track fires, in smoke or chemical-filled areas, or finding people in collapsed buildings.

"They've [the researchers] talked about it for sharks or fish - there has been robotuna that use flapping propulsion in these Strouhal number ranges, and the same principle will definitely be used in MAV research."

The results are so general that researchers predict they could be used to calculate the flight speed of extinct birds or work out the speed of extraterrestrial organisms: "As it's a universal rule, if there are swimming or flying organisms on other planets, then we predict it should apply to them too."