Hi there. I absolutely love the site, and have to thank all of you members for volunteering your time to help increase the level of knowledge in the world.
This question is something I've always wondered about, ever since I was a kid:
Which method of propulsion is more efficient in water - the vertical flexing motion of whales and dolphins, or the horizontal flexing of most fish?

I realise it may be a very Panglossian question, and I know why each group moves the way they do, but as you might almost say the environment they are applied in are the same, they could be compared. I wondered if anyone had done any studies, taking into account other influencing factors such as muscle density, body temperature etc.

An odd question with possibly no merit I know, but I've always wondered


Hi Matt, that is a good question, but sadly not one with an easy answer. The short answer is there should be little difference between them. The theoretical limits of efficience and speed in water are a result of the density of the fluid and the shape of the animal. Provided the animal is as hydrodynamically shaped as possible and can transfer the force of the muscels to the water both should be about the same.

The longer and more complex anser is that it is impossible to know. Unless two species happen to have converged near perfectly apart from that one feature (lateral vs vertical power delivery) then they cannot be properly compared. Different organisms are adapted for diffrent things - efficiency, speed, control, agility, a specific body plan etc. - just compare a sunfish with an eel and a tuna! How would you tell which is 'best' or the 'most efficient', they are all doing different things in different ways, so how do you make a fair comparison? Any differences you would find could be attributed to body shape and style of locomotion rather than a fundamental difference in efficiency over power delivery and it would be pretty much impossible to separate the two.

So in short, there is no reason to expect one to be better than the other and no obvious reason to porevcent them being equally effective, but it might be impossible to test.

I'd just like to jump in and say that robotics has come a long way, it would be relatively easy to experimentally test the hydrodynamic implications of lateral vs vertical power delivery by using robots.

The one possible advantage of horizontal power delivery that I can see is the ability to use boundary effects when swimming near the surface, this should slightly increase swimming efficiency. I'm sure this is not the reason for marine mammals to use the horizontal system - that seems to be based on existing adaptations of the spine for terrestrial locomotion with an upright stance.

I've always wondered why ichthyosaurs didn't use a horizontal system, but I suppose their ancestors had a sprawling stance and maintained lateral undulation as their mode of swimming, like other lizards (e.g. marine iguanas) - does anyone have know of the likely ancestry of the ichthyosaurs?

Last edited by Paolo Viscardi (17th Mar 2008 07:57:14)

I think the ancestry question over icthyosaurs is still debateable in terms of sister-taxa or direct ancestors. However, in wider terms, other close relatives would have still held the lateral body movement seen in lizards and crocodiles, so like dolphin it is just a conservation of movement rather than a specifi drive towards laterla over vertical.

I am not sure why a 'vertical' animal could use boundary effects that a 'lateral' could not? What do you mean?

The boundary effect (also known as the ground effect) is an aero/hydrodynamic feature whereby flow speed/pressure (related) varies near a boundary (eg. air-land, air-water, water-sand). Since pressure differences are what provide the propulsive forces in a fluid, these boundary effects enhance efficiency by providing greater differences in pressure than would be achieved in an unbounded area of the fluid. It's one of the reasons that birds like cormorants will fly low over calm water - they are benefitting from this boundary effect.

Of course, to utilise this effect the aero/hydrofoil needs to be aligned parallel to the boundary,since if it was at right-angles there would be no difference in pressure between the two sides of the aero/hydrofoil.

I'm sure that some benefit is gained simply from the shape of fish, but it would be a small benefit compared to the effect of the propulsive organ being aligned to the boundary.

I expect that rays benefit from boundary effects from the seabed.