We know a human cannot run on water, but could a much stronger and faster animal?

Question

We know a human cannot run on water, but could a much stronger and faster animal?

This viral video shows a moose running across a body of water:

https://www.youtube.com/watch?v=K5-0d00hV1c

Some say the video is fake. Others think the water must be shallow. We probably won't resolve these issues here.

My question is: could this actually happen (in deep water)?

I imagine that in theory, a sufficiently strong animal could kick the water really hard and if it moved across the surface really fast, it could stay above the water.

But could a real animal with a moose's strength and weight do it?

Answer 1

There are three (that I can think of) ways for water to supply an upwards force on something.

1. Buoyancy
2. Surface tension
3. "Thrust" by pushing off of the water

Buoyancy is out. The buoyant force is equal to the weight of the water displaced by the object. Since we are looking at moving across the water's surface, this means that not much water is being displaced: a moose's hooves are not large enough to displace a moose's weight in water.

Surface tension is also out. According to Wikipedia, the surface tension of water is around 71.97 dyn/cm (0.07 N/m), which is insanely small. This is why you only see insects being able to float along the water's surface through surface tension. Additionally, running across the water will probably break any sort of surface tension.

The "thrust" provided by pushing off of the water is also out. First, this depends on the surface area of the hooves as they push off the water, which is not very substantial. Second, based on estimates from this post even for a human this would be impossible. To compare to an animal that can actually do this: the "Jesus Lizard". The details are in the article, but basically this animal has the aid of lighter weight and more surface area and can still only run 10-20 meters before sinking. Third, specifically about the video, the moose is not really pushing off against the water. It's shown to just be trotting, which is definitely not producing enough force to support its weight.

So no, this is not possible for a moose.

Answer 2

I did some calculations based on the this article referenced earlier. (If you still can't open the link in Firefox, it's a bug)

The main concern appears to be the required power output.

To stay above the water, you need, roughly

$ m g=1/3\rho A v^2 $

where $A$ is the area of your foot step, $v$ is the speed of your foot kicking the water, $\rho$ is water density.

And the power output required is $p = m g v$.

The power output an animal is capable of is roughly proportional to its mass, so we want to look at the required $p/m$, which, from the above, is

$ p/m = g \sqrt{m g/(1/3\rho A)} \sim \sqrt{m/A} $

Now we see that the required power output needed per unit mass is roughly proportional to the square root of the size of the animal (assuming the shape is the same) The bigger you are, the harder it is to run on water. So, if a human cannot do it, a moose will find it even harder.

Answer 3

Yes, some human sized animals can run for a few seconds over water - but not humans nor moose.

As other answers pointed out, the animal would need fins and a large power output - that is, large muscles arranged in a way that a large part of its power can be directed to efficiently move the fins.

Dolphins have both, and dolphin running over water (that is moving horizontally on water while keeping their body vertical and nearly all of it out of the water) is a trick usually seen in dolphin shows. It's often called "tail-walking".

(image source)

It is even clearer in video. For example, see https://www.youtube.com/watch?v=A3tbypo870c

Answer 4

A body of water can be crossed, using the thrust from pushing water down. People sometimes cross water in this manner, riding a snowmobile. It is somewhat risky, loose power and the snowmobile sinks. Large animals cannot do this, as they produce too little power. (About 1hp for horse-sized animals). And even if they were much stronger, their feet have a considerably smaller area than a snowmobile's belt. Animal feet kick through the water, instead of forcing large amounts down.

Answer 5

In order to stay on top of water, without using buoyancy forces or surface tension forces, you need to apply a force to the water that counters the gravitational force.

Say a 70 kg human would need to apply a force $F$ of around ~700 N. This human can do that by continuously downwards accelerating $\Phi$ kg of water per second to a speed of $v$ meters per second requiring $P$ Watts.

$$v = \frac{F}{\Phi}$$ $$P = \frac{F^2}{2\Phi}$$

Say a human would be able to apply 250 Watts and needs to do this with a force of 700N then they need to apply this power to accelerate 1 ton of water per second.

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Humans can do this! They only need some help from a device that can help them to apply this force to so much water.

Image from Urs Gaudenz posted on Wikipedia

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Some animals can do it without help of tools. Such as this little one:

Youtube-video

And you have the many water birds that do this.

Answer 6

I must object to the basic premise--humans can run on water--just not at Earth's gravity. (At 10% simulated gravity every test subject could do it, down to only 1 of them at 22%.)

It's basically how fast the feet hit and how big they are. The only way you're going to get much faster is by going smaller (super fast animals accomplish it mostly by long strides, not by moving the limbs faster) and making the feet bigger. (That's how the lizard that runs on water does it.)

Answer 7

Humans, always overlooking the world of the small. It is as vast as the big world, hence strength is relative. So the answer is: insects. As long as they are not winged, etc. which of course can be sure death-trapping appendages, many insects are too light to break the surface tension of water. Yet if you want to narrow it down to strength, one of the mightiest creatures on Earth are: ants.