Size matters when it comes to speed

How fast should an animal be able to move? And why are the biggest animals, which pack more muscle, not the fastest? That’s what Yale scientist Walter Jetz was wondering, so he and his colleagues looked at hundreds of animal species and have come up with a new theory that successfully puts a speed limit on most species…

  • There is a theoretical maximum speed that is expected to increase with body size,  however, in order to actually get to any speed you need to first accelerate, and larger animals take much longer to do so – much like a truck accelerating to 60mph compared to a motorbike or car.
  • Large bodied animals simply do not have sufficient energy to reach their theoretical maximum speed.
  • The general distribution is a ‘hump-shape’ as shown in the plots below. Maximum speed increases with size until we reach a critical mass beyond which the maximum speed reached starts to decrease.

screen shot 2019-01-24 at 10.59.30

  • Data for over 450 species were included in the study, across land, air and water.
  • The study provides insight into evolutionary trade-offs for different species as they evolve to increase their chances of survival.

You can listen to the full interview with the Naked Scientists here.

Image copyright Dawn Key


Thick and sticky fluids

The fourth puzzle in the new feature from Tom Rocks Maths – check out the question below and send your answers to @tomrocksmaths on TwitterFacebook, Instagram or via the contact form on my website. The answer to the last puzzle can be found here.

Viscosity is a property of a fluid on the molecular scale and is a measure of the strength of the internal friction between fluid particles. What this means in practice is that the thicker and stickier the fluid, the higher its viscosity.

Your task in this week’s puzzle is to order the six fluids below by their viscosity, lowest first. The answer will be posted in 2 weeks along with the next puzzle – good luck!

WARNING: answer below image so scroll slowly to avoid revealing it accidentally.



3. Air 1.81 x 10-5 [Pa s]

2. Water 8.9 x 10-4

1. Blood 3 x 10-3 

6. Honey 2-10 [Pa s]

5. Ketchup 50-100 [Pa s]

4. Peanut butter 250 [Pa s]

Spinning a Giant Fish Tank

Yes, you read that correctly. I really did spend a good chunk of the 4 years of my PhD spinning a giant fish tank. This isn’t just any old spinning fish tank though, as you may have guessed, it was specially designed to represent the real-world scenario of a river discharging into the ocean, but on a manageable scale in the lab. So what does such a setup look like? Well, below is a diagram of the tank, taken from my thesis (please excuse my crude drawing in Microsoft Word – it’s harder than you might think).

Let’s start with the tank itself. It’s made from acrylic (basically plastic) and is 1m x 1m with a depth of around 60cm, though it was only filled to 40cm with salt water. As you can see in the diagram, the tank is actually divided up into two sections by an internal barrier. The reason for this was to allow the source structure to be attached to the outside of the tank – I wasn’t allowed to drill holes into the actual tank despite all of my protests… It had to be attached in this way to allow the freshwater from the model river to flow into the main tank which contained the salt water in the model ocean.


The freshwater is stored in the reservoir (a plastic box) attached to the top of the metal structure surrounding the whole setup. This is to provide stability once the whole thing starts rotating, and also I assume to prevent things from flying off and hitting me. The river water flows out of the reservoir down a pipe and enters into the source structure. The amount of water released is controlled by a flow meter and an electronic switch. Once in the source structure, the water begins to accumulate until it resembles a flowing river and then it is released into the ocean (saltwater ambient). The water continues to flow throughout an experiment, much like a real river.

This all happens of course whilst the whole thing is spinning on a giant turntable. Turntable is an appropriate name because it basically just looks like a giant set of DJ decks (with only one disc a metre across). The speed is controlled by a computer and it can go pretty fast, trust me. Before the electronic switch was installed to start and stop the flow of freshwater from the reservoir, I used to have to climb up a ladder and flick the switch manually as it went spinning past. This was fine for the low speeds, but once things started speeding up I couldn’t flick the switch without knocking the entire structure which disturbed all of the measurements I had made. The only answer was to basically climb onto the structure myself and hang off the side whilst it went spinning round at high speed. If anyone ever saw me doing that I’m pretty sure I would have been thrown out of the lab, but hey when science calls ‘you gotta do what you gotta do’.


Now the source structure (shown in the diagram and picture above) was quite the piece of engineering… by which I mean an absolute nightmare to design. Most of the first year of my PhD was spent trying out different designs until finally we found one that gave a discharge that looked like an actual river. If the stream comes out too strong then it looks like a jet – think about squirting a hose in your garden. If it’s not strong enough then it’s just a point source – like a sponge slowly leaking out water. Neither of which represent a river. The trick was to feed the pipe carrying the freshwater into an l-shaped box filled with foam. The combination of the shape, plus the presence of the foam, meant that the box would become sufficiently filled with water before any of it exited into the ocean. It was key that the box filled up above the depth of the opening into the saltwater, so that the depth of the water when it left the source was known (and equal to the depth of the opening). We need to know this initial depth because the depth of the freshwater as it enters into the ocean is incredibly important in determining the properties of the current that forms, but that ladies and gentleman is another story for another day.


All of the articles explaining my PhD thesis can be found here.

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