Size matters when it comes to speed

Why aren't the biggest animals always the fastest?
25 July 2017

Interview with 

Walter Jetz, Yale University


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. Tom Crawford heard how it works...

Walter - 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 that’s where the crux is for the really large bodied animals. They need much, much longer time, given their larger size, to accelerate. That’s why it takes a really large truck longer to get up to 60 mph than it does a small car or, indeed, a motorbike. What we were able to show is that large bodied animals simply don’t have sufficient amounts of energy on board, if you will, to ever get there.

Tom - Yeah. In terms of maximum speeds, I notice that as you mentioned in general, the speed increases with the body mass and the size of the animal and then it begins to tail off and decreases with the large animals. It’s almost like a hump shape?

Walter - Correct. And it’s really fascinating that the relationship applies all the way down to insects well below a gram in size, and then reaches this peak at medium-to-large bodied animals, and then drops off. Our theory does a good job we believe in showing what is maximally possible. But then, how close you’re getting to that will really depend on what your particular evolutionary group that you’re in happens to have tried to maximise over the millions of years in which it has evolved.

If you think about a leopard or cheetah, they are actually smaller than us but they are, of course, the fastest land animals. There you are looking at a group that over millions of years evolved to outrun their prey.

Tom - How successfully did your theory fit the actual data.

Walter - The first author of the study, Miriam Hertz, spent a substantial time trawling through the literature, contacting experts and, in the end, we were able to pull data for about 450 species. Across those 450 species, both on land and air and in the water, we were able to predict the observed variation tremendously well with the theory that we have developed.

Tom - What can we hope to learn from a study such as this?

Walter - Movement is a very fundamental aspect of life. The theory that we developed helps provide base expectation about what is the maximum possible speed for animals of a given size. It offers up a really interesting comparison to then say how far in fact is a given species from that maximum possible speed. It can teach us a lot about the various trade offs that a species group may have gone through in the course of evolution.

Tom - Where do we fit into this as humans?

Walter - We humans are actually not too far above that sweet spot where there is enough time and energy given this body size to get close to the theoretically possible maximum speed.


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