Typically, the faster you want to go, the more energy you have to burn. But it’s not a simple relationship. I can walk home from work, or I can run. But if I’m in a hurry, but not too big of a hurry, is it easier to walk fast, or run slow? Have you ever tried walking as fast as you can? It’s hard! It’s often easier to speed up a little and switch to a slow jog.
The relationship between speed and energy gets even more complicated when you start to look across other animal species. The patterns are different for large quadrupeds versus large birds. Large running birds, like ostriches, have been studied fairly well, but small running birds haven’t been. Small running birds like Lagopus muta here:
This is a ptarmigan, a wide-ranging ground-dweller of a bird. Nudds and colleagues were interested in how these small birds used energy during locomotion, so they put them in treadmills, measured how much oxygen they breathed as they ran, and filmed them so they could see when they were walking or running.
The running is a little complicated, because there are two ways that ptarmigans run. There’s grounded running, where the foot is on the ground more than 50% of the time, and aerial running, where the foot is on the ground less than 50% of the time. It’s faster than grounded running. Buried in the methods is a passing comment that aerial running is also called...
Groucho running. And people think scientists have no sense of humour.
The results are fairly simple to sum up.
The faster the ptarmigans moved, the cheaper it got for them.
Walking used more energy per distance moved than grounded running, which used more energy per distance moved than aerial running.
Ptarmigans are the first birds to show this counter-intuitive result, but they’re not the first animal to show this. Kangaroos show something similar, with the typical explanation that kangaroos are able to use their tendons like springs to store energy. The ptarmigans have some leg tendons that might contribute here, but how big a factor that is remains to be seen.
There are hints of similar patterns of increasing efficiency with increasing speed in roadrunners, but the data there aren’t conclusive. This means either:
- Many small running birds may run more efficiently than they walk, or;
- Ptarmigans alone can run like a boss.
Regardless of how general the ptarmigan pattern is for small birds, it’s still rather different than most other animals. Even the similarity between kangaroos isn’t very strong, because kangaroo hopping isn’t very similar in form to ptarmigan running.
Why would ptarmigans ever walk or use grounded running? It’s easy to see the advantage of moving slowly for walking. You may want to avoid rapid movement to avoid injury or to avoid drawing attention to yourself.
It’s trickier to know when the animals might prefer to use grounded running, which seems to be the worst of both worlds: slow and more costly than aerial running. An answer to that question probably won’t come from lab studies. It will probably require researchers to go into the field and watch a lot of ptarmigan locomotion.
P.S.—Yes, I wrote the entire post just because I wanted to use that silly ptitle.
Nudds R, Folkow L, Lees J, Tickle P, Stokkan K, & Codd J. 2011. Evidence for energy savings from aerial running in the Svalbard rock ptarmigan (Lagopus muta hyperborea). Proceedings of the Royal Society B: Biological Sciences. DOI: 10.1098/rspb.2010.2742
Photo by omarrun on Flickr; used under a Creative Commons license.