Almost every person has walked or run on the beach at some point in their life. Moving on loose sand feels more challenging than walking on a sidewalk. In general, walking on sand requires more work and energy and this is not unique to humans. Now, you may also be familiar with Speedy Gonzales, a Looney Tunes cartoon mouse who runs super-fast all the time. You may wonder how Speedy can always run so fast across all the different terrains that he encounters, but a very similar looking animal to Speedy Gonzales in real life, the kangaroo rat (Dipodomys deserti), may prove that the cartoon's versatility is not so far off reality.

Some researchers have previously tested different types of animals, such as lizards and ostriches, running over solid surfaces versus sandy surfaces. This line of research motivated Joseph Hall (Washington State University, USA), Craig McGowan (University of Idaho, USA) and David Lin (Washington State University) to study a hopping animal, the kangaroo rat, to expand on our current understanding of the effects of different types of ground on animal movements. These little hopping animals are equally adept moving across rocky and sandy terrains, both of which exist in their natural desert habitat. So, the researchers cleverly developed the very first variable terrain rotatory treadmill and put these animals to the test hopping on a solid and a sandy surface while controlling the speed and recording video of the animals’ manoeuvres.

However, before the kangaroo rats were allowed to hop in place, the team had to ensure that the level of sand on the treadmill was sufficient to prevent the animal from touching the solid base beneath. To do this they designed a device that can measure how far the rodents’ feet would penetrate the sand and determined that at least 1.5 cm of sand was sufficient.

Although the six desert kangaroo rats had to move at five different speeds, ranging from 1.25 to 2.25 m s−1, Hall and colleagues only analysed the motion of the little rodents’ hind legs at 1.8 m s−1, the speed at which these animals tend to hop in the wild. Comparing the animals’ hopping styles on solid and sandy surfaces, they found no significant difference in the amount of time the animals’ feet were in contact with the ground or the distance covered during each hop. Also, the researchers found no meaningful difference in the rodent's speed and acceleration between the two different surfaces, although the animals were more crouched when hopping on sand.

Even though the environment has induced changes in the running styles of some animals, it appears that kangaroo rats have evolved so that changes in the surface they hop around on does not affect their speed. They maintain very similar physical hindlimb movements regardless of the firmness of the surface that they are moving over, suggesting that they are finely tuned to their environment. And even though Speedy Gonzales is a runner and kangaroo rats hop, the discovery that kangaroo rats are perfectly tuned to move on hard and soft surfaces could hold the key to Speedy's super-fast reputation.

Comparison between the kinematics for kangaroo rat hopping on a solid versus sand surface
R. Soc. Open Sci.