Many animals take the surfaces across which they move in their stride: some slow down if a surface is less to their liking, while others relish the same material and surge forward. ‘Species sometimes use different approaches for dealing with changes in the surfaces that they are running on’, says Philip Bergmann, from Clark University, USA, who was intrigued by how steppe-runner lizards cope when running on surfaces ranging from fine sand to gravel. However, he adds, ‘No one had really taken a systematic approach, looking at substrates composed of different particle sizes… and no one had studied the effects of the shape of the particles on locomotion’. As steppe-runner lizards have no specialised adaptions that allow them to favour one surface over another and they encounter a wide range of surfaces in their natural homes, Bergmann and Kyle Pettinelli decided to test the lizards’ sprinting prowess on surfaces ranging from 25 μm fine glass beads and sand to coarse gravel and 4 cm pebbles to find out how this running Jack of all trades adapts to different conditions.
Filming the lizards as they scampered along flat sand and gravel runways, Bergmann saw that some lizards were less enthusiastic athletes than others and required more encouragement to pull out their fastest performances. However, when he compared the lizards’ highest speed sprints, they ran fastest on the coarse sand (∼1.1 m s−1), increasing their stride rate rather than taking longer strides, in contrast to the lizards that sprinted on silt and pebbles, which ran at speeds just below 1 m s−1. The lizards also seemed to prefer running on natural surfaces rather than similarly sized smooth glass beads, and rarely recorded speeds of over 0.75 m s−1 when running on the glass beads.
Wondering why the lizards were more competent on the less uniform natural surfaces, Marian Crockett and Erika Schaper compared the density of the glass beads, sand and gravel, and the stability of the surfaces that they produce, and found that the coarse sand should shift least beneath the lizards’ feet as they sprinted over it.
‘Substrate particle size affects both how fast the lizards can run on them and how they move’, says Bergmann, who admits that he was surprised that the lizards performed so much better on coarse sand when they were thought to run competently on most surfaces. However, he suspects that the stable and even surface that is provided by the coarse sand may allow the lizards to produce their top performances, ‘unlike gravel, which makes for a bumpy and uneven surface’.
Having shown that particle size matters, even for an animal that does not seem to have any specialised adaptions for one particular surface over another, Bergmann is now keen to discover how sprinting lizards cope on more realistic terrains where sand, silt and gravel are mixed, and how they adapt when running up sand and gravel hills.
