Bigger animals 'aren't always faster'

"Typically, bigger animals tend to run faster than smaller animals, because they have longer legs. But this only works up to a point. The fastest land animal is neither the biggest nor the smallest, but something in between.

"Think about the size of an elephant, a mouse and a cheetah," Christofer J Clemente, who led the research team, said in the 'Physiological and Biochemical Zoology' journal.

For their research, the team studied monitor lizards to show that that the same principle applies within species as well as across species, and to identify why this is the case.

Because adult monitor lizards vary substantially in size, they are an ideal species for testing how size affects speed. The researchers timed and photographed monitors ranging from two to 12 pounds, as sprinted across a 45-foot track.

The researchers found that the midsize lizards were fastest -- and they discovered why.

Using high-speed cameras and markers placed at key spots on the lizards' bodies, the researchers created computer models comparing characteristics of lizards' running strides.

"We then looked at how the mechanics of the stride changed with body size, and we found that the changes in the stride were consistent with the changes in speed. Above a certain size, lizards were changing the way they ran, perhaps due to a decreased ability of the bones and muscles to support a larger body mass," Clemente said.

The results bolster the hypothesis that large size creates biomechanical constraints, say the researchers.