It is widely assumed that in the animal kingdom faster means better. Speed often implies that an animal is better at escaping from predators, and capturing prey. Indeed, a growing number of studies are linking whole-organism performance traits, such as locomotor speed, to an animal's probability of survival. This suggests that natural selection acts on such traits if they are heritable, which they often seem to be. Recent work by Jerry Husak and colleagues at Oklahoma State University uses collared lizards in a novel approach to understanding the evolution of locomotor performance by addressing whether sexual selection might also shape traits such as running speed. In particular, the research team examined whether reproductive success was positively related to sprint speed in these lizards. If so, this would provide evidence that sexual selection might play a critical, if underappreciated, role in the evolution of locomotor performance in these animals.

To understand the relationship between locomotion and reproductive success, adult and yearling lizards were captured in a small region of Pawnee County, Oklahoma, USA, during the animals' breeding seasons in 2003 and 2004. Captured lizards were first brought into the lab and carefully measured for traits that could impact running speed such as body size and limb length. On days 2 and 3 of captivity, the researchers removed the lizards from their heated chambers and chased them six times along a 3-m trackway. Using video footage of the chases the team calculated the fastest one-meter split from each of the six runs for every lizard, calculating each animal's maximum sprint speed. They also collected blood samples and toe clippings from each lizard, so that they could use the DNA for paternity analyses. They targeted a number of sections, or loci, in the DNA samples to determine which adult males were most likely to be the fathers of the yearling lizards that they had also collected.

By comparing adult sprint speed with the results from the paternity tests, the team found that faster sprinters fathered more offspring. But was this caused by body size or leg length? They found that while larger lizards didn't run faster, longer limbs led to faster lizards. To try and determine what other factors might explain the fast lizards' reproductive success, the team observed the animals in their natural habitat and found that faster runners had larger territories, which could lead to more mating opportunities. They also examined blood testosterone levels to make sure that higher sprint speeds weren't caused by higher levels of this hormone. They found that there was no relationship between blood testosterone and sprint speed, offspring numbers or territory size. In fact no measured trait except maximum sprint speed predicted both reproductive success and territory size in these lizards, suggesting a direct link between running performance and fitness.

Why might faster sprint speeds lead to higher numbers of offspring in these lizards? The link between higher speeds and larger territory size hints that faster males may defend their territories better. Males with larger, better defended territories can attract more females. Faster is better indeed! As more biologists begin to consider links between performance traits and reproductive success, the relative importance of sexual versus natural selection in shaping animal form and function will become clearer.