For many people in Western societies running is an activity that is confined to the gym or watched on television! But running may be a matter of life or death for many terrestrial animals. Running performance is thought to influence an animal's ability to capture prey or to avoid becoming prey itself. The ability to run for longer or faster than others is therefore likely to be extremely favourable, and natural selection would be assumed to produce animals with increasing endurance and speed. A day out at the races watching artificially selected thoroughbred horses reveals the potential improvements that can be made to running performance. Why, then, do large amounts of variation in running ability persist in natural populations of many animals?

To answer this question, Jean-François Le Galliard and his co-workers chose to examine running performance in the common lizard, Lacerta vivipara. They expected endurance running to affect common lizards' survival because running ability is thought to influence social status and competition for basking sites or prey. The team tested the lizards' initial endurance one day after birth by making the youngsters run on a treadmill until they were exhausted. Most lizards could only manage about 200 seconds of running, although some showed exceptional endurance (over 1600 seconds) that even Olympic marathon runners would be proud of. This endurance (or lack of it) was highly heritable. But does this endurance actually affect lizards' survival? To find out, the team released the lizards into enclosures and recaptured them to see how many had survived after one month and after one year. The recaptured lizards revealed that initial endurance did affect survival; low initial endurance lizards were less likely than high initial endurance lizards to be alive a month or a year later. So the weaker lizards had been eliminated by natural selection, but there appeared to be only weak selection for high endurance.

It is surprising that stronger lizards did not have a clear survival advantage over weaker lizards, as we might expect the superior athletes to outperform the lower endurance lizards. Le Galliard and co-workers hypothesised that perhaps the initial differences in endurance measured one day after birth were not maintained as the lizards matured. One possible reason for this could be the availability of food – with plenty of food available, lizards with low initial endurance might improve their endurance by investing in more muscle, enabling them to overcome their disadvantage at birth. But when food is scarce, lizards with low initial endurance may be unable to invest in more muscle and the difference in initial endurance will be maintained. To see if this was the case, the team tested common lizards' initial endurance and then divided the lizards into two groups: they fed one group fully but restricted the other group's diet. As the team had expected, under dietary restriction the initial differences in endurance were maintained, whereas fully fed lizards with high initial endurance lost their original advantage, presumably because the abundance of food allowed the weaker lizards to catch up with the stronger lizards. This suggests that the effect of initial endurance upon survival in the common lizard is dependent upon early environmental conditions.

These results are clearly at odds with the idea that performance measured at birth is a good indicator of lifetime performance. It is likely that these findings are not limited to lizards but that they occur throughout the animal kingdom and may, at least partly, explain some of the variation observed in animal running performance.