Using body condition index can be an unreliable indicator of fitness: a case of sand lizard Lacerta agilis Linnaeus, 1758 (Sauria: Lacertidae)

In ecological and morphological studies, researchers often use body condition index as a measure of an individual's fitness. Our research shows that body condition index may be an unreliable method for this purpose, because changes in lizards' body proportions are not stable during their lifetime. Results obtained from a Polish population of the sand lizard (Lacerta agilis) show that the body length increases faster than the body mass, meaning that the correlation between these 2 parameters is not linear. Herpetologists frequently use the body condition index to compare the fitness of lizards of different ages. Our study demonstrates that this method may not be accurate; in the future, research should strive to develop new ways of measuring lizards' fitness.