Evolution of maximal endurance capacity: Natural and sexual selection across age classes in a lizard

Hypothesis: One of the basic tenets of evolutionary physiology is that physical performances and fitness are tightly linked. Question: Are phenotypes with exceptional locomotor capacity strongly favoured by natural and sexual selection? Organism: A ground-dwelling, actively foraging and non-territorial lizard species, Lacerta vivipara. Methods: We analysed the relationship between morphology (body size and condition) and maximal endurance capacity in three age classes (juveniles, yearlings, and adult males). We then tested whether morphology and endurance capacity predicted variation in annual body growth, annual survival, and reproductive success. Results: The large variation in maximal endurance capacity observed at hatching has a genetic basis. Endurance capacity increased with body size in juveniles and with body condition in juveniles and yearlings. Endurance capacity was not correlated with annual body growth at any age class. Positive, directional viability selection on endurance capacity was detected for juveniles and yearlings, but not for adult males. Endurance capacity was weakly, positively correlated with male reproductive success. Natural selection in juveniles and sexual selection in adult males was non-linear and the strength of selection decelerated with endurance capacity. Conclusion: In the common lizard, selection on maximal performances is non-linear and varies between age classes. This pattern of weak and inconsistent selection could explain the maintenance of considerable genetic variation of locomotor performance within populations.