Larger lacertid lizard species produce higher than expected iliotibialis muscle power output; the evolution of muscle contractile mechanics with body size in lacertid lizards

Abstract

Increases in body size can lead to alterations in morphology, physiology, locomotor performance and behavior of animals. Most studies considering the effects of scaling on muscle performance have studied within species effects, with few studies considering differences between species. A previous review of published data indicated that maximum muscle shortening velocity decreases, but that maximum isometric stress does not change, with increased body mass across species of terrestrial animals. However, such previous analyses have not accounted for the phylogenetic relatedness of the species studied. Our aim was to use phylogenetically informed analysis to determine the effects of body size on isolated iliotibialis muscle performance across 17 species of lacertid lizards. Between one and five individuals were used to obtain mean performance values for each species. We analysed the relationship between each variable and body size, as estimated by snout-vent length (SVL), whilst taking into account the phylogenetic relationships between species. We found that isometric tetanus relaxation time, maximal tetanus stress (force per muscle cross-sectional area) and maximal work loop power output (normalized to muscle mass) all significantly increased with greater SVL. In contrast, fatigue resistance during repeated work loops significantly decreased with SVL and there was no effect of size on tetanus activation time. If we compare our findings to those that would be predicted by dynamic similarity, then as these lacertid species become bigger there is a greater than expected increase in the normalized muscle power output, likely to counter the larger than expected increase in body mass.