BONINE, K.E.*; GLEESON, T.T.; GARLAND, T. Jr.: Phrynosomatid Lizards Show Parallel Variation in Sprint Speed and Muscle Fiber-Type Properties
Phrynosomatids are comprised of 3 distinct subclades ((sand lizards, horned lizards), Sceloporus group)) that exhibit great variation in habitat, behavior, and general body plan. This family also varies widely in maximal sprint speed; sand lizards are exceptionally fast, horned lizards are slow, and the Sceloporus group is intermediate. These differences are paralleled by relative hindlimb span (Bonine and Garland 1999, J Zool Lond 248:255-265). We examined the iliofibularis (IF) muscle to determine if fiber-type composition varies similarly. Sampling 11 species (44 indivs.), we used histochemical assays for myosin ATPase (fast-twitch capacity) and succinic dehydrogenase (oxidative capacity) to classify fibers as fast-twitch glycolytic (FG), fast-twitch oxidative-glycolytic (FOG), or slow-twitch oxidative (SO). Sand lizards have a high proportion of FG fibers (mean 67%) and a low proportion of FOG (29%); horned lizards have lower FG (28%) and higher FOG (61%); members of the Sceloporus group, which are presumed to reflect the phrynosomatid ancestral state, are intermediate for both FG (45%) and FOG (44%). Across all 11 species, %FOG and %FG are strongly negatively correlated (r = -0.95). Analysis with phylogenetically independent contrasts indicates that this relationship is entirely attributable to the divergence between sand and horned lizards. Neither the proportional area of the IF in the hindlimb, nor the cross-sectional areas of FG (11 spp. mean 417 MUm2, s.e. adj. for body mass 13.8 [mean log body mass 1.0045]), FOG (260, 9.5), and SO (137, 12.5) fibers, differ among the 3 subclades. However, mass-adjusted hindlimb cross-sectional area is lower for horned lizards. [NSF Grad. Fellowship KEB, NSF IBN-9724140 TTG, IBN-9723758 TG]
