Comparative analysis of fiber-type composition in the lliofibularis muscleof Phrynosomatid lizards (Squamata)

The lizard family Phrynosomatidae comprises three subclades: the closely re lated sand and horned lizards, and their relatives the Sceloporus group. Th is family exhibits great variation in ecology, behavior, and general body p lan. Previous studies also show that this family exhibits great diversity i n locomotor performance abilities; as measured on a high-speed treadmill, s and lizards are exceptionally fast sprinters, members of the Sceloporus gro up are intermediate, and horned lizards are slowest. These differences are paralleled by differences in relative hindlimb span. To determine if muscle fiber-type composition also varies among the three subclades, we examined the iliofibularis (IF), a hindlimb muscle used in lizard locomotion, in 11 species of phrynosomatid lizards. Using histochemical assays for myosin ATP ase, an indicator of fast-twitch capacity, and succinic dehydrogenase, deno ting oxidative capacity, we classified fiber types into three categories ba sed on existing nomenclature: fast-twitch glycolytic (FG), fast-twitch oxid ative-glycolytic (FOG), and slow-twitch oxidative (SO). Sand lizards have a high proportion of FG fibers (64-70%) and a low proportion of FOG fibers ( 25-33%), horned lizards are the converse (FG fibers 25-31%, FOG fibers 56-6 6%), and members of the Sceloporus group are intermediate for both FG (41-4 8%) and FOG (42-45%) content. Hence, across all 11 species %FOG and %FG are strongly negatively correlated. Analysis with phylogenetically independent contrasts indicate that this negative relationship is entirely attributabl e to the divergence between sand and horned lizards. The %SO also varies am ong the three subclades. Results from conventional nested ANCOVA (with log body mass as a covariate) indicate that the log mean cross-sectional area o f individual muscle fibers differs among species and is positively correlat ed with body mass across species, but does not differ significantly among s ubclades. The log cross-sectional area of the IF vanes among species, but d oes not vary among subclades. Conversely, the total thigh muscle cross-sect ional area does not vary among species, but does vary among subclades; horn ed lizards have slimmer thighs. Muscle fiber-type composition appears to fo rm part of a coadapted suite of traits, along with relative limb and muscle sizes, that affect the locomotor abilities of phrynosomatid lizards. (C) 2 001 Wiley-Liss, Inc.