Abstract
The lizard family Phrynosomatidae comprises three subclades: the
closely related sand and horned lizards, and their relatives the Sceloporus
group. This family exhibits great variation in ecology, behavior,
and general body plan. Previous studies also show that this family
exhibits great diversity in locomotor performance abilities; as measured
on a high-speed treadmill, sand lizards are exceptionally fast sprinters,
members of the Sceloporus group 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 ATPase, an indicator
of fast-twitch capacity, and succinic dehydrogenase, denoting oxidative
capacity, we classified fiber types into three categories based on existing
nomenclature: fast-twitch glycolytic (FG), fast-twitch oxidative-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 56-66%), and members
of the Sceloporus group are intermediate for both FG (41-48%) and FOG (42-45%)
content. Hence, across all 11 species, %FOG and %FG are strongly
negatively correlated. Analysis with phylogenetically independent
contrasts indicates that this negative relationship is entirely attributable
to the divergence between sand and horned lizards. The %SO also varies
among the three subclades. Results from conventional nested ANCOVA
(with log body mass as a covariate) indicate that the log mean cross-sectional
area of individual muscle fibers differs among species, and is positively
correlated with body mass across species, but does not differ significantly
among subclades. The log cross-sectional area of the IF varies among
species, but does not vary among subclades. Conversely, the total
thigh muscle cross-sectional area does not vary among species, but does
vary among subclades; horned lizards have slimmer thighs. Muscle
fiber-type composition appears to form part of a coadapted suite of traits,
along with relative limb and muscle sizes, that affect the locomotor abilities
of phrynosomatid lizards.