Abstract
Lizards are a widely-used model system for studies
of locomotion, yet the morphological and physiological architecture underlying
interspecific variation in locomotor abilities is not fully understood.
The family Phrynosomatidae is particularly suitable for studies of locomotion
because it exhibits great variation in ecology, behavior, general body
plan, and locomotor performance abilities. Phrynosomatidae comprises
three subclades: the closely related sand and horned lizards, and
their relatives the Sceloporus group. Sand lizards are exceptionally
fast sprinters, members of the Sceloporus group demonstrate intermediate
sprint speeds, and horned lizards are slowest. Here, I show that
variation in muscle composition, in addition to relative limb length, predicts
this interspecific variation in sprint speed. I determined fiber-type
composition of the iliofibularis (as a representative muscle important
in hindlimb retraction). Using histochemical assays, I determined
3 fiber-types: fast-twitch glycolytic (FG), fast-twitch oxidative-glycolytic
(FOG), and slow-twitch oxidative (SO). Sand lizards had a high proportion
of FG fibers (64-70%) and a low proportion of FOG fibers (25-33%), horned
lizards had low FG (23-31%) and high FOG (56-72%), and members of the Sceloporus
group were intermediate for both. Hence, %FG and %FOG were strongly
negatively correlated among the 14 species of phrynosomatid examined.
Importantly, this negative relationship also holds across a broader sampling
of lizard taxa from 7 families (23 species total). Reconstruction
of ancestral trait values indicates especially large changes in fiber-type
composition during the evolution of horned lizards. Using phylogenetically
independent contrasts, and correcting for correlations with body-mass,
FG proportion was the strongest predictor of sprint speed, as measured
on a high-speed treadmill, among 14 species of phrynosomatid. Hindlimb
span was also a significant positive predictor. Among 23 species,
hindlimb span was the strongest speed predictor. FG proportion (positive)
and forelimb span (negative) were also significant predictors. Surprisingly,
speed and endurance abilities did not trade-off among species (14 or 23).
Additional morpho-physiological studies will be required to understand
the likely multiple pathways leading to functional equivalence, thereby
allowing circumvention of potential constraints on the adaptation of locomotor
abilities.