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  • College of Natural and Agricultural Sciences

Bonine Ph.D. Abstract Kevin E. Bonine
Ph.D. in Zoology 2001
Dissertation title:  Morphological and physiological predictors of lizard locomotor performance:
                           a phylogenetic analysis of trade-offs
v + 222 pages


    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.