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First published online March 14, 2008
Journal of Experimental Biology 211, 1029-1040 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.015503

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The correlated evolution of biomechanics, gait and foraging mode in lizards

Eric J. McElroy^*, Kristin L. Hickey and Stephen M. Reilly

Ohio Center for Ecology and Evolutionary Studies and Department of Biological Sciences, Ohio University, Athens, OH 45701, USA

View larger version (12K): [in this window] [in a new window]   Fig. 1. Patterns of foraging mode evolution reconstructed on a molecular phylogeny for lizards (Townsend et al., 2004). Black branches (bold text) are sit-and-wait foragers, white branches are wide foragers, and foraging mode for Plestiodon ski. is unknown (grey). Note that WF evolved independently in the ground geckos, at the base of the Scincomorpha, and in the lineages leading to Varanus and the Lacertoidea. Foraging mode reconstruction (branch shading) is based on a larger sample of 110 species (Miles et al., 2007) and from Reilly and McBrayer (Reilly and McBrayer, 2007).

View larger version (31K): [in this window] [in a new window]   Fig. 2. (A–H) Locomotor gaits for lizards that only used running mechanics. Gait is expressed as limb phase vs duty factor (Hildebrand, 1976). Species and sample size are indicated on each panel. Note that by convention the numerical scale on each axis is reversed (Hildebrand, 1976).

View larger version (26K): [in this window] [in a new window]   Fig. 3. (A–G) Locomotor gaits for lizards that used both running and walking mechanics. Closed symbols are running mechanics; open symbols are walking mechanics. Species and sample sizes (bold, running; normal type, walking) are labelled as in Fig. 2.

View larger version (19K): [in this window] [in a new window]   Fig. 4. The evolution of locomotor biomechanics in relation to foraging mode in lizards. (A) Ancestral character reconstruction of phase shift values (from B) are mapped onto the lizard phylogeny from Townsend et al. (Townsend et al., 2004) in A. Branch shading indicates running (black), walking (white), and the grey branches leading to the base of the Gekkota and the Scincidae are equivocal for running or walking. (B) Raw phase shift data indicating running (closed symbols, phase shift 45) and walking (open symbols, phase shift 135) mechanics. (C) Reconstructed patterns of foraging mode evolution from Fig. 1 (black branches are SW lineages, white branches are WF lineages, hatched branch is unknown). Note that walking mechanics evolved each time WF evolved. Asterisks indicate species taken from the literature.

View larger version (17K): [in this window] [in a new window]   Fig. 5. Gaits (means ± s.e.m.) and multivariate differences between species during running (A, for all species) and walking (B, for WF) mechanics. Ellipses surround species means that are not significantly different. (A) When using running mechanics Tupinambis, Eumeces sch. and Eublepharis differed in using significantly larger high duty factor trots than the remaining species. (B) During walking mechanics species clustered into three statistically distinct gait groups. Ameiva and Acanthodactylus use a fast walking gait (lower duty factors) but diverge toward more lateral (lower limb phase) and diagonal (higher limb phase) sequence gaits, respectively. The remaining species clustered into a single group that uses a higher duty factor trotting gait while walking.

View larger version (10K): [in this window] [in a new window]   Fig. 6. Patterns of gait change when shifting from running to walking mechanics in WF lizards. White symbols, walking mechanics (W); black symbols, running mechanics (R). Solid cloud is the ancestral running gait cloud from Fig. 5A. When shifting from running to walking mechanics lizard exhibited four ways of changing position in gait space, based on MANOVAs comparing running to walking gaits for each species: (G1, broken arrows) Varanus and Tracheloptychus switch from the ancestral trotting run to a higher duty factor and lower limb phase trot while walking, (G2, solid arrow) Ameiva exhibits small shifts in limb phase and duty factor in a lateral sequence trot. (G3, dotted arrow) Acanthodactylus maintained a fast diagonal sequence trot and (G4, stippled cloud) Tupinambis, Eumeces sch. and Eublepharis maintained a slow speed high duty factor trot when switching from running to walking.

View larger version (15K): [in this window] [in a new window]   Fig. 7. Evolutionary patterns of foraging locomotor biomechanics and gait in relation to foraging mode in lizards (Fig. 1). From the ancestral condition of sit-and-wait foraging (black branches) with running mechanics (RUN) and a trotting gait (G:TROT), lizards have evolved walking mechanics (WALK) in concert with wide foraging (white branches) several times. Wide foraging species exhibit one of 4 patterns of gait shift (G1, G2, G3, G4, from Fig. 6) involving different shifts in limb phase ( or LP) and duty factor ( or DF). Note that walking mechanics was lost (WALK crossed out) each time foraging mode underwent an evolutionary reversal to SW.