Morphological variation does not influence locomotor performance within a cohort of hatchling lizards (Amphibolurus muricatus, Agamidae)

A causal link between morphology and performance is a central tenet of ecomorphological analyses, but there are few detailed analyses of exactly how morphological variation within a hatchling cohort maps onto locomotor performance, and especially whether or not different tasks favour different morphologies (or vice versa). We measured morphological traits (including body length, mass, head size, limb proportions and fluctuating asymmetry [FA]) on a large sample of laboratory-incubated hatchling lizards (Amphibolurus muricatus, Agamidae), and used principal component analysis to reduce this data set to four major axes of variation (size, shape and two FA axes). Running speeds of each lizard were measured on raceways at four inclines, from level (0°) through to steep (45°). Unsurprisingly, steeper inclines reduced locomotor speeds. Absolute body size was the only morphological trait that was consistently related to sprinting performance, and the relationships were similar at each incline. Within-cohort variation in body shape and FA among this large sample was unrelated to locomotor speeds, thus challenging the common assumption of a causal link between these variables. The only exception was a weak trend for greater hind limb length to enhance locomotor performance more at steep inclines than at shallower angles. In general, our data suggest that different morphological traits do not differentially maximize locomotor performance up variable inclines. Overall, our data provide a cautionary note about the generality of causal connections between within-cohort morphological variation and locomotor performance under different environmental contexts.