Anti-predatory behaviour evolves under strong action of natural selection because the success of individuals avoiding predation essentially defines their fitness. Choice of anti-predatory strategies is defined by prey characteristics as well as environmental temperature. An additional dimension often relegated in this multilevel equation is the ontogenetic component. In tegus Salvator merianae, adults run away from predators at high temperatures but prefer fighting when it is cold, while juveniles exhibit the same flight strategy within a wide thermal range. Here we integrate physiology and morphology to understand ontogenetic variation in the temperature-dependent shift of anti-predatory behaviour in these lizards. We compiled data for body shape and size and quantified enzyme activities in hindlimb and head muscles, testing the hypothesis that morpho-physiological models explain ontogenetic variation in behavioural associations. Our prediction is that juveniles exhibit body shape and muscle biochemistry that enhance flight strategies. We identified biochemical differences between muscles residing mostly on the LDH: CS ratio, being hindlimb muscles more glycolytic than jaw musculature. Juveniles, which often use evasive strategies to avoid predation, have more glycolytic hindlimb muscles and are much smaller when compared to adults being 1-2 years old. Ontogenetic differences in body shape were identified but marginally contributed for behavioural variation between juvenile and adult tegus, and variation in anti-predatory behaviour in these lizards reside mainly on associations integrating body size and muscle biochemistry. Our results are discussed in the ecological context of predator avoidance by individuals differing in body size living at temperature-variable environments, where restrictions imposed by the cold seem compensated in specific phenotypes.