Interspecific allometry of locomotor morphology, gait kinematics and energy metabolism is well described, but the underlying reasons for these patterns are not yet fully understood. Between disparate species, a plethora of confounding morphological and physiological variables precludes identifying the independent effects of body size, particular morphological characteristics or gait kinematics on metabolic costs. The main objective of my thesis is to elucidate the links between these integrated components of locomotion by exploiting a range in morphological and physiological similarities and differences in a single species to have resulted from selective breeding.My model species, the leghorn chicken (Gallus gallus domesticus), is selectively bred for egg laying productivity and varieties of different body size which are physiologically and geometrically similar: standard (large) and bantam (small). At sexual maturity, leghorns also exhibit male-biased sexual size dimorphism and males and females differentially bias tissue allocation to pelvic limb muscle and viscera/reproductive organs, respectively.Using respirometry measurements, I first demonstrate that between males of standard and bantam leghorn varieties, the cost of transport is independent of body mass. Using limb posture, muscle architecture and kinematic measurements, I provide the first empirical evidence in support of a potential mechanism behind a lack of scaling in the cost if transport at the intraspecific level.I also provide the first evidence of a greater metabolic cost of transport in a male compared to a female in any species. I demonstrate that the sexual dimorphism in energy metabolism cannot be accounted for by comparison at dynamically similar speeds, limb posture, muscle architectural properties or the rate of muscle force generation. Furthermore, by comparing with a younger cohort of standards, I demonstrate that sex differences in locomotor energy metabolism manifest before the onset of sexual maturity and must, therefore, result from alternative sub-organismal sexual dimorphisms. I suggest that these differences may relate to physiological specialisation for to economical egg-load carriage and performance in inter-male aggressive combat. I also provide the first detailed comparison of the gait dynamics of the sexes in any species. I demonstrate that female leghorns walk with proportionally more foot-ground contact than their conspecific males. Furthermore, these kinematic differences are concomitant with the increase in anatomical mass at sexual maturity loading the swing and stance muscles of males and females, respectively. Perhaps the most striking finding in this thesis is that, while small cursosrial birds generally walk with crouched limbs, female standard leghorns walked with upright limbs. Just as larger species tend to have upright limbs to support a disproportionate amount of weight, compared to smaller more crouched species, these females may do the same due to artificial selection for increased egg production. Together, the locomotor measurements of the leghorns presented this thesis appear to support a recent hypothesis, that animals select postures and gaits to optimize muscle mechanical work and power demands and minimize active muscle volume.