Fig. 3. Fractional increases in net metabolic rate at different speeds in trunk-loaded guinea fowl. Net metabolic rates were calculated by subtracting the resting metabolic rate of the bird sitting quietly in a darkened box on the treadmill from the rates measured at the different speeds. Fractional increases were then calculated by subtracting the unloaded net metabolic rate from the loaded net metabolic rate and dividing by the unloaded net metabolic rate. The broken line indicates the fractional increase in total mass (body mass plus load) produced by loading the trunk. Values are means ± 1 s.e.m. (N=6).