Fig. 5. Cost of transport (J kg^–1 m^–1) decreases with body mass among >90 different species of birds and mammals (Langman et al., 1995; Taylor et al., 1982) but does not change with body mass within horses. For clarity, only the mouse and elephant data points are shown for the inter-specific relationship, although the line is derived from data representing >90 species. Open squares are mean values for each size range of horses from the present study and closed squares are mean values from the literature (N=3, Hoyt and Taylor, 1981; N=5, Eaton et al., 1995; N=4, Minetti et al., 1999; N=4, Pagan and Hintz, 1986; N=7, Potard et al., 1998). The cost of transport was calculated as the linear slope of the rate of oxygen consumption versus trotting speed for horses. We assumed an energetic equivalent of 20.1 J ml^–1 O₂. The scaling relationship for the horse data was calculated using a least-squares linear regression, following Taylor et al. (1982). The 95% confidence limits of the exponent for the horse line are ±0.138.