Fig. 10. Mean metabolic response to trunk loading or to head-supported loads in walking mammals and guinea fowl. The ratio of net loaded to net unloaded metabolic rate is plotted as a function of the ratio of loaded (body mass plus load) to unloaded body mass (body mass only). The solid lines with a slope of 1.0 are included for reference. (A) All the data. (B) Measurements in which the mass ratio was less than 1.5 times the unloaded body mass. Solid circles, five human studies with the load applied in back packs (Soule et al., 1978; Pierrynowski et al., 1981; Duggan and Haisman, 1992; Lloyd and Cooke, 2000; Quesada et al., 2000). Open circles, human data for loads carried in a waist pack (Griffin et al., 2003). Open inverted triangles, two studies of human males carrying loads on their heads: American men (Soule and Goldman, 1969); Indian men (Datta et al., 1975). Open squares enclosing x, African women carrying head loads (Jones, 1989). Open circles enclosing +, Napalese porters carrying loads resting on their backs with a tump line around the head (Bastien et al., 2005) (G. J. Bastien and N. C. Heglund, personal communication). Open circles enclosing x, two studies of children carrying back packs (Hong et al., 2000; Merati et al., 2001). Plus signs, large quadrupeds (horse, Brahman cattle and water buffalo) (Lawrence and Stibbards, 1990). The asterisk indicates the ratio for guinea fowl walking at 0.5 m s^-1. Net metabolic rates were calculated as the active metabolic rate minus the resting rate. For human studies in which resting metabolic rate was not given, we used an approximate value of 1.5 W kg^-1, which was based on the available data for standing humans in references cited here.