Locomotor economy and the origin of bipedality: reply to Steudel-Numbers.

We are pleased to have the opportunity to respond to Dr. Steudel-Numbers’ comments on our earlier work regarding the energetics of hominid bipedality. Energetic models are becoming increasingly important in the study of hominid evolution (e.g., Aiello and Wheeler, 1995; Elton et al., 1998; Leonard and Robertson, 1992, 1994, 1997a,b; Steudel, 1994, 1996). Consequently, ongoing discussion is needed on how to best apply information on comparative energy metabolism to address evolutionary questions. Steudel-Numbers is critical of our use of World Health Organization (FAO/WHO/UNU, 1985) norms (derived from McDonald, 1961) for determining energy costs of human locomotion. Some of this criticism is based on her misunderstanding of the methods of respiratory and metabolic physiology. Specifically, she refers to the Douglas bag technique as being a “closed circuit” method of calorimetry because the expired air samples are being collected. In fact, “open” vs. “closed” circuit refers to whether the inspired air is taken from the open environment (open circuit) or whether the system is closed off from external air with subjects rebreathing from a pre-filled container (spirometer) of oxygen (closed circuit) (see Consolazio et al., 1963). The Douglas bag technique is thus an open circuit method of calorimetry. Various open circuit methods can be used to measure energy expenditure under field and laboratory conditions. Portable respirometers (such as the Kofranyi-Michelis (K-M) meter, and the more recently developed Oxylog, Cosmed K4, and Aerosport KB1-C and VO2000 units) allow for the measurement of energy expenditure while subjects are freely moving. McDonald (1961, p. 741) notes that portable respirometers were used in some of the studies he compiled. Research with these portable meters has shown that the cost of wearing the units is very small. Consolazio et al. (1963), for example, found that the average energy costs of walking on a treadmill with the K-M meter at speeds of 2.0 and 3.5 miles per hour were not significantly different from those obtained without wearing the meter. Similar results have been obtained with the more recently developed portable units, with differences in energy expenditure relative to the Douglas bag method being within 62–3% (McNeill et al., 1987; King et al., 1999; McLaughlin et al., 2001). Overall, although somewhat different research protocols were used in the studies compiled by the WHO, the alternative methods for studying respiratory metabolism are well-established and have been shown to produce reliable and comparable results (Consolazio et al., 1963; McLean and Tobin, 1988). The criticism that the different studies drawn on for the WHO norms did not all use the same methods is one that could be raised for most all comparative studies of locomotor energetics. Indeed, an evaluation of the individual sources drawn on in the work of Taylor et al. (1970, 1982), Rodman and McHenry (1980), and Steudel herself (1994, 1996) shows that they do not all rely on the same energetic methods. Lack of uniformity in methods is clearly a limitation that is common to this type of comparative research. We chose to use the WHO values because they were based on a large sample of studies that had been critically evaluated by a group of experts in human nutrition and energetics. The fact that individual studies have produced results that deviate from these norms is hardly surprising. Much research has shown that human populations may differ from standard norms for activity-specific energy costs (e.g., Katzmarzyk et al., 1996; PanterBrick, 1992; Panter-Brick et al., 1992; Spurr et al., 1996, 1998; Strickland and Ulijaszek, 1990). Nonetheless, these norms provide a broad comparative base that we believe is preferable to values derived from a single study. Indeed, prior to our work (Leonard and Robertson, 1995, 1997a), the standard reference used by Steudel (1994) and others (e.g., Taylor et al., 1970; Rodman and McHenry, 1980) to quantify human locomotor energy costs was the study by Margaria et al. (1963), which was based on two individuals (both males). Steudel-Numbers is also critical of our discussion of sex differences in energy costs of locomotion in humans. In our previous work, our estimates of locomotor costs suggested that bipedal females had greater energetic savings than bipedal males. We briefly attempted to explain this (Leonard and Robertson, 1997a, p. 307) by noting the research indi-