1School of Anthropology, University of Arizona, 2Department of Anthropology, Hunter College, 3Department of Anthropology, Stanford University, 4Department of Anthropology, Yale University, 5Archaeology Unit, University of Dar es Salaam, 6Department of Anthropology, Durham Univerity
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Recent work suggests that the Hadza (hunter-foragers in Tanzania) expend similar amounts of energy per day compared to urban-living populations, and individuals do not reduce energy expenditures with age. However, it is unclear how variation in activity levels contributes to overall energy use. To investigate daily activity levels, we measured heart rates (HR) continuously in a sample of 39 individuals from two Hadza camps over two field seasons. Measurements were taken over consecutive days (ranging from seven to 14 days) from dawn to dusk. We converted raw HRs into percentages of age-adjusted maximum HR (MHR) using equations from the literature. We calculated the amount of time spent in seven different HR zones each day (from less than 40% MHR to greater than 90% MHR, increasing in 10 percent increments). Age had a significant effect on time spent in HR zones (MANOVA p = 0.01), with older adults spending more time in higher HR zones than younger individuals. Sex had no significant effect on time spent in HR zones (p = 0.13). In most individuals (~70%), time spent in the high aerobic zones (>70% MHR) on consecutive days has a negative autocorrelation, indicating most individuals alternate hard and easy days. We explore the implications of this daily energy use strategy and suggest that it is only possible when food sharing among both kin and non-kin allow for sufficient rest following long, aerobically active foraging bouts. We also explore the physiological and behavioral causes of age-related variation in activity measured by HR.
This study was funded by NSF BCS 0850815.