Energetic Competition Between Neandertals and Anatomically Modern Humans

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The effects of climate on energy expenditure, which include climatic influences on human metabolic physiology and variation in the level of physical activity required for subsistence under different environmental conditions, have been the focus of a considerable amount of research. In general, this work shows that human foragers inhabiting colder climates tend to have higher energy expenditure than those in warmer climates, both in terms of maintenance energy needs (basal metabolic rate, or BMR) and the amount of energy spent obtaining resources. Recently, several authors have applied these findings to the fossil record, with results that indicate that Neandertals would likely have had very high daily energy expenditure (DEE) as part of their adaptive response to the thermoregulatory and subsistence challenges of life in cold climates. These studies imply that anatomically modern humans would have had substantially lower energy requirements than, and thus a competitive advantage over, Neandertals in Europe. Estimates of DEE do not exist for anatomically modern humans in cold climates, however, begging the question of whether they might have required similarly high amounts of energy to survive in glacial Europe. Here we present an exploratory analysis of climatic effects on energy expenditure in Neandertals and anatomically modern humans, using a new method. This method uses mean annual temperature along with body size, sex and age to predict BMR, thereby directly incorporating the effects of climate on metabolic physiology. We used this method to calculate BMR in a sample of Neandertals and Pleistocene modern humans, obtaining paleo-mean annual temperature values for fossil sites from data available from the Oxygen Isotope Stage 3 Project. We then estimated DEE from these BMR values, using climate-specific physical activity level (PAL) values based on extant human foragers living in different environments. Our model suggests that Neandertals would have had substantially higher energy needs than anatomically modern humans in similar climates, on the order of 100–350 kcal per day, which corroborates the results of previous studies using different methods. This difference is in large part due to greater body mass in Neandertals, which may have related mainly to a higher proportion of muscle mass compared to anatomically modern humans. Greater muscularity in Neandertals would have provided them with greater thermoregulatory capability, may have served as insulation, and was also probably part of a subsistence strategy of close-range encounters with large mammalian prey. If greater muscle mass served these ends in Neandertals, modern humans must have used other means of dealing with insulation and subsistence in order to maintain less-massive bodies. There is archaeological evidence consistent with this interpretation, suggesting that anatomically modern humans may have brought with them into Europe improved methods of insulating themselves, as well as broadened subsistence techniques. Lower adult energy needs could have provided modern humans with reproductive advantages in the form of reduced birth spacing, greater survivorship, or both. This would likely translate into a competitive advantage over Neandertals who had higher, and thus harder to meet, energy demands.