Thermo-imaging bipedalism on the savanna: Chimpanzee locomotion at Fongoli, Senegal and implications for the evolution of hominin bipedalism

Abstract

Pressures associated with savanna habitats, such as heat stress, are often cited to explain adaptive traits in hominins, like the evolution of bipedalism. Wheeler’s (1984) physiological model, for instance, predicts that bipedal posture reduces contact with UV radiation by lessening the total body area exposed to direct sunlight. Chimpanzees occupying a savanna mosaic environment provide a unique opportunity to assess aspects of these hypotheses, such as how thermal stresses impact furred apes in a hot, arid and open environment. This study followed chimpanzees at the Fongoli site in Senegal, as they are thus far the only habituated savanna woodland community. Chimpanzees here display more bipedal behavior than is observed at other sites and are known to soak in pools and use caves to cope with heat stress. Apes here are, therefore, good living primate models to use in assessing selective pressures associated with hot, arid savanna environments. This research tested the hypothesis that body temperature differs between the array of positional behaviors used by West African chimpanzees throughout their home range; more specifically, bipedalism was predicted to correlate with a lower chimpanzee body temperature when compared to quadrupedal positions. Using a thermo-imaging camera to record individual body temperatures, I evaluated the heat load related to various postural and locomotor behaviors, specifically that of bipedalism. Results reveal lower body temperatures associated with bipedalism when compared to positions like sit and quadrupedal stand. These data offer support to models of hominin evolution that suggest bipedalism confers a thermoregulatory advantage