Primates are exceptional among mammals for their climbing abilities and arboreal lifestyles. Here we show that small primates (less than 0.5 kilogram) consume the same amount of mass-specific energy (COTTOT) whether climbing or walking a given distance. COTTOT decreases with increasing body size for walking but does not change for climbing. This divergence of COTTOT is likely due to fundamental differences in the biomechanical determinants of the costs of climbing versus walking. These results have important implications for understanding the origins of primates, suggesting that small early primates may have been able to move into a novel arboreal niche without increasing metabolic costs.