In this study, we investigated how relative lower-limb length (LLL; leg length adjusted for body height) affects heat loss and changes in body temperature during running. Although longer legs are often thought to increase body surface area relative to mass (which could help with thermoregulation), previous studies suggested that their actual effect on total surface area might be small compared to the effect of trunk dimensions. Our goal was to test whether having relatively longer legs improves thermoregulation during exercise, and whether this effect is due to differences in body surface area distribution. We recruited 37 adult male runners and had them run on a treadmill for 40 minutes in a warm, controlled environment (25°C, ~41% humidity). We measured their core temperature (via ingestible sensors), skin temperature (via infrared thermography), and oxygen consumption (via indirect calorimetry). We calculated mean body temperature and standardized heat loss (heat loss divided by heat production), and used a geometric model to estimate body and limb surface areas. We found that individuals with relatively longer lower limbs had a smaller increase in mean body temperature during running and higher standardized heat loss. However, relative LLL had no significant effect on total body surface area relative to mass. Instead, individuals with relatively longer legs had a higher proportion of their body surface area in the lower limbs, which may explain the increased heat loss, since these regions have higher skin temperatures and greater airflow during running. These findings suggest that relatively long lower limbs may provide a thermoregulatory advantage during endurance running, even if they don't affect total body surface area relative to mass. This thermoregulatory effect could represent an additional selective pressure contributing to the elongation of lower limbs in the genus Homo at the beginning of the Pleistocene.