Huddling is the key energy saving mechanism for emperor penguins to endure their incubation fast during the Antarctic winter, but the underlying physiological mechanisms of this energy saving have remained elusive. The question is whether their deep body temperature may drop in association with energy sparing, taking into account that successful egg incubation requires a temperature of about 36°C and that ambient temperatures of up to 37.5°C may be reached within huddles. Using data loggers implanted into five unrestrained breeding males, we present here the first data on body temperature changes throughout the breeding cycle of emperor penguins, with particular emphasis on huddling bouts. During the pairing period, core temperature decreased from 37.5±0.4°C to 36.5±0.3°C, associated with a significant temperature drop of 0.5±0.3°C during huddling. In case of egg loss, body temperature continued to decrease to 35.5±0.4°C, with a 0.9°C decrease during huddling. By contrast, a constant core temperature of 36.9±0.2°C was maintained during successful incubation, even during huddling, suggesting a trade-off between the demands for successful egg incubation and energy saving. However, such a limited drop in body temperature cannot explain the observed energy savings of breeding emperor penguins. Furthermore, we never observed any signs of hyperthermia in huddling birds that were exposed to ambient temperatures as high as above 35°C. We suggest that the energy savings of huddling birds is due to a metabolic depression, the extent of which depends on a reduction of body surface areas exposed to cold.