Timing of winter phenotype expression determines individual chances of survival until next reproductive season. Environmental cues triggering this seasonal phenotypic transition have been rarely investigated, although they play a central role in the compensation of climatic fluctuations via plastic phenotypic adjustments. Initiation of winter daily torpor use – a widespread energy saving phenotype – could be primarily timed according to anticipatory seasonal cues (anticipatory cues hypothesis), or flexibly fine-tuned according to actual energy availability (food shortage hypothesis). We conducted a food supplementation experiment on wild heterothermic primates (grey mouse lemurs, Microcebus murinus), at the transition to the food-limited dry season, i.e. the austral winter. As expected under the food shortage hypothesis, food supplemented individuals postponed the seasonal transition to normal torpor use by 1-2 month(s), spent 4 time less torpid, and exhibited minimal skin temperature 6°C higher than control animals. This study provides the first in situ experimental evidence that food availability, rather than abiotic cues, times the launching of torpor use. Fine-tuning of the timing of seasonal phenotypic transitions according to actual food shortage should provide heterotherms with a flexible adaptive mechanism to survive unexpected environmental fluctuations.