Free-living greylag geese adjust their heart rates and body core temperatures to season and reproductive context.

Animals adaptively regulate their metabolic rate and hence energy expenditure over the annual cycle to cope with energetic challenges. We studied energy management in greylag geese. In all geese, profound seasonal changes of heart rate (fH ) and body temperature (Tb ) showed peaks in summer and troughs during winter, and also daily modulation of fH and Tb . Daily mean fH was on average 22% lower at the winter trough than at the summer peak, whereas daily mean Tb at the winter trough was only about 1 °C below the summer peak. Daily means of Tb together with those of air temperature and day length were the most important predictors of daily mean fH , which was further modulated by precipitation, reproductive state, and, to a minor degree, social rank. Peaks of fH and Tb occurred earlier in incubating females compared to males. Leading goslings increased daily mean fH . Our results suggest that in greylag geese, pronounced changes of fH over the year are caused by photoperiod-induced changes of endogenous heat production. Similar to large non-hibernating mammals, tolerance of lower Tb during winter seems the major factor permitting this. On top of these major seasonal changes, fH and Tb are elevated in incubating females.