Biologging allows us to record core body temperatures in mammals living free in their natural habitat, where they are subjected to complex stressors. In species that do not go into torpor, changes in the pattern of the daily rhythm of core body temperature can be quantified using cosinor analysis. Changes in energy intake and expenditure result in changes in the daily amplitude of the body temperature rhythm, and those changes are correlated with changes in animal performance, including growth and reproduction. Kangaroos displayed a gradual reduction in the minimum morning body temperature with little to no change in the nighttime (active) body temperature as spring progressed into the dry summer. Sheep restricted to 70% of their normal food intake exhibited increases in the daily amplitude, with decreases in the nighttime minimum, of body temperature. In free-living rabbits, the initial amplitude of the body temperature rhythm predicted the number of pregnancies that a female had during the subsequent seven months. In alpacas, shearing, during benign ambient conditions, induces a large increase in amplitude of the body temperature rhythm, and the individual monthly growth could be predicted by the amplitude of the daily body temperature rhythm. Homeothermy seems to be the default condition in well nourished and hydrated larger mammals, while disturbances to energy or water balance, associated with decreased individual performances, often lead to heterothermy. We suggest that the characteristic of the individual daily T_b rhythm provides a sensitive index of fitness.