The body temperature rhythms in laboratory mice C57Bl/6 and common greenfinches (Chloris chloris) and the moments of feeding by common starlings (Sturnus vulgaris) have been compared with the intensity of fluctuations of 40К radioactive decay. It was found that body temperature changes in greenfinches and mice positively correlate with the intensity of fluctuations of 40K radioactive decay. The method of superimposed epochs shows that the increase of body temperature in mice, reflecting the phase of the beginning of sleep/wake cycle activity, as well as food intake by starlings are observed at an increase in the intensity of 40K radioactive decay. Thus, animal activity in the ultradian range of periods may be related to external quasi-rhythmic physical influences, rather than determined only by endogenous processes. Given the extremely low dose of natural 40К exposure, a factor that can cause observed radioactivity fluctuations may be biotropic.