Monitoring rodents' life history by body temperature and activity

European hamster populations are rapidly declining all over their distribution area. One of the suggested reasons is a strong and ongoing decrease in the reproductive output of about 77% since the nineteen twenties. The cause of this decline is, however, unknown. To find out why the reproduction rate has decreased, precise information on the phenology of reproduction and litter‐sizes is necessary. In this study we show that recordings of body temperature (T b ) and activity of European hamster females provide accurate information on the reproductive phenology as well as individual health and death. In contrast to non‐gravid females, thermograms and actograms of gestating and lactating females clearly show arrhythmic patterns. Parturition is indicated by a profound hypothermia. The duration of the decline part correlates with the litter‐size. Thus, the timing and number of litters as well as the litter‐size can be precisely determined in T b recordings. Moreover, after parturition females show rhythmic patterns of activity and T b only after separation from the pups, no matter at which postnatal day that happens, allowing conclusions whether at least one pup could be raised to independence. In a pilot study we recorded T b in wild European hamster females in a Dutch population which is regularly restocked by captive bred animals. These data support the hypotheses that European hamsters currently reproduce too less and too late in order to keep the population stable. A helpful side‐effect of activity and temperature recordings is that they allow to determine the exact date and time of death of transmitter‐tagged individuals and thus conclusions about the reason. With the exception of the sensor implantation, monitoring T b is a non‐invasive method to study the life history of small mammals. Monitoring reproduction of small mammals might be a helpful tool to understand the massive global decline of species.