Life history written in blood: erythrocyte parameters in a small hibernator, the edible dormouse

The oxygen delivery system is one major determinant of the performance of vertebrates and responds sensitively to a variety of internal and environmental factors. To understand physiological mechanisms underlying variations of fitness, we investigated effects of demanding conditions associated with certain life-history events, food availability, and population density on the oxygen delivery system in free-ranging edible dormice (Glis glis). We sampled blood (n = 248) and urine (n = 319), performed an erythrocyte haemogram and visually determined the presence of haemoglobinuria. Reproduction leads to increased mortality in edible dormice and our study now reveals severe haematological impairments during reproduction that were associated with nutrient and energy deficits and stress. These effects were even more pronounced in subsequent reproductive years, indicating prolonged physiological impairment. Under limited food availability, the rate of erythrocyte generation was reduced. This seems to be part of an energy saving strategy instead of representing a poor body condition as survival probability in this species is high in years of low food availability. A high prevalence ratio of haemoglobinuria (up to 85%) at the end of the active season indicated amplified erythrocyte destruction through haemolysis. This may be the result of a preparative mechanism to avoid massive oxidative damage during the long hibernation period. Most ecophysiological studies so far focus on single erythrocyte parameters on a short time scale, which could be misleading. Our results clearly highlight that a wide-array RBC approach is a powerful tool for investigating mechanisms underlying physiological performance and fitness, also for other vertebrate taxa.