Hibernation behavior of Rana lessonae and R-esculenta in their natural habitat

We studied the hibernation behavior of the water frog Rana lessonae and its hybridogenetic associate R. esculenta in their natural habitat during thre e successive winters. Animals caught in pitfall traps at a fenced pond were individually marked with PIT tags and some (n=36) were additionally equipp ed with radio transmitters. Of the animals caught, 85% left the fenced pond for hibernation. More R. esculenta remained inside the fenced area compare d to R. lessonae. R. lessonae emigrated earlier in autumn and came back lat er in spring than R. esculenta, but the distance to their hibernation sites did not differ. Both species left the fenced pond earlier in the year when ambient temperatures were lower. All radio-tracked animals hibernated in w oodland, 3-7 cm below the surface in soil, under moss, fallen leaves or sma ll branches. Soil temperatures at the actual hibernation sites were signifi cantly higher than at randomly chosen control sites. A surprising finding w as that most frogs changed their hibernation sites during winter, and often more than once. Movements were more frequent in the warmer first half of t he winter than in the cooler second half, but some animals were active even on days with mean temperatures below 1 degrees C. These results show that both species do not spend the whole winter torpid in one particular hiberna tion site bur move around, especially at higher temperatures. Most of the a nimals lost weight during the winter, and the weight loss was greater in fe males than in males and higher in warm than in cold winters. To what extent weight loss and survival is influenced by the chosen hibernation sites and the amount of movement during winter, and whether this contributes to the differences in species and sex ratios found in mixed populations, needs mor e investigation.