Conditions controlling the onset of breeding migration of the Japanese mountain stream frog, Rana sakuraii.

I studied the conditions controlling the timing of breeding migration of the Japanese mountain stream frog Rana sakuraii, an explosive breeder, over 9 years (1992, 1993 and 1999-2005). I analysed two factors: the daily cumulative temperature (DCT) during hibernation and the triggering temperature on the day of migration onset. Frogs hibernated in shallow running water in streams in December. A total of 53,155 breeding migrating adults were captured by traps. Every year, breeding migration was induced by a rise in daily maximum water temperature to about 5 degrees C. However, its date was limited to 1 February at the earliest, and the onset needed a DCT (from 20 January, using 3 degrees C as the threshold for daily effective temperature) of at least about 15 degrees C. Earlier (e.g. in mid- or late January), even if the maximum temperature rose to 5-8 degrees C, migration did not begin. Moreover, even in early February, if the maximum temperature rose to 5 degrees C, if it had been too cold in January and the DCT was low, migration would not begin until mid- or late February. Thus, the earliest date of readiness for migration varied from 1 February to mid-February, depending on the winter DCT. Logistic regression analyses showed that both factors, the DCT and the daily temperature, were significant. I propose that in temperate-zone amphibian explosive breeders, both the passing of an essential number of days and an essential DCT during hibernation are prerequisites for the onset of breeding migration before the daily temperature rises to the threshold.