Diel activity patterns during autumn migration to hibernation and breeding sites in a Japanese explosive breeding frog Rana sakuraii

摘要

Relatively few studies have investigated diel activity patterns in amphibians; in particular, such studies focusing on autumn migration are lacking. I investigated the diel activity patterns during autumn instream migrations (to hibernation and breeding sites) of Rana sakuraii, an explosive breeding frog, at mountain headwaters in Japan. Successive censuses at 2-h intervals, over a period of 24 h, were conducted 15 times during 4 years (2001, 2002, 2013 and 2014). The diel activity pattern was not significantly different between the sexes. It was clearly bimodal nocturnal (almost like a crepuscular type), with the first peak appearing at 1600-2400 h and the second peak appearing at 0400-0800 h. The diel activity was highly correlated with day time, but not with water and air temperatures. However, it was controlled primarily by illumination rather than by the day time, because relatively many frogs migrated even during the day when it was very dark due to overcast or rainy weather and during the midnight time (0000-0400 h) when the environment was slightly illuminated by moonlight owing to clear weather. In contrast, the diel activity pattern during the winter breeding migration was clearly unimodal, with the peak at 1600-2400 h, probably because the ambient temperature during the second peak was too low, even though the illumination was suitable. The results presented herein demonstrate that R. sakuraii requires an optimum ambient illumination; consequently, its diel activity shows a bimodal nocturnal pattern.