Conditions controlling the timing of the autumn migration to hibernation sites in a Japanese headwater frog, Rana sakuraii

Abstract

AbstractStudies on amphibian autumn migrations to hibernation sites are few, and those investigating the conditions controlling the timing of migrations are even fewer. I investigated an explosive‐breeding frog, Rana sakuraii, in Japanese mountain headwaters in 6 different years, focusing on the start and end of autumn migrations. Autumn migrations began in late October. Initially, frogs moved into the streams from terrestrial sites around the stream sources, and thereafter moved downstream entirely underwater. Start days were always the first rainy days after the minimum air temperature had dropped to about 6°C or lower (minor start days) or about 4°C or lower (major start days) regardless of the air temperature on the start days. In early or mid‐December, in‐stream migratory movements ended when the maximum water temperature dropped below the 5°C threshold (the same threshold temperature initiating the winter breeding migration). I hypothesize that, in many temperate‐zone amphibians, the timing of autumn migration to hibernation sites is basically dependent on three common conditions: (1) the experience of a prerequisite minimum air temperature (PMAT) before the start day; (2) the decrease in day length to a threshold value; (3) the first rainy (or high humidity) day after (1). Further, I hypothesize that the threshold ambient temperature inducing the end of the autumn migration is almost the same as that inducing emergence from hibernation, that is, the start of the winter breeding migration for explosive breeders, and that this is the daily maximum, and not the daily minimum.