Some effects of external factors upon the reproductive behavior of the grass frog Rana t. temporaria L. (Ranidae, Anura).

The reproductive period of Rana temporaria can be subdivided into a prespawning period and a spawning period. During the prespawning period only a few Rana temporaria, most of them males, are active in the spawning biotope. There is no calling activity, and deposition of spawn rarely occurs. The time of onset of the prespawning period is variable. In this period the frogs are highly responsive to a drop in the temperature of air and water, burying themselves in the bottom or leaving the spawning waters.By contrast, many animals participate in the spawning period. Migration to the spawning ground takes place chiefly at night. The frogs engage in mating activity immediately upon arrival, and both males and females depart as soon as they have spawned, migrating away in directions corresponding to those of the immigration. During the spawning period there is no demonstrable response of the grass frogs to weather conditions. Rana temporaria begins its spawning period at the so-called "set time" only in meteorologically normal years. When meteorological conditions are unusual, the spawning period can begin as much as 10 days earlier or later.Possible external timing signals for the onset of migration include temperature in combination with day length, and a drop in atmospheric pressure accompanied by rainfall.