Lutra 47(2)_Spoelstra et al_2004

The European souslik in the natural light-dark cycle: what is the signal for afternoon retreat?

European sousliks (Spermophilus citellus) are strictly diurnal with a very precise daily activity rhythm in field conditions. This activity rhythm is regulated by the internal biological clock which in turn is synchronized by the natural light/dark cycle (entrainment). Due to their regular retreats into the darkness of their burrows, fast light intensity changes in natural light cannot be used for resetting of their biological clock. Careful analysis of retreat behaviour at the end of the day shows that neither a specific light intensity level nor a specific light intensity decrease can explain the day-to-day variations in the timing of retreat. If a general decrease in light intensity is a signal indeed, its effect must be phase dependent since a light intensity decrease by the solar eclipse on August 11, 1999, in the middle of the day failed to induce retreat behaviour.