149 ACTA ZOOLOGICA BULGARICA Acta zool. bulg., Suppl. 2, 2008: 149-154 Daily Activity Pattern in Free-living European Ground Squirrels Spermophilus citellus (Mammalia: Rodentia) from Northwestern Bulgaria Yordan S. Koshev, Maria A. Kocheva Institute of Zoology, Bulgarian Academy of Science, 1, Tsar Osvoboditel Blvd., 1000 Sofia, Bulgaria; E-mail: bgsouslik@gmail.com Abstract: The study describes the daily activity of the European ground squirrel in an experimental plot, situated in a heavily grazed pasture near the town of Knezha, Northwestern Bulgaria. Above ground activity was recorded with a visual scanning procedure in summer and autumn of 2006. In summer animals emerged on an average 1.94 h after the civil twilight at dawn and the active period lasted about 11 h. In autumn the daily activity started 3.43 h after the the civil twilight at dawn and lasted about 7.3 h. The activity pattern in summer was bimodal, while in autumn only one clearly expressed peak was observed. The influence of climatic factors (temperature, humidity and claudiness) as well as of time of the year was analysed. Activity patterns were influenced mainly by the time of the year, as it represented consecutive periods of the annual activity of the ground squirrels. Keywords: daily activity pattern, environmental effects, European ground squirrel, souslik, Spermophilus citellus, Bulgaria Introduction The literature on daily activity patterns in small mammals is still dominated by experiments, carried out under laboratory conditions. The time budget of these animals under natural conditions still remains poorly known. The European ground squirrel (Spermophilus citellus L. 1766) is ideal for indirect (for example counting of borrow entrances) and direct observa- tion (for example, observation of daily activity) as it is diurnal and highly visible in the short grass mead- ows. The species has negative trend of populations and it is included as vulnerable in the IUCN Red List of Threatened Species (AMORI 1996). In Bulgaria, which is the core of their distri- bution (MITCHELL-JONES et al. 1999), the ground squirrels inhabit open grasslands. Being diurnal, they search for their diet of seeds, leaves, flowers, and occasionally insects and eggs during the day. In the morning they go out when the dew is lifted, and could be seen almost until sunset (STRAKA 1961). Ground squirrels are real hibernators and they re- main continuously underground from around August or September to March (MILLESI et al. 1999). In spring and summer, the animals reproduce and subsequently prepare for hibernation in autumn and winter. The reproductive cycle is accompanied by changes in physiology and behaviour (MILLESI et al. 1998, MILLESI et al. 1999 a, b). The energetic de- mands of the animals vary during the annual cycle and could affect the above ground activity patterns.