Key words: Muscardinus avellanarius, tail loss, demographic parametersCaudal autotomy – the ability to shed the tail– is a common and well-recognised antipre-dator defence mechanism in reptiles andamphibians. A review of the occurrence ofthis phenomenon in rodents is presented byShargal et al. (1999). These authors state thattail autotomy occurs in at least 35 species andhas evolved separately in eight rodentfamilies, with no clear pattern in systematics,geography, or habitat use. Two major typesof tail loss are recognised in rodents: simul-taneous loss of an entire part of the tail, whenthe tail breaks across vertebrae, and sheddingof a portion of the tail skin alone withsubsequent loss of the denuded part of thetail (Mohr 1941). These two types are calledtrue autotomy and false autotomy, respec-tively (Dubost and Gasc 1987; McKee andAdler 2002).In general, earlier studies of tail loss were notaccompanied by field research, and onlyanatomical/histological aspects of this phe-nomenon were studied (e.g., Go¨gl 1930;Mohr 1941). Several later studies dealt alsowith some ecological aspects of tail loss inrodents: Florida mouse (Peromyscus florida-nus)(Layne 1972), black rat (Rattus rattus)(Michener 1976), common spiny mouse(Acomys cahirinus) and golden spiny mouse(A. russatus)(Shargal et al. 1999) as well asCentral and South American spiny rats(Proechimys)(Dubost and Gasc 1987;McKee and Adler 2002).False tail autotomy in dormice (Gliridae) is awell-known phenomenon, which is men-tioned in some generalised publications onthese rodents (e.g., Vietinghoff-Riesch 1960;Storch 1978; Airapetyants 1983; Morris 1997;Rossolimo et al. 2001). However, onlyanatomical/histological aspects of tail autot-omy have been studied in dormice (Go¨gl1930; Mohr 1941), any ecological aspects ofthis phenomenon were not reflected. The aimof the present study was to analyse thefrequency of occurrence of tail autotomy inseparate demographic groups, annual survi-val rates of tail-less individuals and someother ecological aspects of tail autotomy inthe common dormouse (Muscardinus avella-narius) populations.Studies of two isolated common dormousepopulations were carried out in Lithuania intwo locations: at study site A (south-westernLithuania, Sˇakiai district; 55103