Mammalian Biology Zeitschrift fu È r Sa È ugetierkunde www.elsevier.de/mammbiol Short communication The size and number of harems in the polygynous bat Pipistrellus nathusii (Keyserling and Blasius, 1839) (Chiroptera: Vespertilionidae) By M. CIECHANOWSKI and T. JARZEMBOWSKI Department of Vertebrate Ecology and Zoology, University of Gdan Ásk and Department of Medical Microbiology, Medical University of Gdan Ásk, Gdan Ásk, Poland Receipt of Ms. 16. 07. 2003 Acceptance of Ms. 03. 11. 2003 Key words: Pipistrellus nathusii, mating groups, Poland The mating system of European bats of the genus Pipistrellus (Kaup) is characterised by resource defence polygyny with territor- ial males and accompanying harems of females (Gerell-Lundberg and Gerell 1994). One of their representatives, Pipis- trellus nathusii (Keyserling and Blasius) per- forms regular long-distance migrations (Pe- tersons 1990; J arzembowski 2003), which strongly affects mating phenology in this species (Gerell-Lundberg and Gerell 1994). Among several aspects of biology of its mating period, roost preferences (Fur- mankiewicz and Szkudlarek 2001) and ad- vertisement behaviour (Gerell-Lundberg and Gerell 1994; Furmankiewicz 2003) have been studied thoroughly, while avail- able studies dealing with social organization during this period have been rather descrip- tive, often with no statistical verification (Heise 1982; Schmidt 1984; Rachwald 1992). Practically no attention has been paid to factors determining the size of harems, which could partially evaluate differences in the reproductive success among particular males. The aim of this study is to analyse the varia- tion in size and number of mating groups (harems) of P. nathusii and their relation with the number of potential mating part- ners in a particular area. The study was conducted in two study plots on the Baltic Sea Coast (N Poland): the So- bieszewo Island (the western part of the Vistula Spit) near Gdansk (54°20¢ N, 18°47¢ E) and Maøa Pias Ánica in the Dar- z Çlubska Forest (54°40¢ N, 18°13¢ E). All sites were located in managed forests (mostly one-age plantations), dominated by Scotch pine Pinus sylvestris . On the Sobieszewo Is- land 60 bat boxes of the modified Strat- mann type were hung up in 1994. Addition- ally, 50 bird nestboxes were found there. All boxes were controlled every 5 days between May and September from 1995 to 1998. In years 1996±1998 boxes were not checked during the nursing period (II decade of June±II decade of July). In the Darz Çlubska Forest, 34 bat boxes of the modified Strat- mann type were hung up on 30. 04. 1998. They were controlled every 10 days in July- September (1998±1999) or every two weeks in May±June (only 1999). Bats captured in the boxes (according to the licenses of the Ministry of Environmental Protection) were sexed and aged. Sexual activity of 1616-5047/04/69/04-277 $ 30.00/0. Mamm. biol. 69 (2004) 4 ´ 277±280