Animal Reproduction Science 112 (2009) 362–370 Vaginal oestrus during the reproductive and non-reproductive period in European ground squirrels Anna Strauss a,∗ , Ilse E. Hoffmann a , Manfred Walzl b , Eva Millesi a a Department of Behavioural Biology, University of Vienna, Althanstr. 14, 1090 Vienna, Austria b Department of Theoretical Biology, University of Vienna, Althanstr. 14, 1090 Vienna, Austria Received 22 November 2007; received in revised form 26 April 2008; accepted 21 May 2008 Available online 27 May 2008 Abstract Most hibernating animals are thought to be monoestrous because reproductive activity is subject to strong time constraints. In previous studies, female European ground squirrels (Spermophilus citellus) turned out to have elevated oestradiol levels during late lactation and after weaning, indicating ovarian activity during summer. Therefore, we monitored vaginal cytology, endocrine changes, and vulval development in semi- free-living female European ground squirrels throughout one active season. Vaginal oestrus, defined by the predominance of cornified cells in smear samples, was found during the mating period shortly after vernal emergence. This phase was followed by metoestrus, characterised by the appearance of leukocytes, and a subsequent anoestrous phase. During weaning or postlactation, a second vaginal oestrus was documented in all experimental females, again followed by a metoestrous and an anoestrous phase lasting until hibernation. In line with the second vaginal oestrus, plasma oestradiol concentrations peaked during postlactation. Proges- terone levels were elevated from gestation to postlactation, and titres were marginally higher during vaginal oestrus in summer than in spring. Vulval swelling was more pronounced during the first than the second vagi- nal oestrus. The second oestrous cycle was non-reproductive, as males were sexually inactive with regressed testes during summer. We assume that the second oestrous cycle and the accompanying endocrine changes have beneficial effects on prehibernatory fattening and reproductive performance in the subsequent season. This might allow females to become oestrous immediately after emergence from hibernation in spring. © 2008 Elsevier B.V. All rights reserved. Keywords: Spermophilus citellus; Oestrous cycle; Vaginal cytology; Oestradiol; Progesterone ∗ Corresponding author. Tel.: +43 1 4277 54466; fax: +43 1 4277 54506. E-mail address: anna.strauss@univie.ac.at (A. Strauss). 0378-4320/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.anireprosci.2008.05.068