In previous studies, a second oestrus cycle including an active luteal phase during the non-breeding season in summer has been documented in both free ranging and captive female European ground squirrels. As the onset of prehibernation fattening and peak progesterone levels coincided, potential positive effects of progesterone secretion on the course of the fattening phase were assumed. In the season of 2014, we monitored reproductive performance of 9 females kept in semi-natural enclosures. In

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... contrast to previous years, none of the studied females showed elevated progesterone titres except during gestation, although results of vaginal cytology demonstrated the occurrence of a vaginal oestrus phase in most individuals during summer. This may indicate ovarian activity without the formation of active corpora lutea as a main source for progesterone. Reproductive outcome was very low, as only 3 out of 9 females reproduced and 2 of them weaned their litter successfully. In the others, no indications of advanced gestation or lactation could be detected. Due to the restricted sample size, this study was focused on non-reproductive females. Additionally, our data were compared to unmated females of a previous study which were kept in the same outdoor enclosure without any contact to males. Individuals lacking a pronounced progesterone secretion during summer showed a prolonged fattening phase with significantly lower body mass increase rates compared to females of the previous study with active corpora lutea. Although females with low progesterone concentrations exhibited a prolonged fattening phase, immergence body mass did not differ between the compared years. Thus, reaching a certain threshold in body fat content might initiate the hibernation period. The result on lower fattening rates in non-reproductive females of this study were reflected by feeding experiments showing no differences in foraging behaviour in non-reproductive females before and during the fattening period. Although the reason for reproductive fai [...]