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Follicular Development in European Ground Squirrels (Spermophilus citellus) in Different Phases of the Annual Cycle

E Millesi, Behavioural Biology, University of Vienna, Vienna, A-1090, Austria
A Strauss, Department of Behavioural Biology, University of Vienna, Vienna, Austria
T Burger, Department of Theoretical Biology, University of Vienna, Vienna, Austria
I Hoffmann, Department of Behavioural Biology, University of Vienna, Vienna, Austria
M Walzl, Department of Theoretical Biology, University of Vienna, Vienna, Austria

Correspondence: Eva Millesi, Email: eva.millesi{at}univie.ac.at

Abstract

In seasonally breeding mammals, in particular hibernators, reproduction underlies severe energetic and temporal constraints to enable the allocation of sufficient body-fat reserves before winter. Thus, the timing of conception in spring can be crucial in terms of reproductive success. This study investigates follicular development in European ground squirrels (Spermophilus citellus) in three phases of the annual cycle: at vernal emergence, after weaning the offspring and shortly before hibernation. The animals were kept in outdoor enclosures within the natural habitat of the species. They were captured in weekly intervals, weighed and reproductive status was determined. Unilateral ovariectomy was scheduled such that the three periods were sampled. Numbers and diameters of tertiary follicles and corpora lutea in each ovary were determined, and plasma oestradiol and progesterone levels were analysed. The highest numbers of tertiary follicles, including Graafian follicles, were found in ovaries at spring emergence. During postlactation the number of tertiary follicles was lower, and active corpora lutea appeared in the investigated ovaries. Shortly before hibernation, active corpora lutea were present, but luteolysis had started in some individuals. Both oestradiol and progesterone secretion peaked after the termination of lactation and decreased before hibernation. The results demonstrate a second oestrus cycle in European ground squirrels after weaning, including an active luteal phase. This non-reproductive oestrus cycle with its endocrine output is an intriguing phenomenon. It may positively affect both prehibernatory fattening and reproduction in the subsequent season.