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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 (TF) and corpora lutea (CL) in each ovary were determined, and plasma oestradiol and progesterone levels were analysed. The highest numbers of TF, including Graafian follicles, were found in ovaries at vernal emergence. During post-lactation, the number of TF was lower and active CL appeared in the investigated ovaries. Shortly before hibernation, active CL 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.

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In European ground squirrels (Spermophilus citellus) as in other obligate hibernators, physiological processes underlie strict time constraints. Especially for the juveniles growth and prehibernatory fattening are temporally very limited. The aim of this study was to determine the effects of specific nutritional factors on development in juvenile males. Male ground squirrels were investigated for a 7-week period starting at natal emergence. Individuals were assigned to 1 of 3 experimental groups and were provided with a diet either high in proteins or polyunsaturated fatty acids (PUFAs). The third group was used as a control and was fed with a standard rat diet. Food quality affected growth in that males of the high-protein group had significantly larger head lengths in week 4, 5, and 6 after natal emergence than those of the other 2 groups. However, body mass did not differ significantly between the groups. All juvenile males showed an increase in plasma testosterone levels from week 1-2 to week 3-4 post-emergence, thereafter testosterone secretion remained stable until week 5-6. The daily testosterone increase after the first 2 weeks was significantly higher in the PUFA group compared to the other 2 groups. The results underline the importance of diet composition for growth rates and the timing of gonadal activation during early development in this species.

Timing of reproduction constitutes a major decision in life history delimited by resource availability and mortality risk. Hibernating species are subject to additional temporal constraints. Both sexes have to cope with a short active season in completing gonadal development, reproduction, and prehibernatory fattening. Males have to adapt to the strict seasonality and must be reproductively competent [47] Acta Theriologica 53 (1): 47-56, 2008. . 2008. Testis development and testosterone secretion in captive European ground squirrels before, during, and after hibernation. Acta Theriologica 53: 47-56.In some hibernating species, an extended euthermic period before spring emergence has been reported during which testicular maturation occurred. In this study, we investigated whether male European ground squirrels Spermophilus citellus (Linnaeus, 1766) develop scrotal testes before or after the termination of hibernation. The course of testis development and testosterone concentrations were examined in young-of-year male ground squirrels (n = 4) before, during, and after their first hibernation. In the environmental chamber, all animals showed regular hibernation patterns with decreasing torpor bout lengths prior to the end of hibernation. Four weeks before hibernation ended, testosterone levels were elevated during spontaneous arousals. Testosterone concentrations peaked during the first 2 months post heterothermy and decreased thereafter. In 2 experimental males, testes descended from the inguinal region into the scrotum shortly before the end of heterothermy. The state of testis development and testosterone secretion corresponded to that found in semi-free-living males at spring emergence. Testis size increased in active animals to maximum widths during weeks 3 and 4 post heterothermy. Scrotal skin pigmentation was absent until heterothermy ended and developed during the first month thereafter. The data suggest that male S. citellus do not need a pre-emergence euthermic period for reproductive development.