Follicular development in European ground squirrels (Spermophilus citellus) in different phases of the annual cycle

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 (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.

Effects of Natural Environmental Conditions on the Ovarian Histometric Dynamics of Five Striped Squirrels (Funambulus pennanti)

“…February and March is the estrous period, where February being the peak estrous period. Thus present study correlates well with the morphological and histological observations of ovaries in Indian palm squirrel, wild ground squirrel (Citellus dauricus Brandt) and European ground squirrels (Spermophilus citellus) (Seth & Prasad;Millesi et al, 2008;Bao et al, 2014). …”

Validation of Fecal Progesterone Analysis for Predicting Pregnancy in Siberian Flying Squirrels（<i>Pteromys volans</i>）

“…In ground squirrels, such as the Cape ground squirrel Xerus inauris, the California ground squirrel Spermophilus beecheyi, the European ground squirrel Spermophilus citellus and the woodchuck Marmota monax, high progesterone concentrations have been detected during lactation, although these species produce only one litter per year [38][39][40][41]. The estrus cycle is reinitiated during lactation period, because examination has revealed the presence of neither corpus luteum nor their remains during early lactation [23]. In arboreal squirrels, Tait et al [42] investigated progesterone concentrations of the gray squirrel Sciurus carolinensis during pregnancy and lactation.…”

“…However, we have little information on the animal's basic reproductive physiology, including its estrus cycle and pregnancy. Progesterone concentrations are generally measured to study reproductive physiology, and for this purpose plasma samples are frequently used in mammals [23,24]. However, repeated blood sampling of Siberian flying squirrels is considered unsuitable because of its impracticality.…”

The common iliac artery in the ground squirrel (Citellus citellus)

“…The effects of TSH-releasing hormone on neurons of the brain were studied in hibernating and active ground squirrel (Belousov and Belousova, 1993). So were studied the fine structure of photoreceptor cells of the ground squirrel (Hollenberg and Berstein, 2005), the follicular development in European ground squirrels in different phases of the annul cycle (Millesi et al, 2008). Brown adipose tissue has been widely investigated (Cinti et al, 1997;Kochan et al, 1999;Konishi et al, 2000;Murakami et al, 2001).…”