Follicular development and hibernation in European ground squirrels

Obligate hiberntors such as European ground squirrels are severely limited by time constraints. Adult females mate shortly after emergence from hibern ation, often within one or two days. Follicular maturation therefore has to occur either before or during winter. In this study oestrogen levels of Fe males were monitored throughout the active season in the field. These data were supplemented with information on individuals in captivity during the a ctive season and hibernation. Adult females had high oestrogen levels short ly after the termination of hibernation whereas yearling oestradiol levels were lower and increased later. In adult individuals oestrogens peaked post -lactation and decreased rapidly with the onset of pre-hibernatory fattenin g. During hibernation oestrogens remained baseline in all individuals. Ovar ies of juvenile and adult females were collected and analysed shortly befor e and after hibernation. in juveniles, follicles remained primordial throug h hibernation without reaching an antral stage. In older females differing numbers of intact tertiary follicles were detected. These data suggest that in European ground squirrels follicular maturation is initiated after lact ation and terminated at pre-hibernatory Fattening. During hibernation gonad al activity is suppressed and follicles are arrested. Females which complet e follicular development in summer become oestrous earlier in spring. This is part of an adaptive scheme which can affect reproductive effort and succ ess.