Estrus and estrogen changes in mated and unmated free-living European ground squirrels

The course of behavioral and vaginal estrus and patterns of circulating est rogens were followed in free-living European ground squirrels (Spermophilus citellus) after their emergence from hibernation. Normally mating females were compared to a second group in an area where males had been removed fro m the population before female emergence. Both groups showed vaginal estrus , but the patterns differed. Mating shortened vaginal estrus to a 3-day per iod compared to 8 days in unmated females. The extent (cell number) of cell cornification during estrus and the cellular components (percentage distri bution) of metestrus did not differ between the two groups. Females in the area without males had significantly higher estrogen levels during estrus a nd metestrus compared to those in the control area. European ground squirre ls were found to be monestrous, as none of the unmated females reentered es trus after metor diestrus was detected. The prolongation of vaginal estrus in unmated females can be viewed as either a physiological inevitability or an adaptation to low mate availability. The extension is still relatively short compared to other sciurid species and perhaps a product of constraint s producing a strict time frame for reproduction. (C) 2000 Academic Press.