Estrus and Estrogen Changes in Mated and Unmated Free-Living European Ground Squirrels Eva Millesi,* ,1 Susanne Huber,² Katharin Pieta,* Manfred Walzl,* Walter Arnold,² and John P. Dittami* *Institute of Zoology, University of Vienna, Austria; and †Research Institute of Wildlife Ecology, University of Veterinary Medicine, Vienna, Austria Received July 26, 1999; revised November 29, 1999; accepted December 20, 1999 The course of behavioral and vaginal estrus and patterns of circulating estrogens were followed in free-living Eu- ropean ground squirrels ( Spermophilus citellus) after their emergence from hibernation. Normally mating fe- males were compared to a second group in an area where males had been removed from the population before female emergence. Both groups showed vaginal estrus, but the patterns differed. Mating shortened vag- inal estrus to a 3-day period compared to 8 days in unmated females. The extent (cell number) of cell corni- fication during estrus and the cellular components (per- centage distribution) of metestrus did not differ between the two groups. Females in the area without males had significantly higher estrogen levels during estrus and metestrus compared to those in the control area. Euro- pean ground squirrels were found to be monestrous, as none of the unmated females reentered estrus after met- or 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 com- pared to other sciurid species and perhaps a product of constraints producing a strict time frame for reproduc- tion. © 2000 Academic Press Key Words: ground squirrel; vaginal cytology; estrus; estrogen changes. In many mammals seasonal events such as repro- duction occur under strong time constraints. Variation in their timing often results in dramatic differences in individual survival and reproductive success (Michener, 1984). In line with this, mating in the Eu- ropean ground squirrel, Spermophilus citellus, occurs in a well-defined time frame in spring, within 3 weeks after females emerge from hibernation (Millesi, Huber, Dittami, Hoffmann, and Daan, 1998). In previous studies we have shown that the latency to enter estrus after hibernation differed among individual females (Millesi, Huber, Everts, and Dittami, 1999a). Some females mated within 2 days after emergence, while others waited up to 3 weeks. The timing of mating and conception produced pronounced differences in re- productive output: early emerging females had larger litters and more male offspring than their late-emerg- ing counterparts (Millesi et al., 1999a). Mating and conception were delayed among most yearling fe- males, whereas among older females latency to mate was positively correlated with reproductive effort during the previous season. Females that lactated for longer periods one season had longer latencies to be- come estrous the next (Huber, Millesi, Walzl, Dittami, and Arnold, 1999). In earlier studies we also docu- mented that juveniles in our study population were born in early May after a 4-week gestation period. They remained in the natal burrow for about 1 month and weaning began in mid June depending on litter size and the mother’s condition (Millesi et al., 1999a). Adult and yearling females entered hibernation in August, hence the time for molt and prehibernatory fattening was limited. As a consequence, late-repro- ducing females weaned their offspring earlier to have ample time to prepare for hibernation. Furthermore, late-born juveniles had less time overall to grow and fatten for hibernation. Timing of conception is thus an important compo- nent of life history strategies among European ground squirrels, as well as many other mammals. For this reason we examined the role of immediate environ- mental cues in its control. Potentially important fac- 1 To whom correspondence should be addressed at Institute of Zoology, University of Vienna, Althanstr. 14, A-1090, Vienna, Aus- tria. Fax: +43-1-31336778. E-mail: eva.millesi@univie.ac.at. Hormones and Behavior 37, 190–197 (2000) doi:10.1006/hbeh.2000.1574, available online at http://www.idealibrary.com on 0018-506X/00 $35.00 Copyright © 2000 by Academic Press All rights of reproduction in any form reserved. 190