Female hormonal profiles and vaginal cytology in a ground squirrel species with prolonged hibernation.

Hibernating mammals have annual cycles with strict timing of all processes including reproduction, yet female reproductive physiology is only poorly studied in such species. We investigated the estrous cycle in yellow ground squirrels (Spermophilus fulvus), which hibernate up to 9 months. Using vaginal cytology and serum progesterone and estradiol profiles, we identified proestrus, estrus, metestrus, and anestrus in the cycle. As in other rodents, predominance of cornified cells marked estrus, and an increase in the leukocyte number reflected metestrus. The only estrus in a year started, and then, pregnancy occurred mostly within 3 days after spring emergence. In one female, we noticed a copulatory plug for the first time for this species. The progesterone level increased from proestrus/estrus to pregnancy and decreased in the postlactation period. Estradiol concentration did not change significantly throughout the estrous cycle but varied among females and was consistently high in some of them. The hormone levels did not vary with female age despite smaller body size in yearling females than in older ones. During postlactation, progesterone concentration diminished toward hibernation, and we could not detect signs of prehibernation activation of the female reproductive system. Probably, gonadal preparation in S. fulvus females proceeds at periodic arousals during hibernation. Our data showed fast transitions between phases of the female estrous cycle and early maturation of juvenile females, consistently with the hypothesis of the fast life-history strategy of S. fulvus. [ABSTRACT FROM AUTHOR]