Data for 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 poorly studied in these species. We investigated the estrous cycle in yellow ground squirrels (Spermophilus fulvus), which hibernate up to 9 months. On the basis of vaginal cytology and serum progesterone and estradiol profiles, we identified proestrus, estrus, metestrus, and anestrus in the cycle. Similarly to 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 the 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 as compared with older ones. During postlactation, progesterone concentration diminished toward hibernation, and we failed to 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, thus supporting the hypothesis of fast life history strategy of S. fulvus.