Fertility and fecundity decline with advancing age in female mammals, but reproductive aging was decelerated in Siberian hamsters (Phodopus sungorus) raised in a short-day (SD) photoperiod. Litter success was significantly improved in older hamsters when reared in SD and the number of primordial follicles was twice that of females held in long days (LD). Because anti-Müllerian hormone (AMH) appears to inhibit the recruitment of primordial follicles in mice, we sought to determine whether the expression patterns of AMH differ in the ovaries and serum of hamsters raised in SD versus LD. Ovaries of SD female hamsters are characterized by a paucity of follicular development beyond the secondary stage and are endowed with an abundance of large eosinophilic cells, which may derive from granulosa cells of oocyte-depleted follicles. In ovaries from 10-week-old SD hamsters, we found that the so-called ‘hypertrophied granulosa cells’ were immunoreactive for AMH, as were granulosa cells within healthy-appearing primary and secondary follicles. Conversely, ovaries from age-matched LD animals lack the highly eosinophilic cells present in SD ovaries. Therefore, AMH staining in LD was limited to primary and secondary follicles that are comparable in number to those found in SD ovaries. The substantially greater AMH expression in SD ovaries probably reflects the abundance of hypertrophied granulosa cells in SD ovaries and their relative absence in LD ovaries. The modulation of ovarian AMH by day length is a strong mechanistic candidate for the preservation of primordial follicles in female hamsters raised in a SD photoperiod.

Golden (Mesocricetus auratus) and Siberian (Phodopus sungorus) hamsters are widely used as animal models for seasonal reproduction; but M. auratus shows no developmental delay in short days until after sexual maturity, whereas P. sungorus juveniles delay development in short days. As the photoperiodic response of Phodopus campbelli is not well established, litters of the two Phodopus species were gestated and reared under long days (14 h light:10 h darkness) or short days (10 h light:14 h darkness) until 70 days of age. As expected, under short photoperiod P. sungorus showed reduced body, testes, epididymides, uterus, and ovary weight; antral follicles and corpora lutea were absent and vaginae remained closed. Animals moulted to winter pelage, and low concentrations of each of leptin, testosterone, and prolactin were present in male serum. Phodopus campbelli juveniles also responded to the short photoperiod as measured by reduced body, testes, epididymides, and ovary weight. The summer pelage persisted. However, both sexes of P. campbelli developed functional reproduction under 10 h light:14 h darkness. All females had a patent vagina by 10 weeks; ovaries contained antral follicles and corpora lutea, and uteri were not reduced in weight. In males, the concentrations of testosterone, leptin, and prolactin were not reduced by short photoperiod. Developmental patterns in the three species of hamster, therefore, differ and are not predicted by relatedness or latitude of origin. Other ecological traits, such as predictability of summer rainfall, ambient temperature, and differential responses to social cues might be important.

The naked mole-rat (NMR, Heterocephalus glaber) is renowned for its eusociality and exceptionally long lifespan (> 30 y) relative to its small body size (35–40 g). A NMR phenomenon that has received far less attention is that females show no decline in fertility or fecundity into their third decade of life. The age of onset of reproductive decline in many mammalian species is closely associated with the number of germ cells remaining at the age of sexual maturity. We quantified ovarian reserve size in NMRs at the youngest age (6 months) when subordinate females can begin to ovulate after removal from the queen’s suppression. We then compared the NMR ovarian reserve size to values for 19 other mammalian species that were previously reported. The NMR ovarian reserve at 6 months of age is exceptionally large at 108,588 ± 69,890 primordial follicles, which is more than 10-fold larger than in mammals of a comparable size. We also observed germ cell nests in ovaries from 6-month-old NMRs, which is highly unusual since breakdown of germ cell nests and the formation of primordial follicles is generally complete by early postnatal life in other mammals. Additionally, we found germ cell nests in young adult NMRs between 1.25 and 3.75 years of age, in both reproductively activated and suppressed females. The unusually large NMR ovarian reserve provides one mechanism to account for this species’ protracted fertility. Whether germ cell nests in adult ovaries contribute to the NMR’s long reproductive lifespan remains to be determined.