REPRODUCTION, PREGNANCY, AND CIRCULATING REPRODUCTIVE HORMONES IN THE LACTATING SIBERIAN HAMSTER (PHODOPUS SUNGORUS)

The Siberian hamster (Phodopus sungorus) has a post-partum estrus in which it may mate, ovulate, and become pregnant. The pregnancy proceeds as a normal pregnancy while suckling young. Delayed implantation does not exist in this species. If the female fails to become pregnant during the post-partum estrus, at least one and possibly two other estrous cycles follow in which she may mate and become pregnant. Of 88 litters born after the first litter from 20 pairs of Siberian hamsters during their reproductive life span the days elapsing/mean number of young (± SEM) were as follows: 18–23 days (5.85 ± 0.3, n = 33), 24–29 (5.16 ± 0.4, n = 19), 30–35 (4.25 ± 0.6, n = 8), 40–49 (4.33 ± 0.4, n = 16), 50–52 (5.80 ± 1.0, n = 5), 62–65 (4.00 ± 1.2, n = 3), 70–78 (2.75 ± 0.6, n = 4). No correlation could be found between the number of young nursing and the number of days between litters. Circulating plasma levels of progesterone (P), prolactin (Prl), luteinizing hormone (LH), and follicle-stimulating hormone (FSH) were radioimmunoassayed from lactating pregnant and non-pregnant Siberian hamsters and from lactating Syrian and Chinese hamsters at 2, 5, 10, and 15 days post-partum. At 5 days post-partum, both pregnant and non-pregnant Siberian hamsters exhibited significantly higher levels of P than Syrian or Chinese hamsters. Prl levels were significantly lower in Siberian hamsters at 2 and 5 days post-partum when compared with those in Syrian hamsters. Selected rank-order comparisons showed a strong negative correlation (P<.001) between P and Prl for pregnant lactating Siberian hamsters.