Changes in Serum Steroid Levels in Female Red Pandas

Serum was collected from five female adult red pandas for 23 consecutive times during their estrus and gestation periods. The collected serum was analyzed through a radioimmunoassay to examine changes in estradiol (E2) and progesterone (P) content during the reproductive period of the red pandas. The results revealed the following: (1) When the female red pandas entered their estrus periods, the serum E2 content increased rapidly and peaked several times. Only one peak was consistent with mating behavior. The range of the estrus period length was 14–48 days, with the average length being 28.26 ± 13.24 days (n = 5). Peaks in serum E2 content were observed during the gestation period of pregnant red pandas and during the luteal phase of nonpregnant red pandas. (2) The serum P content increased during the gestation period of pregnant red pandas and luteal phase of nonpregnant red pandas. The P content started to increase after mating. Accordingly, we can deduce the following: (1) multiple estrus periods occur in red pandas during the reproductive season; (2) follicle development still occurs in the gestation period and luteal phase; (3) no evidence of delayed embryo implantation was found in captive breeding red pandas in Fuzhou, China, according to the change trend of the P content.