Serum progesterone and estradiol-17 beta concentrations in captive and free-ranging adult female Japanese black bears (Ursus thibetanus japonicus)

Progesterone(P-4) and estradiol-17 beta (E-2) concentrations were measured in serum samples obtained from 23 captive and 23 free-ranging adult female Japanese black bears. We then determined the relationship between changes i n these sex steroid hormones and pregnancy. In all captive bears, which inc luded animals of both known and unknown reproductive status, serum P-4 conc entrations were low from April to July, then tended to become higher after August. The levels then became much higher still in November and December, but returned to low levels in March. Serum P-4 concentrations in eight capt ive pregnant bears, which had parturitions the following spring, increased gradually from August (0.5-2.4 ng/ml) to October (0.9-3.6 ng/ml), and achie ved significantly higher maximum levels in December (7.2-18.0 ng/ml). There after, serum P-4 concentrations tended to decrease (3.5-6.4 ng/ml in Januar y and 0.3-0.7 ng/ml in March). In all captive bears, serum E-2 concentratio ns varied from April to October but showed low levels in November and Decem ber, and became high in January. Serum E-2 concentrations in the eight preg nant bears were high in May (95.6-191.4 pg/ml) and varied from August to Oc tober (35.6-143.3 pg/ml). Subsequently, serum E-2 concentrations in Decembe r dropped to significantly lower minimum levels (5.3-11.9 pg/ml) and increa sed again in January (67.6-153.1 pg/ml). Among the free-ranging bears, the data on serum P-4 concentrations in eight bears led to expectations of preg nancy, whereas serum E-2 concentrations showed no distinct evidence related to pregnancy. These results, particularly in captive pregnant bears, indic ate that a marked increase of P-4 in December might be accompanied by react ivation of the corpus luteum preceding implantation. Furthermore, changes i n E-2 concentrations suggested the possibility that a decline in December a nd an increase in January are associated with implantation and parturition, respectively.