Serum progesterone, oestradiol, luteinizing hormone and prolactin profiles in the female black bear (Ursus americanus)

Identifying steroid and pituitary hormone profiles in the female black bear (Ursus americanus) throughout pregnancy may provide a greater understanding of the reproductive cycle and indicate which hormones are required for implantation. Our objective was to assess endocrine activity in black bears oestrus onset, at oestrus, during pregnancy and after parturition. Serum samples were obtained from 12 captive, 16 uncollared and five radiocollared free-ranging female black bears from March through the end of December and assayed for serum progesterone, oestradiol, luteinizing hormone (LH) and prolactin (PRL). In captive bears, progesterone concentrations were low at days 0–10 after oestrus and increased significantly days 25–35 and 45–52 after oestrus. Oestradiol concentrations were high at oestrus (day 0) and days 4–10 after oestrus and then decreased days 25–35 and 45–52 after oestrus. LH concentrations were not significantly different throughout the sampling period. Changes in PRL concentrations pattern were similar to those of oestradiol, with elevated levels at oestrus and days 4–10 after oestrus, followed by a significant decrease 45–52 days after oestrus. In non-collared free-ranging bears, progesterone concentrations increased gradually after mating with a further significant increase in November–December. Oestradiol concentrations were highest in March (before mating) and in June (during mating) followed by a significant decrease in July (early delay period) and November–December (peri-implantation period). LH concentrations were low until November–December and then increased significantly. PRL concentrations were low in March (before mating), increased significantly during the mating season in June, decreased slightly in July, and were low in November–December (peri-implantation period). In radiocollared free-ranging bears, serum progesterone concentrations were elevated in pregnant bears in December and extremely low in lactating and non-lactating bears in March. Oestradiol levels were slightly higher in pregnant bears in December than in non-lactating or lactating bears in March. PRL concentrations were considerably higher in lactating bears in March than in pregnant bears in December. Our results suggest that: (1) serum progesterone concentrations are low, but detectable during the early delay implantation period and greatly elevated during the peri-implantation period; (2) serum oestradiol concentrations are elevated at oestrus and decline during the delay period; (3) LH may be involved in luteal activation; and (4) the decline of serum PRL concentrations during short days may be necessary for implantation to occur.