Endocrine and electrophoretic profiles during pregnancy and nonpregnancy in captive female black bears

Abstract

We collected blood samples from captive adult female black bears (Ursus americanus) at 10-day intervals during September–March in 1987–1988 and 1988–1989 (6 bears/year) in Virginia. Ten of 12 bears hibernated for an average of 92 days. Serum progesterone concentrations slowly increased from 2 to 7 ng/mL during October and November, then increased 2- to 3-fold 58 ± 5 days before parturition in five bears that produced cubs. After the implantation peak, progesterone gradually declined, reaching undetectable levels 1–2 days postpartum. Similar but more variable progesterone profiles were observed in five of seven bears that did not produce any observed cubs. Progesterone was also assayed in 38 active wild black bears to relate to reproductive status in the den. Results suggested that pseudopregnancy, implantation failure, or fetal resorption may occur in black bears. Mean serum estradiol-17β concentrations were 18.5 ± 0.7 (SE) pg/mL in captive bears, with pregnant bears showing no change between implantation and parturition. Polyacrylamide gel electrophoresis of serum proteins was not a reliable indicator of pregnancy.