Serial ovarian ultrasonography in wild-caught wood bison (Bison bison athabascae)

The objectives of this study were to determine the feasibility of daily examination of wild-caught wood bison and to characterize the ovarian function using serial transrectal ultrasonography and blood hormone analysis. Ten 2-year-old wood bison heifers obtained from Elk Island National Park were placed in a corral adjacent to a handling system designed for restraining bison. The handling system was left open to the corral allowing the bison to explore it freely for 2 months. Active acclimation followed for a 2-week period, during which the bison were herded daily through the handling system and rewarded with whole oats. Finally, the bison were restrained in the handling system and rewarded with whole oats upon release. Once conditioned, daily transrectal examination of the ovaries was completed in 100% of attempts for 30 days (January-February) using a B-mode scanner with a 5 to 10-MHz linear array. Follicle size and numbers were recorded, and individual follicles were identified serially. Blood samples were collected daily and the serum was analyzed for FSH concentrations. Nonrandom changes were detected in the number of follicles ≥4 mm in diameter per day (P < 0.05). Each peak in follicle numbers was associated with the development of a single dominant follicle. The interval between the emergence of successive dominant follicles was 6.8 ± 0.6 days (mean ± SEM). The maximum diameter of the dominant follicle was 9.9 ± 0.4 mm. In conclusion, wild-caught wood bison were amenable to daily examination and blood sampling, and ovarian dynamics were characterized by wave-like development of anovulatory antral follicles. The demonstrated success of this approach to the study of ovarian function will be useful for characterizing the annual reproductive pattern in wood bison, which is necessary for the development of bison-specific protocols for controlling ovarian function for species conservation.