Toronto Zoo’s program is producing more female wood bison – and it’s not a bull

Eric Zweifelhofer is a postdoctoral researcher.
  • Wood bison walk around their enclosure at the Toronto Zoo in 2015.

Wood bison once numbered in the hundreds of thousands, but due to overhunting and disease, their population has drastically declined and they are currently classified as threatened under the Species at Risk Act (SARA).

Toronto Zoo and University of Saskatchewan researchers have collaborated over the past 14 years to implement cutting-edge reproductive technologies, including semen collection, artificial insemination and embryo transfer to help conserve wood bison, Canada’s largest land mammal, with males weighing over a year. tonne.

This breeding season, our goal was to increase the proportion of females born in managed care using sperm carrying the X chromosome (female).

In mammals, the sperm fertilizing the egg determines the sex of the offspring: X chromosome = female, Y chromosome = male. In zoos, female and male wood bison are housed separately under certain circumstances, as males may exhibit aggressive behaviors that are difficult to manage.

Using reproductive technologies, a single semen collection from an individual bison bull could be used to artificially inseminate over 300 females. However, each year a female can produce only one calf, which makes the female the limiting factor and therefore the preferred sex for a conservation breeding program.

For our recent study, semen was collected from 2,000-pound mature wood bison at the University of Saskatchewan. Using technology originally marketed in dairy cattle, the X-carrying (female) sperm were separated from the rest of the semen sample. The sorted sperm were then placed in small straws, each containing millions of sperm, frozen and finally airlifted to the Toronto Zoo in September.

Female wood bison at the zoo have undergone hormone treatments to synchronize their ovarian cycles so they can all be artificially inseminated with sex-sorted sperm at the time of ovulation to maximize pregnancy success.

To determine if the sex-sorting technique would result in a difference in the sex ratio of bison, half of the females were inseminated with sex-sorted semen while the other half was inseminated with unsorted semen.

Similar to human pregnancy monitoring, we used ultrasound to confirm bison pregnancies by visualizing heart rate 30 days after insemination and to detect fetal sex approximately 70 days after insemination. At the end of the breeding trial, we had four pregnant bison, one of which was from sex-sorted semen and was a confirmed female.

Combined with results from the concurrent trial at the University of Saskatchewan, the trials resulted in nine women from 10 pregnancies with semen sorted by sex. These results highlight the potential value of implementing semen sex sorting as a herd management tool for wood bison conservation programs.

We look forward to seeing four tan calves bounding around the Toronto Zoo’s wood bison habitat in mid-June performing “zooms” around their moms. For me, seeing these calves will make all the hard work worthwhile and show that the research we are doing can really make a difference to species conservation.

Eric Zweifelhofer is a postdoctoral fellow split between the Toronto Zoo and the University of Saskatchewan.

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Ryan H. Bowman