It may seem an unusual proposal for a dwindling wildlife species, whose numbers we usually want to increase. But a study just published in the Journal of Zoo and Wildlife Medicine reveals that artificial contraception can work to keep American bison from reproducing, apparently with no ill effects on the bison.
Most biologists working on bison want the critters to be fruitful and multiply as much as they can healthily: the species' numbers are down by about 99.5 percent from their historic levels, estimated to be somewhere around 60 million animals in 1492. But this study came about in special circumstances, as scientists struggle to manage an introduced population of bison in a limited habitat: Santa Catalina Island, off the Southern California coast.
Brought to the island in 1924, Catalina's bison had thrived to the point where their numbers threatened the well-being of wildlife that actually belonged there. But an innovative experiment launched in 2009 suggests that through birth control, the island's bison can be humanely kept to a level the island can support.
Catalina's bison herd reached about 600 animals during the 20th Century, and the pressure the multi-ton grazers put on the island's vegetation was considerable. Until 2009, the Catalina Island Conservancy -- which manages 88 percent of the island -- had been rounding up excess bison and shipping them to the mainland. The Conservancy had calculated that the island could support somewhere between 150 and 200 of the animals.
But roundups were huge undertakings, creating hazard and stress for both the bison and their managers. And between those roundups, bison numbers just grew again.
So in 2009, the Conservancy's scientists started injecting female bison with porcine zona pellucida (PZP), a veterinary contraceptive that had previously been used on other ungulate animals such as horses and deer.
"Shipping the bison to the mainland was costly, and it raised concerns about the stress on the animals during shipment and the expansion of the herd beyond ecologically sustainable numbers between shipments," said study co-author Julie King, the Conservancy's director of conservation and wildlife management. "We launched the contraceptive program because it is a humane and cost-effective solution to managing the herd and protecting the Island's resources."
The results, according to the study, have been promising. Before PZP, about two in three female bison would bear young each year. That dropped to one in ten the first year PZP was used, and around one in thirty the second year. By last year, just one new bison was born on Catalina: in honor of his uniqueness, the Conservancy dubbed the calf "Uno."
As administering contraceptives to bison needs to be done at long rage with a dart gun, the process doesn't afford much opportunity for a thorough checkup beforehand. Fortunately, PZP seems not to have a significant effect on calves if it's administered to a bison cow that turns out to have already been pregnant when she got darted. The trick is now to see how long it takes the drug to wear off, should Conservancy biologists determine they can let nature take its course again.
"The bison contraception program is a good example of trying to reach a balance with cultural, aesthetic or recreational needs and uses and cost-effective natural resource management to maintain the health of the ecosystem," said John J. Mack, the Conservancy's chief conservation and education officer.
"By proving the effectiveness of this humane approach to herd management, this research will be a benefit to bison herds throughout the U.S.," added the Conservancy's CEO and President Ann Muscat. "It also lays the groundwork for further contraceptive studies in other wild species."