When Nancy Kilian, a self-described horse lover, spotted a brown-and-white pinto mare standing among the sagebrush and juniper trees near Carson City, Nevada, a couple of weeks ago, she didn’t hesitate.

She loaded her darting rifle, aimed at the female’s rump, and fired.

“She jumped a bit,” Kilian recalled, “because it stings, but settled down. I knew I was doing her a favor.”

The favor: Injecting the one-year-old animal (whom Kilian recognized from a photo) with a contraceptive vaccine that triggers her immune system to reject fertilization. Kilian or another member of the American Wild Horse Preservation Campaign, a California-based nonprofit, will give the mare a booster in January via the same method.

Mustangs are a type of wild horse descended