The mountain lion known as P-47 survived fires, freeways and hostile ranchers. But in March, the 3-year-old big cat—tracked by California biologists since his kitten days—succumbed to a hidden hazard: an insidious form of food poisoning. Six anticoagulant compounds—chemicals used to kill rodents...

It seemed like a good idea at the time: Buy a house with a two-acre property, let our energetic herding dog have the run of the place and spend blissful summer days digging side by side in the dirt with her. And it was blissful, watching Mattie carve out her napping spots behind the ferns and tall...