Matt Miller, an Idaho-based science writer with The Nature Conservancy, says rodents and other small livestock represent a low-impact meat alternative to carbon-costly beef. Miller, who is writing a book about the ecological benefits of eating unconventional meats, visited Colombia several years ago. At the time, he says, conservation groups were expressing concern about local ranchers clearing forest to provide pasture for their cattle — activity that was causing erosion and water pollution. "They were encouraging people to switch from cattle to guinea pigs," Miller says. "Guinea pigs don't require the land that cattle do. They can be kept in...