Mark Zuckerberg has upped the ante in his dietary adventure. As we reported last May on Fortune.com, the Facebook CEO pledged that this year "the only meat I'm eating is from animals I've killed myself." He started out by slaughtering a pig, goat and chicken. Now the Silicon Valley billionaire has expanded his menu. Zuckerberg has learned to hunt, according to people close to him. He got a hunting license and recently shot and killed a bison.

Since one of his goals is to eat "a lot healthier foods," as Zuckerberg, 27, told me last May, the choice is a logical one. Bison meat, which is tender and sweeter than beef, is also lower in fat and cholesterol. Through a Facebook spokesman, Zuckerberg declined to comment. But last Thursday at F8, the company's developers conference, he delivered a hint about his culinary evolution. As he gave his keynote speech, Zuckerberg's Facebook page loomed on a big screen behind him. On the left side of the page: a photo of a juicy-looking sandwich and two words: "Bison Burgers."