It all began one day a dozen or so years ago when Mrs. Martini's husband, head keeper of the Lion House at the Bronx Zoo, brought her a new-born lion cub and asked her to save its life. A year or so later three tiger cubs in need of a foster mother's care found refuge in the Martinis' small apartment in the Bronx. A black leopard baby followed--and Mrs. Martini was launched upon a career that was hugely to her liking.