Mexican gray wolves at Brookfield Zoo

4 of 13: Until 1998, when reintroduction efforts began, Mexican gray wolves were considered extinct in the wild. (There once were approximately 4,000 wolves in their historic range.) In May 1976, the species was listed on the Endangered Species List by the U.S. Fish & Wildlife Service (USFWS). ( Photo / Jim Schulz/Chicago Zoological Society)

REPLAY SLIDESHOW