06 March 2009
Vol 323, Issue 5919

About The Cover

Cover image expansion

COVER Gray and black wolf pups howling in Wolf Park in Battle Ground, Indiana. In North American gray wolves (Canis lupus), a mutation that alters coat color from brownish-gray to black, which derives from past hybridization with domestic dogs, is increasing in forested habitats because of positive selection. See page 1339). Image: Monty Sloan