White-nose syndrome, a fungus that has decimated bat populations across eastern North America, may be spread primarily by female bats. That’s the conclusion of a new study from Pennsylvania, which tracked the genetics of little brown bats (Myotis lucifugus, pictured above), a species hit particularly hard by the disease . The plague began sweeping through eastern Pennsylvania in January 2009, but western corners of the state remained disease-free for another 2 years. It has spread hundreds of kilometers from this pocket in the northeast Appalachian Mountains, while some parts of nearby western Pennsylvania remain untouched even today. The scientists examined eastern hibernating colonies positive for white-nose syndrome in 2009 and western habitats that were negative through 2011 to 2012, according to a report published in the May-June issue of the Journal of Heredity. By comparing mitochondrial genes, which are maternally inherited, versus nuclear genes passed by both parents, the team could assess how both sexes traveled through Pennsylvania . Traits in the nuclear DNA did not significantly vary, suggesting males freely move and mate across the state. Maternal traits, in contrast, segregated between eastern and western sites. The researchers were surprised that female movements also paralleled the intensity of how white-nose syndrome coursed through the state. Studies show that female little brown bats tend toward philopatry—sticking around their place of birth. But if they do leave home, the results suggest that they follow geographical landmarks, such as the Ridge-and-Valley Appalachians. Maternal traits flowed along the state’s eastern mountains into an additional colony in West Virginia, but receded at the Appalachian Plateau in western Pennsylvania. The results don’t peg female bats as the primary carriers of white-nose syndrome, but simply suggest they influence the timing and pattern of spread, given the disease map matched that of maternal traits. If this gender bias holds as the disease creeps into the Midwest , conservation officials may want to focus treatment strategies on female bats, the researchers say.