Young raccoon - Bill Buchanan/USFWS.

John Heinz National Wildlife Refuge is 1 of 44 Important Mammal Areas designated by the Pennsylvania Wildlife Federation. The designation was awarded noting the refuge as supporting northern river otter use on occasion and being the last potential location for the marsh rice rat (Oryzomys palustris) in the State.

Numerous mammals are known to inhabit the refuge. The gray squirrel (Sciurus carolinensis) is a common species found throughout upland habitats of the refuge, where it plays an important role in seed dispersal. Other common open space species supported by the refuge include the northern shorttailed shrew (Blarina brevicauda), the meadow vole (Microtus pennsylvanicus), white-footed mouse (Peromyscus leucopus) and several other rodent species, as well as raccoons (Procyon lotor), mink (Mustela vison), skunks (Mephitis mephitis), opossums (Didelphis virginiana), and eastern cottontail rabbit (Sylvilagus floridanus) (PNHP 2008). Woodchuck (Marmota monax) and red fox (Vulpes vulpes) have been observed attemptting to burrow dens into dikes. 

Muskrat (Ondatra zibethica), long-tailed weasel (Mustela frenata), and least shrew (Cryptotis parva) are fairly common. Recent records also indicate beaver (Castor canadensis), American mink (Neovison vison), and river otter (Lontra canadensis) occur occasionally on the refuge. It is also likely that the refuge sees occasional use by coyote, which have been documented on adjacent property at Philadelphia International Airport. Bats are frequently observed on the refuge during warmer seasons and a formal species diversity and population survey would provide valuable information on recent declines of these important creatures due to white nose syndrome and habitat disturbances.

White-tailed deer (Odocoileus virginianus) are another mammal supported by the refuge. Refuge staff regularly conduct on-the-ground deer population surveys.

Two nonnative species present include the Norway rat (Rattus norvegicus) and house mouse (Mus musculus). Also, feral domestic house cats pose a serious invasive mammalian predatory threat to all small native wildlife (birds, mammals, reptiles and amphibians).