"Boyd walked the shore looking for muskrat sign"
Creative Commons AttributionMuskrat (Ondatra zibethicus) - Credit: Jumpingmaniac

The muskrat is a semi-aquatic rodent found over most of Canada and the United States and a small part of northern Mexico. They mostly inhabit wetlands, areas in or near salt and fresh-water marshlands, rivers, lakes, or ponds. 

The animal is named after the yellowish, musky-smelling substance that it excretes (probably as a means of communicating with other muskrats during its breeding season).