On another layer, using a one pixel brush at 50% or less opacity, pick up color from the edge of the squirrel and stroke individual hairs. (using the ALT key changes the brush to the eyedropper tool)Again be careful to vary the length and direction of these strokes. Have some of them cross over other hairs for a more realistic appearance. Keep in mind also the varying length of fur that exists on different portions of the animals body.