Like many science labs, the Conservation Biology Institute inside the Smithsonian’s National Zoo is brimming with vials. Maybe it’s because I was six months pregnant when I visited, but the samples being stored in these vials, held at the institute’s animal nutrition department, seemed especially priceless.

These are milk samples—exotic animal milk samples, to be exact, and the National Zoo has the largest and most diverse collection of them in the world.

One upright freezer holds a dozen or so little bottles filled partway with a brownish substance and sitting in a tray marked “fur seal.” Cardboard boxes with the word “marmoset” scribbled on them line the shelves of a walk-in freezer. Around the corner, a chest freezer is set at