In a camping tent in the Peruvian jungle, four scientists are crowded around a tiny patient: An Amazonian rodent that could fit in the palm of a human hand.

The researchers placed the small-eared pygmy rice rat into a plastic chamber and piped in anesthetic gas until it rolled over, asleep. Removing the creature from the chamber, they fitted it with a miniature anesthetic mask and measured its body parts with a ruler before gently pulling hairs from its back with tweezers.

The hairs, bundled into a tiny plastic bag, would be carried to a nearby lab at the Los Amigos Biological Station for testing to determine whether the rat is yet another victim of mercury contamination.