Message lizards are Darwinist fabrications which are used to carry messages. They are used on the Leviathan to carry orders and other messages, and they are also used in Japan to carry messages among civilians.
Although they are "not properly cold-blooded," they are not very effective in cold weather, as Deryn notices when the Leviathan crashes in Switzerland. When Deryn makes a sliding escape, the lizard that she sent ahead of her manages to jump out of the way when she goes by. When nervous, they will also sometimes repeat random snatches of conversation. The lizards travel around the Leviathan in tubes, which causes them to sometimes pop out when not expected. Dr. Erasmus manages the lizards on the airship, and has one on a dissecting table in the lizard room. Alek and Deryn visit the lizard room when Alek is trying to find a way to escape from the Leviathan.
Count Volger is always suspicious of the lizards, refusing to say anything secret when the creatures are anywhere nearby. However, he dresses as one for the Zoological Society fancy dress party. Though Alek is at first uncomfortably with the creatures, he eventually uses them to send messages frequently.