Plague rats.
Galvani's Speculation is an audiograph found in Dishonored, recorded by Doctor Galvani.
Transcript[]
Obviously the plague rat is distinct from the ordinary rat, but in what respect? In size, and the coarseness of its fur. And I believe, in intelligence although the experiments there are not complete. Coriander's zoological survey describes only the ordinary rat, which means plague rats have only been here for five or seven years at most. This was not a gradual migration. Could they have been introduced on purpose, perhaps by a foreign power?
Location[]
The audiograph can be found on Galvani's desk in his laboratory at his manor during the mission High Overseer Campbell.