An animal detection unit detects cold-blooded animals and small mammals traversing a tunnel or other constricted area. The animal detection unit includes an elevated threshold that extends across a width of the tunnel or other constricted area. A photo emitter generates a light beam. The photo emitter is located at a first end of the elevated threshold. A photoelectric receiver receives the light beam. The photoelectric receiver is located at a second end of the elevated threshold. The light beam is low enough with respect to the elevated threshold that cold-blooded animals and small mammals moving over the elevated threshold are raised in height sufficiently to block the light beam as the cold-blooded animals and small mammals move over the elevated threshold. A camera is mounted on the frame. The camera is aimed and focused to capture visual recordings of the cold-blooded animals and small mammals as they move over the elevated threshold. The photoelectric receiver sends a trigger signal when an animal blocks the light beam. The camera captures a first visual recording of the animal when the photoelectric receiver sends the trigger signal.