That got me interested in how well lizards hear. He does seem to react to sounds when he's awake, but is he just seeing things and reacting to those?
In my research I realized that animal hearing is really interesting, so I decided to make a weekly series about animals and their hearing.
First up: lizards!
Loki has a visible ear (actually his tympanic membrane). It looks like a hole in his head, outlined below in purple:
Lizards' tympanic membranes might look like Loki's, or they might look like an iguana's, almost level with the skin. Some are even like human's, recessed deep in the skull. They don't have ear flaps like humans do. The tympanic membrane picks up sound vibrations from what the reptile is in contact with - the ground, a basking rock, etc.
Most lizards seem to hear about the same. (Even ancient ones!) The green iguana and other lizards with tympanic membranes can pick up sounds in a range of 500-4,000 Hz. They show peak sensitivity at 24 dB. Compare that with humans: our typical range is 20-20,000 Hz and our sensitivity is at 120 dB. So, lizards cannot hear as well as a human, but they are probably more sensitive to ground movement than we are.
Reptile Hearing - Melissa Kaplan
San Diego Zoo