The outer ear
It is composed of the pinna as well as an auditory canal and ends at the tympanic membrane. Each of these has their own roles and they are as follows:
1. The pinna
This outer part plays the main role in capturing audio signals as well as helping in cooling in case of temperature rise to avoid overheating or a heat stroke. Here are the main functions of the pinna:
Picking an audio signal
The size of the pinna in rabbits varies from the small ears in Netherland dwarf to those long large ears that the English lop has.
The up-eared (non-lopped) rabbits, can rotate their pinna by about 270 degrees to detect the source or direction from which of any sound is coming from as well as hear two different sounds concurrently.
They can be lowered backward, stand upright, rotate sideways or even forward and they can hear even faint sounds that human beings may not be able to hear. The pinna can also be part of a rabbit's ear body language.
Besides picking vocal sounds, the rabbit’s big ears provide a large surface area that helps to regulate their body temperature.
To help in thermoregulation, the pinna has a dense and extensive blood vessel network that can receive a lot of blood. Also, it has little or short fur with the inner part hardly having any fur.
When the temperature rises, more blood flow to the pinna since the blood vessels here dilate and the excess heat is dissipated before the cooler blood can flow back to the body. Sometimes, these critters may slowly move their ears back and forth to enhance cooling.
In cold weather, the blood vessels on the pinna constrict and reduce the amount of blood flowing to them and consequently reduce heat loss.
Finally, during very hot weather, a damp piece of cloth is rubbed on the ears or little water sprinkled to enhance heat loss by evaporative cooling.
2. The ear canals
The remainder of the outer ear is nothing other than the ear canal that delivers audio signals (sound waves) onto the tympanic membrane (eardrums) making it vibrate. Anatomically it has a bent, i.e., it goes vertically downwards then horizontally onto the eardrums.
The canals are prone to debris and earwax accumulation which can encourage infections (offer breeding grounds).
Whereas your pet might be able to clean any debris and wax using their back toenail, you need to help them remove excess wax, especially for the lopped-ear breeds. It is easy and safe since these animals have a bend as we have already mentioned.
Finally, this portion is often a place where you will find ear mites. However, sometimes these mites and the crusting symptoms they cause may extend outwards to the pinna and other body parts.
The middle ear
Also referred to as to the tympanic cavity, this part is filled with air and it is separated from the outer ear by the tympanic membrane. It has three small ossicle bones, the hammer (malleus), anvil (incus) and stirrup (stapes) that for a chain. The main role of the ossicles is to transfer the sound waves or vibrations picked from the eardrums as they vibrate to the inner ear, which is filled a liquid fluid via the oval window.
The ossicles amplify sound while if the sound is too much, the stapedius muscle will help impede too much vibration to avoid inner ear damage.
To help balance pressure on the inner part of the tympanic membrane and the outer part, there is a valve controlled narrow passage or tube known eustachian tube that connects the inner ear and throat.
Although the eustachian tube is valve controlled and plays the vital pressure balance role, it is often a gateway for infectious bacteria to go into the middle ear to cause various diseases including otitis media (pus-producing).
If the eustachian tube gets closed or clogged after infection by otitis media, pus buildup can generate so much pressure that will cause eardrum rupture and hence pus will be coming from your rabbit’s ear canal.
The inner ear
The inner ear is made up of a maze of passages and tubes often referred to as to the labyrinth composed of the vestibular system and the cochlea which a spiral or snail-shaped tube.
The labyrinth has two parts, the bony labyrinth and inside it, there is the membranous labyrinth. The membranous labyrinth is filled with the fluid call the endolymph while the outer walls on the membranous labyrinth and the bony labyrinth (between these two parts of the labyrinth), there is the perilymph.
1. The cochlea
The cochlea plays the role of processing the sound vibrations transmitted from the ossicle via the oval window before these signals can be transmitted to the brain. The inner side of the cochlea has the basilar membrane which has varying stiffness and thickness from one end to the other one to give each part different resonating frequencies.
The organ of Corti that lines the basilar membrane has hair-like structures, the sensory cells.
How are these sensory cells important? “When a sound wave reaches the portion of the basilar membrane with the same resonant frequency, its associated hair cells send nerve impulses to the brain to give a perception of the sound's pitch”  via the auditory nerve.
Should there be excessive endolymph fluid movement, often due to a loud sound, some hair cells on the organ of Corti may die causing partial deafness and your pet may not recognize your voice if you try calling it.
Once the auditory cells have the impulse, they send it to the brain for interpretation and a rabbit can detect which sound it was.
2. The vestibular apparatus
The vestibular apparatus helps sense spatial orientation, equilibrium, and coordinated motion. It ensures that this pet can maintain balance. It has as the utricle, saccule and three semicircular canals that works together to maintain balance and proper body coordination.
Saccule and utricle – static equilibrium
The saccule and utricle detect head position relative to gravity and this helps a rabbit detect if its head is tilted. It has tinny hair-like structures that have microscopic calcium carbonate on their top, the otolith crystals.
During movements, the weight of calcium carbonate crystals will push, pull or bend these hairs to the direction of gravity or opposite the direction of head movement and this action will make the nerves at their base sense and relay the movement to it to the brain via the vestibular nerve.
The information generated from these nerves will help the rabbit know if it has flopped, is upside down or if it is on the correct posture. To help keep the head in the normal position, there is a ‘righting reflex action’ that signals the neck muscles to relax or contract to ensure the head is in its normal position.
Semicircular canals – dynamic equilibrium
On the other hand, the three semicircular canals help the rabbit to know in which direction the head is moving in three a dimensional space as well as give information about spinning or rotation.
They are the horizontal, superior and posterior semicircular and each of these canals are oriented in different panes in such a way that each is perpendicular to the other two, and they are filled with endolymph fluid and have hair cells.
When the head moves forward or backward, upwards or downwards, sideways or spins, the fluid moves in the same direction, so does the hair-like cells. However, due to inertia, there is a lag on the fluid and hair-like cells will bend.
Once they bend, the hair cells will then transmit this information to the brain via the vestibulocochlear nerve and the rabbit is able to know which direction it is moving into and the brain decodes it accordingly to help keep balance.
Therefore, the rabbit can detect back, forward, turning or tipping of the head sideways as well as spinning movements.
What if the labyrinth has a problem?
If the labyrinth is working well, your rabbit can keep the correct orientation to its environment and maintain balance.
However, if it is missing, gets damaged or affected by a disease, injury, and so on, your rabbit may receive a false signal on its spatial orientation and it may not be able to maintain normal balance.
When a rabbit has an impaired labyrinth, he or she will have muscle relaxation on the damaged side (let us say the right side) while those on the opposite side (the left side) of a rabbit’s head will tense. This will make him feel like his head is tilted towards the left side.
However, there is no actual movement, and this is only perceived since the brain will have received a false signal from the damaged labyrinth and the rabbit will attempt to fix this perceived misorientation.
Therefore, to correct this perceived misorientation, a righting reflexing action will make your bunny hold his head tilted to the right side or twisted down towards the affected side (tilt or twist depending on which of the three semicircular canals received the strongest stimulation).
Since this is a false signal from the brain, this pet will hold his head in this position, try to roll or even have nystagmus movement (rapid side to side eye movement). Also, circling or rolling in rabbits with neurological disease including E. cuniculi is often noted.