Model organisms with soft skulls, such as the mouse and the neonatal rat, have traditionally been difficult to secure firmly in a stereotaxic instrument.
The stainless steel ear bars used for adult rats are too heavy to position precisely enough to provide a secure hold while still avoiding flexing the sides of the skull inward (which compresses the airways and impedes blood flow.) Further, the neonatal rat pup has no external opening of the auditory canal into which to insert the ear bar points.