Testing the orbital lubrication hypothesis: the Harderian glands in burrowing skinks (Reptilia: Squamata)

Abstract

The Harderian gland is an orbital gland thought to be a source of corneal lubricant, supplementary to the other orbital glands. This study investigated the possible role of skink Harderian glands in corneal lubrication. It was hypothesized that if these glands play a role in corneal lubrication, then the structure of these glands would be affected by structural orbital modifications. We examined the Harderian and lacrimal glands of five species of Australian skinks (Lygosominae), two of which possessed orbital modifications in the form of a transparent immoveable eyelid (spectacle) and skull reduction. All species possessed well-developed posterior lacrimal glands, but no anterior lacrimal glands. Anatomically, the Harderian glands were smaller in the burrowing species relative to the nonburrowing species. No other obvious species-specific differences were observed. The absence of any differentiation at the microscopic level suggests that although there is some change in the relative amount of secretant produced, the nature of the secretion studied by classical histochemistry remains essentially unchanged. However, at higher taxonomic levels, the size and structure of the Harderian gland may be taxon-specific and unrelated to the orbital environment. Thus, orbital lubrication may not necessarily be the sole function of the Harderian gland.