Chapter 1, The Reptilian Spectacle - UWSpace - University of ...

snakes have no need of a spectacle as they do not all fit the theory’s requirement of nocturnal activity
patterns and deserticolous habitat.
In discussing the protective nature of the spectacle, one should obviously consider its
mechanical properties. Unfortunately these properties have eluded rigourous and systematic inquiry
but have nevertheless elicited anecdotes such as Walls’ and Johnson’s quotes above and the following
anecdote of my own: While studying a marbled gecko (Gekko grossmanni), a nocturnal arboreal
species, I lightly rubbed my hand quite by accident against its spectacle. Remarkably (and regretfully),
this resulted in a deformation on the spectacle surface, resembling a tear, which was clearly observable
with a slit lamp. Fortunately, the gecko’s damaged spectacle was improved after the following moult,
and completely renewed after a second moult, the new stratum corneum showing nothing of the earlier
blemish. As this indicates, the delicate gecko spectacle does not provide the same degree of robustness
against ocular trauma as a snake’s. No such deformation ever occurred by rubbing or abrading their
spectacles, which attests to their durability. This fragility of the gecko spectacle may draw suspicion to
the belief that the spectacle evolved for the purpose of mechanical protection, but again it must be
borne in mind that current incarnations imply nothing of the original function. And while an arboreal
gecko’s habitat and lifestyle, nocturnal or not, may make it less likely to suffer insults to the eye than a
snake that uses its head to push through anything in its path, and while it is equally endowed with the
ability to regularly replace damaged spectacle scales, it nevertheless emphasizes the need to consider
alternative theories of the function of spectacles.
1.5.2 Minimization of Evaporative Water Loss
Arnold (1973) proposed another such theory by suggesting that spectacles and windowed eyelids may
minimize evaporative water loss from the cornea. According to Reichling (1957, cited in Arnold 1973),
water loss from the tear layer could account for up to 20% of the water loss in Lacerta agilis, quite a
high proportion considering this lizard has small eyes proportional to its body size. Evaporation from
the surface of the eye would be of greater significance to smaller species with large eyes that are
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