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

Walls 1942; Chou & Cullen 1984) and fishes (Walls & Judd 1933; Walls 1942; Kennedy & Milkman
1956; Muntz 1973), which are thought to function as barriers to UV and/or to increase contrast.
Whatever the functional significance of its yellow spectacle, it certainly restricts vision in the UV-A
and attenuates a significant portion of the blue region. The somewhat brown colouration of the
spectacles of the hognose snake spectacle as well may function as a modest UV filter. The nature of the
colouration in either of these species is not known, but may be contributed by pigments deposited in
the scale during keratogenesis. Alternatively, it may result from staining by the animals’ substrate, such
as by tannins or quinone pigments.
The function of a thicker spectacle scale may relate to the spectacle’s protective role against
injury. Snakes and geckos probably owe their spectacles to a single common ancestor (see Chapter 1),
but in other squamates such as scincids, the presence of a spectacle seems to correlate roughly with
body size (Walls 1942; Greer 1983). This suggests that the value of a spectacle is less for larger
species. While some species of python and boid achieve much greater sizes than colubrids, many are
comparably sized as are crotaline vipers, indicating that body size within a family is unlikely to have
influenced spectacle scale thickness. There is significant overlap in the habitats occupied by the
colubrids and vipers in this study, so this as well is unlikely to influence scale thickness. Where the
colubrids overall differ from vipers, boids and pythons is in their speed, which makes them more
susceptible to ocular insults during locomotion (Masticophis flagellum is especially fast), and in their
lack of an efficient envenomation mechanism or larger size, making them more vulnerable to predators
and uncooperative prey. This latter point has been observed in a population of island tiger snake
(Notechis scutatus) with a disproportionately high incidence of blindness due to injury by adult gulls’
attempts at protecting their nests (Bonnet et al. 1999).
3.4.3 Activity Patterns & Ocular Morphology
An attempt to deduce relationships between diel activity patterns and λ50% would be somewhat
confounded by several of the sampled species not being conveniently classifiable as strictly diurnal or
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