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

nocturnal (Brattstrom 1952; Klauber 1997). The nominally nocturnal Boa constrictor, for example, is
occasionally active diurnally (Martínez-Morales & Cuarón 1999; Chiaraviglio et al. 2003; Romero-
Nájera et al. 2007), and some species of colubrids (eg. black rat snakes, Elaphe obsoleta) and crotaline
vipers (eg, Crotalus atrox, Crotalus viridis) have been shown to shift their activity patterns according
to latitude and seasonal temperatures (Gauthier 1967; Landreth 1973; Golan et al. 1982; Sperry et al.
2010), effectively flip-flopping between being diurnal and nocturnal or crepuscular.
We can nevertheless consider individual species with clear habits. The colubrid coachwhip
snake (Masticophis flagellum) is an active diurnal predator inhabiting prairie, open pine forest and
semi-arid habitats (Greene 1997). With a large minimum pupil size restricted by the protrusion of the
lens through the pupil (van Doorn, unpubl. obs.) and no eyelids to shield them from incident radiation,
the eyes of a coachwhip would be exposed to very high doses of UV. One might then speculate that its
spectacle scale has such a sharp cutoff around 350 nm to permit high sensitivity over most of the
visually relevant UV-A spectrum while shielding the eye from the most damaging shorter wavelengths.
However, the coachwhip spectacle scale’s high λ50% may equally be a side-effect of the necessity for a
thicker scale (up to 50 µm) to shield it from abrasive substrates, from ocular injury during strikes, or
from a prey item disagreeing with its fate.
The rather low λ50% of crotaline vipers, regardless of habitat and diel pattern, suggests the
spectacle scale has not evolved a protective function against UV in this subfamily. Unlike coachwhips
and many other colubrids (including many in this study such as Pituophis, Elaphe, and Lampropeltis),
crotaline vipers have pupils that can constrict to far smaller apertures than a coachwhip as the lens does
not protrude through the pupil (Beer 1898; Michel 1932). These small apertures are in themselves
protective of harmful radiation to the eye. Many arboreal boids and pythonids, also with small
minimum pupillary apertures, would receive a far lower dose of potentially harmful UV wavelengths
than deserticolous crotaline vipers, yet they differ little in their spectacles’ λ50% with only the erycine
rubber boa and the semi-aquatic, mostly nocturnal anaconda having spectacle scales with measurably
higher UV cutoffs. Many elapids (but by no means all) also have minimum pupillary diameters
constrained by the protrusion of the lens (Duke-Elder 1958; Greene 1997). This restriction could
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