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

virtue of both being characteristic of colubrids. The question then remains of why colubrid spectacle
scales are thicker and have higher λ50% than other families, Elapidae and specific boids excluded. To
speculate on this, a consideration of the functional effects of λ50% and thickness is called for.
3.4.2 Functional Differences in Spectacle Scale Transmittance Spectra &
Thickness
Functional differences between transmittance spectra would relate to both the perceptual capabilities of
these animals and to the protection against harmful radiation afforded by the λ50%. Some species of
snake and gecko have been shown to possess UV-A-sensitive cones (Loew 1994; Loew et al. 1996;
Sillman et al. 1997; Sillman et al. 1999; Sillman et al. 2001; Davies et al. 2009; Yang 2010; Hart et al.
2012), suggesting that the visual perception of UV-A wavelengths is a common trait throughout these
taxa. While the UV-A spectrum spans a broad region from 315-400 nm, it should be borne in mind that
vision in this region will be restricted to the specific spectral sensitivity of an animal’s short-
wavelength sensitive (SWS) cones (or medium wavelength cones (MWS) with absorbance spectra
extending into the UV-A), which varies between species according to their expressed opsins. The
retinal absorbance spectra of three snakes included in this study have been characterized (Thamnophis
sirtalis (Sillman et al. 1997), Python regius (Sillman et al. 1999), and Boa constrictor (Sillman et al.
2001)) with each being found to possess a UV-sensitive visual pigment with an absorbance peak
around 360 nm, well above the cutoff frequency of the spectacle scale (T. sirtalis: 338 nm; P. regius:
309 nm; B. constrictor: 314-317 nm). Although UV perception is widespread and used in navigation
(Coemans & Vos 1992; Hawryshyn 2010), foraging (Viitala et al. 1995; Siitari et al. 1999), and
communication with and recognition of conspecifics (Fleishman et al. 1993; for a review, see
Honkavaara et al. 2002), short wavelength radiation has also been implicated in ocular pathologies
such as cataractogenesis and photochemical retinal damage (Sliney 1986; Taylor 1989; Wu et al.
2006), although most of these data are from mammals with little data available for reptiles.
The conspicuously yellow spectacle scale of the snouted cobra stands out in recalling the
yellow lenses and corneas of some diurnal terrestrial vertebrates (Walls 1931; Walls & Judd 1933;
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