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

2.4 Discussion
The purpose of this study was foremost to document and characterize blood flow dynamics in the
snake spectacle under various conditions according to factors both endogenous (when at rest and
during moulting) and environmental (when a threat is visually perceived) and, in doing so, determine if
these dynamics could support a mechanism for mitigating visual clarity loss due to the blood vessels.
Three characteristics of spectacle blood flow were apparent from these experiments: (1)
regardless of whether the animal is at rest or disturbed, blood flow is discontinuous, except during the
moulting phase; (2) the visual perception of a potentially threatening organism induces a reduction in
the proportion of time during which blood flow occurs in the spectacle; (3) spectacle blood flow during
the integument renewal phase remains strong and, though flow can stop briefly if the animal is
disturbed, the vessels do not constrict or empty. While any visual consequences owing to the presence
of the spectacle blood vessels are speculative, the discussions here will assume that they are, based on
the facts that (1) no terrestrial vertebrate has non-retinal blood vessels in the transmissive regions of
the eye and (2) Lüdicke’s findings that the spatial distribution of spectacle blood vessels in Ahaetulla
nasuta, a foveated vine snake, minimized the density of the vascular meshwork in the foveal and
binocular fields. Only one study has measured the acuity of a snake (Baker et al. 2007), which was
found in the midland banded water snake (Nerodia sipedon pleuralis) to be approximately 4.9 cycles/
degree (cpd) using evoked telencephalic potentials. As a reference, the acuity of cats and dogs
measured with similar techniques are respectively 3.2-6.5 cpd and 12.6 cpd (Berkley & Watkins 1973;
Odom et al. 1983), and that of rats is ~0.44-1.2 cpd depending on specific technique used (Boyes &
Dyer 1983). The water snake’s acuity is thus quite good for a small eye and Baker et al. (2007)
commented that larger-eyed snakes like coachwhips may well achieve higher acuity results. Although
the acuity of coachwhips has not been measured, their large eyes, their ecology and their behaviour all
imply a high reliance on vision (Greene 1997), suggesting that their visual clarity would be negatively
impacted by blood flow through the spectacle vasculature. One anatomical advantage the coachwhip
may have in minimizing the resolution of the spectacle vasculature in the retinal image is its
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