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

comparatively large pupil size even at full constriction (pers. obs.), because a larger aperture
minimizes depth of field (Green et al. 1980). In tandem with the short focal lengths of snake eyes
(Sivak 1977; Howland et al. 2004), species with small pupillary apertures at full constriction, such as
some pythons and boas (Greene 1997), are more likely to perceive the spectacle vessels.
The cyclical nature of flow through the spectacle vessels may act to reduce their negative
impact on vision, particularly in light of the transparency of the blood vessel walls (Mead 1976). When
the vessels are absent of flow, they are hardly visible even with a slit lamp. The animals’ perception
and observation of potential threats therefore depends partly on the likelihood that the spectacle vessels
were empty at the time the threats presented themselves. While these blood vessels are a permanent,
immobile fixture of the spectacle, their location within the visual field will shift when the eyes are
turned. As a result, visual adaptation to the blood vessels (eg. from Troxler’s effect in which stationary
targets appear to fade or disappear, Troxler 1804) would only occur when the eyes remain still for
extended periods (Lettvin et al. 1968). It was found that at rest the coachwhip’s eyes remain perfectly
steady (pers. obs.), far more stable than a human subject’s whose eyes exhibit constant minute shifts in
gaze direction. This remarkable ability of snakes to maintain a steady gaze may thus eliminate their
perception of the spectacle vessels as well as benefit their perception of motion through Troxler’s
effect.
Although likely to be subject to sympathetic innervation, the factors responsible for timing the
resting cycles of dilation and constriction of the spectacle vessels remain unknown. However, as
cutaneous vasculature, these vessels may be involved in thermoregulation (Bartholomew 1982). In
pilot experiments, the proportion of spectacle flow appeared to be related to some degree on ambient
temperature, with lower temperatures resulting in longer periods without flow (pers. obs.). This is
consistent with the animals being moved from a warm terrarium (25-31ºC) to a lower ambient
temperature, which would result in cutaneous vasoconstriction to minimize heat loss and maintain core
temperature post-transfer (Morgareidge & White 1969; Rice & Bradshaw 1980; Bartholomew 1982).
The rapid recovery in the proportion of blood flow after removal of the threat suggests a neural
mechanism is involved in control of spectacle blood flow. The question remains whether the observed
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