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

1.5 Adaptive Significance of the Spectacle and its Evolution
“[The spectacle] is quite insensitive to touch. The cobra, python, and other snakes all allowed me to
touch it, and even polish it with a rag, so as to get a clear view of the fundus, without any attempt at
resistance or even sign of discomfort” - George Lindsay Johnson, 1927
While the validity of this claim of insensitivity remains untested, particularly given that the spectacle is
innervated, this quote nevertheless sums up nicely the spectacle’s protective character. The spectacle of
extant snakes unquestionably serves a protective role as attested by the severely abraded snake
spectacle in Figure 1-12. Of course its current adaptive significance, as with any trait, makes no
implication of the selective pressures on its early evolution. Thus, a number of theories have been put
forth to explain the evolution of the spectacle.
1.5.1 Mechanical Protection
Perhaps the most frequently recognized theory is that of mechanical protection from blowing sand and
against obstacles during close-crawling, subterranean, and nocturnal locomotion (Rochon-Duvigneaud
1916; Walls 1934, 1940, 1942). In legless or short-legged organisms, the eyes will be exposed to any
number of large obstacles in their path as well as to small rocks, twigs and other sharp, pointed, or
abrasive objects that would pose quite a threat to an exposed cornea, particularly under restricted
visual conditions such as at night. Arid environments carry the risk of blowing sand and dust, foreign
particles that can cause not only discomfort, but serious harm to eyes, nictitans and eyelid conjunctiva.
Snakes and geckos likely inherited their spectacles from their respective common ancestors (discussed
further below), so they all possess them regardless of habitat, size and diel activity, but among other
spectacled squamates, spectacles and windowed eyelids are indeed found most frequently in smaller
species, in those that burrow, in nocturnal species, and in species that inhabit dry and semi-dry
microclimates (Storr 1971; Greer 1983; Walls 1934; Walls 1942). Walls asserted that most extant
26