X. Bonnet et al., Why do snakes have eyes? The (non-)effect of blindness in island tiger snakes (Notechis scutatus), BEHAV ECO S, 46(4), 1999, pp. 267-272
Large(to >1 m), diurnally active tiger snakes (Notechis scutatus) are abund
ant on Carnac Island, near the coast of Western Australia. Our behavioural
and mark-recapture studies provide the first ecological data on this popula
tion, and reveal a surprising phenomenon. Many adult tiger snakes have had
their eyes destroyed, apparently during nest defence, by silver gulls (Laru
s novaehollandiae). This loss of vision did not reduce the snakes' body con
dition (mass relative to length), or their rates of growth or survival (mea
sured over a 12-month period). Blind male snakes trail-followed females, an
d mated successfully. Thus, destruction of a major sensory modality had no
detectable effect on these predators. This result is strongly counter-intui
tive, but mirrors an earlier report of congenital blindness (without ill-ef
fects) in American viperid snakes. Similarities between the two systems (is
land populations, highly venomous snakes, reliance on sessile prey) clarify
the circumstances under which the loss of vision does not reduce an organi
sm's viability. These natural experiments support Gans' hypothesis of "mome
ntarily excessive construction" in that the snakes possess a complex organ
system that they do not actually require for successful feeding, survival o
r reproduction.