When snakes perceive a threat, they optimize their vision by reducing the flow of blood to their eyes, according to a new study from the University of Waterloo.
Scientists Kevin van Doorn and Jacob Sivak report their study of the coachwip snake in the latest edition of the Journal of Experimental Biology, finding that the reptile's visual blood flow pattern is dynamic, changing depending on what's in its environment.
"Each species' perception of the world is unique due to differences in sensory systems," said van Doorn, who works with Waterloo's School of Optometry & Vision Science.
Snakes do not have eyelids, but a clear scale known as a spectacle over each eye; the spectacles essentially function as protective eye wear, protecting the organ while allow allowing clear vision.
While van Doorn was studying a separate part of the snake's eye, illumination from his instrument indicated something unusual: the spectacles contain a network of blood vessels. Curious about whether the blood vessels obscured the snake's vision, van Doorn began to examine whether the blood flow patterns changed under different conditions.
"When the snake was resting, the blood vessels in the spectacle constricted and dilated in a regular cycle. This rhythmic pattern repeated several times over the span of several minutes," the researchers said in a statement.
But when the snakes were presented with stimuli it perceived as threatening, the researchers observed marked change in the spectacle's blood flow pattern.
"The blood vessel constricted, reducing blood flow for longer periods than at rest, up to several minutes. The absence of blood cells within the vasculature guarantees the best possible visual capacity in times of greatest need," they said.
A third experiment on the snake's blood flow when shedding its skin was preformed next. During shedding, the researchers found that the eyes stayed dilated and that blood flow was continuous and strong, quite unlike the blood cycle observed when the snake was in a normal state of rest.
"Together, these experiments show the relationship between environmental stimuli and vision, as well as highlight the interesting and complex effect blood flow patterns have on visual clarity. Future research will investigate the mechanism underlying this relationship," the researchers said.