Snakes control blood flow in their 'spectacles'
Snakes can control blood flow in their "spectacles", scientists have found.
The animals are known for their lack of eyelids, and instead have a transparent scale covering their eyes for protection.
These "spectacles" contain blood vessels, and researchers investigated how this arrangement does not obscure vision.
They found blood flow changed when the snakes perceived a threat, allowing for optimal vision.
The study, by scientists from the University of Waterloo School of Optometry in Canada was published in the Journal of Experimental Biology.
The eyes have it
"Reptilian spectacles are essentially the result of eyelids that fused and became transparent during embryonic development," said lead researcher Dr Kevin van Doorn.
"And because these are scaly animals we're talking about, a large scale covers its surface and is referred to simply as the 'spectacle scale'."
He said that spectacles were "ubiquitous" among snakes, were also found in many geckos and other varied lizard species, and could be seen in the shed skins of the reptiles.
"I was actually investigating another aspect of the snake eye, but the illumination used by my instrument was just perfect for exposing the vessels," said Dr van Doorn.
He was surprised to see the blood vessels, which despite first being reported in the 19th Century are little-known by many herpetologists.Fight-or-flight response
"Our research was based on the premise that perhaps spectacle blood flow could be adjusted to minimise the vessels' effect on vision," said Dr van Doorn.
While inspecting the spectacles of coachwhip snakes in the lab, Dr van Doorn found that blood flow was indeed dynamic and not constant.
"Instead, there are cycles of flow and no flow - dilation and constriction of the vessels - in resting animals, such that, for a significant proportion of the time, the absence of blood cells within the vasculature means that the clarity of their vision is likely spared," he said.
And it was the researcher's own presence in the lab that delivered further results.
"I was happily observing the spectacle blood flow in one of my snakes when I turned away to adjust my instrument; when I turned back, flow had stopped," he said.
"It took me a moment, and several repeats of adjusting my instrument, to realise the spectacle blood flow was responding to my own activity."
Dr van Doorn suggested the response could be an element of a broader reaction to a perceived threat that occurs through the whole body - or it could be part of a fight-or-flight response that is known to affect blood flow to the skin.
"Regardless of these uncertainties, the fact remains that vision is spared some of the time while the snakes are at rest and for most of the time when visually engaged," he said.