Does eyesight have an impact on the lives of snakes?

Spotted bush snake has eyes suited for daytime hunting

The preamble.

Having received my “brief” from the editor for this article. I was looking forward to a slow amble through the research material at my disposal – books and memories going back over the years – and the odd peek at Google if necessary. But then some relevant experiences I’ve had since I’ve been working with snakes started to interfere with my plan, raising some interesting anomalies, things I hadn’t really considered investigating. I’d planned to cover the transition from blind to sighted as simply as possible, but as I progressed, I got the feeling that it might not be that simple.

With that in mind, once I’ve covered what I’d planned to say about the subject, I’ll come back to where I am now, to summarise. It certainly won’t be in the way intended, some of you might agree with me. I’d welcome your responses.

The evolution of eyesight in snakes.

It’s generally been accepted, subject to ongoing revisions and research, that our snake has been evolving over the last 130 million years or so. Along the journey, it’s been morphing from what was once a blind, subterranean lizard, into the creature that lives amongst us now, a vital cog in the slowly turning wheel that ensures our environmental health.

It makes sense, that perhaps the original model, the fossorial lizard, had little need for any form of eyesight. Hunting for food in the pitch-darkness of the underground tunnels and caverns seemed to have been based, as with most animals that live in permanent darkness, on the ability of the hunter to sense its prey, rather than see it. During the ensuing evolutionary period however, in view of the radically changing food requirements of these new creatures now living mostly above ground, the bulk of them needed to adapt to their more terrestrial lifestyles, where eyesight could be a necessity. Some of the changes taking place in the new reptilian world, did not apply throughout, and there were changes aplenty, some of them quite radical, but we’ll stick with the snakes.

The Olive House Snake is mainly a diurnal hunter

Puff adders all have elliptical pupils and will ambush prey day and night.

The new eyes.

Unlike humans or lizards, snakes cannot close their eyes. Some people might perceive a wide-eyed snake to be staring at them, or, as is quite commonly believed – and touted by some of the less credible fear-mongers who abound – to be ‘hypnotising’ them, when in fact, it might simply be enjoying 40 winks. Each eye lies behind a transparent protective ‘shield’ which is an integral part of the entire keratinous skin that the snake will continue to renew as it grows. The outgrown skin will be replaced (sloughed) in conjunction with the growth of the snake – often in one entire piece, head to tail including the eye lenses. The new skin that lies below that being discarded, is composed of larger scales – including new and clear lenses for the eyes – all catering for the continued growth. Keratin is a modified protein that can be found in many animals, in components such as fingernails, hair, Rhino horns and snake skins.

Can it see me?

Here’s a hint for those who might find themselves unexpectedly coming across an immobile snake staring at them, because it happens quite often, from the garage rafters, the garden path, or in the toilet cubicle at 03.00 am. The snake might actually be taking a nap where you see it, even though its eyes are wide open. Look at the head and watch to see if the snake’s highly flexible and sensitive tongue starts to move or not. A snake that has noticed unusual movement in its field of vision, will normally try to obtain additional pheromonal (chemically scented) information from the area of the movement, without necessarily having to move the head to look at the moving object. This tongue movement will tell you that your snake is not sleeping. Puff adders – Bitis arietans – who are packed full of amazing sensory devices, are masters of the art. You can then act accordingly – having previously read and understood the rules that apply in any snake encounters – mentioned several times in previous articles. I don’t want to get too technical, but here are a few guidelines for those who feel the urge to get up close and personal with our friend. Don’t forget the glasses, it might be a spitter, and don’t get too familiar – they are wild animals.

The pupils.

The eyeball within the eye socket looks much like a normal eyeball, yet the pupil is more purposefully adapted. It could be specifically for those species that hunt at night, needing nocturnal vision, therefore it’s more likely to have an elliptical pupil – often referred to as a ‘cat’s eye pupil’ – which can be dilated, if necessary, to allow in more light. Those that feed mainly in the daylight hours, have diurnal adaptations to suit, often visible as a fully round pupil that comes in an amazing range of colours, sometimes dark and sometimes ringed by a colourful surrounding iris.

Although these are the generally accepted versions, there will always be exceptions, and we should accept that a diurnal hunter would not be averse to the odd skulk around the neighbourhood after dark. Our Rhombic Night adder – Causus rhombeatus – is an example of one of those exceptions. Although its pupils are round, it mainly hunts at night, when its primary food sources, the toads and other frogs, are on the move.

Night adder. Round pupils but hunts whenever food is available.

A Night Adder with its favourite snack.

We are told that most snakes do not have the ability to focus on their target as do humans and other animals, but depending on the species, some snakes have the ability to move the pupil forward or rearward within the eye socket, in order to fine tune the focus, Cobras – Naja spp. – for example, whereas there are others which might have what I’d now term ‘entry level’ vision, without this capability.

The Vine snake – Thelotornis capensis – has an unusual modification of its pupil, which is horizontal, enabling the snake to obtain a more defined 3-dimensional view of its prey, something that most other snakes are not capable of doing. There is a behavioural trend that snake watchers will have noticed that which I’d call the ‘triangulation’ of the head when stalking prey, that of gently moving and tilting the head from side to side, presumably allowing the snake to reasonably accurately determine the exact distance and location of its prey, away from its current position. The snakes that adopt this strategy would probably be the hunter-gatherers, the bush snakes and grass snakes, where it’s a case of smell it, follow the scent, get up close, aim and shoot. Try controlling your movements so as not to alert the snake and watch the stalking Variegated bush snake – Philothamnus semivariegatus – atop its favourite spot – your kitchen pelmet – as it surreptitiously approaches its weekly gecko snack.

Vine snake in classic pose. Head and pupils adapted for 3 dimensional vision.

Spotted bush triangulating.

Based upon observations and common behaviour in these circumstances, I’ve always assumed that snakes cannot determine exactly what it is that’s moved and attracted their attention, which is why ‘rule Number One’ for everyone, will always be ‘stand still’! In doing so, we are not representing a threat, thus becoming ‘invisible’ to the snake, giving me a perfectly good reason to question the impact of eyesight in the lives of these animals! Once they’ve established the presence of a moving object, they will activate their numerous senses, to determine the position and potential of the object. The eyesight does not seem to be a great help in this regard. If they cannot determine what it was that moved and if they understand this to be in proximity, they will react accordingly. Some will remain immobile to enhance their camouflage – once again, Puff adders to the fore – some might exit the scene rapidly and seek shelter, and others, who might feel threatened, could begin exhibiting visible or audio warning signals – hissing, striking out, coiling, and uncoiling, hooding, tail rattling, spitting etc.

These actions tell us how conscious snakes are of the potential dangers surrounding them, in addition to confirming that the animal cannot clearly determine what it is that has moved.

Imagine if we reacted in the same way every time something approached us!

Short snouted sand snake. Round pupils. Point, shoot, kill!

Herald. Cat’s eye pupils, but hunts when food is available

Wild snakes that are locally resident and acclimatised to regular, but non-threatening, movement in their proximity, seem to be able to adapt to some extent. I’m reminded of some adders, that have lived happily and totally unconcerned, under the floorboards of the outside dog kennel. Dogs didn’t seem too fussed either.

Although data obtained from observations of captive or ‘pet’ specimens is not necessarily a true reflection of the life of a wild snake, there is one indication relative to their eyesight that might help us avoid making the wrong assumptions: that being the number of pet owners who get bitten during feeding time. Captive snakes often move towards movement near their enclosures, presumably indicating that the snake is aware that food is on its way, rather than an identification of a specific food item. As I’ve said, when I saw the title of this article, I was confident that with the benefit of past research and hands-on experiences, I’d be able to get the message across. Whew, was I wrong!

Southern Brown egg eater twilight hunting with dilated pupils

Southern brown egg eater with cat’s eye pupils. Hunts at any time.

So where am I going with this?

After I’d covered the scientific minutiae of the eyeball, I’d planned to recall some of my in-field experiences and take another look at the impact aspect.

Despite the noise and social media heroics, we generally see very little in the way of snake activity in the wild. They are naturally secretive, and exposure on their part is seldom deliberate. Like most wild things in nature, they are subject to predation 24/7, and take precautions accordingly. Most creatures are aware that they are permanently in mortal danger – something they have inherent in their being; from the second they arrive.

Snakes cannot ‘hear’ sounds like most other creatures can. Some can clinically determine movement within their proximity, by feeling vibrations transmitted through their body pits and scales (sensitive structures that most of them have) and via the jawbone, to the diagnostic system. They will react to these transmissions long before they witness the cause. No eyesight required. Their tongues collect pheromonal signals – airborne or tactile – which are transmitted via Jacobsen’s organ (explained in earlier articles), to the brain for analysis. No eyesight required. A snake that is seeking out a suitable partner for the onward growth of the family clan, will be attracted to its target by the pheromonal output – the ground based, and airborne trails – laid by the potential mate. No fancy kit or music required, just a sense of purpose. In effect, it seems to me, that throughout the entire process leading up to the completion of the mission, little or no eyesight is required.

Brown House snakes have cat’s eye pupils but hunt any time.

A summary – sort of.

Whilst putting my thoughts on paper and trying to relate everything to the subject, I decided to try and link the value of the snake’s generally not so clinical eyesight, to the various scenarios I could recall. It wasn’t long before I started hitting walls.

I’ve often asked myself, how did the Night adder miss the toad with its first strike, when it was only millimetres away from its prey? More than once I’ve seen Boomslang – Dispholidus typus – botch the timing of a nesting bird strike. I’ve heard of Puff adder bites inflicted on path-walkers in the veld, who assured helpers that they hadn’t stood on the puffie. Perhaps debatable, but of those who were accidentally ambushed, didn’t the snake know that the human was considerably bigger than the field rat it zapped last week. Lack of practice?

Back to the beginning. Although I wanted to write about the impact of eyesight in the life of the snake, I almost couldn’t find a valid reason for them having eyes!

I learned more, and as you’ve noticed, I still can’t give you a decent answer. Work in progress perhaps?

About the author

Pat was born in the UK, educated and brought up in Rhodesia/Zimbabwe, and came to South Africa in 1983. He developed an early interest in the study of the behavioural aspects of insects and animals, rather than the deeper scientific side, and he’s worked with snakes for most of his life. Pat conducts educational walks and talks throughout SA and into neighbouring countries where snakes are grossly misunderstood and needlessly killed. He writes articles for a number of publications, and has written the book ‘Getting to Know the Neighbours’, which is all about understanding snake behaviour and is aimed at teaching others how to come to terms with them.

Pat can be contacted at: herpet@snakecountrycc.co.za.