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10 Things You Didn't Know About Snakes

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    10 Things You Didn't Know About Snakes

    1. Vision






    ​Snakes are peculiar among vertebrates when it comes to the structure of their eyes, even differing from their lizard cousins. In order to focus on a subject, the lens in the eyes of vertebrates changes curvature by means of ciliary muscles, these muscles were lost in ancestral snakes meaning an alternative method of focusing was required. Snakes will instead move the lens forwards and backwards by using muscles within the iris to apply pressure to the vitreous fluid inside the eye, forcing the lens to move forwards, and relax those muscles to allow the lens to passively move back.

    Snakes generally have good vision, and can even see in colour, although diurnal snakes are dichromatic or trichromatic, meaning that some species who are active during the day possess either 2 or 3 pigmented light receptors responsible for colour perception; the normal condition in humans is Trichromacy, and in mammals is dichromacy, while reptiles and birds normally posses 4 light receptors, and can often see in the Ultra-Violet range. The reason why snakes have poorer colour vision when compared to other reptiles can be once again explained by the regression of the eye during the early evolution of the ancestors of snakes. The light receptor cells of the eye are divided into two groups, rods and cones. Rods are responsible for vision in dim light while cones work best in well-lit conditions and are responsible for colour vision. The photoreceptors that regressed during the fossorial phase of early snakes were in effect rebuilt to accommodate the requirements of the species moving into a day-time lifestyle. This was achieved by repurposing some of the existing rods into cones.













    ​2. Hearing






    Because snakes are anatomically peculiar in not having an outer ear, it was assumed for a long time that snakes could not hear, it is now known that this is not the case. The outer ear may have regressed due to a fossorial ancestry, but the inner ear still remains intact and functional. Snakes can hear both ground vibrations and airborne sounds, although they are restricted to a limited range of sound frequencies, from approximately 50 to 1000 Hz. By comparison, humans can hear in the range of 20 to 20,000 Hz. Hearing in mammals is assisted by three smalls bones in the middle ear called the ossicles, consisting of the malleus, incus and stapes. The equivalent in reptiles is the articular, quadrate and columella.

    In mammals, the eardrum is connected to the stapes, and in reptiles it is connected to it’s equivalent, the columella. The two other bones are located in the middle ear in mammals, but in reptiles they are apart of the jaw. Because snakes lack eardrums, the columella leans against the quadrate bone for transmission of vibrations (including airborne sound vibrations) for hearing. What this essentially means is that snakes hear through their jaws.













    3. Jacobson's Organ






    ​Also called the Vomeronasal Organ, this sensory organ is the primary means of how snakes gain information about the world around them. Terrestrial snakes are able to smell in the usual manner, by inhaling through their nostrils (however, aquatic snakes do not possess a sense of smell) but the Jacobson’s organ provides much more detailed information about the odours they are detecting. The tongue (which generally lacks taste buds and does not seem to directly contribute to sensory perception in most cases) is flicked out to sample the air and collect particles, these particles are transferred to the Jacobson’s organ located on the roof of the mouth when the tongue is retracted. The forked nature of the tongue allows the snake to determine the direction of a certain smell. This is further enhanced by the fact that the Jacobson’s Organ is a paired structure and that information collected in each organ is processed separately.








    4. Organs






    ​Because of the elongated nature of snakes, their internal organs also require elongation, specifically in regards to the lungs, liver, kidneys, and testes. The heart is also elongated and comparatively slender in arboreal species, many of which are exceptionally thin.

    The lungs of snakes are particularly interesting. The left lung is present in many species but greatly reduced in size and no longer serves any purpose, in some species the left lung has been lost entirely. The right lung, by contrast, is large and elongated and varies in size massively between species, being anywhere between 8-83% the length of the snakes body. The lung is divided into two functionally different regions; the “vascular lung” which is used in respiratory gas exchange (O2 and CO2), and the “saccular lung” which has no respiratory function. The saccular lung functions only as an air sac and may help the snake maintain structural support of its body, especially when the snake stretches to span gaps. Alternatively, the saccular lung may just play a role in maintaining posterior body form.







    5. Reproduction






    It is often assumed that all snakes lay eggs, there are in fact three modes of reproduction; Oviparity (females give birth to eggs) Ovoviparity (eggs are produced within the female but hatch before giving birth to live young) Viviparity (live birth – no egg is produced). Generally speaking most reptiles are oviparous (egg producers), but certain environmental pressures, such as cool environments or environments which provide very few suitable nesting sites have given rise to live bearing snakes. This allows the snake to keep the young at proper temperatures by constantly adjusting it’s position for optimal thermoregulation, and by extension maintain suitable temperatures for the young she carries, as well as not having to worry about finding a safe and suitable place to lay her eggs. There are, however, disadvantages such as decreased mobility, leaving her unable to hunt or eat effectively and limitations to how many offspring she can produce.

    At least one species of snake has an even more bizarre mode of reproduction. The Brahminy Blind Snake (Rhamphotyphlops braminus) (also called the Flowerpot Snake) lays eggs, but all the offspring are female. This species of snake has no males and instead relies on asexual reproduction, effectively cloning itself.





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    ​6. Spurs and Legs





    ​Snakes are specialised animals, which are descended from four-limbed ancestors. Some snake families possess vestiges of posterior limbs (pelvic girdles), although in modern snakes the pelvic girdle (representing vestiges of the femur and/or pelvis) is no longer in contact with the vertebral column or is missing altogether.

    In the Boas and Pythons these vestiges are characterised by external protrusions of keratinised tissue called spurs. Spurs help males to clasp onto the female for breeding. Interestingly, the mechanisms of limb development may not have been completely lost in snakes. It has been observed that hindlimb development halts in Python embryos as a result of mutations in the limb-specific enhancer of a gene known as Sonic Hedgehog (Shh). This gene disrupts the genetic circuit that drives limb growth in snakes.









    7. Energy Conservation





    ​Snakes are periodic feeders owing to the fact that they have slow metabolisms, meaning they have low energy requirements. Because they are able to eat such large prey they do not need to eat often. Generally speaking, snakes ingest 6 to 30 meals per year to meet their overall energy needs. If mammals do not eat they will generally perish within a few days or weeks, whereas some species of snakes in captivity can survive for more than a year without a meal. Some rattlesnakes are able to survive without food for up to 2 years!

    Digestive efficiencies of snakes vary between 85-95% while some species are able to gain up to 99.8% of the potential chemical energy of their food. The efficiency of their digestive capabilities may be linked with the snakes’ intermittent feeding behaviours.

    Gut passage time can vary from a few hours to days in mammals but snakes are clearly champions when it comes to the maximum passage time for faecal matter. Heavy bodied terrestrial species such as Viperid and Boid species retain faecal material for months at a time. For example, the maximum defecation interval for the Gaboon Viper is 420 days! The accumulation of faeces may help with anchoring the lower body while the snake strikes.






    Gaboon Viper and Long-nosed Tree snake have different morphologies and feeding strategies, resulting in different energy requirements and gut passage times (The ambushing Gaboon Viper requires less feeds and defecates far less frequently while the active arboreal snake species defecates within 24 hours to lighten themselves for locomotion).




    8. Locomotion






    ​One of the most remarkable features of snakes is how they are able to move so effortlessly through a large variety of landscapes without limbs. Although other animals have evolved elongated, limbless bodies (such as certain amphibians and lizards) non have developed these attributes with such apparent perfection.

    All snakes can swim by undulating in the water, but sea snakes have specific adaptations to aid in their movements such as vertically flattened, streamline bodies and broad paddle-like tails. Yellow-bellied Sea Snakes are interesting in that they can even swim backwards.

    Another group of snakes have adapted to flattening their bodies to help them get around; the Paradise Flying Snake. This species of arboreal snake lives up in the canopy and flattens its body horizontally instead of vertically to help capture the air during gliding, effectively turning itself into a parachute. Flying snakes have such remarkable control in the air that they can even change direction mid-glide.

    Terrestrial snakes are highly accomplished climbers, this is partly owing to the fact that snakes do not just “slither” to get around (also known as lateral undulation), but adopt several strategies to move, including a mode of locomotion called ‘Concertina Locomotion’, which utilising stationary body parts to push or pull the remaining body forward. Snakes can also anchor themselves to a surface using their belly scales like little fingertips to keep a grip. This is extremely useful when climbing challenging surfaces.



    (a) A corn snake ascending a tree. (b,c) Scales are used to grip tree bark asperities. Snake scales at their (d) minimum and (e) maximum angles of attack (flat). (f,g) A snake climbing an inclined surface. Sliding is prevented by emergency braking associated with lifting of the body (Marzi et al, 2012).





    The gliding Paradise Flying Snake and aquatic Olive Sea Snake flatten their bodies along different planes (flying snakes flatten horizontally to catch the air like a parachute and sea snakes flatten vertically to better streamline themselves while swimming).




    9. Sociality





    ​Reptiles are often assumed to be a-social with interactions restricted to territorial defence and courtship behaviour, snakes most of all are seen as anti-social solitary animals, but modern biologists are uncovering increasing evidence to the truly complex nature of reptile sociability.

    It appears that some squamates (lizards and snakes) are more similar to mammals and birds in their social behaviour then what could have been previously imagined. Several species of Skinks from the genus Egernia will form monogamous pairings and stay in family groups consisting of the two biological parents and their offspring. Some species of skink, such as the Great Desert Skink will also cooperatively build large elaborate burrows for their families. Monogamy is not restricted to this genus but can also be observed in the Shinglebacks (or Sleepy Lizards) and Desert Night Lizards.

    In regards to snakes, the best-studied snakes for their social behaviour are the rattlesnakes. Rattlesnakes will express many of the same social behaviours as seen in the social lizards, as well as conspecific alarm signals to alert others of the same species to danger and the formation of nurseries where females aggregate and cooperatively care for the young. What’s more fascinating is that these snakes can recognise their family members, even after years of separation and will demonstrate social preferences, habitually avoiding certain individuals they don’t like while seeking out the company of specific individuals. It would seem that rattlesnakes make ‘friends’ with each other. Unfortunately research is severely lacking in regards to social behaviour across a broader range of species, largely because snakes are secretive animals and very difficult to study in the wild. Judging by the current trends of discoveries made about social reptiles it would be more surprising if social behaviour wasn’t more widespread amongst the reptiles.





    Rattlesnake mother with her newborn. Photographed by Richard Sanderson.




    10. Intelligence





    ​It is a common misconception that reptiles lack cognitive abilities. This may be because it is not often studied in reptiles, and even less so in snakes, but there is strong evidence to suggest that reptiles (including snakes) have surprisingly advanced cognitive abilities.

    Chelonians (turtles and tortoises) tend to take the spotlight for cognitive research, largely because they are thought to have changed little in the last 225 million years, representing animals who lived closest to the time of the reptile-mammal split, and therefore allows us insight into the mental capacities of ancient ancestral reptiles. Cognitive experiments on turtles have demonstrated that reptilian learning and memory capabilities, as well as behavioural flexibility in problem solving and social cognition may closely parallel those observed in mammals and birds.

    Snakes have been directly studied for their spatial learning and memory capacities, corn snakes directly demonstrated that they are capable of learning spatial and memory based tasks rapidly. Indigo Snakes were conditioned to operate a contact relay and press a key, they have shown that their rate of response to operant conditioning is similar to comparable studies of rats that had been trained to press levers or disc-pecking pigeons.

    It’s worth bearing in mind that although intelligence is studied more in mammals and birds, they are descendants of the same reptilian ancestors as modern reptiles. Evolution can explain the diversity of species but has no grounds for determining the cognitive capacities of animals.



    The Eastern Indigo Snake, considered one of the more intelligent species of snake.






    References


    Bull, C.M., (2000). Monogamy in Lizards. Behavioural Processes 51:7-20

    Clark, R.W., (2004). Kin Recognition in Rattlesnakes. Proceedings of the Royal Society of London Series B Biology Letters 271, S243-S245

    ​Hellmuth, H., Augustine, L., Watkins, B., and Hope, K., (2012). Using Operant Conditioning and Desensitization to Facilitate Veterinary Care with Captive Reptiles. Vet Clin North Am Exot Anim Pract.15 (3), pp. 425-443

    Holtzman, D.A., Harris, T.W., Aranguren, G., and Bostock, E., (1999). Spatial Learning of an Escape Task by Young Corn Snakes, Elaphe guttata guttata. Animal Behaviour 57 (1), pp 51-60

    Kardong, K., (1998). Vertebrates: Comparative Anatomy, Function, Evolution (Second Edition). McGraw-Hill

    Kleinginna, P.R., Jr. (1970). Operant Conditioning in the Indigo Snake. Psychonomic Science, 18 (1), 53-55

    Leal, F. and Cohn, M.J., (2016). Loss and Re-emergence of Legs in Snakes by Modular Evolution of Sonic Hedgehog and HOXD Enhancers. Current Biology, 26 (21), pp. 2966-2973

    Lillywhite, H., (2014). How Snakes Work: Structure, Function and Behaviour of the World’s Snakes. Oxford University Press


    Marzi, H., and Hu, D.L., (2012). Friction Enhancement in Concertina Locomotion of Snakes. Journal of the Royal Society Interface. 9 (76), pp. 3067-3080

    Melissa (2014). Squamate Sociality: Surprisingly Like Birds and Mammals. [Online] Snakes.ngo

    Secor S.M, Stein E.D, Diamond J., (1994). Rapid Upregulation of Snake Intestine in Response to Feeding: A New Model of Intestinal Adaptation. The American Journal of Physiology 266, G695-G705

    Vitt, J., and Caldwell P., (2014). Herpetology (Fourth Edition). Elsevier

    Wilkinson, A., and Huber, L., (2012). Cold-Blooded Cognition: Reptilian Cognitive Abilities. The Oxford Handbook of Comparative Evolutionary Psychology


    Yu-Chung Lin, Ji-Chuu Hwang and Ming-Chung Tu, (2003). Does the Saccular Lung affect the Cantilever Ability of Snakes? Herpetologica 59 (1), pp. 52-57



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