World Snake Day Special – #AskHSS

July 16th is World Snake Day! To celebrate, we’ll be doing a special two-part blog post. We asked our followers on Instagram @herpsocsg for their burning questions about snakes! Thank you to all who contributed your questions – we’ve answered them below. Without further ado, let’s dive straight into #askHSS!

GENERAL QUESTIONS
How many species of snakes are there?

According to the Reptile Database, there are more than 3700 snake species in the world (as of August 2019). In Singapore, we have at least 67 species of snakes, of which five are introduced and one is a visitor!

How long have snakes roamed the earth?

The origin of snakes is still heavily debated. Fossilised jaw bones of early snakes have been found from the Mid-Jurassic around 167-143 million years ago (Caldwell et al., 2015). Some other studies suggest they originated later in the Early Cretaceous around 128.5 million years ago (Hsiang et al., 2015). It is difficult to pinpoint the exact period they first arose because snakes do not fossilise well!

What are some major misconceptions about snakes?
A juvenile Puff-faced Water Snake (Homalopsis buccata)
Photo by Shivaram Rasu

Many people believe that snakes are inherently aggressive and will actively try to bite people. However, snakes very much prefer to escape from danger, and usually only bite if provoked. That being said, it is best to observe snakes from a distance to give them the space to move away.

Another common idea people have is that snakes are slimy! In reality, snakeskin is quite dry and smooth to the touch.

Have snakes become more common or rare in Singapore since the 1990s?

There are many anecdotal reports of people encountering snakes in their kampungs, which often bordered forested areas. However, it is very difficult to gauge the population of snakes in any environment because of their elusive nature. Few studies are done on this in Singapore. However, deforestation and human encroachment of natural habitats can threaten forest dwelling snakes. In the meantime, other species are forced to adapt to rapid urbanisation as well.

How do I go about looking for snakes? Do I just hope I run into them?

A good place to look for snakes would be the Central Catchment Nature Reserve, Bukit Timah Nature Reserve and Sungei Buloh Nature Reserve. However, snakes can be very shy and are very well camouflaged, so they might be hard to spot!

It is always helpful to understand the ecology of the snakes you are searching for. Arboreal snakes will be found on trees! Fossorial snakes will be in the leaf litter. Ultimately, the best way to find snakes would be to go out as much as you can, and you will eventually be able to spot them more easily. Practice makes perfect!

IDENTIFICATION TIPS
How do you distinguish between Big-eyed Whip Snakes and Oriental Whip Snakes?
Left: Big-eyed Whip Snake (Ahaetulla mycterizans). Right: Oriental Whip Snake (A. prasina)

Both of these species do look very similar! They are both thin, bright green snakes that love trees. Miralles & David (2010) produced a detailed account of how to differentiate them, and we have summarised some of these features in the following table.

Big-eyed Whip SnakeOriental Whip Snake
Size of the eyesLarger (relative to head)Smaller 
Shape of the headUpper surface of snout is convexUpper surface of snout is flat or depressed
Anal plateUndividedDivided
ahaetulla identification
A pictorial drawing of the differences between the (A) Big-eyed Whip Snake and the (B) Oriental Whip Snake.
Adapted from Miralles & David (2010) (Used in accordance with Fair Use)
How do I tell the difference between a snake and a legless lizard?

While they look superficially similar due to their apparent lack of limbs, both groups represent different evolutionary lineages. Snakes are derived from a single common ancestor, while the loss of legs in lizards has occurred multiple times independently! This is nicely summarised in the following figure from the University of California, Berkeley.

legless lizards
Figure adapted from University of California, Berkeley. Check out their article here!

There are some defining features that you can use to differentiate the two! Legless lizards have eyelids and external ear openings, while snakes have neither.  Snakes also have enlarged ventral scutes on their bellies, while legless lizard scales are uniform in size. Finally, the lower jaws of snakes are split in the middle, to enable them to swallow large prey. However, in lizards, these jaws are fused. 

Fun fact: The Basilisk in Harry Potter and the Chamber of Secrets is probably a legless lizard, rather than a snake. It has eyelids, ear holes, a fused lower jaw and uniform scales throughout its body! Can you spot these features in the video?

PHYSIOLOGY OF SNAKES
Do snakes have good eyesight?

It depends! Snakes are very diverse, hence good eyesight may or may not be adaptive. For instance, burrowing snakes such as the Brahminy Blind Snake (Indotyphlops braminus) live most of their life underground. They do not need good vision, and have eyes that only detect the presence and absence of light.

However, diurnal snakes have better vision. Whip Snakes possesses some of the best visual acuity in snakes. Their horizontal pupils and eye positions allow a 45-degree overlap to produce binocular vision. They also have a region of high visual sensitivity known as a fovea, which is rare for snakes (Walls, 1994)!

lizard snake eye
Simplified diagram of some differences between the structure of eyes in lizards and snakes.
Figure adapted from Held Jr. (2014).

Despite being related to lizards, snakes have very different eye structures. They lack eyelids, have lost their fovea and have vessels that run over the retina (Held Jr., 2014). It is thought that the ancestors of snakes were burrowing organisms, which lost their limbs and vision. When snakes re-emerged to the surface, their visual systems had to be ‘rebuilt’ from scratch! 

Do snakes have ears?

Snakes do not have external ears like lizards, but they do have similar internal ear structures. On land, most tetrapods including lizards register sound via changes in air pressure. An external membrane helps to pick up pressure changes in the air, and small bones attached to this membrane react to these changes. Their movement causes a displacement of fluid in the inner ear, enabling us to “hear”.

Snakes lack this external membrane, so such pressure changes are detected through the quadrate bone (shown in blue below), which is directly joined to the inner ear elements (shown in green below) (Christensen et al., 2012).

jaw bone of snake.jpg
Micro-CT scan of Python regius. Green: Columella auris; Blue: quadrate; Red: mandible
Figure adapted from Christensen et al., 2012

This set-up seems to suggest that snakes can only detect vibrations from the ground, but in reality, snakes are still sensitive to airborne sound! A study with Royal Pythons showed that sound can translate to vibrations in the skull, which can influence a snake’s behaviour (Christensen et al., 2012). Hence, snakes can “hear”, but simply in a different way from us.

How do snakes mate and reproduce?

Snakes have a ‘courtship’ phase, similar to many other animals. Females release pheromones when they are sexually receptive, and males will follow this trail. Multiple males may court a single female, and form a mating ball. This has been recorded for active species such as the Paradise Tree Snake (Chrysopelea paradisi) and Elegant Bronzeback (Dendrelaphis formosus).

Male snakes have hemipenes that they insert into the cloaca of the female. The ova are then fertilised internally – though some snakes are able to store sperm for more favorable conditions. 

Some snakes can reproduce just by making clones of themselves in a process known as parthenogenesis. One example would be the Brahminy Blind Snake – an all-female species! Other snakes such as pythons, file snakes and boas usually reproduce sexually, but can produce offspring if there are no males available.

Most snakes are oviparous, which means that they lay eggs. However, several snake families have evolved to be viviparous, where the embryo develops in the body of the parent before being born live.

What substance covers a snake’s skin?

Snakes are covered in scales, which are derived from keratin. This is the same material that hair and fingernails are made of!

What are their first line of defense?

Snakes have many defense strategies, and it is hard to say which is the most important. Many snakes rely on camouflage to blend in with their surroundings, which make them exceptionally hard to spot! Whip Snakes resemble vines on a tree. Some nocturnal snakes, such as the Sunbeam Snake (Xenopeltis unicolor), have iridescent scales that can break up their outline if approached by a predator in daylight. 

Sunbeam Snake (Xenopeltis unicolor)
Photo by Serin Subaraj

Other snakes opt for warning signals to deter predators from attacking. Majority of these species are venomous. Some of these warning signals could be in the form of bright colors (e.g. Banded Malayan Coral Snake), or threat displays (e.g. King Cobra).

The striking warning pattern of the highly venomous Banded Malayan Coral Snake (Calliophis intestinalis)
Photo by Law Ing Sind

Non-venomous snakes can even mimic the patterns and behaviours of venomous snakes. For instance, the harmless Pink-headed Reed Snake is thought to mimic the highly venomous Malayan Blue Coral Snake (Baker & Lim, 2012). This can deter potential predators for enough time to allow the harmless snake to make a quick getaway.

Do snakes ever knot themselves up by accident?

No, snakes do not knot themselves up.

new meme
Photo of Shore Pit Viper by Toh Wei Yang.

However, such behaviour is seen in some sick snakes! Inclusion Body Disease (IBD) can occur in boid snakes, resulting in a compromised central nervous system. Affected snakes may exhibit symptoms where they tie themselves into ‘knots’ (Chang & Jacobson, 2010).

IBD python.jpg
Such an abnormal position of the snake is indicative of the disease!
Figure adapted from Chang & Jacobson (2010).

That’s all for now! Do let us know if you enjoyed this edition or if you have any feedback on our responses. Share this blog post with your friends and family!

Lookout for part 2 of the World Snake Day celebration – our ‘species spotlight’!

References

Baker, N. & K. K. P. Lim, 2012. Wild Animals of Singapore.A Photographic Guide to Mammals, Reptiles, Amphibians and Freshwater Fishes.Updated edition. Draco Publishing and Distribution Pte. Ltd. and Nature Society (Singapore).

Caldwell, M. W., Nydam, R. L., Palci, A., & Apesteguía, S. (2015). The oldest known snakes from the Middle Jurassic-Lower Cretaceous provide insights on snake evolution. Nature communications, 6(1), 1-11.

Chang, L. W., & Jacobson, E. R. (2010). Inclusion body disease, a worldwide infectious disease of boid snakes: a review. Journal of Exotic Pet Medicine, 19(3), 216-225.

Christensen, C. B., Christensen-Dalsgaard, J., Brandt, C., & Madsen, P. T. (2012). Hearing with an atympanic ear: good vibration and poor sound-pressure detection in the royal python, Python regius. Journal of Experimental Biology215(2), 331-342.

Held Jr, L. I. (2014). How the snake lost its legs: curious tales from the frontier of evo-devo. Cambridge University Press.

Hsiang, A. Y., Field, D. J., Webster, T. H., Behlke, A. D., Davis, M. B., Racicot, R. A., & Gauthier, J. A. (2015). The origin of snakes: revealing the ecology, behavior, and evolutionary history of early snakes using genomics, phenomics, and the fossil record. BMC evolutionary biology, 15(1), 87.

Miralles, A., & David, P. (2010). First record of Ahaetulla mycterizans (Linnaeus, 1758)(Reptilia, Squamata, Colubridae) from Sumatra, Indonesia, with an expanded definition. Zoosystema, 32(3), 449-456.

Walls, G. L. (1944). The vertebrate eye and its adaptive radiation.

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