Snakes are reptiles related to lizards, crocodiles and turtles. One could define them as elongate, legless, carnivores reptiles of the suborder Serpentes that can be distinguished from legless lizards by their lack of eyelids and external ears. There are more than 2,700 different species of snakes in the world, of which, approximately 300 species have potent venom in their arsenal of attack and could kill humans.
Snakes are ectothermic, amniote vertebrates covered in overlapping waterproof scales. Many species of snakes have skulls with many more joints than their lizard ancestors, enabling them to swallow prey much larger than their heads with their highly mobile jaw. In order to accommodate their narrow bodies, snakes’ paired organs (such as kidneys) appear one in front of the other instead of side by side, and most have only one functional lung. Some species retain a pelvic girdle with a pair of vestigial claws on either side of the cloaca.
Living snakes are found on every continent except Antarctica and on most islands. Fifteen families are currently recognized comprising 456 genera and over 2,900 species. They range in size from the tiny, 10 cm long thread snake to pythons and anacondas of up to 7.6 metres (25 ft) in length. The recently discovered fossil Titanoboa was 15 metres (49 ft) long. Snakes are thought to have evolved from either burrowing or aquatic lizards during the Cretaceous period. The diversity of modern snakes appeared during the Paleocene period.
Most species are non-venomous and those that have venom use it primarily to kill and subdue prey rather than for self-defense. Some possess venom potent enough to cause painful injury or death to humans. Non-venomous snakes either swallow prey alive or kill by constriction. They are mostly flesh eaters and they swallow their prey whole.
Locomotion in snakes
Snakes use at least five unique modes of terrestrial locomotion. The kind of locomotion a snake uses in any particular instance depends on several factors such as the kind of surface it is crawling on and its speed. In fact, an individual snake can use most or all of the five modes, and can even use two modes in different parts of the body. Below are descriptions of the various modes of snake locomotion and a brief bibliography.
But first, the locomotor mode of most limbless lizards is Simple Undulation. Simple undulation is characterized by waves of lateral bending being propagated along the body from head to tail. The bends push laterally against surface objects, but do not deform locally around them, and usually slip out of contact quickly; in this way, simple undulation differs from the more complex Lateral Undulation of snakes (see below). Most elongate and limbless lizards use simple undulation when crawling on the surface of the ground; however, a few species use the more complex mode of lateral undulation that snakes use.
Lateral Undulation is the common serpentine locomotion of snakes. In lateral undulation, as in simple undulation, waves of lateral bending are propagated along the body from head to tail. But lateral undulation is unique in that whenever a bend contacts a surface object, such as a rock or stick, it exerts force against it and deforms locally around it. Whenever a snake pushes against multiple objects simultaneously, the lateral force vectors cancel each other, leaving a resultant vector that propels the snake forward; postural adjustment around each object gives the snake even finer control over the direction of force exertion. Force exertion against each object is inversely proportional to the number of objects being pushed against simultaneously by the snake, but total force is roughly constant for a given speed and substrate. In lateral undulation, the large dorsal muscles are activated sequentially along the body. The muscles are active unilaterally in each bend, from the convex part of a bend forward to the straight or concave part of the bend. As the snake progresses, each point along its body follows along the path established by the head and neck, like the cars of a train following the engine as it moves along the track (although the propulsive mechanism is very different); thus, sliding friction is a critical component of lateral undulation. The local adjustment of curvature around each point of contact with an external object indicates a high degree of sensory-motor control, unique to snakes and a few species of limbless lizards.
Sidewinding is used by many snakes crawling on smooth or slippery surfaces, but is best known in the sidewinder rattlesnake (Crotalus cerastes) and a few desert vipers of Africa and Asia. Sidewinding is similar to lateral undulation in the pattern of bending, but differs in three critical ways: First, each point along the body is sequentially placed in static (rather than sliding) friction with the substrate. Second, segments of the body are lifted off the ground between the regions in static contact with the ground. Thus, the body sort of rolls along the ground from neck to tail, forming a characteristic track (that is proportional to body length) in sand; after being lifted off the ground and set down again a short distance away, the front part of the body begins a new track while the rear part of the body completes the old track. Third, because of the static contact and lifting of the body, the snake travels roughly diagonally relative to the tracks it forms on the ground. Muscle activity during sidewinding is similar to that in lateral undulation except that some muscles are also active bilaterally in the regions of trunk lifting.
Concertina locomotion involves alternately pulling up the body into bends and then straightening out the body forward from the bends. The front part of the body then comes to rest on the surface and the back part of the body is pulled up into bends again, and so forth. The bends may push laterally against the sides of a tunnel or vertically against the ground to keep the body from slipping. Thus, static friction is critical to concertina locomotion. Concertina locomotion is used in crawling through tunnels or narrow passages and in climbing. In concertina locomotion, blocks of muscles are activated simultaneously, and unilaterally, in regions of bending and of static contact with the sides of a tunnel.
Rectilinear locomotion is movement in a straight line. It is used mainly by large snakes such as large vipers, boas, and pythons. In rectilinear locomotion, the belly scales are alternately lifted slightly from the ground and pulled forward, and then pulled downward and backward. But because the scales “stick” against the ground, the body is actually pulled forward over them. Once the body has moved far enough forward to stretch the scales, the cycle repeats. This cycle occurs simultaneously at several points along the body. Static friction is the dominant type of friction involved in rectilinear locomotion. Unlike lateral undulation and sidewinding, which involve unilateral muscle activity that alternates from one side of the body to the other, rectilinear locomotion involves bilateral activity of the muscles that connect the skin to the skeleton. One set of these muscles lifts the belly scales up and pulls them forward and another set of muscles pulls the them downward and backward.
Slide-pushing involves vigorous undulations of the body that slide widely over the surface. Slide-pushing is used when a snake on a smooth surface is startled and tries to escape quickly, but slips over the surface. In slide-pushing, irregular bends of the body and tail press vertically on the surface at different points; although the body slips on the surface, it pushes down with enough force to move the center of mass in a quasi-regular, often step-like, pattern. Thus the snake progresses irregularly by slipping along the ground. Sliding friction is most important in slide-pushing, although there may be occasional moments of static contact. The patterns of muscle activity during slide-pushing are unknown.
Perception in snakes
Snake vision varies greatly, from only being able to distinguish light from dark to keen eyesight, but the main trend is that their vision is adequate although not sharp, and allows them to track movements. Generally, vision is best in arboreal snakes and weakest in burrowing snakes. Some snakes, such as the Asian vine snake (genus Ahaetulla), have binocular vision, with both eyes capable of focusing on the same point. Most snakes focus by moving the lens back and forth in relation to the retina, while in the other amniote groups, the lens is stretched.
Snakes use smell to track their prey. It smells by using its forked tongue to collect airborne particles then passing them to the Jacobson’s organ or the Vomeronasal organ in the mouth for examination. The fork in the tongue gives the snake a sort of directional sense of smell and taste simultaneously. The snake keeps its tongue constantly in motion, sampling particles from the air, ground, and water analyzing the chemicals found and determining the presence of prey or predators in its local environment.
The part of the body in direct contact with the ground is very sensitive to vibration, thus a snake can sense other animals approaching by detecting faint vibrations in the air and on the ground.
Pit vipers, pythons, and some boas have infrared-sensitive receptors in deep grooves between the nostril and eye, although some have labial pits on their upper lip just below the nostrils (common in pythons), which allow them to “see” the radiated heat. Infrared sensitivity helps snakes locate nearby prey, especially warm-blooded mammals.
Determining the Sex of Snakes
It’s hard to image today that only a few decades ago, most keepers had no idea what was the gender of the snakes they kept. In a very few species, such as the leaf-nosed snake of Madagascar, the sexes have different appearances. However, most snakes do not have physical characteristics that will visually identify their sex. There are several commonly used methods of sex determination that are now employed to reveal that important information to a keeper.
The gender of an adult snake can be determined by introducing a smooth, blunt, lubricated slender probe into its cloaca, and pushing the probe against the posterior wall of the cloaca to see if it can be freely and gently pushed into the base of the tail. This is referred to as the “cloacal probing technique,” or, more often, simply as “probing.” When a keeper says “I probed my snake and it is a male,” they are referring to the cloacal probing technique. This technique was described in 1933 by Blanchard and Finster, and in detail by Fitch (1960), but really did not come to the attention of most keepers until the publication of two short papers by Josef Lazlo (1973, 1977). Prior to that time, most keepers had no certain way to determine the sex of their captive snakes.
When probing a snake to determine gender, it’s necessary to select a suitable sized probe. One should use the largest probe that could be inserted into the hemipenes of a male. One mistake made by many keepers is to use a probe that is too small in diameter. Usually this does not affect the outcome, but in some cases and some species, the determination made by a small probe is uncertain, as a small probe may pass relatively deep into the hemipenial homologs of a female.
The probe is inserted into the cloaca and directed against the posterior wall of the cloaca to determine if it can be passed into the tail, and if so, how far. This technique is based on the fact that a probe introduced into the cloaca can be slid a greater distance into the base of the tail of a male than into the tail of a female. The probe passes inside the inverted hemipenis of a male snake. The measure of the penetration of the probe into the base of the tail is the number of subcaudal scales spanned by that distance, counted from the vent posterior to the scale at the level of the maximum penetration of the probe. In most male snakes, a probe can be inserted a distance spanning 8-16 subcaudal scales.
Determining the gender of a snake by probing is not a surgical procedure and does not require sterile technique. The hemipenis of a male snake is introverted into the body, meaning it folds in on itself, pulled outside-into the tail by a hemipenial retractor muscle that passes up the inside of the everted hemipenis to attach to the tip. When the muscle contracts it pulls the hollow hemipenis into the body. It works sort of like when the finger of a glove is pulled into the hand of the glove. When a probe is inserted into the tail of a male snake, it is actually being inserted into a space surrounded by the external surface of the hemipenis. Snakes are not easily harmed or physically damaged by this sexing procedure, but it is necessary to be gentle and use judicious force.
The posterior cloacal wall of most female snakes has two outpockets that pass a short distance into the base of the tail. In most snake species a probe can be introduced into these pockets only a very short distance, usually only the distance of 1-2 subcaudal scales into the base of the tail. There are some species in which the females may probe deep relative to most female snakes; for example, diamond python females may probe as deeply as 8-9 subcaudals.
These outpockets are female hemipenial homologs, the developmental equivalent of hemipenes in male snakes. These paired funnel-shaped structures become increasingly narrow, ending in connective ligaments that continue toward the tip of the tail to insert on posterior subcaudal vertebrae, as do hemipenis retractor muscles.
In general, female hemipenial homologs do not evert, as do male hemipenes. The biggest exceptions to that rule that we have observed are blood pythons and short-tailed pythons; females have big well-developed hemipenial homologs that they can evert. Many “male” blood pythons have been missexed females that were observed to stick out their “hemipenes” when they were upset. Were comparison possible, the everted hemipenes of male blood pythons are bigger, with more structure and more vascularization than the relatively smooth, pale, everted hemipenial homologs of females.
The gender of some snakes, particularly large individuals, may be difficult to determine with certainty when using the cloacal probing method because of the funnel shape of the hemipenial homolog, which allows probes of different diameters pass to different depths. This condition probably exists to some degree in all female snakes, but the size and length of the hemipenial homologs in some snake species may accommodate the relatively deep passage of a sexing probe, as is seen in diamond python females. Sometimes keepers will choose to use a probe that is too narrow on the incorrect assumption that this will make probing the snake easier. In some species a narrow probe will probe deeply into females, and consequently increases the uncertainty of the determination. It’s a better technique to use the largest probe that could be expected to comfortably pass into the hemipenes of the snake if it were a male; such a probe will pass the shortest distance into a female, usually alleviating any confusion.
Smaller diameter probes also increase the chance of puncturing the female hemipenial homolog. This apparently happens with some regularity, and whenever sexing snakes, especially pythons, we have found it strongly advisable to probe both sides of the cloaca. Most people are right-handed and they tend to direct probing to the left side of the cloaca only. We occasionally find the hemipenial homolog on this side to be perforated in captive female snakes that are missexed as males; generally we find the hemipenial homolog on the right side of a female to be intact. We haven’t observed perforated hemipenial homologs to cause medical problems; however, it does not appear that they ever repair.
One other pointer that may help to ascertain sex in the confusing cases that are sometimes encountered when probing snakes: males typically probe to identical depths on each side, while females may probe unequal depths, varying several subcaudals between the right and left sides, owing to differences in size, condition and stretch between female hemipenial homologs. The hemipenes of males tend to be much more uniform in structure and probe to equal depths.
Hatchlings of all the snake species with which we have ever had the opportunity to try can be sexed using a method known as “popping.” The thumb of one hand is placed on the anal scale of the baby to be sexed and used to gently pull the scale forward, exposing and slightly opening the vent. The thumb of the other hand is placed on the subcaudal surface (underside) of the tail near the base. The thumb on the underside of the tail squeezes the tail against a finger and, using the thumb in a rocking motion, pushes gentle pressure toward the vent. The increased internal pressure in the base of the tail generated by the pressure of the thumb causes the hemipenes of the males to pop out.
The hemipenes of hatchling males are little pink or reddish rods that pop out, one on each side of the cloacal opening; on most species they have a visible red blood vessel on their medial surfaces. A females may evert her cloaca and erect her scent gland papillae in response to the pressure. The scent gland papillae may appear similar to hemipenes (especially if no comparison is possible,) though they are always smaller, and they may have a tiny red tip, but no blood vessel is visible. Snakes can be sexed with certainty using the “popping” method only during the first few weeks after hatching. After that time the young males gain sufficient muscle control of their hemipenes to make uncertain any “females” identified by this method. Hatchling snakes of all species are delicate and it’s critical to their physical well-being to gently restrain them when they are being sexed, and to not subject their spine to excessive compression or stretching. The use of an appropriate-diameter clear-plastic tube is a very safe and appropriate means to restrain delicate snakes, biting snakes, and venomous snakes.
Some adult male snakes will allow themselves to be “popped.” Here an adult male ball python allows the bases of his hemipenes to protrude in response to pressure on his tail. Upon seeing even this much of the hemipenes, a keeper can be assured that this is a male snake. The problem is that adult males may not allow any even partial eversion of their hemipenes in response to pressure. The brown substance on the hemipenial base on the right side of the photo is the “musk” from the one of the paired scent glands that are in the base of the tail of nearly all snakes. Pushing on the base of the tail often compress the scent glands in the tail of either sex, causing them to expel a small amount of their secretion. Note the spur of this male ball python, visible on the margin of the cloaca on the right side of the photo. Actually, for a male ball python, it’s a slender spur showing little wear. It does have a lot of inward hook, typical of males.
Sexually dimorphic characters
Throughout the snake kingdom, most species show only minor, if any, external difference between the sexes. However, scattered throughout the family tree of the snakes are occasional examples of sexual dimorphism. There are a few species that exhibit dramatic gender-specific observable characters.
Probably the best known of these is the cloacal spurs of the Boidae. A spur typically consists of a spur base which is capped with a spur claw; species with spurs have one spur at each side of the anal scale. In some smaller species, the spur is reduced in size and set into a fold of skin at the lateral margin of the anal scale, essentially unnoticeable. In some larger species of pythons, spurs are large, strong and very visible upon inspection.
In most of the boas, pythons and sandboas, the spur is used by the males during courtship to stimulate and position the females. In most of the species of the Boidae, the spur of the male is relatively larger, thicker at the base, and with a more pronounced inward hook. However, adult males use their spurs vigorously and for extended periods during courtship and combat, oftentimes the spurs of males are worn, the tips missing, and they may appear as actually smaller than the less-used spurs of females.
In most cases, the size and shape of spurs are an indicator of the sex of a boa or python, but this is not always an accurate means to determine sex with certainty. We have seen large adult, sexually-active male pythons with no apparent spurs, and occasional older females may have what seem to be quite large spurs.
In some sandboas and in rosy boas, the spurs of males are small but visible, while females have no visible spurs.
Male and female snakes leaf-nosed snakes (Langaha nasuta) have very different nose appendages. Some pitvipers have different colored sexes. The sexes of most vipers and pitvipers have different numbers of subcaudal scales.
MORE TO COME