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.

Cloacal probing

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 detali 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.

As pictured here, 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.

Restraining Snakes During Probing

Most snakes do not care to allow themselves to be sexed without a struggle. Determining the sex of a snake by cloacal probing is best done as a two person job-a "holder" and a "prober". There are a variety of methods that can be successfully used to hold a snake for this procedure, our only comment is that it's the responsibility of the holder to see that the snake being sexed does not bite the prober. Snakes being probed have an uncanny way of sort of winding back and forth in some state of agitation, to end up biting not the person restraining him, but the person who is not paying attention to the foreparts of the snake, and instead is concentrating on the tail of the offended snake. It's disconcerting to all parties.

Perhaps the easiest and most certain way to accomplish safe restraint ("safe" meaning "safe for all parties") is by placing the anterior three-fourths of the snake in an appropriate-diameter clear-plastic restraining tube. A set of these tubes in graduated sizes, is an important item of equipment for any snake collection.

Cloacal Popping

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.

The sexes of some species have different tail lengths, with the males having longer tails ostensibly to accommodate the hemipenes. Most terrestrial colubrid snakes have significant tail length differences between the sexes. The sexes of sandboas have noticeably different tail lengths. Most arboreal snakes and other snake species with muscular prehensile tails do not show much difference between the sexes. In general, however, differences in tail length are not a satisfactory means of determining sex.

Often the adult size of an animal may give some indication of the gender of the animal. If you visit the zoo and see an immense reticulated python, it's almost certainly a female. However, a magnificent specimen of a western diamondback rattlesnake is probably a male. Females are the larger sex in most species of sandboas, pythons, boas, garter snakes and some Old World pitvipers. At the other extreme, males are considerably larger than females in rattlesnakes, kingsnakes, king cobras, and some of the ratsnakes.

Behavior

Sometimes snake behavior is an indicator of gender. If you put two unidentified adult snakes together and they combat, then it's likely that they are both males. Of course, if no combat is observed, they could be a male and female or both females. And it's always possible that they could both be males, as combat is unknown to occur in many snake species, and it doesn't always occur even in species known to vigorously combat.

Likewise, courtship and even observed copulation doesn't necessarily mean that the observed snakes are a male and a female. Some female-female dominance behavior looks sort of like courtship and male-male copulations have been observed in pythons, kingsnakes, and vipers.

The careful observer may notice shed skin from hemipenes in the water bowl, that would be pretty conclusive evidence that a male is inhabiting the enclosure. Sometimes solitary males will leave small deposits of sperm in their water bowl or in little spots around the cage. When observed, this is pretty conclusive evidence that the inhabitant of the cage is a male.