Joel,

Funny that you should ask this one today.  Just yesterday I was teaching a seminar to other veterinarians how to do a physical exam and sex snakes (not something they teach in vet school).  This seminar came out of several vets asking me how to sex snakes and trying to get me to help them over the phone.  However, not only this seminar came out of the conversations; I also rediscovered how impossible it is to try and teach someone this technique without actually demonstrating it and then walking them through it on their own.

Probing is the most accurate way of sexing snakes.  The theory is that male snakes have paired hemipenes caudal to the vent, located on each side of midline on the ventral side of the tail.  Think of them as inside out socks.  When they use one or the other, they simply relax the sphincter muscle at the base of the hemipene and use the increased blood pressure to evert and fill them to engorgment.  Probing is simply determining whether or not that "sock" is there or not, as females either don't have them or, as is the case in many species, have much smaller, non-invertible, pockets.  At the base of these pockets is the musk gland (this is important).  It may sound simple enough, but much care must be exercised.  I have seen many male and female snakes with trauma to their hemipenes/musk glands and bleed, and even some females have had their musk glands punctured.  The problem is that this area is home to all forms of bacteria, and if there is damage or a wound is generated, bacteria can enter and incite an infection in the snake.  This infection and subsequent inflammation can have serious consequences, such as males can lose the ability to breed, systemic infections can ensue, and even death can be a result.  This all can happen even if you don't see the trauma.

Common problems in probing can result in not having a smooth enough probe, not enough lubrication being used, the probe diameter being too large or too small for the snake, too much force being used, or the probe inserted into the wrong area or the wrong way.  I also caution people not to probe a snake more than once in a three-day period.

One show I worked at a proud new owner showed me an adult snake they had just bought; They had it probed by at least a half dozen breeders there at the show to definitively find the sex before showing it to me.  Unfortunately, they had gotten both male and female probing results from different breeders.  I examined the snake, and although it was easy to probe it as a female, it turned out that at least one of the breeders had probed too deep and ruptured the musk gland, allowing a probe to insert to the end of the tail under the skin.  This snake then had to have surgery to repair the damage and to prevent a systemic infection had to follow up with a course of antibiotics.  Needless to say, this owner was mad at all the people claiming to be "experts", as it was the owner that had to foot the bill to bring the snake back to health. 

As a result, I won't even try to teach you to probe over the Internet.  What I recommend is that you purchase a top-quality set of probes (at least 5 in the set with a hatchling probe) and find a veterinarian or experienced herpetoculturist to teach you, one on one.  This would be the best way.

Popping is another common method of sexing snakes.  As the story goes, there is no true negative, only a true positive.  I have probed many a snake that popped as a female and turned out to be male.  You can also have an eversion of the musk gland sac in some species that will pop as a male, but really be a female (but this is rare).  Rule #1- never pop a snake that is more than a month old- serious damage can result, and if it is a male, it may never be able to breed.  Rule #2- If you must do it, have someone very experienced do it to show you, and then practice on your own animals for years before you do it for anyone else.  Because of the high risk of damage, I never, ever, pop a client's snake, no matter how young.

Again, I don't dare try to teach you this technique, either.  Because of all the injured snakes I've seen from this manner of sexing, I don't think it's a technique that should be used.

Lastly, you can sex a snake by looking at the tail shape and count the number of scutes (ventral scales).  For each species of snake, there is a "type individual" with a catalog of the number of scales on each part of the body.  Males of most species have more scutes after the vent, and also have a much slower taper of the tail back to the tip.  Females tend to have a smaller tail, with a faster taper after the vent.  This is best done when you have more than one individual to compare to each other.  Granted, if you aren't careful, it's easier to make a mistake, but there's absolutely no risk of hurting the animal by using this technique.  You have a perfect situation- two snakes, one of each sex.  Compare the tails carefully, consult with books or experienced breeders to find the scute counts and tail shape for males and females, and you'll be just as accurate as if you probed.

In summary, probing and popping are both techniques that shouldn't be taught long-distance.  Find someone in your area or herp group that is experienced with probing and owns a very good set of probes (they should be well-used, too!), and ask them to show you how to probe.  Then, get your own set and try it out.  Otherwise, get some information and compare tail lengths.

Good Luck and Happy Herping!

Dr. Adolf Maas, DVM