Double "D" Reptiles

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Sex Determination in Reptiles

This section is aimed at assisting reptile keepers, breeders and general science enthusiasts in understanding how the sex of a reptile is determined. There is no tried and true general rule for this except to say that it happens in one of 2 methods. Eventually, this very factor may become a major determining factor in splitting the scientific divisions in a manner not done before. Certainly, we have split them based upon manner of birth and other generally noticeable differences. As more becomes known on this subject, what is to stop scientists from using information based upon offspring sex determination to be yet another dividing factor? Only time, and more research, will tell.

A large amount of the information contained in this document is available from various sites on the Internet, but very little of it has been listed on one site at the same time. Another part, and possibly the most important, was gleaned from research done by Dr. David Chizar, Dr. Hobart Smith and others as they tried to explain separate instances of unusual reproduction in reptiles. We'll discuss this a little further down in the article. Let's just say that their findings were controversial and enlightening at the same time considering that while the herping community knew a great deal about some animals, there was virtually nothing known or understood about others. Once you grasp the depths of their studies, the rest seems to fall into place. I won't try to quote them, but let me give them credit here for the outstanding work they did in making the unknown known and understandable.

Temperature controlled Gender

As was mentioned above, some reptiles have their gender determined by the temperatures at which their eggs are incubated. The most well known instance may be in crocodilians; crocodiles, alligators, caimans and others. Among other reptiles for which temperature controlled gender are at least some turtles, tortoises and geckos.

Generally, when determining the male/female ratio when temperature sexing, cooler temperatures result in higher female production while warmer temperatures result in higher male offspring being produced. The strange thing about this can be found in Leopard Geckos where even higher temps can result in more females being produced, but quite often they are sterile and will not reproduce as they mature. This is often the result of accidental overheating of the incubator or, all too often, the way breeders supply pet stores for relatively low prices. The eggs hatch faster at higher temps, so when breeders are looking to hatch a large number of eggs in the fastest time possible, for quick money in the neighborhood pet supply business, these higher temps pose no obstacle or threat to their continued income.

More data needs to be collected, over long periods of time in order to determine just how broadly temperature sexing of eggs extends. The unfortunate part of this is that too many people breed their animals, incubate their eggs in the safest (and most efficient manner) and raise the offspring systematically so as to not run any risk. The risk mentioned here is the loss of offspring and money, which can sometimes be used to restock the food supply for the parent animals later. Until more breeders are willing to take risks with offspring and their possible income from offspring, keep detailed records of every little event in their pets life and report all of the results, good and bad, then the current information available on reptiles may never be updated which would allow us, as their keepers in captivity, to take the best possible care of them so that they can live their lives to the fullest.

List of species for incubation temperature controlled gender

Leopard Geckos

Non-temperature Controlled Gender

For the greatest number of reptile species, including snakes, their gender is not controlled by the temperatures at which the eggs are incubated but by the genetic codes supplied by the parent animals. Reptiles produce sex chromosomes in much the same way that mammals (such as humans) do. One gender has 2 of the same chromosomes and the other gender has different chromosomes. Thus the genes split for reproduction and each parents genetic material is supplied for the offspring singularly.

In humans, scientists code these as X and Y chromosomes. Female humans have 2 X Chromosomes and must provide an X for reproduction. Male humans have an X and a Y chromosome and so may provide either for reproduction while never knowing which. By this method, it is the human male genetic material that determines the gender of the offspring.

Research by noted scientists such as Dr. David Chizar and Dr. Hobart Smith, among others, has determined that the genetic make-up in snakes, at least some lizards, and possibly all reptiles, is the opposite. Their findings were finally published in an early issue of Fauna magazine along with another startling discovery we will touch on shortly. They determined that the chromosomes for snakes, which we here will label Z and W, are found in just the opposite situation. Specifically, females have the different chromosomes (1 Z and 1 W) and ultimately determine the sex of the offspring while the males (2 W chromosomes) simply provide the other half for the genetic gender code.

When the eggs are being formed in the female, the chromosomes split and either the Z or the W chromosome is incorporated into the egg while the extra is either re-absorbed or expelled from the body naturally. When mating occurs, the male sperm fertilizes the egg with the only component it can possibly provide, even though the chromosomes have split. The result is either a male or female offspring with a genetic code provided from each parent animal.

Species not specifically listed in the list above are not currently known to be gender determined based on incubation temperatures.

The big surprise Parthenogenesis!

The problem that Drs. Chizar and Smith had was a number of odd occurrences either in their labs or other locations/situations that they were familiar with. It was the same problem that has been recorded multiple times in populations of western fence lizards (believe this is correct) that were entirely female. The problem that had plagued scientists was how these animals were able to produce living offspring without the presence of a male of the species. This is known as parthenogenesis and was thought to be limited to this one species. (Of course, this was convenient because it didnt disturb the current balance of their knowledge, which had reptiles reproducing using the same genetic model as mammals.) This way of looking at reptile gender encoding was totally disrupted when Dr. Chizar entered his lab one morning to find a female Timber rattlesnake, Crotalus horridus in her enclosure with a new baby rattlesnake. This female was a research animal and had not been with a male of her species for roughly 10 years or more!

At first, it looked like a record for reptile sperm retention. DNA samples were taken along with finally determining that the offspring was male, and the results shook the scientific world to its reptilian foundation. DNA samples from the mother and offspring looked basically identical except for a few minor coding signatures that were totally missing. Along with Dr. Smith, samples of other occurrences of the same thing were compared with quite similar results. Parthenogenesis in snakes had been confirmed!

The problem this now presented was figuring out exactly how this phenomenon happened. They had to look again at a number of offspring, a couple of which had survived and a good number that had been stillborn, and recheck the gender. It began to appear that all viable offspring had in fact been male with no complete female offspring produced.

If more information on this event, and other similar events, can be found, we will post it here as soon as possible.