This article looks at the effects of incubation temperature on sex.

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Most people are aware that in species such as Leopard Geckos, there is a strong effect of incubation temperature on the sex of the hatchlings, but most people seem pretty unsure about what effects it may or may not have on bearded dragons. Frequently, there are dragons up for sale in various reptile adverts and for sale sections, that are "incubated for female" - but when asked, the breeder doesn't actually know how to "incubate for female", and there is no consistent temperature given between various breeders who claim to incubate for female. This article will hopefully explain in some detail the science behind "incubating for" and explain why - under normal conditions - this isn't possible for beardies.

Apologies if some of this is a little bit too "sciency" - there will be a couple of "concluding" paragraphs at the end, for anyone who just wants the information, and not the background stuff etc.
 

In mammals (and humans) there are two different types "sex chromosomes" - X and Y. You get one from your mother, one from your father. If you get XX you are a girl (XX is known as the "homogametic" combination, as the sex chromosomes are of the same type). If you get XY, you are a boy (aka "heterogametic").

Many reptiles and birds do it differently. In bearded dragons, the chromosomes are called Z and W, and it is the opposite way round. ZZ (homogametic) gives a "genetic" male, and ZW (heterogametic) gives a "genetic" female. The reasons for putting "genetic" in inverted commas will become apparent later.


The University of Canberra research team first identified the sex chromosomes - until 2005, it was known that bearded dragons had genetically-determined gender, but no-one had managed to identify the sex chromosomes, because they were too small. Once they could be identified, and the Z and W could be distinguished, it was possible to check whether a bearded dragon had a "phenotypic" (physical) gender (hemipenes, ovaries etc) that matched up to its "genetic" gender.

 

It is now believed that the Z chromosome carries a gene which is crucial for male development. It probably encodes an enzyme, which catalyses a reaction to produce something that causes "maleness". This is a similar situation to humans:  by default, humans develop as females, and require a gene from the Y chromosome (known as "sry") to develop testes, which then release hormones causing development into a male. In the absence of this gene, we follow a "default" pathway of female embryonic development.

The reason that the Z-chromosome gene probably encodes an enzyme is that it appears to be temperature-sensitive, with an optimal temperature range, believed to be around 21c (70f) - 34c (93f). Within this range, it functions as normal.

In order to become physically "male", the embryo needs a high dose of whatever the end-product of the enzyme reaction is. I will refer to is as "sex-determining factor" - SDF - from now on for convenience. There is a threshold value for this - above threshold, and you get a male; below threshold and you get a female as that is the default developmental pathway. A ZZ male has two copies of the gene; a ZW female has only a single copy. This means that the dosage to the male will potentially be doubled. A genetic female will never receive enough of the product from the single Z chromosome to develop as a physical male, whereas under normal circumstances a genetic male - with two copies of the gene - will receive enough to develop as a male. However, under some conditions it is possible for a genetic male to not receive enough of the SDF to develop as a male. In these cases the embryo will follow the "default" pathway, and become female.

Genetic Z-chromosome defects aside - which are often lethal anyway - the only way for a genetic male to not receive enough SDF is if the enzyme that catalyses its production is working suboptimally - i.e. outside of its optimal temperature range. This means that the amount of SDF produced is below threshold, so although genetically the embryo is male, it will develop as a female, with female reproductive organs etc.

Eggs incubated above 93f (the upper end of the enzyme's optimal activity range) will occasionally not receive sufficient SDF to develop as males. Above 36c (97f), the research team found that 100% of hatchlings were physically female, with a gradual change of percentage physical females in between. There was a high amount of mortality at that temperature, due to the incubation temperature being so extreme. They tried analysing the results assuming that all mortalities were "genetic, physical males", and still found that significantly more than 50% of eggs would have contained females. Of the physical "females" that hatched, 51% were found to be genetically male. They had proved temperature-dependent sex reversal (TDSR) was possible in bearded dragons, for high temperatures. As the enzyme's optimal range is believed to have 21c as a low-end of the range, they have not been able to prove any form of TDSR for low temperatures - no eggs incubated below 22c hatched, so there were no babies to analyse.
 

If you have a genetic male trying to produce eggs, there may be meiotic failures due to chromosomes not lining up properly, which would cause infertility. This may not be the case, as ZZ chromosomes can line up during sperm production.

If viable eggs were to be produced, they would all be genetically male - both parents are ZZ, so all babies would be as well. Unless the eggs were incubated at very high temperatures to ensure TDSR, all babies would be genetic and physiological males.






1. It is possible to "incubate for female", but at much higher temperatures that those commonly used for incubation. I have never found anyone who would incubate beardie eggs above 97f (indeed, I've never met anyone incubating above 90f, due to the increased risk of birth defects and in-egg mortalities...) At 97f, there are very high mortality rates, and birth defects are not uncommon, due to other, non-sex-determining enzymes being affected by the high temperatures. TDSR females can apparently start to appear from around 93f.

2. The key temperature-sensitive phase of incubation appears to be the middle third of egg development.

3. Any "male" eggs which produce physical "females" due to incubation temperature should theoretically have reproductive problems - either egg non-viability, or 100% male offspring. As the research is less than a year old, I don't think they have had chance to test these theoretical ideas - it may be that more papers are published over the next couple of years, when the team gets the chance to grow some TDSR "females" to breeding size, and breed them with normal males, to see what offspring (if any) are produced.

4. There have been reports of so-called "hot" female bearded dragons. This is a well-known phenomenon in some female leopard geckos which have been incubated at a high temperature (intended to produce males). I have written another article looking at this: Temperature-dependent Sex Determination in Leopard Geckos. The leopard gecko mechanism predicts an increased sensitivity to testosterone caused by increased egg incubation temperature. If a similar effect was present in bearded dragons, this would explain aggressive, stereotypical "male" behaviour by "hot" females.



This means that whilst it is possible to "incubate for female" in a lab setting, a lot of the eggs will fail, and a lot of the hatchlings will have problems associated with too high an incubation temperature, so it is a dangerous and (given the theoretical infertility / 100% male offspring problems) ill-advised strategy. Additionally, many of the TDSR "females" produced may have behaviour problems that would make them entirely unsuitable for breeding / living with other females. This pretty much defeats the point of attempting to produce more females in the first place.


 

** Thus babies advertised as "incubated for female" will almost certainly have the usual 50:50 chance of being male or female, unless the breeder risks high mortality and birth defects by incubating at an extreme temperature. **

As this is fairly recent research (2007), at the time of writing there has not been sufficient time for the team to grow on the TDSR "female" babies, to see whether they are able to produce viable eggs, and whether all of those eggs would turn out to be genetic males as predicted.






"The dragon lizard P.vitticeps has ZZ/ZW micro-sex chromosomes" - T.Azaz, A.Quinn et al. 2005 - Chromosome Research.

"Temperature sex reversal implies sex gene dosage in a reptile" - A.Quinn et al. 2007 - Science vol. 316.

"Effect of hormonal manipulation on Sociosexual Behaviours in adult female E.macularius" - Flores and Crews 1995, Texas University - Hormones and Behaviour 29.