I've been flicking through some of the research undertaken by Alex Quinn et al. at the University of Canberra, Australia, and thought I'd put some thoughts on it up on here. It will later form part of an article for my website, but for now I don't have the time to write the full article. It looks at the effects of incubation temperature on the sex of the babies in Bearded Dragons.

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 Classifieds 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.

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.


As a bit of background:

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 "homogemetic" combination, as the sex chromosomes are of the same type). If you get XY, you are a boy ("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 Canberra research team first of all 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 "physical" gender (hemipenes etc) that matches up to its "genetic" gender.


Their latest findings, published in April last year, show that there is a temperature-dependent element to gender:

Basically, 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, we develop as females, and require a gene from the Y chromosome - "sry" to develop testes, which then release hormones causing development into a male. In the abscence of this gene, we follow a 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 "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). 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 be doubled. A genetic female will never receive enough of the product from the single Z chromosome to develop as a physical male. However, it is possible for a genetic male to not receive enough of the SDF to develop as a male - in which case, 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 top-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 females, with a gradual change of %females in between. Obviously 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 "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.


This of course throws up some interesting problems:

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 males.

At this point I'd like to say well done to anyone still reading, who hasn't just skipped straight to the conclusion - during my re-read I was very tempted to miss out all of the middle section...


Just to conclude, for anyone who didn't fancy reading all the stuff above:

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 BD eggs above 97f (indeed, I've never met anyone incubating above 90f, due to the risk of birth defects...). 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.

2. The key temperature-sensitive phase of incubation is the middle third of egg development.

3. Any "male" eggs which produce females due to incubation temperature should theoretically have reproductive problems - either egg inviability, 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.


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 fertility/100% male offspring problems) ill-advised strategy.

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.

I hope that some people find it interesting and helpful (and apologies for the length - I've just re-read it all and it took a while...)

Andy


A couple of references I used to write this:

"The dragon lizard P.vitticeps has ZZ/ZW micro-sex chromosomes" - T.Azaz, A.Quinn et al. 2005 - Chromosome Research 13:763-776

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

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Thanks sharing this

It's great, and it is very helpful

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Wow I found that really interesting! Thanks.

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interesting.
97 f to cause what we might consider a hot female eh? thats higher than i was under the impression of.

about 7 degrees higher to be honest lol.

ALways interesting to read this stuff.

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I don't know if such a thing as a "hot" female exists in beardies like it does in leos. The impression I've got from the research papers is that the SDF dosage is kind of "final", and will shape their physical characteristics as well as their mental ones - although they would be genetically ZZ, they would have a female-wired brain, and would otherwise act as normal.

If the Quinn research group do decide to grow some of their TDSR "females" up to adult size, they may publish a follow-up paper looking at behaviours and social interactions.

As the optimal range for the SDF is believed to go up to around 93f, you could start to see TDSR females from that temperature upwards - just not 100% TDSR, which they found doesn't happen until around 97f. Of course a 90f incubator could have a hot spot a couple of degrees higher, so I suppose it is perfectly possible (although the odds against are pretty large) of getting a TDSR in a cheaper "90f" incubator... Of course, with the increased risk of birth defects and egg failures at 90f, it would be much safer to not get near that temperature at all.

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ah well we have a female beardie that is defo fitting the hot description.
We had a book out of german origin about 18 months ago that had a bit about it, im not sure what its called mind.

I thought the genetically male but develops as a female with female organs implies what is for aswell as leos what a hot female is/would be.

if not, then in leos what is the difference that makes hot females possible?

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I don't know what the difference is in leos, but there was some suggestion that certain reptiles (I don't know if leos would be included or not) don't actually have sex chromosomes, and their gender is determined entirely by temperature. Of course, it could just be the case that, like the BDs, the sex chromosomes are too small to have been detected thus far.

If it was a similar mechanism for leos, then surely any incubation temperature that produced more than 50% females would include some TDSR females, so you would get "hot" females every time you incubated for female, regardless of the temperature being too high?

Just out of interest, how does your "hot" bearded dragon behave? Is it just generally male-hating? Or everything-hating?

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extremely unsociable to other dragons of both sexes.

large pores [but no bulge]
when angry goes gully blacl down entire underside of body from bearrd to belly and tail.
Lots of head bopping nowadays.
Very jealous beardie.


On leos.. not sure but its low incubation like 81-84 or soemthing for female.... 85-88 male.. and then above 89 female again but with general hot behaviour and appearance as far as im aware.

back to beardies, [well kinda general] I thought it was that the a genetic male is gona be male but a genetic female having both x and y or whatever [sorry im not good with specifics] will then develop as either depending on temps, with as that article sugests are much wider range for difference than soemthing like leos.

On leos, I guess your right ye if its entirely dependant on temps then maybe they arent either genetically..but then how do hot females come about? if its not forcing a change? of any type?

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I've finally got some free time on my lunch break to dig out a paper on leos...

"Effect of hormonal manipulation on Sociosexual Behaviours in adult female E.macularius" - Flores and Crews, Texas University. Hormones and Behaviour 29, 458-473, 1995

The basic premise of their experiment was to incubate a group of eggs at 79f (they got 100% female), and another at 91f (producing 25% female). They grew the females to adulthood.

Their results showed that the "hot" females showed increased aggression to other leos ("hot" or not).

They also performed ovariectomies on some of the females. If done just after hatching, it increased aggression at adulthood; if performed at adulthood, it decreased aggression from that point onwards. This suggests that a hormone produced from the ovaries between hatching and adulthood is responsible for changing the brain structure to "female", at normal temperatures.
An interesting by-product of this is the discovery that males can tell whether or not a female has ovaries present. If she is intact, he will attempt to court her. If she has undergone ovariectomy, he will either ignore her or attack her.



They suggest that "hot" females are caused by either:

1. A change in the hormonal environment within the egg - the high temperature causes e.g. increased testosterone action.

2. A higher temperature causes increased expression of Testosterone receptors within the brain, making the "hot" females more sensitive to Testosterone.


They believed that 2. is more likely. They treated a group of 79f females with Testosterone, and a group of 91f females with Testosterone, and found that the 91f group were much more sensitive to it (measured as increased likelihood of aggression after treatment).


The article cited Crews' 1988 paper that found that "hot" females are "masculinised" - they show male-pattern growth, and a more male-like endocrine physiology (hormones etc).

The article also found that "hot" females can't distinguish between 91f and 79f females, and will attack both indiscriminately. However, males can tell the difference, and will court 79f females, whilst attacking 91f females. This suggests that there is something more than just over-expression of Testosterone receptors in the brain in the "hot" females.

It's possible that the "masculinised endocrine physiology" includes increased production of Testosterone, which can affect skin-bound lipids, and thus be detected by the male.



From reading this, it seems that it is Temperature-dependent Sex-Determination (TSD) in leos, not the TDSR seen in beardies. The "hot" characteristic of female leos seems to be independent of their actual sex-determination. There is just a reduced chance of them being female at 91f. I would assume that the differential egg conditions/differential levels of Testosterone receptor expression would be the same for males/females incubated at 91f.
In males it would either be obscured by them already producing Testosterone, or not measurable due to the extreme unlikelihood of producing males at lower temperatures.

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thanks for this - I found it really interesting. I thought I'd read somewhere before that there wasn't really a temperature at which you could breed for a particular sex for BDs. The odds are that even at normal temps you will get a mismatch between the numbers of males and females, thus leading to people thinking they've been successful. Or could it also have something to do with the fact that young BDs are very difficult to sex?

I do hope no one tries to incubate at high temps just to get females!

Anyway - thanks for this. And if you hear more in the future about whether the females can breed please post that too! Thanks for sharing!

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