This article looks at how gender is determined in Leopard Geckos.

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After writing the Article on Temperature Dependent Sex Reversals (TDSR) in Bearded Dragons and posting it on a couple of forums, a discussion came up about the issue of potential "hot" Bearded Dragons, and whether any parallels could be drawn to the well-known phenomena of "hot" females in Leopard Geckos.

In the course of writing the TDSR Article, I came across a few interesting bits of research into Leopard Geckos, and the effects of incubating at different temperatures. This is also my first non-Bearded Dragon Article...


 

 

Flores and Crews' study in Texas in 1995 focussed around finding explanations for aggressive behaviour in females which hatched from male-dominant clutches of eggs. Current scientific knowledge (as of February 2008) suggests that Leopard Geckos, in common with many other reptiles, don't have sex chromosomes. Instead, hermaphrodite eggs are laid, and the physical sex is determined by incubation temperature. Unlike in mammals, there is no "genetic sex" with Leopard Geckos. This view may be subject to change in the future, however - the sex chromosomes in Bearded Dragons for instance were so small that they were only discovered in 2005. However, the relative "ease" with which Leopard Gecko gender can be determined by incubation temperature, and the lack of problems that all "female-incubated" Geckos seem to have with relation to gender bias in their clutches, suggests that even if sex chromosomes are present, they are far more gender-flexible than the sex chromosomes currently known to science. The potential problems with gender-bias are discussed towards the end of the TDSR Article so I won't go into them here.

 

 

 

In the course of the 1995 study, two groups of eggs were incubated, and raised to adulthood. One was incubated at 79f, and produced 100% females. The other was incubated at 91f, and produced 25% females and 75% males. The females hatching out of the 91f eggs showed increased aggression to other Leopard Geckos as adults (regardless of whether the other Gecko was male or female), compared to the females from the 79f eggs.

They then tested a group of 91f females and a group of 79f females with testosterone, to see how it would affect aggression levels. The increase in aggression was much more for the 91f females than for the 79f females, suggesting that the 91f females are more sensitive to the effects of testosterone. Flores and Crews proposed that during incubation, the increased heat somehow causes either increased expression of testosterone receptors in the brain, or increased sensitivity of them. The increased reaction to the same dose of testosterone suggests that it is not just a case of 91f females producing more testosterone than 79f females.

 

Firstly, male Leopard Geckos can tell the difference between a 79f female and a 91f "hot" female. They generally attempt to court the 79f females, but will either ignore or attack the 91f females. This suggests that there is some sort of outwardly-visible signal that allows males to tell the difference between "hot" females and regular females. Two hypotheses have been proposed to account for this:

1. As well as increased expression / sensitivity of testosterone receptors in the brain, there is also an increase in testosterone production, which can possibly interfere with lipids in the membranes of skin cells, allowing it to be detected by the male.

2. There is some sort of body-language / other communicatory cue that the researchers failed to pick up on, but which the male Leopard Gecko is sensitive to.



Secondly, some of the 79f females underwent Ovariectomies. The group found that if this was performed at hatching, adult aggression levels increased. This suggests that the Ovaries are responsible for producing something (probably hormones) whilst the Gecko is a juvenile, which causes development of a female brain, or reduced sensitivity to testosterone. The lack of Ovaries in those Geckos who underwent Ovariectomy at hatching meant that they did not receive this hormone to prevent aggression as adults.






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

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