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Andy Edge
Hades Dragons
February 2008
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 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 physiological 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 (see the
TDSR
Article for an explanation of potential problems), suggests that even
if sex chromosomes are present, they are far more gender-flexible than the sex
chromosomes currently known to science.
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
it. 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.
A couple of other interesting points came out of the study:
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. Two hypotheses has 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.
References:
"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.
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