Unlike species such as chameleons, which display a high degree of sexual dimorphism, male and female blue-tongued skinks look virtually identical. Gender determination is difficult unless you observe social interactions, observe one or both of your lizard’s hemipenes, or find a seminal plug. Positive identification of a blue-tongued skink's gender is not always possible.
Hemipenes
All male lizards have paired reproductive organs, termed hemipenes; lizards carry their hemipenes inverted, inside their tails. Lizards evert their hemipenes during mating activity and sometimes during elimination. Failing to see a lizard evert hemipenes is not necessarily indicative of a female -- observing hemipenes only confirms that an animal is male.
Seminal Plugs
Male lizards produce seminal plugs inside their inverted hemipenes. Sometimes, a lizard will eject these plugs; you may find them in the cage or water bowl. Seminal plugs are white to grey in color, and vary in consistency. It won't matter if you house two lizards in a single habitat unless you witness one of the creatures ejecting the plug.
Behavior
Behavior and social interactions can provide clues to the gender of blue-tongued skinks. Often, animals of the same gender are antagonist or combative with each other. In mixed gender pairing, the male will often pursue and attempt to mate with the female, who may or may not accept his advances.
References
Photo Credits
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