Visible differences between male and female reptiles are often subtle, if present at all. While a number of methods exist to differentiate between the sexes, they differ from one taxa to the next. Some methods for determining gender can be used by anyone, while other techniques, like popping and probing, require expertise or veterinary assistance.
Fortunately, many species have distinctive, external clues that can be used to differentiate males from females. Many male lizards are adorned with spikes, crests or dewlaps that are lacking or reduced in females of the same species. Species featuring this dichotomy, termed sexual dimorphism, include green iguanas (Iguana iguana), basilisks (Basiliscus sp.) and anoles (Anolis sp.). Additionally, males of some lizard species have visible bulges from the hemipene -- a bi-lobed reproductive organ -- at the base of their tails or enlarged femoral pores on the ventral region of their legs. Aquatic turtles can often be distinguished by the presence or absence of long fingernails on the front legs; males have very long fingernails, which they use to stimulate the cheeks of receptive females.
Often, the quickest way to determine the gender of a reptile is to ask another individual of the same species. Most lizards and turtles are able to determine the sex of conspecifics on sight, and snakes can easily smell the differences between the genders. By making a supervised introduction of the two animals, you can observe their behavior and deduce their genders in some cases. Males of many species will immediately sense their rival and behave antagonistically, whereas females often simply ignore each other. This is especially the case with lizards such as bearded dragons (Pogona vitticeps) and panther chameleons (Furcifer pardalis). In the case of a mixed-gender pair, the male may pursue or seek to dominate the female. It's important to separate animals before they become combative or stressed to prevent illness or injury.
Though not infallible, size can be a significant clue for mature reptiles. As a general trend, adults of species that feature male-to-male combat produce larger males than females, and the reverse is true in species without male-to-male combat. Accordingly, very large bearded dragons (Pogona vitticeps), frilled dragons (Chlamydosaurus kingii), rattlesnakes (Crotalus sp.) and green iguanas (Iguana iguana) are most likely males, whereas very large boa constrictors (Boa constrictor ssp.) or Burmese pythons (Python molorus bivvitatus) are likely females.
Probing and Popping
When all else fails, snakes can be usually be probed or have their hemipenes everted by someone trained to do so. Everting the hemipenes of a snake or lizard is termed "popping." Some lizard species have been successfully probed or popped, but it isn't as reliable a technique. In the case of probing, a stainless steel rod of appropriate diameter will be inserted into the vent of your reptile. In males, the probe will pass into one of the inverted hemipenes and penetrate rather deeply; in females, the probe passes into an accessory gland, which is broader and less deep than the male. Realize that either of these techniques can cause pain and irreversible damage if performed incorrectly, so consult with your veterinarian.
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