One of the first problems encountered in trying to breed Gilas in captivity is the difficulty in determining the sex of individual animals. Many techniques have been proposed, and many have turned out to be of little use.
Gila monsters have evolved in many unique and fascinating ways. Understanding their natural history will not only increase our appreciation of this misunderstood creature, but will greatly aid in their successful captive propagation.
There are differences in body form, but these differences are difficult to quantify and they are not consistently expressed. Mature males typically have a stouter build and a broader head. Mature females, by comparison, have a narrower head and a more pear-shaped body. Be aware that these are general trends. There are animals that appear in-between these extremes and are, therefore, difficult to accurately sex from appearance. These differences based on body form are somewhat subjective and some experience is required to become good at the technique. I don't consider using body form a consistently accurate method of determining sex in Gilas.
Behavior has been used to identify the sex of Gilas. The best way to use this technique is to have a known male and observe his reaction to other animals placed with him. During the breeding season, males will act aggressively towards one another in a very ritualized form of combat. Again, be aware that these interactions can be misinterpreted. Any combination of sexes can act aggressively towards one another when first introduced. That is why it is recommended to have a known male as a reference when attempting this technique. For example, if you placed two Gilas of unknown sex together and they fight, they could actually be two females and not two males. During the breeding season a known male will usually act with interest and not aggression towards a female (although, there are exceptions). He will do a lot of tongue flicking and following her around. Although experience will increase the accuracy of this technique, I don't consider it a consistently accurate method either.
Currently, the only simple, accurate and safe method available for determining the sex of Gilas utilizes ultrasound examination to image the gonads. The testes in a mature male are clearly visible on ultrasound. In addition, in the mature female, follicles are visible in the ovaries regardless of the time of year the examination is made. Since this technique is so easy to master, as long as your vet has an ultrasound and the willingness to try it he or she could probably sex Gilas accurately on the first attempt.
The ideal time to perform ultrasound sex determination is from the fall through the spring. During this period of time, the developing female ovarian follicles are larger and are easier to see. However, with experience, ultrasound sex determination can be accurately performed at any time of the year.