Gila Monster Mating

Male Gila monster approaches a female in her burrow,
A male Gila monster enters a female's burrow.

Many individuals have recommended keeping multiple males and females together in a group during breeding season. This is both because of the difficulty in determining sex as well as the thought that combat between males will facilitate successful breeding. It is the opinion of the author that this approach can be counterproductive to success.

In the wild, Gilas demonstrate a structured social system including establishment of dominance through male-male combat. The hypothesis is that in free-ranging Gilas as well as in other squamate reptiles, the “winner” of these male-male fights gains access to females. However, in the typical captive group setting the dynamics of the male-male interactions are very different from what occurs in free-ranging Gilas. In a typical captive setup, males lack an avenue for escape. Dominant males will often fight one another incessantly at the exclusion of all other activities, including mating. Male combat can be a useful tool to stimulate a reluctant male to breed, but it must be managed carefully to be successful in the captive environment.

Gilas will breed successfully without male combat. I house the Gilas separately and introduce females into the male’s cage individually. If you try combating males, do so with extreme caution to prevent injuries. Biting seems to be a component of combat in captive males that is less likely in free-ranging Gilas.

Camera with IR lighting allows 24 hr. monitoring.
A close look shows the camera and IR lighting that allow 24 hr. monitoring of reproductive behavior. In addition, fluorescent lighting provides a basking area in the spring.

The timing of successful copulation is related to the body temperature after emergence from winter cooling. If animals are kept with a consistent body temperature of 30 C (86 F) from the end of winter cooling, they will typically mate in 4-6 weeks. In the wild, mating occurs later than it typically does in captivity. In my captive group, I mimic the temperatures of wild Gilas and mating occurs in my animals during the same time that it typically occurs in the wild.

A single male can mate with multiple females. However, I strongly recommend an even ratio of males to females in a breeding colony. A one-to-one gender ratio will great aid breeding efforts.

Copulation has been observed to last from 15 minutes to as long as 2.5 hours.

Once a female is gravid, she should be separated from her cage-mates if she is not already alone. This will prevent unnecessary stress to her and will prevent cage-mates from eating her eggs.

Careful observation reveals distinct changes in female behavior from copulation to oviposition. The first stage is marked by behavior that I characterize as "quiet but alert." She will spend long periods of time motionless but very alert, frequently basking in a warm part of the cage. A couple of weeks before oviposition, she will become very active, digging about the cage. By this time she should be provided with a nest box. I use damp sphagnum moss as an egg-laying medium. A short time before oviposition, the gravid female will retire to this nest box and remain relatively inactive.