Most desert lizards are territorial which means they will protect some part of their home range from others of the same species. Research in the Mojave Desert of California has documented that large male chuckawallas will actively defend large rocky outcrops from other male chuckawallas, but will allow several females and even juvenile males to inhabit their area. Although this is commonly thought to be a male behavior, females can also be territorial.
Desert lizards exhibit a variety of behaviors to discourage other males from trespassing. Head bobbing, accompanied by flattening the body to give the illusion of larger size, is observed in many species. If the visiting male does not leave the area, as most will do, then a fight will ensue. The resident male is usually the winner and the trespasser quickly leaves.
In captivity, lizards are forced to live in a confined area that is usually much smaller than the territory they occupy in the wild. If more than one male is present, the animals will often fight until one individual establishes dominance and then aggressive behavior will decline. In effect, an uneasy truce is declared because the submissive animal can not flee the area as it would do in the wild.
Although this truce may appear as a harmonious relationship to us, it can be extremely stressful to the submissive lizard. A hierarchical relationship is not natural to most lizards, since they will most often separate themselves in the wild with each defending its own territory. The submissive animal will often suffer stress that is exhibited by an increase in the size of the adrenal glands, a decline in the efficiency of the immune system, decreased growth, and increased mortality. Researchers at the San Diego Zoo found that young male iguanas exhibit decreased growth rates if they are even in the same room with a large male iguana. They detect the other males by sight and smell.
The best approach to dealing with desert lizards is to keep males separate from each other. Captivity in itself is stressful and the presence of aggressive males can increase f the stress and decrease the health of your lizards. If animals must be kept together for short periods, provide hiding places and rock piles that will allow them to avoid viewing each other if they so desire.
Once again, the best approach is to keep males separate. In addition, females will often show aggressive behavior toward other females if they are gravid. Gravid female spiny-tailed lizards are very aggressive toward both males and females and can inflict serious injuries upon their cage mates. Separate gravid lizards if any signs of aggression are shown.