Sharing the enclosure with the spiny lizards is a breeding group of Common Chuckwalla, Sauromalus ater. This is the most widespread species, ranging over rocky desert areas of much of the south western USA and Mexico. There are four other species, three on islands off the coast of Baja California and Mexico, which are classed as either Near Threatened or Endangered, mostly as a result of their limited range rather than any specific new threats. The remaining species, the Penisular Chuckwalla S.australis, is classed as Least Concern , as is the Common Chuckwalla.
Chuckwallas are part of the same clade of iguanid lizards as the much larger Green Iguana from the rainforests of Mexico and further south, and like them are primarily vegetarians, although they will take a few insects. The most important plant in their habitat, on whose leaves they rely when nothing else is available, is the Creosote Bush Larrea tridentata. This is the characteristic evergreen shrub covering much of the desert areas of the south west, and gets its name from the smell of the foliage. Given the opportunity, they will also eat leaves and flowers of desert annuals, especially during the brief period after the rains when the desert blooms. They do most of the feeding for the year during the few months between emerging from hibernation in spring and the heat of midsummer, when they are most obvious. During the heat of summer they mostly retire into the depths of the rock crevices that are their usual homes. Although they will drink rarely, they do not need to, and obtain all their water from the plants they eat. Given the opportunity, they will put on weight fast and rely on their body fat to sustain them through the harsher periods of the year when they cannot feed.
Chuckwallas are diurnal, and are extremely fond of basking. As one of the largest lizards in North America after the Gila Monster, they are prey for numerous other animals. Coyotes, foxes, and other mammals will take them, as no doubt will snakes, but lizards basking in the open in particular have to beware of being picked off by birds of prey. In order to avoid this fate, they are extremely alert, and are surprisingly agile despite their shape. When alarmed, they retreat to rock crevices and then hyperinflate their lungs, effectively wedging themselves into their crevice and making themselves extremely hard to dig out.Depending on available resources, which in turn depend on the rains, Chuckwallas are reasonably sociable. In captivity, females and juveniles associate well enough, but adult males cannot be kept together without conflict. They mate soon after leaving hibernation, and the small clutch of 6-10 eggs are buried in suitable soil. The incubation is around 80 days, depending on temperature. In order for the new hatchlings to acquire the gut flora needed to digest their nutrient poor diet they need access to faeces of the adults.
In captivity chuckwallas become tame easily, and require similar care as a bearded dragon. They must have a hot basking spot in order to properly maintain their body temperature and digest their food, and a hide area they can wedge themselves into. Bristol Zoo has a tame individual that is used in their small animal encounters, and he always behaves perfectly.
(Creosote bush from Wikipedia, rest of the photos are mine)