That you did. You saw the desert iguana, Dipsosorus dorsalis, which can grow up to 16 inches in length, including the tail. All members of the Iguanid family of lizards are vegetarians and large.
Perhaps the desert iguana’s size is remarkable because in late March it just appears, in all its grandness, with no warning at all. All of a sudden there are these big white lizards crossing the street or sitting under a bush in your yard. The Arizona Sonoran Desert Museum reports that desert iguanas appear in March, breed in April and May, and lay 2-10 eggs between May and early July. Hatchlings emerge in late July through August. These dates may be later for the high desert.
Before our iguanas are visible they are hibernating. In February 2004 a trench was dug near the West End Cottage at the 29 Palms Inn. About 18 inches down we exposed a hibernating desert iguana. A rude awaking, but (s)he lived.
The desert iguana is light in color, usually pale gray or whitish with a reticulated brown pattern on its back and sides. The tail, which can be as long as the body from snout to vent, has rows of brown spots. The sides of both sexes blush pink during the breeding season. Their coloration against a sandy background, in tandem with stillness, allows the lizard to escape predation much of the time.
The desert iguana has a higher tolerance for heat than other lizards and can be active in the middle of a summer day. When they are not resting for long periods in the shade of a creosote bush, you can find them perching on large rocks or sand mounds.
I have heard several people observe that iguanas lack a fear of humans. They go about their business never seeming to care that you are standing close by. This is worth some research.
Let’s start with why they might be so big. They might be so big because they, like all herbivorous lizards, have very large partitioned colons that provide several advantages (stick with me on this) when it comes to eating abundant, easily obtainable but low quality food, i.e., plants.
High quality food, like insects, cost a lot of energy to get and digest. Low quality food costs less to get and there is help with the digestion. The lizard’s large colon provides increased surface area to absorb nutrients and water; and the partitions provide important microhabitats for dense nematode faunas and bacterial and protozoan populations. These fauna, essential in the digestion of vegetable matter, appear to be of mutual benefit for all. The nematodes do the initial heavy labor of mechanically breaking down the green matter into smaller pieces and producing useable waste products. The waste products (vitamins, cellulose, volatile fatty acids, etc.) regulate the composition and abundance of the colonic microbes, which do the final breakdown. The nematodes feed on the microbes. (Remind you of a cow, or for that matter, a termite?)
All this activity needs both space (for the colon) and time to operate. Herbivorous lizards are big and lazy. Metabolically their digestion is from 30 to 70% efficient (70-90% for carnivorous lizards). But who cares, they spend only about 1% of their time feeding and 90% resting. This is the dream of all true couch potatoes.
Dipsosorus dorsalis has a good nose. Research has demonstrated that the desert iguana can detect chemical deposits from snakes and tell the difference between snakes that eat iguanas and those that do not. This ability provides confidence when foraging for food or when entering burrows. It may also provide confidence when moving in the vicinity of your shoes.
They also have amazing eyesight. They secrete a scent from the femoral pores (found on the underside of the thighs on the rear legs) to aid in recognition of their kind. Studies show that these secretions also fluoresce and that the lizard has vision in these wavelengths.
What does all this have to with fear of humans—probably nothing. Desert Iguanas can’t afford a nervous temperament because most of their life must be spent digesting their favorite creosote flowers and other fibrous flora. Their size, color, and stillness help to protect them from predators while they digest. In addition, they can recognize friend and foe by smell and sight in order to avoid trouble. These amazing animals are totally equipped for survival!
~ Pat Flanagan