Venomous lizards of the desert: studies of Gila monsters and beaded lizards have uncovered an array of surprising characteristics, from odd fighting rituals, to extreme energy efficiency, to a venom useful in treating diabetes

Natural History, July-August, 2004 by Daniel D. Beck

In June 1986 my brother Jon and I pulled out of Tucson for a road trip to Chamela, Jalisco, in the heart of Mexico's searingly hot tropical dry forest. It was no tourist trip. I was a graduate student, eager to begin the first intensive field study of the beaded lizard (Heloderma horridurn). I had my gear, my letters of permission, and four years of field experience observing the beaded lizard's sister species, the Gila monster (H. suspectum). Along with those essentials, my baggage also included a variety of assumptions about the creatures I was studying, assumptions drawn from my own limited experience and from the conventional wisdom about "monstersaurs," as the two species of Heloderma are known. I assumed, for instance, that Gila monsters are strictly creatures of daylight. The vast majority of monstersaur activity I had observed in southwestern Utah's Mojave Desert, the site of my previous work, had taken place during the day, particularly in the morning.

As a result, seeing a Gila monster was the last thing on my mind when I turned in on our first night. We had set up camp in a quiet spot in the desert north of the town of Guaymas in the Mexican state of Sonora. To avoid overheating while I slept, I pulled my sleeping bag up only to my waist. Soon I was dreaming. I had a fairly typical dream--for a herpetologist. In it I found a beautiful green iguana and picked it up, holding it in the prescribed manner, one hand supporting the body, the other grasping the neck, to avoid being bitten. My dream turned real as I woke to find that I actually was holding something. It took me a second to take it all in, but there in my hands was a large adult Gila monster. I was holding it gently, but snugly, just as I had held the iguana in my dream. I sat up and stared at the lizard, trying to disentangle dream and reality. The Gila monster had crawled into my sleeping bag, and I had picked it up in my sleep. "Jon," I called out to my brother, "a Gila monster." Shaken from slumber, he replied, "Shut up and go back to sleep, you're just dreaming." "No, look," I hollered excitedly until he had to pay attention. "I can't believe it--a Gila monster!"

Gila monsters are rarely encountered in the wild, even by experienced field biologists, let alone by a man in a dream. As the shock wore off, I was forced to confront one of those assumptions I had carried to Mexico with me--the assumption that Gila monsters are strictly daylight animals. I had been fooled into thinking I knew the species' full pattern of behavior, when in fact I had witnessed only a fragment of the big picture.

Monstersaurs seem to have inspired an awful lot of this kind of thinking, what I call "overinterpreting experience" Perhaps that's not surprising; the animals almost invite confusion. To look at a Gila monster-four legs, long tail, triangular head, blunt snout--one might think it's simply a lizard like any other. But monstersaurs actually share a more recent ancestor with snakes than they do with most other lizards. It might be even more accurate to think of monstersaurs as "snakes with legs" than as another group of lizards. But the expression "snakes with legs" doesn't capture their intriguing qualities either. Several lines of ongoing research show that these animals display unusual metabolic levels, remarkable aerobic abilities, and dramatic dominance contests, all of which are connected in a pattern that offers a whole new twist on what it means to be a lizard.

In thinking about monstersaurs, snakes do provide a tempting point of reference. Consider an experience I had in April 1984, a few years before I woke up with a Gila monster in my hand. During the preceding summer at my study site in the Mojave Desert, I had outfitted several Gila monsters with internal radio transmitters. That spring, after one of "my" males emerged from winter dormancy in March, I began following him closely. Like Elmer Fudd, the lizard went hunting for cottontail rabbits in their nests. His meticulous search of boulders and burrows paid off; by mid-April he had added a third of a pound, or 22 percent of his fall weight, to his lumpy frame. For in his feces attested to his success as a rabbit hunter.

One April day, though, rather than hunt, he walked nearly a mile in a straight line to a shelter that a female had occupied the preceding summer. In the next three days he visited two more shelters, before settling into one that another male had used the previous spring. A few days later a second male Gila monster confronted him outside his new den. The two grappled and twisted in a strenuous series of ritualized postures. Just past dusk, they separated, having spent three hours in nearly continuous combat. The struggle left each lizard coated with a thin film of blood, and each appeared exhausted. The resident male crawled back into his shelter while the intruder lumbered out of sight. Three days later, a female joined the victor in his den.

The entire episode of ritualized combaq: was strongly reminiscent of the entwining combat "dance" of snakes. Moreover, like many snakes--but unlike any other lizards--Gila monsters and beaded lizards are venomous. Those similarities make it tempting to lump the monstersaurs with snakes, yet the differences between the two groups are every bit as pronounced.