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ASU’s DeNardo explores true monster tales

Dale DeNardo

Dale DeNardo

Dale DeNardo, an assistant professor in the School of Life Sciences and veterinarian for Arizona State University’s animal care and technology department, has read more than his share of tall tales about Gila monsters. He tracks the creatures in the desert northwest of Tucson and studies them up close in his laboratory. He says the facts about this desert lizard are every bit as amazing as the fiction.

To date, most of the studies on Gila monsters have been conducted outside Arizona. The work was done on the edge of the animal’s range in the red-rock desert of southern Utah and the Chihuahuan Desert of western New Mexico. Surprisingly few research projects have been carried out in the Sonoran Desert, the heart of the lizard’s territory.

DeNardo has concentrated his efforts in this region and his findings are adding exciting new information to the body of knowledge about this fascinating – and elusive – creature. In some cases, his data are reversing some of the conventional wisdom that scientists have long taken for granted.

An animal physiologist, DeNardo explores how Gila monsters resolve the tremendous challenges posed by the Sonoran Desert’s climatic extremes. Rainfall that is seasonal and sparse. Temperatures routinely soar above 110 degrees F in summer and drop below freezing in winter.

Gila Monster
Monster Measures

The Gila monster is the largest and only venomous lizard found in the United States. The big lizard’s teeth look like tiny grooved daggers. Despite a fierce reputation, Gila monsters actually are quite reclusive. They spend most of their time underground living in desert burrows stolen from mice, squirrels, or other small rodents.

Scientific name: [Heloderma suspectum]

Weight: 3 to 5 pounds

Length: 1.5 to 2 feet, head to tail

Diet: Small rodents, birds, eggs, lizards, frogs, scorpions, spiders

Color: Dark bands and blotches on beaded orange or pink background

Gila monsters are ectotherms. Like other reptiles, they rely on their surroundings to maintain a preferred body temperature. But the Gila monster’s preferred body temperature is 85 degrees F. That is exceptionally low when compared to other desert lizards whose preferred body temperatures generally range in the mid-90s F.

DeNardo’s research findings show that Gila monsters have resolved this hot-cool contradiction by dipping into a grab bag of ingenious survival strategies. He says the big lizards function at their optimal body temperatures in cool, underground burrows.

Gila monsters can remain lodged underground for months at a time. They survive on reservoirs of fat and water in their bodies. They must eventually surface to replenish their stores and reproduce.

One of the first things they do is search for food. Gila monsters are expert nest raiders. But foraging for bird and desert tortoise eggs as well as newly born rabbits and desert rats is no easy task.

“If you’ve ever hiked out in the desert you know that you don't just come across bird and rodent nests on a regular basis,” DeNardo says. “Gila monsters spend a lot of time roaming the desert searching for food. It can take days, sometimes more than a month, to find a nest. That leaves them at risk for temperature extremes, either during the day when it can be too hot or at night when it’s far cooler than their preferred body temperature.”

DeNardo outfits his study animals with radio transmitters and temperature gauges. The technology helped him discover that Gila monsters solve this problem in some very clever ways.

They minimize the time they spend on the desert’s surface by gorging on food when they find it. Some Gila monsters have been known to clean out a nest of juvenile rabbits in a single feeding.

“The big lizards can eat up to 100 percent of their body weight at one time,” DeNardo says. “Think about how you feel after you’ve eaten a big Thanksgiving dinner. Most likely you've eaten two to three pounds of food. But that’s probably only two to three percent of your body weight.”

Much of this food energy is stored as fat reserves in the lizard’s body cavity and sausage-like tail. Gila monsters draw sparingly from these energy reserves when they retreat to underground burrows. They might stay in the burrows for months at a time during winter.

Gila monsters are also capable of slurping and storing vast quantities of water. DeNardo’s work has revealed that Gila monsters emerge from their spring dens with a bladder so bursting with fluid that it fills the area from their pelvis all the way up to their lungs.

This portable water supply allows the lizards to roam far and wide without fear of dehydration. But, since water is heavy, Gila monsters will jettison their fluid burden when other sources are readily available in the wild. In the Sonoran Desert, for example, Gila monsters have a second peak of activity after the summer monsoons stimulate another spurt of breeding. This time, however, the animal's bladder is so empty it is nearly undetectable.

DeNardo thinks he knows why.

The summer monsoon brings infrequent but predictable rains. The lizards don't need to haul their own supplies. Gila monsters survive dry bouts between storms by withstanding levels of dehydration that would kill most humans.

Gila monsters are well adapted to the boom-and-bust cycles of desert rainfall. DeNardo says that they can lose 200 grams, or up to 40 percent of their body weight, much of which is water. After a storm they can quickly regain more than half of what was lost in a single drink.

“They really know how to take advantage of a little puddle of rain,” DeNardo adds.

The Sonoran Desert’s “hot season” lasts from April through October. To minimize water loss, Gila monsters are most active at night when temperatures dip to 70 to 80 degrees F. The search for food and underground shelter is constant. Males engage in wrestling marathons with other males during the breeding season. But not all of these life-maintaining tasks can be done at night.

To keep their bodies from dangerously overheating in the searing desert sun, some lizards are known to keep cool by panting.

DeNardo’s research findings show that Gila monsters are apparently unique among lizards. They “sweat” by evaporating water across the membranes of their cloaca, the reptile equivalent of a mammal's anal canal. This cooling method is extraordinarily effective but uses tremendous amounts of water.

As a result, Gila monsters have evolved an ingenious form of thermostat control. When the lizards become dehydrated they cannot afford to lose too much water. So they turn up the temperature at which the cloacal cooling kicks in, thereby carefully calibrating their body’s response to external conditions.

The lizard’s control of temperature and water is impressive. But one of the most critical factors in Gila monster survival is the availability of suitable burrows.

At the onset of winter, Gila monsters tend to locate their holdouts in south-facing rock outcrops. These areas offer protection from the season’s cold. They also provide warm basking sites when the lizards emerge in early spring.

During the summer months, however, the lizards tend to retreat into far cooler earthen burrows. Among the most desirable are packrat middens.

Packrats build large nests heaped high with the joints of cholla cactus and other desert debris. They are remarkably well insulated. DeNardo and his team have placed thermometers in these refuges and found that temperatures in a packrat nest that is dug 6 to 12 inches into the ground are comparable to those taken from depths of 2 to 3 feet into the ground.

So precise are the lizard's needs for just the right kind of refuge that animals will use them over and over again depending on the season.

“Refuge choice is a limiting factor for Gila monsters,” DeNardo explains. “Not every little hole works for them. Some holes may be too small or not have enough room for them to turn around. Some may go too deep or not deep enough.”

Over millions of years, Gila monsters have made fine-tuned adjustments that allow them to persist in the desert’s harsh environment. But times are changing. The increasing intrusion of humans into once pristine desert areas is the most recent challenge to their existence. Whether Gila monsters will survive the challenge remains unanswered.

Currently, Gila monsters are not officially listed as either a rare or endangered species. But their populations exist in highly localized pockets, many of which are experiencing a surge of human development.

“Gila monsters are pretty widespread,” DeNardo says. “They are not in danger of extinction. But they’re getting hit hard in terms of habitat loss.”

By Adelheid Fischer. Excerpted from the Spring 2005 ASU Research Magazine. For more information about Gila monster research at ASU, contact Dale F. DeNardo, Ph.D., School of Life Sciences, at (480) 965-3325 or send e-mail to


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