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Myths and Monsters

by Alex Hawes

"What is this black menace that kills everything it sees and hears?" implores the solemn narrator in the trailer to the 1959 film, The Giant Gila Monster. "No human mind could imagine the enormous destructive power of this maddened, killing thing. If you're young people in love, look out! If you're driving down a lonely road, you're as good as dead."

Gila monster
A Gila monster at the Zoo's Reptile Discovery Center. (Jessie Cohen/NZP)
In this splendidly cheesy B-movie, a rampaging, train-engine-size Gila monster (played by a Mexican beaded lizard, the Gila monster's closest living relative) menaces a small Texas town. The beast, a freak result of an overzealous pituitary gland, swallows a hitchhiker, overturns a fuel truck, and, most disturbingly, interrupts a local sock hop. In the end, the film's hero blows up the beast using a hot rod packed with nitroglycerin.

Gila monsters (Heloderma suspectum) haven't fared much better in real-life confrontations with people. From Western scientists' first written description of Gila monsters in the mid-19th century to the recent development boom in the U.S. Southwest, the bogeyman in the monster's closet has been human.

No record exists of a real Gila monster killing a person in more than 50 years, yet fear of this animal's lethal bite undoubtedly fuels the historic hysteria surrounding it. Gila monsters and Mexican beaded lizards (H. horridum) are the only known venomous lizards in the world, and over the years they have been falsely credited with the ability to spit or belch toxic potions, withstand the crush of 50-pound boulders, and wither surrounding vegetation with a single drop of their deadly saliva.

Ignorance about these reclusive creatures is not confined to popular culture. "There is more nonsense in the scientific and medical literature on the Gila monster than on all the other venomous reptiles in Arizona combined," write Charles Lowe, Cecil Schwalbe, and Terry Johnson in the 1986 book The Venomous Reptiles of Arizona. The biggest scientific myth may be that Gila monsters—which are descended from tropical ancestors—are maladapted to the arid, subtropical Sonoran Desert of Arizona and northern Mexico. Their adaptations for energy conservation and thermoregulation suggest otherwise.

"They've been here as long as the Sonoran Desert's been here," says Dale DeNardo, an environmental physiologist and attending veterinarian at Arizona State University in Tempe. "They must be adapted well."

Life in the Slow Lane

Gila monsters and beaded lizards are descended from an ancient reptile clan, the Monstersauria, once found across North America, Europe, and Asia, beginning about 100 million years ago. Today, they constitute the entire Helodermatidae family, which takes its name from the Greek words helos, meaning "nail stud," and derma, meaning "skin." According to the fossil record, modern Gila monsters have changed little during their 20,000-plus years as a species. Adults typically reach lengths of 16 to 24 inches, from snout to tail tip. In the wild, the average weight for an adult Gila monster is just over a pound, and on rare occasions an adult may weigh two pounds.

Biologists traditionally split Gila monsters into two subspecies, reticulate and banded, according to regional variation in skin pattern. They range from the southern reaches of the Mojave Desert in Utah, Nevada, and California, through their core habitat in the Sonoran Desert of Arizona and northern Mexico, to the extreme western edge of the Chihuahuan Desert in New Mexico—and from scrub plains near sea level to pine forests a mile high.

Gila monsters spend as much as 95 percent of the year holed up in rocky crevices, badger burrows, packrat middens, and other prefabricated dwellings, according to herpetologist Daniel Beck, who wrote the definitive 2005 treatise, Biology of Gila Monsters and Beaded Lizards. In winter months, six or more Gila monsters may share a single shelter to seek protection from the elements or predators. They emerge from winter shelters between January and March to bask and sometimes hunt, but their activity peaks in April and May as they seek mates, food, and deeper, cooler shelters for the hotter months.

Field researchers find it difficult to tell male and female Gila monsters apart at first glance. However, males display higher stamina and slightly larger skulls, which are both possible adaptations for their dramatic breeding-season wrestling bouts. These duels may last as long as 12 hours, but the payoff is worth it: The victor gets access to females and their shelters. Mating typically takes place in spring within the privacy of underground retreats, and six to eight weeks after copulation, females lay clutches averaging a half-dozen eggs. After the breeding season, adult lizards return to an asocial lifestyle, confronting the difficulties of desert survival in solitude. There is no evidence that the female incubates the eggs or that either parent is involved in caring for the young after they hatch. The eggs remain underground through the winter and the young hatch the following spring.

Gila monster skin
The bumpy skin of one of the Zoo's Gila monsters. (Jessie Cohen/NZP)
Helodermatid lizards are the only nest-predator specialists in their desert habitat. A single meal, such as a half-dozen quail eggs, can satisfy a Gila monster's energy needs for up to four months. Typically, reptiles, amphibians, and other ectotherms burn 20 to 40 times less energy than comparably sized mammals, but the Gila monster is the Toyota Prius of the animal kingdom. With metabolic rates among the lowest of any lizard measured, relative to weight, Gila monsters can draw upon the fat of a feast slowly and go months between meals. By storing fat in their sausage-shaped tails and abdomens, Gila monsters can spend less than 100 hours a year active at the surface—a typical work week for a high-metabolism Wall Street broker. In contrast, humans and other endotherms—animals that control their internal body temperatures with metabolic energy—need to eat much more frequently. Even eating-contest champions, who can bolt nearly 20,000 calories worth of hot dogs in 12 minutes, are only swallowing enough to last an average man eight days.

One might be tempted to call Gila monsters sluggish, at least from a safe distance, yet they demonstrate surprising stamina. Beck has fitted helodermatid lizards with oxygen masks and run them on treadmills to measure their aerobic capacity. Although the lizards could only muster top sprint speeds of barely one mile per hour, each species was capable of ambling along at a steady half-mile-per-hour pace for great lengths of time. One beaded lizard, a true Energizer Bunny, continued for so long—nearly four hours—that the exasperated lab technician relented and unplugged the treadmill.

Regulating heat and hydration is a year-round concern for Gila monsters and all ectotherms. "Temperature kills much more quickly than starvation," says DeNardo. Gila monsters can become paralyzed or convulsive if stranded aboveground at temperatures exceeding 108°F—a mark frequently surpassed in the Sonoran in summer. Beck has never witnessed Gila monsters willingly active at triple-digit temperatures. Instead, they escape heat exhaustion and dehydration in cool burrows.

This low tolerance of high heat has led some to question Gila monsters' desert credentials. The Sonoran Desert is a mere eight million years old; helodermatids had evolved up until that point in tropical deciduous forests similar to those inhabited by modern Mexican beaded lizards. Perhaps Gila monsters are truly tropical at heart. However, keeping cool in the desert gives them one crucial benefit: Metabolic rates drop with body temperature. A decline from 80 to 60°F, according to Beck, cuts a lizard's metabolic rate in half. Like your laptop, a Gila monster in hibernate mode consumes practically no energy—and it needs little food.

A Taste for Toxin

There may be a reason why so few nest predators exist in the desert: empty nest syndrome. During the spring or during the summer monsoon, Gila monsters diligently forage for the eggs of quail, doves, lizards, and tortoises, as well as for newborn nestling rabbits and rodents. The lizards must gorge themselves when possible, then rely on their slow-motion metabolism to pull them through the lean months. It is truly feast or famine.

Raiding nests is no picnic. Ground squirrels (Spermophilus spp.), desert tortoises (Gopherus agassizii), and other protective parents vigorously defend their young. First, though, Gila monsters must find the nests, which are often cleverly camouflaged. "In eight years [studying Gila monsters in the field], I've seen three bird nests and one cottontail [rabbit] nest," says DeNardo. "I wouldn't make a good Gila monster." With their acute sense of smell, Gila monsters may decipher chemical trails left by small animals that lead them to their prey.

Capturing defenseless nestlings and motionless eggs requires little agility—and no venom. Helodermatids bear potent toxins not for killing quarry, but to defend themselves against coyotes (Canis latrans), badgers (Taxidea taxus), and other predators encountered during their long, slow treks for food or mates.

Gila monsters' skin pattern of mottled black splotches set against a field of coral pink, vermilion, or orange conceals them well amid the desert vegetation. But when a predator does discover a Gila monster, the lizard freezes, then slowly backs away while hissing and presenting its tail. ("It's better to have a third of a tail eaten than to lose your head," notes Jon Davis, a doctoral student in DeNardo's laboratory.) Meanwhile, it begins coating its teeth with toxic saliva. If attacked, it retaliates with the tenacity—and bite—of a bulldog. One literally needs a crowbar to pry off a gnawing Gila monster. This biting strategy ensures the delivery of sufficient venom to stun or immobilize an assailant.

Venomous snakes like rattlers and cobras typically strike large-bodied prey before retreating out of harm's way to let the poison kick in. Lizards, however, lack this strike capacity—which, Beck writes, may explain why only two venomous lizards exist. Gila monsters must chomp down on an attacker, keeping it perilously close at hand, as tension in their jaw muscles delivers venom into the wound through their special grooved teeth.

Helodermatid poison appears most effective against endothermic animals. It can reduce blood flow and cause symptoms ranging from internal hemorrhaging to vomiting, respiratory failure, and cardiac arrest. These toxic chemicals may stimulate not just excruciating pain in a victim, but heightened memory of the biting episode—a powerful souvenir to prevent future encounters.

Gila monster
There are no known deaths of healthy people from Gila monster bites. (Jessie Cohen/NZP)
A Gila monster bite has never been known to kill a healthy person, given appropriate medical care. Toxicologist Findlay Russell of the University of Arizona in Tucson extrapolated from studies of mice dosed with Gila monster venom that it would require 32 to 70 milligrams of helodermatid toxin to kill a 150-pound man. Gila monsters, however, store no more than about 20 milligrams at a time—perhaps enough to kill an infant, according to toxicologists, although children are much less likely to be bitten by a Gila monster than by a rattlesnake. (For comparison, certain snakes may store up to 850 milligrams in their venom glands, although they typically deliver a small fraction of this with each bite.)

Nonetheless, stories abound of deadly encounters between man and monstersaur. In David Brown and Neil Carmony's entertaining 1999 book Gila Monster: Facts and Folklore of America's Aztec Lizard, the authors highlight a host of historic accounts, some stretching the limits of believability. In one of the last alleged fatalities (March 1953), a Nevada man, following two beers, five scotch-and-sodas, and six martinis, played "Russian roulette" by sticking his finger in and out of a Gila monster's mouth. (He lost.) Another anecdote involves a parachutist blown straight into a lizard's waiting jaws.

Being bitten by a Gila monster is apparently quite an accomplishment, requiring either excessive alcohol, terrible luck, great stupidity, or all three. The Samaritan Regional Poison Center in Phoenix, Arizona, treats about one bite case per year on average. The advice, if bitten, is simple: Remove reptile! Daniel Beck writes, helpfully: "A thin, flat lever inserted between the lower jaw and the flesh and turned 90 degrees may work to quickly release the jaws. ...I do not recommend trying to remove the lizard by applying a flame to its chin or by using dangerous solvents such as gasoline."

Yet the poison so feared by many desert dwellers is helping people far more than it is hurting. Gila monster venom contains a peptide called exendin-4 that resembles a hormone humans release during digestion. In Gila monsters, the concentration of exendin-4 in the bloodstream surges thirtyfold following a meal, perhaps to help them stomach large meals. This detail attracted the attention of John Eng, an endocrinologist at the Bronx Veterans Affairs Medical Center in New York. Partnering with Amylin Pharmaceuticals, Eng and his colleagues investigated and then patented exendin-4 as a treatment for type 2 diabetes.

Roughly 21 million Americans have type 2 diabetes, an adult illness associated with obesity that can cause blindness, kidney failure, and in rare cases, death. Diabetics can't effectively produce insulin; without it, dangerous levels of glucose build up in the body. In conjunction with other medications, exendin-4 stimulates the pancreas to produce sufficient amounts of insulin. It is also less likely than other diabetes treatments to create excessively low levels of glucose, which can lead to comas or even death.

In September 2002, Amylin Pharmaceuticals sold rights to a synthetic version of exendin-4 to Eli Lilly and Company for $325 million. The drug, marketed now under the trade name Byetta, received Food and Drug Administration approval in April 2005. In addition to regulating insulin production, Byetta has been found to suppress appetite, a significant benefit for sufferers of this weight-triggered ailment. In one trial, patients prescribed Byetta shed an average of five pounds over six months, while diabetics taking traditional insulin injections gained four pounds on average.

Another chemical in Gila monsters, a hormone called gilatide, may one day help people suffering from memory ailments like Alzheimer's and attention deficit disorder. Analysts predict that helodermatid-inspired drugs could soon generate pharmaceutical revenues in the billions.

The Witching Hour

Sadly, Gila monsters are more often bulldozed than thanked for their contributions to mankind's welfare. People and their cars, pets, and homes pose the greatest threat to Gila monster survival. The population DeNardo and Davis studies in rural Pinal County, Arizona, for example, is feeling the squeeze of sprawl from Phoenix to the north and Tucson to the south. "This area will probably be all houses in ten years," says Davis.

Davis and DeNardo won't publicly disclose the location of their study site in Pinal County, due to the threat of poaching. The impact of poachers on Gila monster populations remains unclear, but the lizards are popular in the exotic pet trade and can fetch up to $1,500 each on the black market, according to Davis. While not considered endangered, Gila monsters nonetheless receive full state protection across their range, shielding them from unlicensed collection, transportation, and killing. The Arizona Game and Fish Department estimates that up to 40 Gila monsters are poached every year in the Tucson region alone. The agency has begun implanting microchips to track live, decoy lizards.

Gila monster
Suburban sprawl is reducing the habitat of Gila monsters. (Jessie Cohen/NZP)
Harm can come from good intentions, too. Every year, dozens of Gila monsters and hundreds of rattlesnakes are moved away from residences in Phoenix and Tucson to supposed safety. Brian Sullivan and his colleagues at Arizona State University followed 25 "nuisance" Gila monsters that had been relocated within the Phoenix region. They found that lizards brought to undisturbed desert habitat more than a kilometer [more than half a mile] from their point of capture subsequently traveled six times the usual distance per day, perhaps in a vain attempt to find their way home. All either went missing or died—victims, potentially, of starvation or increased exposure to predators.

Gila monsters seem to do best if simply left alone. "It feels good to take a Gila monster to the beautiful desert, release it, and go home smiling. But just because it feels good, doesn't mean it's right," says DeNardo.

One evening, I follow another Gila monster researcher, Patrick Emblidge, through rocky terrain just north of St. George, Utah. Emblidge—along with Beck, his advisor at Central Washington University in Ellensburg, Washington—is examining Gila monster habitat use in the desert surrounding St. George. It is a balmy, 97°F July night. We track one of his four radio-implanted individuals up a steep sandstone outcrop. The chirps emanating from his radio lead us to the softball-size entrance of a darkened burrow, where our Gila monster is sensibly holed up for the moment.

St. George's warm climate and proximity to Zion National Park, the Grand Canyon, and other national treasures have drawn snowbirds by the thousands. The city, in fact, grew faster than any other metropolitan area in the U.S. between 2000 and 2006, adding nearly a thousand new residents a month. "[Gila monsters] are getting crushed between development to the north and their elevational limits," says Emblidge. Ironically, the individual lizard whose saliva led to the discovery of exendin-4 likely came from St. George.

One further irony: The best hope for the Gila monster may be a species whose eggs it hunts, the desert tortoise. Extraordinary measures have been taken recently to protect portions of this threatened tortoise's habitat, including the creation in 1996 of the 62,000-acre Red Cliffs Desert Reserve outside St. George. The reserve may also be one of the last safe places left for Gila monsters in the Mojave. Desert tortoises, which gravitate toward creosote-dominated flatlands, don't perfectly overlap with Gila monsters in the Mojave. Emblidge hopes, however, that his project will help guide those protecting desert tortoises to select habitat that likewise benefits Gila monsters.

Emblidge's project includes another inspiring goal: to share his research with St. George's most impressionable minds. Emblidge and Beck have teamed up with local schools to teach first and fourth graders radiotelemetry skills, graphing techniques to plot shelter locations, and, above all, the beauty and value of these rare creatures.

"It's good to increase awareness where possible," says Emblidge. And not to believe everything you see at the movies.

Posted by Emily (Verdeshes) - information from FONZ

Gila monster
Heloderma suspectum
Lethal Lizard
Once the Gila monster clamps down on its prey, it doesn’t let go easily. Venom flows through ducts in its lower jaw, seeps into the wound and paralyzes the prey.
Gila monster
© MSA 2005

Range: southwestern United States, Mexico
Habitat: deserts
Range Map

Posted by Emily (Verdeshes) - information from FONZ

Gila Monster Taxonomy
Order: Squamata
Family: Helodermatidae
Genus/species: Heloderma suspectum


The Gila monster is one of only two species of venomous lizards (its cousin, the Mexican beaded lizard, is the other). It can bite quickly and hold on tenaciously. Rather than injecting venom through hollow fangs like venomous snakes do, Gilas have enlarged, grooved teeth in their lower jaw. When they bite, their powerful jaws chew the venom in through capillary action along the grooves in these teeth. Gila monster venom is about as toxic as that of a western diamondback rattlesnake. However, a relatively small amount of venom is introduced in a Gila bite.

The dorsal coloration of the Gila monster is black with pink or orange. In the southern subspecies, the reticulated Gila monster, the light markings, or bands, are broken up to form a reticulated pattern. In the northern subspecies, the banded Gila monster, the light markings generally form an unbroken band across the back.
Gila monsters spend most of their lives hidden below the ground. There are three things that make them particularly well suited for the harsh environment in which they live. First, they are large lizards (the largest in the U.S.) measuring up to about 22 inches (56 cm) in total length, and are able to store more energy than smaller lizards. They store fat in their tail and in their bodies. Second, they are capable of eating relatively large meals. They have been observed in the wild eating meals up to one third of their body weight. Third, Gilas have low resting metabolic rates. Their low metabolic rates, and the ability to eat large meals, combined with their large capacity to store fat, make frequent searching for food unnecessary. Therefore, they are rarely seen above ground. It has been suggested that Gilas may consume their entire yearly energy budget in three or four large meals.

Most of their above-ground activity occurs in three months in the spring. Not only is this when mating occurs, but it is when their main source of food (vertebrate nests) is most abundant. They are diurnal but most activity occurs in the morning. Gila monsters have a home range of about one square mile (1.6 km2). They are usually solitary animals, but do gather in communal areas in the spring for mating.

Distribution and Habitat

The range of the Gila monster is primarily in Arizona and Mexico, the extreme southeastern corner of California, the southern tip of Nevada, and the southwestern corners of Utah and New Mexico. It is named for the Gila River in whose drainage it is a common resident.

Gila monsters are desert dwellers, living near washes and arroyos and in semiarid rocky regions of desert scrub or grasslands. Gila monsters also seem to prefer rocky foothills and avoid open flats and agricultural areas. They can live in elevations up to 5,000 feet (1,500 m).

Diet in the Wild

Gila monsters most often prey on small birds and mammals, eggs, lizards, frogs, insects, and carrion.

Zoo Diet

They are fed about two mice each, twice a month.


In late April to early June, courtship and male-to-male combat take place. Females lay two to 12 leathery eggs that overwinter below ground and hatch ten months later in the next spring. Hatchlings are about six inches (15 cm) long and are miniature replicas of their parents.

Life Span

They may live 20 or more years in zoos, possibly longer in the wild.


Listed as threatened under the United States Endangered Species Act and listed on CITES Appendix II. They are threatened by habitat destruction: overgrazing, truck farming, and the planting of cotton. They are also protected under Arizona law.

Fun Facts

With very few natural predators, they may spend up to 98 percent of their lives in or at the mouth of a burrow. Gila monsters enjoy a mixed reputation with Native Americans. While the Tohono O'Odham and the Pima believed that the lizard possessed a spiritual power capable of causing sickness and the Apache believed that its very breath could bring death, the Seri and Yaquai believed in the healing powers of the lizard's hide.

A component of Gila monster venom called Exendin-4 is currently being investigated as a promising new drug to treat type-2 diabetes. This peptide stimulates the secretion of insulin in the presence of elevated blood glucose levels. It also has the effect of slowing gastric emptying. Phase I clinical studies have recently begun with this exciting experimental drug.

Posted by Emily (Verdeshes) - information from FONZ

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