Skip to main content

Bigger Bladder key to Sonoran Survival

November 28, 2007

Key insights about one of the Sonoran’s most elusive resident species, the Gila monster, Heloderma suspectum, are trickling out from Arizona, including opening eyes of supporters of the National Zoo in Washington, D.C., thanks to research by Dale DeNardo, ASU’s veterinarian and assistant professor in the School of Life Sciences, his graduate student Jon Davis, and their colleagues.


A story, featured in the November/December issue of Smithsonian Zoogoer, unveils many of the adaptations employed by Gila monster to promote desert living, but stops short of revealing the tricks that Davis and DeNardo have found tucked under its belt: a bladder that acts like a canteen and the ability to dump excess heat like an organic swamp cooler via evaporative cooling from the cloaca.


DeNardo and Davis’s fascination with the novel nether regions of these lizards arose from concerns around human impact on wildlife in our fragile desert regions. Denardo says, “We were interested in how urban development in the Phoenix area might impact our wildlife. So we wondered what it was that most limited the Gila monster’s range to the Sonoran Desert?”


The only venomous lizard in the U.S., Gila monsters primarily inhabit the Sonoran Desert of Arizona and northern Mexico, though a few small pockets persist in parts of southern Utah and Nevada, as well as eastern New Mexico. This desert-dwelling predator lacks sweat glands; however its heat advisory kitbag includes some behavioral quirks, in addition to its unusual physical adaptations. During hot summer days, Gila monsters, like rattlesnakes, retreat to burrows and largely forage at night when temperatures are cooler. However, both scientists believe that it is the combination of the Sonoran Desert’s significant monsoon season and the Gila monster’s physiological adaptations that determine the Gila monster’s distribution and survival.  


How did these ASU physiologists stumble across these phenomena? Weirdly enough, it was the animal equivalent of that classic Western image: the dying man staggering out of the desert. Davis noticed that his radio-transmittered reptiles, coming out of the long periods of drought prior to the summer monsoons, looked to be on the edge of disaster: skinny and desiccated. However, within hours of the rains, these same animals had bounced back, water-wise, and then some.


“These lizards were lugging around a bladder so full that it filled their abdominal cavity, making up nearly 20% of their body weight.” Davis says. To put this in perspective, it means that a 160 lb. human would need to consume and retain more than four gallons of water. According to Denardo, this translates into a considerable built-in canteen, essential for desert life.


DeNardo and Davis point out that the size of the bladder actually reduces endurance, which may limit this active forager’s ability to hunt. Fortunately their prey – cottontail and rodent nestlings and bird and reptile eggs - aren’t fast moving. Even so, the scientists were left to ponder why the animals didn’t just void the extra load?


 “We did some tests using radio-labeled water introduced into the bladder and found that the water could be absorbed across the urinary bladder and into the bloodstream,” Davis explains. This canteen, he says, allows the Gila monster to weather 95 days without access to water. Just long enough to survive the rigors of the Sonoran Desert – but not nearly long enough for the Gila to survive in more arid regions, like the Mojave, where the absence of monsoon rainfall leads to a drought lasting five months or longer.


On any given day, or rather in this case, night, you can find Davis at one of a number of sweltering study sites scattered across the Sonoran Desert. Finding the elusive Gila monster, without a radio-transmitter, is darn near impossible. Davis can search for days to find one individual - unless of course he’s made his way to his study site at the exclusive Stone Canyon Golf Course in Oro Valley. Here researchers have found as many as 115 Gilas over the course, so to speak, of three years. Most of the Gila monsters at this man-made oasis are fat (which is conspicuously stored in their tail) and hydrated –due to the monsoons and greater access to stable sources of water and food.


So is it bad or good that urban development has come to this area? DeNardo points out that it is studies like those being done by Davis and the rest of his group, supported by Arizona Game and Fish and the National Science Foundation, which “help us all start to put together the big picture.”


“Taking an animal whose habits have been clouded in mystery and doing research on physiology, ecology, evolution - in terms of adaptation- and putting it all together tells us what an organism really is and the better we understand that process for the Gila monster, the better our understanding of all creatures,” DeNardo says.


DeNardo claims the most valuable feature of his and Davis’s studies; work published earlier in the year in the peer-reviewed Journal of Experimental Biology, is that it reveals these physiological adaptations and shows the ecological significance of them to a wider audience. He believes it is critical to have a strong understanding of why an organism is where it is: “If I understand that, then it gives me insight into what it takes to preserve not just Gila monsters, but other Sonoran Desert animals.”  


“We have an obligation to make sure that we don’t become one of the last species on the planet,” DeNardo adds. 


                        #          #          #

 Margaret Coulombe is with the School of Life Sciences, 

To hear Dale Denardo talk about Gila monsters and research in the DeNardo laboratory, go to School of Life Sciences Science Studio podcast:, or visit: