Gila monsters’ range will shrink with extreme warming

Extreme warming in the future may shrink the range of venomous Gila monsters in the U.S. Southwest.

“We see a much stronger reduction in range for this species,” said Steven Hromada, a postdoctoral scientist at Fresno Chaffee Zoo in central California.

But a close examination of the landscape features and protections afforded in their current and future range can help wildlife managers plan to help the iconic lizard persist into the future.

Gila monsters (Heloderma suspectum) can live for decades, though it isn’t clear exactly what age they might reach in the wild. They are among only a handful of lizards known to carry venom strong enough to affect humans. Their bites can lead to extreme pain and even hospitalization. But their venom also has benefits—proteins carried in Gila monster saliva led to the development of the weight-loss and diabetes drug Ozempic.

Gila monsters face habitat loss, and Nevada considers the species a protected conservation priority. The reptiles are nest predators, subsisting on eggs from Gambel’s quails (Callipepla gambelii) and Mojave desert tortoises (Gopherus agassizii) as well as baby desert cottontail rabbits (Sylvilagus audubonii). After rummaging through a nest, they will retreat underground to digest for some time.

For a lizard, Gila monsters move relatively slowly. “They know they’re pretty tough, and their venom is a defense mechanism,” Hromada said. While researchers don’t really know how they disperse—or how far they move after hatching—the species usually stays in the same place for most of its life, making individuals vulnerable to shifting climate patterns.

As part of a larger project to learn more about their habits around the Las Vegas area, Hromada and his colleagues published a study recently in Ecology and Evolution on how projected future climate scenarios might shift the range of the lizards in the Mojave Desert.

Gila monsters past and future

The team first modeled Gila monster distribution based on previous surveys and radiotelemetry tracking that Hromada and his colleagues conducted in three different periods over the past 25 years in the Mojave Desert. They purposely didn’t publish information on these sites as poachers sometimes collect the species illegally from the wild for the pet trade.

The researchers then examined greenhouse gas emissions scenarios that the Intergovernmental Panel on Climate Change produced to see how the weather patterns would change in the Mojave through 2082. Finally, they looked more finely at the landscape to see if any obstacles could influence the way that Gila monsters may disperse in the future in response to changing climate.

Under the lower emissions scenarios, Hromada said the situation doesn’t drastically change for the species. The models reveal that some new suitable areas for the species open up at higher elevations, and they don’t lose too much of the area where they are currently found.

But in higher emissions scenarios, Gila monsters’ range shrinks in the Mojave due mostly to an increase in average heat above what the species can typically tolerate. As a result, populations will likely become more isolated from each other, and the range that currently sits along the Colorado River at lower elevations in particular probably won’t be suitable any longer.

Gila monsters are relatively drought-resistant. If the climate is too dry, they may extend their winter hibernation by months. Just the same, scientists don’t know much about the impacts of long-term or more frequent droughts, Hromada said.

But as areas become hotter in general, plants become drier. This creates a problem for the prey that Gila monsters rely on. Surveys conducted as part of the larger research on the species suggested these lizards were more common in areas near permanent water sources, likely because rabbits and quail are also more common there. In contrast, in one particularly dry area that Hromada’s colleague surveyed years earlier, each Gila monster sighting took roughly 400 hours of searching.

“The areas that are likely to become less suitable for the species are those that are predicted to be hotter in future climate scenarios,” Hromada said.

Planning future Gila monster conservation

In some cases, their ability to disperse to areas with more suitable temperatures as the climate warms appears to be blocked by landscape features like cliffs and roads. While the lizards don’t seem to shy away from crossing the roads, they face danger from vehicle strikes in these areas.

“Knowing that dispersal is going to limit the habitats this species has in the future is going to be important,” Hromada said.

He hopes that land managers will use the results of this study to plan future conservation for Gila monsters. “A lot of the distribution of the species falls into protected areas, but some of the areas fall within general [U.S. Bureau of Land Management] land that is not zoned as areas of critical concern, or a national monument,” Hromada said.

Some of this BLM land may be currently used for off-road vehicles or may be open to mining permits or other uses, for example. Reclassifying some plots could help ensure future conservation of this species, especially if the projected range of the species under changing climate scenarios overlaps with other species of concern like the Mojave desert tortoise.

“There’s potential for multi-species habitat planning here,” Hromada said.