For almost 25 years, Mike Cardwell has been studying rattlesnakes in the desert using radiotelemetry devices. With surgically implanted transmitters inside the snakes, he can track both their location and their internal temperature in the wild.
Before 2023, the hottest internal temperature Cardwell ever recorded in a living snake was around 97 degrees Fahrenheit.
But after a streak of daytime temperatures topping 110 degrees in July 2023, two of the diamondback rattlesnakes he was tracking north of Tucson died while sheltering underground.
Cardwell suspected they overheated.
"I'd never had a rattlesnake die underground during the summer," said Cardwell, who holds a master's degree in science from University of California-Sacramento and serves as a consulting biologist for Arizona Poison and Drug Information Center.
"Snakes tolerate getting too cold much better than they do getting too hot," Cardwell said. "Everybody thinks that reptiles love the heat. They love being warm, but they don't love being hot."
Despite their sharp fangs and deadly venoms, the rattlesnakes struggle to defend themselves against a new threat to their survival: extreme heat.
And it's not just the snakes that are feeling the heat. From elusive jaguars to saguaro cactus, the changing climate is taking its toll on some of Arizona's most iconic species, even those who have adapted to the desert heat over thousands of years.
Human intervention has helped to curb the effects on some of the most imperiled native species, and in some cases, even succeeded in recovery efforts, but the overarching threat of climate change persists.
Arizona is already feeling its impacts. Last summer was the state's hottest on record, with an average temperature of 82.1 degrees. The average temperature in Phoenix was 98.9 degrees, making it the warmest year on record in America's hottest city.
It was also Earth's hottest year ever recorded, reaching a critical milestone: the first year Earth has warmed 1.5 degrees Celsius above pre-industrial levels. World leaders and scientists hoped to avoid surpassing this threshold under the Paris Agreement.
With a changing climate comes a host of environmental and meteorological concerns: extended heat waves, wildfire, drought, habitat degradation and disruptions to migratory patterns.
As endangered squirrels become isolated on sky island peaks and parched pronghorn search far and wide for a rare drink of water, Arizona's unique species have evolved and adapted to the harsh environment, but they now face new challenges as the desert heats up.
Rattlesnakes, like most reptiles, are ectothermic creatures, also known as cold-blooded, meaning they cannot independently maintain their internal body temperature and are dependent on environmental conditions to regulate it.
In the heat of the day, desert-dwelling snakes will retreat into shade or rodent burrows to shield themselves from the scorching sun.
While temperatures on the surface rise and fall throughout the day, peaking in the heat of the afternoon and then cooling down in the nighttime, the temperature inside a burrow remains relatively constant.
But when a heat wave strikes, sending both daytime and nighttime temperatures soaring for multiple days in a row, the underground temperature can start to rise.
After the mysterious deaths of the two diamondback rattlesnakes, Cardwell dug up temperature data from a 20-year-old study in the Mojave Desert to help develop a mathematical model for measuring what might be happening inside the desert burrows.
Using the average daily temperatures, the model showed the underground temperature was around the thermal maximum for snakes -- around 104 degrees Fahrenheit. Hot enough to kill.
Extreme heat is intensifying in Arizona, as temperatures are warming earlier in the spring and persisting later into the fall. Heat waves are increasing in temperature, frequency and duration, pushing native species like rattlesnakes to their biological limit.
In 2024, Phoenix experienced 113 consecutive days with temperatures at or above 100 degrees, the longest streak ever recorded. Tucson experienced 112 days with highs of at least 100 degrees, another record.
Even outside of metro areas -- which tend to be warmer because of the urban heat island effect as pavement and buildings absorb and reemit heat -- the entire state is trending warmer. In the last 130 years of record-keeping, the top five warmest years took place in the last decade.
"When we have really hot air temperatures for multiple days in a row, the burrow temperature can catch up to the surface," said Cardwell. "And if it's hot enough, there's the real possibility the snakes can't get deep enough to escape."
In July 2024, Kinley Ragan braved summer temperatures as she and a volunteer hiked through mountains in southern Arizona, checking on field cameras installed earlier in the year. As a field research program manager for the Phoenix Zoo, her work collecting data on the animals moving through the wildlife corridor is a core part of the zoo's conservation mission.
Her latest project, monitoring 50 camera traps in the understudied Atascosa highlands region in southern Arizona resulted in a remarkable discovery.
One of the field cameras had captured the image of an endangered ocelot, the first record of the elusive feline in the region in over 50 years.
The discovery highlighted the lengths some mammals will travel to find resources during sweltering summer months.
"It's not just heat that they're dealing with, it's also lack of groundwater, lack of surface water, decreased shade, habitat connectivity loss, habitat loss," said Ragan. "What really becomes important is the ability to move, to go to either higher elevations or to where there's more water or where there are resources."
Like Phoenix residents heading up to Flagstaff on a summer weekend, some desert mammals' strategy to beat the heat is to simply move to a cooler place.
As the region's megadrought continues, roaming mammals, like black bears and mountain lions, may start to occupy larger ranges, widening their territory in the search for water and other resources.
"We already have evidence of species moving north," Ragan said. "However, that's under the assumption that they can get there."
Barriers like highways, fencing, border walls and other development projects threaten to leave roaming species without the option to expand, ultimately leading to localized extinction.
Funnel fencing and wildlife overpasses over busy roadways are tools to help increase habitat connectivity and reinforce wildlife-human coexistence.
But for some species, migrating to another location is an impossible task.
The Mount Graham red squirrel is an endangered subspecies of American red squirrel that was thought to be eliminated from the wild in the 1950s. In the 1960s, a few individuals were discovered to be living exclusively on Mount Graham.
Long-term drought has degraded the old-growth forest where the squirrels live. The lack of snowpack and moisture leaves the trees vulnerable to invasive insect infestations and wildfires, which are the major threats to the species.
In 2017, the lightning-ignited Frye Fire burned across the mountain, reducing the population of Mount Graham red squirrels from 200-300 down to only 35, according to the U.S. Fish and Wildlife Service.
While wildfire season in Arizona used to arrive in spring and reliably end by the monsoon, fire seasons have turned into fire years as the blazes become more erratic and frequent.
Temperatures and aridity are increasing and precipitation isn't rising to match it, so the landscape is getting drier. An excess of dry fuels and vegetation is compounded by fire weather conditions that are becoming more common year-round.
Global warming triggered by people burning fossil fuels for energy has worsened fire conditions, causing warmer temperatures and making dry years more extreme. These conditions can make fires spread more easily and be more difficult to put out.
On top of worsened fire conditions, humans are responsible for the majority of fire starts when recreating or traveling.
Recovery efforts, like a captive breeding program at the Phoenix Zoo, have helped to bolster the Mount Graham red squirrel's population, but conservationists fear that the species may be one wildfire away from eradication in the wild.
Drought in the wild: It's a thirsty world for Arizona wildlife. How a state agency keeps water basins full
Every year, the Arizona Game and Fish Department hauls thousands of gallons of water to the driest and most remote areas of the state to help wildlife populations survive the drought. Over the years, the demand for water in the desert has grown.
"This last year was probably our busiest because we had pretty much the least amount of rain that I've ever seen," said Joe Currie, habitat planning program manager at the Arizona Game and Fish Department. Currie has worked on the agency's water hauling program for 28 years.
The water catchment systems are designed to collect rainwater and make it available to wildlife in large basins. If there isn't enough rain in any given year, state wildlife managers have to fill the systems manually.
Normally, the water hauling program lasts from April to September, but in 2024, the trucks continued to operate continuously throughout the winter.
The department's wildlife water development program began in the 1940s as a way to bolster quail populations for hunting season, but in an era of megadrought and increasing temperatures, the program has grown from just a handful of systems around Phoenix to over 3,000 catchments across the state.
In some cases, the water provided by the program may be the only resource to stave off fatal dehydration across an entire region.
"Some places which have been devoid of water, they are very, very dependent on it," said Currie. "Without those waters, we'd have parts of our landscape that are devoid of wildlife."
Large ungulates, like mule deer, elk, white-tailed deer, bighorn sheep and the endangered Sonoran pronghorn, are among the most dependent species on the water catchment system because they need to drink water almost daily.
"Cabeza Prieta Wildlife Area, where the Sonoran pronghorn are, is heavily, heavily dependent on artificial waters," said Currie. "Especially now, since it's experiencing hotter and drier, longer periods of time."
Arizona has always been warm and dry because of its arid climate. It has two rainy seasons, the summer monsoon and the winter, which produces snow in higher elevations. Many animals have adapted to water scarcity, with seasonal migrations, nocturnal habits and burrowing during the worst of the heat to offset water loss.
But these adaptations may not be enough, as climate change and prolonged drought are pushing animals' limits.
The Southwest is experiencing an ongoing 30-year drought. Since 1994, the region has experienced drier-than-normal conditions. Although drought is a natural part of life in the desert, the current drought is considered one of the worst in the last 1,200 years.
On average over the last 30 years, Arizona recorded about 11 inches of rain a year, an inch below the long-term average. Hot temperatures exacerbate drought impacts, and vice versa, making surviving in the desert even more difficult for wildlife.
Warmer temperatures increase evaporation, drying out rivers, reservoirs, soil and vegetation, and speeding up Earth's water cycle as more water goes back into the air for precipitation. Winters are getting warmer, resulting in less snowfall. Snow that makes it to the ground is melting earlier and at a faster pace, reducing meaningful runoff.
Despite an increase in workload, Currie says the program isn't going anywhere as long as people believe that wildlife is inherently valuable.
"People hold a value to wildlife, and they know that this water is pretty important. So they're not going to allow this to go away unless there's just no other alternative," said Currie.
From license plates to postcards, there are few Arizona symbols as iconic as the saguaro cactus. Growing up to 40 ft tall and living for over 100 years, the keystone species provides a food source and habitat for multiple Sonoran Desert species. Even the white blooms that crown the saguaro are the Arizona state flower.
But the beloved species is becoming an increasingly rare sight in Phoenix, where rising temperatures and the urban heat island effect are threatening to overheat the saguaro.
Cactuses have evolved with adaptations that allow them to grow successfully in extremely arid environments. The spines that grow on cactuses are modified leaves that prevent water loss through transpiration, and the ribbed structure of some cactuses allows them to expand as they collect and store water in their fleshy stems.
One of the saguaro's most effective adaptations is its metabolism. The pores that allow for gas exchange caused by photosynthesis remain closed during the day to prevent water loss. At night, when temperatures drop to around 85-90 degrees, the pores will open.
"The problem is when the temperature at night is still 100 degrees, then the cactus have a hard time with photosynthesis. And that's when they can get in trouble," said Raul Puente-Martinez, a research botanist at Phoenix's Desert Botanical Garden.
The death rate for saguaro at the Desert Botanical Garden has increased in recent years, according to a 2025 study. Between 2015-2020, the average death rate at the garden was 1-2%, but that number increased to 7% in 2023. In 2024, the rate fell down to just over 3%.
After the older saguaros die, it could take decades for younger cactuses to take their place if they are able to withstand the heat and reach maturity.
"It took 30 million years to get those adaptations, but right now, the climatic changes are much faster," said Puente-Martinez. "There is a big risk that they might not have enough time to readapt again to the new norms on heat waves."
As heat waves affect plants, the effects are felt by the pollinators that rely on their blooms for food.
Every fall, migrating monarch butterflies travel thousands of miles through Arizona to overwintering sites in Mexico. If high temperatures disrupt or destroy blooming plants along their route, it can result in population loss for the beloved species, which is currently being considered for federal protected status.
"A single monarch makes the whole fall journey, it's gonna travel thousands of miles depending on where it's going, and they need nectar resources on that journey," said Natalie Melkonoff, an insect ecology program manager at the Desert Botanical Garden. "So like those longer summers or heat waves late in the fall could reduce nectar resources."
By 9 a.m. on July 7, the temperature in Phoenix had already crept up to 99 degrees, and native wildlife living along the Phoenix Zoo's Arizona Trail were taking advantage of some relatively cool weather.
Before temperatures maxed out at 111 degrees that day, coyotes scurried through sprinklers, while javelinas sat in cool, shaded mud wallows in their enclosure. The Mexican wolves were treated to a freshly cleaned pool, and even the jaguars got to chew on frozen meat-pops to help keep them cool.
Keepers monitored the animals' behavior for signs of dehydration or overheating, while public-facing employees kept a watchful eye on the brave visitors who sweated it out alongside the wildlife until they ultimately sought refuge in one of the zoo's cooling shelters.
Zoo life can be a lux experience for these Arizonan species, but for their wild counterparts living in the Sonoran desert, there is no escape from the region's extreme heat and extended drought. That day, the temperature was 10 degrees hotter than average, pointing to the larger warming trend across the desert Southwest.
When summer arrives in Phoenix, city officials respond to the needs of an urban population coping with rising temperatures. Cooling and hydration centers begin to open to the public while hiking trails at scenic mountain parks start to close. The officials with the Phoenix's Office of Heat Response and Mitigation plan to tackle the challenges facing a city at the forefront of extreme heat.
For more stories about heat and climate: Sign up for AZ Climate, The Republic's weekly environment newsletter.
Similarly, Arizona's native flora and fauna have their own obstacles to survival in one of the world's most unforgiving landscapes. Still, wildlife advocates, caretakers and scientists remain optimistic about the future.
They believe the public plays a key role in protecting the desert's iconic wildlife.
"There's a lot of reason for hope for a lot of these species," said Tara Harris, director of conservation and science at the Phoenix Zoo. "Some of the really difficult threats are ones where you don't know what the solution is going to be. And climate change is certainly one of those."
"The million-plus people that come here to this zoo, whether those people go on to be conservation biologists or just concerned citizens, everybody has an important role to play and hopefully we have lots of different ways to get at these problems," said Tara Harris.
During the summer months, the gates at the Phoenix Zoo close to the public at 1 p.m.
As visitors filter out, Bugsy the ringtail (a small raccoon-like mammal native to the Southwest) sits curled up on a ledge next to the vent pumping cold air into his enclosure. Like so many people and creatures in Phoenix, the Arizona state mammal just wants a little relief from the heat.
John Leos covers environmental issues for The Arizona Republic and azcentral. Send tips or questions to john.leos@arizonarepublic.com.
Hayleigh Evans writes about extreme weather and related topics for The Arizona Republic and azcentral.com. Email her with story tips at hayleigh.evans@arizonarepublic.com.
Environmental coverage on azcentral.com and in The Arizona Republic is supported by a grant from the Nina Mason Pulliam Charitable Trust.
Follow The Republic environmental reporting team at environment.azcentral.com and @azcenvironment on Facebook and Instagram.