Meals for monsters

Friday, October 7, 2011
story and photos by Pete Zrioka

Gila monster

Their name sounds terrifying. But the lizards known as Gila monsters are actually quite shy. Their size and bite are the only monstrous things about them. Sometimes growing up to two feet long, Gilas are the largest lizards native to the United States.

The Gila monster is one of only two venomous lizards in North America. Its bite is painful, but no humans have ever died from it. Gilas will only attack humans when they feel threatened. And while their size might seem scary, it also makes them sluggish.

But their monstrous mass makes Gila monsters perfect research subjects. Christian Wright and Karla Moeller are biology Ph.D. students at Arizona State University. They are trying to understand how Gila monsters adapt and survive in the changing desert environment.

Moeller studies differences between adult and juvenile, or young, Gila monsters. She wants to know how those differences can affect their behavior. She says juvenile animals haven’t been studied very much. But if there are major differences between young animals and adults, biologists need to take this into account.

The size of Gila monsters makes them convenient for Moeller’s research. Adults usually grow to about a foot-and-a-half in length. Juveniles are bigger than a lot of adult lizards from other species.

Adult and juvenile Gila monsters Adult and juvenile Gila monsters

Because of this, the researchers can easily implant radio transmitters in the animals to track them in the wild. They also implant temperature sensors called iButtons in both adult and juvenile Gilas.

“The devices we use are small and fit very nicely in the body cavity without hurting the animal,” says Moeller. “We’ve had animals in our study for over eight years that have had iButtons and transmitters in them and they’re perfectly healthy.”

The iButtons record the animals’ body temperature every hour. By comparing body temperature with the air temperature at the site, the researchers can tell if the lizards are above or below ground.

Gila monsters are ectotherms, which means their bodies cannot regulate heat. A Gila’s body temperature changes with the temperature around it. So the creatures will bask in the sun to warm up, and burrow in the ground to cool off.

The iButtons let the researchers monitor the lizards’ activity. Meanwhile, the radio transmitters track their locations. The researchers keep track of approximately 15 adult Gila monsters, located in an area about 30 miles north of Tucson.


Wright studies how adult Gila monsters cope with changes in food supply. He wants to predict how they will respond to the changing climate of the Sonoran Desert. Climate scientists predict that the region will have higher temperatures and lower annual rainfall in the next 100 years.

“Precipitation is directly linked to food availability, so I’m looking at how they respond to food availability in the wild and using that to predict how future climate changes could impact their survival,” says Wright.

Wright conducted a feeding study on the wild population. He gave half of the adult lizards a meal and the other half nothing as a control. Since Gila monsters use their bladders to store water and keep fat reserves in their tails, they can go long periods of time without eating or drinking. Wright wanted to know if a large meal would affect their behavior and physiology in the wild.


“They spend a lot of their time inactive, but getting more food means they have more energy, allowing them to search for more food, or potential mates,” says Wright. “Or that might mean they’ll reduce their activity because, ‘Oh, I’ve got a full belly, I don’t need to search for food and I can stay underground and conserve energy.’”

Juvenile Gila monster

Wright is still analyzing the study data, but so far the information shows no difference between the two groups. This suggests food availability has little bearing on how Gila monsters conserve water. Because there were no differences in mass or fat stores, animals with more energy may be spending it, rather than conserving it like the unfed animals.

Wright also conducts studies in the lab. He is trying to find out how meals affect the hydration levels of the animals.

“They go long periods without water – up to three months. A lot of work has shown they can tolerate extended droughts. But that’s just looking at water,” says Wright. “I wanted to look at how food plays a role in hydration.”

To find out, Wright fed one group in the study but not the other, watering neither. By analyzing blood samples, Wright found that the fed animals were getting very little water from their meals.

“Although they are a desert-dwelling species, they are dependent on the winter and monsoonal rainfall pulses to survive,” says Wright. “This means that if precipitation patterns change enough, Gila monsters may not be able to get the water they need.”

This could be disastrous for the lizards, which are already considered a threatened species.

Moeller also looks at how Gila monsters manage their water, but she compares juveniles and adults.

“We know adults can hold water in their bladders and pull from that throughout the summer to survive,” she says. “But the juveniles are smaller and have a higher surface-area-to-volume ratio. This means they’re going to lose more water relative to the adults.”

So how do juveniles make it through the drought? To answer this, Moeller must figure out how much water the animals lose at different temperatures.

She puts the lizards into a temperature-controlled chamber. She then runs air across their bodies and compares the humidity of the air going in and the air going out. Humidity is a measure of how much water vapor is in the air. It tells Moeller how much water the animals are losing at various temperatures.

Putting a Gila monster in a chamber Putting a Gila monster in a temperature-controlled chamber

Then she combines this data with information about Gilas in the wild. The iButtons will tell her how the free-ranging animals are behaving, and whether there are differences between the adults and the young.

All of the research contributes to our understanding of conservation. In particular, it will help scientists understand how climate change affects survival.

“There’s a lot of work that’s currently being done about how climate change is going to impact animals, not only if they can survive but how they’re going to cope,” says Wright. “People tend to focus on the cute and cuddly animals, but climate change is going to impact more than just them – and those impacts could be dramatic.”