How Often Do Gila Monsters Eat? A Few Big Meals a Year

A feast-and-fast lifestyle

The Gila monster lives in deserts where food is abundant for a short time and scarce for most of the year. Its answer is to feed heavily when it can and then do almost nothing.

Most feeding happens in spring, from about March to June, when birds, rabbits and rodents are breeding and nests are full of eggs and young. A Gila monster can eat an enormous amount in one sitting: young lizards can swallow about half their own body weight, and adults roughly a third. For a person, that would be like eating 20 kg (45 lb) of food at once.

Gila monsters do not stick rigidly to a schedule, and they will eat whenever they find suitable food. But in many years a few big meals are enough, and a lizard may spend only a handful of days above ground hunting. In one study in southwestern Utah, Gila monsters were active on the surface on only about 10 days during an active season of roughly 90 days.

After a heavy meal, a Gila monster usually retreats to a shelter to digest, which can take days. Later in the summer, when nests are empty and the ground is hot, feeding becomes much less common, and from late autumn to late winter the lizards do not eat at all while they rest underground in brumation, the reptile version of hibernation. Over a full year, a wild Gila monster may therefore spend far more time fasting than feeding.

What do Gila monsters eat?

Gila monsters are nest raiders. Their main foods are:

• Eggs of ground-nesting birds such as quail, and of lizards, snakes and desert tortoises
• Nestlings, including young birds that cannot yet fly
• Baby mammals, such as newborn rabbits, ground squirrels, mice and woodrats in their nests
• Occasional extras, including lizards, insects and carrion

Large items may be crushed, while small ones are swallowed whole, usually head first. Gila monsters are good climbers and can reach nests in shrubs and cactus.

How Gila monsters find food

Gila monsters hunt mainly by smell. Like snakes, they flick a forked tongue to pick up scent particles from the air and ground and carry them to a sensory organ in the roof of the mouth. This lets them follow scent trails to buried eggs or a rabbit’s nest hidden under a bush.

After eating, a Gila monster often keeps searching the same spot, flicking its tongue to check for more eggs or young. A nest of quail eggs or a litter of baby rabbits can be eaten in one visit. Because their prey rarely fights back, they seldom need to use their venom while feeding; see Is the Gila monster venomous?

The fat-storing tail

The key to going so long between meals is the tail. A Gila monster stores fat in its short, thick tail, a bit like a camel’s hump. After a good spring the tail is plump and rounded. As the months pass and the lizard lives on its reserves, the tail slowly shrinks.

The tail’s condition is therefore a rough guide to how well a wild Gila monster has fed. Females that have just laid eggs are often thin, because egg production uses a lot of stored energy.

A very slow metabolism

Gila monsters also need less food than most lizards because their metabolism is unusually slow. Studies have found that they use less than half the energy expected for a lizard of their size. They spend around 90 percent of their lives resting in cool burrows or rock shelters, where they burn very little energy.

Water is managed just as carefully. A Gila monster can store water in its bladder and reabsorb it when needed, and it drinks from puddles after rain. Spending most of its time underground, out of the sun, also cuts water loss.

How Gila monsters compare with other reptiles

Many reptiles can go long periods without food, because, unlike mammals and birds, they do not burn energy to keep a constant body temperature. Large snakes such as pythons may go weeks or months between meals. The Gila monster is unusual among lizards in how far it takes this strategy, relying on a short spring feast for most of its yearly energy. For more on how reptiles control body heat, see Are reptiles cold-blooded?

This lifestyle suits a harsh desert, but it has a downside. Gila monsters reproduce slowly and depend on healthy nesting populations of birds and mammals. Loss of desert habitat to development reduces both their food and their shelters, one reason the species is listed as Near Threatened. Other desert specialists with their own ways of saving energy and water are featured in desert animals.