Jo Price explains all you need to know about the Gila monster, including how its venom has helped save lives
Named after the Arizona Gila River basin, where they were first discovered, Gila (pronounced hee-lah) monsters are one of only a small number of venomous reptiles and the largest lizards in the USA. The largest lizard in the world is the komodo dragon
Gila monsters have a frightening reputation, says Jo Price, especially in local folklore, as they can deliver an agonising bite, but they pose little risk to humans if left alone.
Where do Gila monsters live?
Gila monsters are found in south-west USA and north-west Mexico, with the core of their range in Arizona and Sonora. The total adult population size is unknown but according to the International Union for the Conservation of Nature (IUCN), it is at least several thousand.
These reptiles have a home range of about 2.6km2 and inhabit dry areas such as desert grassland and scrubland, canyon bottoms, dry creeks and rocky slopes. They spend more than 90 per cent of their time in underground shelters, only emerging to hunt or bask. Though solitary, they do occasionally share these refuges with other Gilas.
How big are Gila monsters
Gila monsters are just over half a metre in length – about 56cm
What do Gila monsters eat?
Gilas can consume up to a third of their bodyweight in one meal. They have a strong bite and swallow most of their food whole. Using their forked tongues to pick up scents, they raid nests to prey on small birds and eggs, and also eat small mammals (particularly young rabbits and ground squirrels), other reptiles, insects and carrion.
They don’t need to feed frequently as they store fat in their thick tails and stocky bodies, which explains why they can spend so much time underground to escape the intense heat.
When do Gila monsters use their venom?
They are more likely to use their venom on predators – coyotes, snakes and birds of prey – than on prey as they can easily handle their choice of food.
The venom is released from glands in the Gila monster’s lower jaw and flows through their grooved teeth when they clamp down and chew. The venom attacks the nervous system, causing severe pain and a drop in blood pressure. When these reptiles bite, they secure a firm hold and usually hang on.
How has the lizard’s venom saved lives?
While studying Gila monster venom, scientists discovered a hormone called exendin-4. It’s similar to the human hormone GLP-1, which increases insulin production in the pancreas. This led to the development of a new drug called exenatide (the venom’s hormone in synthetic form) to treat type 2 diabetes. Exenatide mimics the effects of GLP-1, boosting the human body’s ability to release its own insulin.
When and how do Gila monsters breed?
Gila monster breeding season usually begins in late spring to early summer. Adult males compete for mating opportunities by engaging in wrestling matches, and mating reportedly takes place underground and in sheltered sites. Females then dig holes, lay clutches of approximately two to 12 oval-shaped eggs and cover them. This tends to occur before the onset of the monsoon, to reduce the chance of the eggs drying out.
When do Gila monster young emerge?
Based on observed incubation times in captivity, hatchlings should appear after four months. However, a study by Arizona State University suggests wild Gila monster young overwinter, remaining in their nests for seven to 10 months after hatching. Hatchlings from eggs laid in July did not emerge from their nests until late April through to early August the following year.
It is thought that a later emergence gives the best chance of survival. If the Gilas emerged in early spring, their prey (the kits of rabbits, for example) would be too big for them to eat. In summer, other lizards begin to lay eggs that young Gila monsters can consume. Another benefit of biding their time is that temperatures are warmer when the reptiles are foraging under the cover of darkness. A hatchling is about 16cm in length and will grow up to 56cm, reaching maturity at three to five years.
Do Gila monsters face any threats?
The species is listed as Near Threatened by the IUCN. It is in decline due to habitat loss, driven by urbanisation and agricultural development throughout much of its range. Also, though collection is prohibited, populations have been illegally exploited by commercial and private collectors. The species is listed on CITES Appendix II and some areas of its range are protected from development, such as national parks and federal wilderness areas.
Why do Gila monsters have beaded scales?
The genus Heloderma means ‘studded skin’ and accurately describes the bony deposits in the Gila monster’s skin. These are known as osteoderms and cover the lizard’s bulky black body, apart from the underside. The protective bead-like scales, alongside pink or orange markings, warn other animals to keep their distance.
Are there any subspecies?
Two subspecies are recognised. In the southern subspecies, the reticulated Gila monster (Heloderma suspectum suspectum), the reptile’s bands are broken up to form a reticulated pattern. In the northern subspecies, the banded Gila monster (Heloderma suspectum cinctum), markings generally form bands across the back
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