Lizards 1: The Gila Monster

The first lizards that a visitor to Bristol Zoo will encounter are two young Gila Monsters (Heloderma suspectum) as they pass through Twilight World. Unfortunately, although these are fascinating animals, most visitors walk straight past as they are not exactly the most active of animals, in fact they generally behave as though they were stuffed. However, when readers of this blog next see an exhibit, I hope they will at least check them out.

Gila Monsters take their name from the Gila Basin in Arizona, which is in the center of the species range, which extends north into Nevada and south into parts of Mexico. Further south in Mexico it is replaced by the very similar Beaded Lizard, H.horridum,  which can be found south into Guatemala. The two species occupy different habitats – the Gila Monster is a desert animal while the Beaded Lizard is a denizen of woodland. Both species tend to be secretive and slow moving, tending to spend much of their time in burrows, rock crevices, or other retreats, emerging only to look for food by day or night depending on the season.

Both species have a slightly unusual diet – most of their food is in the form of eggs of birds or reptiles, or nestling birds and rodents. They will also catch frogs or other animals, but their physiology is adapted to a routine of feeding very infrequently. When they do feed, they eat as much as possible, and then store surplus fat in their thick tails, which serves as an energy store during the dry summers and winter hibernation, when food is often unavailable.

Gila Monster reproduction has been little studied in the wild, but it appears that males may compete over females shortly after emerging from hibernation. Females can lay up to 12 eggs, depending on the size of the female, which are buried in the ground in midsummer. Eggs have a long incubation period, and the young emerge in April the following year. In captivity the incubation is much less, as in the wild the incubation is interrupted by winter hibernation while still in the egg.

The famous venom of the Gila Monster is probably as much for use in defence as in killing its prey, which are usually incapable of physical resistance anyway. Although it is now known that some, and possibly all, monitor lizards also have toxic saliva, that of the two species of Heloderma are delivered into the bite wound by specialised grooves in their teeth. The venom is from modified salivary glands in the lower jaw, and although few (possibly none) no human fatalities can be definitely ascribed to the venom, a bite is extremely painful and requires hospital treatment. Given the very sluggish nature of the animal, getting bitten requires considerable effort on the part of the victim – on the level of picking one up and waving your finger under its nose. Despite this, in the UK for an individual or institution to keep one requires a licence from the local authority under the Dangerous Wild Animals Act.

Gila Monsters have been bred on several occasions at Bristol Zoo. To do so they were hibernated for several months, before being warmed up and introduced to each other. Actual husbandry for Gila Monsters is quite straightforward – they are not active animals requiring large amounts of room, and although fairly large for a lizard they are simple to feed on thawed rodents. At present their conservation status is listed as Near Threatened – the main threat is habitat destruction as a result of the growth of cities in the dry areas of the south western United States. People still kill them out of fear in some areas unfortunately, mostly as a result of exaggerated myths about how dangerous they are.

Next time, more lizards from the southern US.