Added 27 March 2012. Updated 2 April 2012: updated Bibliography.

A Brief Look at the


Beaded Lizard and Gila Monster

The Family Helodermatidae is represented by just one small genus from North America, Heloderma. Despite this these lizards (or at least the Gila Monster) have been well known for many years, partly for their large build and rather unique appearance and partly for being renowned (at least until recently) as the world's only venomous lizards.

In appearance both lizards are unmistakeable for any other lizard or indeed each other, having the characteristic “beaded” skin. Although related to both the Family Varanidae (monitors) and Family Anguidae (slowworms, glass lizards and alligator lizards), they cannot be confused with either. Steel lists common characteristics of both varanid and helodermatid anatomy, as well as some differences. Smith summarised the characteristics of the genus as follows: tubercles present on sides and back of head and body, each enclosing a bony particle; small plate-like scales on ventral surface, arranged in rows and in general similar in appearance to the ventral plates of Gerrhonotus and Ophisaurus; broad, stout body; short, blunt, fat tail; union of skull bones with bony skin tubercles; internal groove present on all teeth; venom glands.

The venom of Gila Monster is normally not fatal, but should not be underestimated. Although no deaths were recorded in the twentieth century, local Indians were always aware of the lizards' venomous capabilities, and there were apparently at least two or three fatalities in the nineteenth, in the days when alcohol, the contemporary state of medicine and lack of knowledge of the creatures made for a poor mixture. A Colonel Yeager died after apparently being drunk one morning and goading a Gila Monster he was holding. Even in the twentieth century, a French scientist who was bitten by one experienced fainting fits and other side effects for up to five months after the bite. In general, animal venom can also cause damage to the human body in the form of necrosis or damage to organs, so a bite from even a normally non-lethal animal is to be avoided. The venom glands are located in the lower jaw, as opposed to those of snakes which are located in the upper jaw. In contrast to snakes, the dentition of helodermatid lizards means that they cannot inject the venom and therefore have to work it into the victim by chewing. However the venom appears to be mainly for defensive use rather than for subduing prey. Prey itself consists of small mammals including rodents, and the eggs of birds and reptiles. In captivity the lizards seem especially fond of eggs.

Gila Monsters have been and are still kept in captivity, although they may require permits depending on local or national legislation due to their venom. It is also illegal to collect Gila Monsters in the US without a permit, while the export of Mexican wildlife (inc. the Beaded Lizard) is prohibited. For these reasons the keeping of these lizards is really for specialists who have the willingness and skill to breed them. Although the animals may become docile and apparently quite lazy, this should not be mistaken for tameness and a lowered willingness to bite: Smith notes that despite their apparent lethargy Gila Monsters can snap their heads sideways very rapidly in an attempt to bite. An interesting observation is that at least some Gila Monsters like to soak in their water bowl for hours, perhaps surprising for creatures of a desert area, although they are found in arid rather than true desert dwellers and appear to require rainfall at least during certain seasons. Switak recommends not leaving a water container in the terrarium for long periods: it is worth checking this advice against other sources. The bulky tail functions as a fat storage device, and for this reason Gila Monsters and Beaded Lizards can aestivate or hibernate in inclement or unfavourable conditions. Although very capable of digging, according to Smith they usually occupy mammal burrows instead. Smith also recorded a longevity of 19 years for one captive, way back in the 1940s. An interesting characteristic noted by both Smith and Switak is the propensity of the Gila Monster at any rate to sleep sometimes on its back with its legs widely spread. Like monitors, Heloderma species like to bask in the sun: equally like monitors, captives exposed to natural sunlight will lose their docility temporarily, which makes handling them outside a matter for extreme caution. After a few days back in the vivarium the docility apparently returns. Husbandry suggestions can be found in Rogner, Sprackland and Switak (see Bibliography).

In palaeontological terms the helodermatids go back as far as the Cretaceous (genera Paraderma and Gobiderma from the US and Mongolia respectively), and the genus Heloderma has been around since at least the Miocene Era. The genus appears to have been more widespread in prehistoric times since the bones of another related genus, Eurheloderma, were found in France in rocks from the Oligocene-Eocene era. For this reason Steel considers the living helodermatids to be relict species.

Scientific Name

Common Name





H. horridum

Beaded Lizard

Mexico ( S Sinaloa, Nayarit, Jalisco, Michoacán to Oaxaca, Guerrero, Moreles, Chiapas), Guatemala

Max 1m/3': SVL about 35% of TL

The larger of the two species, found in the Pacific drainage of Mexico and an isolated range in Guatemala. Mainly a forest dweller. Scalation details: subcaudal scales 73-87. Dentition details: 6-7 maxillary teeth; teeth retained on palatine bone in roof of mouth. Caudal vertebrae: 40. Other: claws proportionately long. Coloration: dorsum of head virtually all black; juveniles have 6-7 bands on tail. Reproduction: up to 12 eggs laid July-August.

. h. horridum

Mexican Beaded Lizard

Mexico (S Sinaloa, Nayarit, Jalisco, Michoacán to Oaxaca, Guerrero and Moreles)

Largest subspecies, found in tropical deciduous or thorn forest, or occasionally savannah or pine or oak forest. Scalation details: supranasal separated from postnasal by 1st canthal; usually <8 scales across top of head; 11 or fewer scales along midline of head between internasals and occiput. Coloration: juveniles have similar pattern of light and dark markings; adults variable, but generally black or brown markings are more extensive than in exasperatum; yellow dots, bars or irregular spots, more or less bilaterally symmetrical.

H. h. alvarezi

Black Beaded Lizard, Chiapan Beaded Lizard

Mexico (Chiapas)


An inhabitant of tropical deciduous forest. Coloration: almost entirely black or dark-brown with adulthood; faint yellow bands on ventral surface of body and tail.

H. h. charlesbogerti

Motagua Valley Beaded Lizard

SE Guatemala

Only described in 1988. Found in tropical deciduous valley forest. Scalation details: females have enlarged preanal scales. Coloration: head and neck uniformly black; dorsum of body dark with pale yellow spots and blotches across posterior area; no complete bands, rings or rosettes forward of vent.

H. h. exasperatum

Rio Fuerte Beaded Lizard

Mexico (S Sonora, N Sinoloa)


Primarily an inhabitant of deciduous forest, found at altitudes up to 1,000-3,500'. Scalation details: supranasal usually in contact with postnasal; up to 8 scales across head between superciliaries; 2nd supralabial always in contact with nasal or prenasal. Coloration: light, yellowish or pinkish ground colour equal to or exceeding black or dark brown coloration.

H. suspectum

Gila Monster

USA (SE California, S Nevada, SW Utah, Arizona, New Mexico), Mexico (Sonora)

Max <60cm/24": SVL about 45% of TL

The smaller of the two species, found in the arid parts of its range. Scalation details: subcaudal scales <62. Dentition details: 8-9 maxillary teeth; no palatine dentition. Caudal vertebrae: 25-28. Other: splenial bone on inside of lower jaw overlaps coronoid bone above it; pair of enlarged preanal scales in front of vent. Coloration: head mottled with pink except for snout; juveniles have 4-5 bands on tail. Reproduction: up to 12 eggs laid July-August.

H. s. suspectum

Reticulate Gila Monster

USA (SW New Mexico), Mexico (Sonora)

Coloration: darker; juvenile pattern lost by adulthood; pink and black mottling or blotching; 4-5 bands on tail which may increase in width to encroach on lighter bands, or be interspersed with rows of lighter scales; lighter caudal bands have dark markings..

H. s. cinctum

Banded Gila Monster

USA (SW Utah, SE Nevada, NW Arizona, SE California)

Coloration: pale; juvenile rings retained in adults as 4-5 black or dark brown bands on tail, including tip, either broader than intervening light bands or mottled with pink and yellow; 4 black double cross-bands containing lighter spots on dorsum; darker individuals with salmon coloured (rather than pink) areas may occur.


Not read by the author, but probably of interest to anyone who wants to research and/or own one of the helodermatids:

Biology of Gila Monsters and Beaded Lizards, Daniel D Beck, University of California Press 2005 (ISBN: 0520243579).


Wikipedia has a useful article on Heloderma.

Another useful Wikipedia article on Heloderma suspectum, including current drug research based on the species' venom.

Stephen L Angeli's site. Carries good detail and some very good photographs, including one of a Beaded Lizard climbing a tree.

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