The beaded lizard (Heloderma horridum) is a species of venomous lizard found principally in Mexico and southern Guatemala. Along with its congener, the Gila monster (Heloderma suspectum), it is the only lizard to have evolved an overt venom delivery system, and has long been considered the only venomous lizard. The beaded lizard is larger than the Gila monster but has duller coloration, black with yellowish bands of differing width depending on the subspecies. A specialized predator that feeds primarily upon eggs, the primary use of its venom is still a source of debate among scientists. However, this venom has been found to contain several enzymes useful for manufacturing drugs in the treatment of diabetes, and research on the pharmacological use of its venom is ongoing.

Threatened throughout its range by overcollection and habitat loss, it is a CITES protected animal. The Motagua Valley subspecies (H. h. charlesborgeti) is one of the rarest lizards in the world, with a wild population of fewer than 200 animals.

Taxonomy

The beaded lizard has one close living relative, the Gila monster (H. suspectum), as well as many extinct relatives in the Helodermatidae whose evolutionary history may be traced back to the Cretaceous period. The genus Heloderma has existed since the Miocene, when H. texana ranged over most of North America.^[3] Because the Helodermatids have remained relatively unchanged morphologically, they are occasionally regarded as living fossils.^[4] Although the beaded lizard appears closely related to the monitor lizards (varanids) of Africa, Asia and Australia, the wide geographical separation and unique features not found in the varanids indicates that the beaded lizard is better placed in a separate family.^[5]

The species was first described in 1829 by Arend Wiegmann as Trachyderma horridum, however, he renamed it Heloderma horridum six months later.^[6] Its generic name Heloderma means "studded skin", from the Ancient Greek words hêlos (ἧλος)—the head of a nail or stud—and dérma (δέρμα), meaning skin. Its specific name, Horrĭdum, is the Latin word meaning rough or rude.

There are four subspecies of beaded lizard:

• Mexican beaded lizard (Heloderma horridum horridum) (Wiegmann, 1829)^[7]
• Black beaded lizard (H. h. alvarezi) Bogert and Martin del Campo, 1956^[8]
• Rio Fuerte beaded lizard (H. h. xasperatum) Bogert and Martin del Campo, 1956^[9]
• Motagua Valley beaded lizard (Heloderma horridum charlesbogerti) Campbell and Vannini, 1988^[10]

Description

Closeup of a Helodermatid's skin, composed of beadlike scales

Adult beaded lizards range from 24 inches (61 cm) to 36 inches (91 cm) in length. It is substantially larger than the Gila monster, which only reaches lengths of 12 inches (30 cm) to 16 inches (41 cm). Although males are slightly larger than females, the animals are not sexually dimorphic. Both males and females are stocky with broad heads, although the males tend to be broader.^[11] The beaded lizard's scales are small, bead-like and non-overlapping. Except for the underside of the animal, the majority of its scales are underlaid with bony osteoderms.^[11]

Their base color is black and marked with varying amounts of yellow spots or bands, with the exception of H. h. alvarezi, which tends to be all black in color. The beaded lizard has a short tail which is used to store fat so the animal can survive during months of estivation. Unlike many other lizards, this tail does not autotomize and cannot grow back if broken. The beaded lizard has a forked black tongue which it uses to smell, with the help of a Jacobson's organ; it sticks its tongue out to gather scents and touches it to the opening of the organ when the tongue is retracted.^[3]

Habitat and range

The beaded lizard is found in the Pacific drainages from Southern Sonora to Southwestern Guatemala and two Atlantic drainages, from Central Chiapas to Southeastern Guatemala.^[11] Their habitat is primarily tropical deciduous forest and thorn scrub forest, but are found in pine-oak forest, with elevations from sea level to 1500 meters. In the wild, the animals are only active from April to mid-November, spending about an hour per day above the ground.^[12]

The nominate subspecies H. h. horridum is found in Mexico, from Sonora to Oaxaca. The Rio Fuerte beaded lizard (H. h. exasperatum) is found from southern Sonora to northern Sinaloa. The Black beaded lizard ( H. h. alvarezi) is found in the northern Chiapas and the depression of the Río Lagartero in Huehuetenango to northwestern Guatemala.^[12] The ranges of these three subspecies overlap, making them sympatric.^[13] The Motagua Valley subspecies (H. h. charlesbogerti) is the only allopatric one, separated from the nearest population (H. h. alvarezi) by 250 km of unsuitable habitat.^[13] The Motagua Valley beaded lizard is the most endangered of the subspecies if not of all lizards; it is found only in the dry valley of the Río Motagua in the north-east of Guatemala; it is believed less than 200 of these animals exist in the wild.^[12]

Ecology

Diet

The beaded lizard is a specialized vertebrate nest predator feeding primarily on bird and reptile eggs. A semi-arboreal species, it is found climbing deciduous trees in search of prey when encountered above ground.^[14] It will occasionally prey upon small birds, mammals, frogs, lizards, and insects. Steve Angeli and Robert Applegate, noted captive breeders of the beaded lizard, have remarked that captive specimens, both wild-caught and captive-born will often refuse any food, except for eggs.^[11][15]

A pair of Mexican beaded lizards at the Buffalo Zoo. The specimen on the right is in the process of molting.

Venom

The venom glands of the beaded lizard are modified salivary glands located in the animal's lower jaw. Each gland has a separate duct leading to the base of its grooved teeth. When biting, the beaded lizard hangs on its victim and chews in order to get its venomous saliva into the wound. Although its jaw grip is strong its unsocketed teeth are easily broken off at their bases. The beaded lizard's venom is a weak hemotoxin and although human deaths are rare, it can cause respiratory failure. It consists of a number of components, including L-amino acid oxidase, hyaluronidase, phospholipase A, serotonin, and highly active kallikreins that release vasoactive kinins. The venom contains no enzymes that significantly affect coagulation. Almost all documented bites (eight in the past 100 years) have resulted from prodding captive animals with a finger or bare foot.^[16]

While invertebrates are essentially immune to the effects of this venom, effects on vertebrates are more severe and varied. In mammals such as rats, major effects include a rapid reduction in carotid blood flow followed by a marked fall in blood pressure, respiratory irregularities, tachycardia and other cardiac anomalies, as well as hypothermia, edema, and internal hemorrhage in the gastrointestinal tract, lungs, eyes, liver, and kidneys. In humans, the effects of bites are associated with excruciating pain that may extend well beyond the area bitten and persist up to 24 hours. Other common effects of bites on humans include local edema (swelling), weakness, sweating, and a rapid fall in blood pressure. Beaded lizards are immune to the effects of their own venom.^[17]

The Mexican beaded lizard's base color is black, with yellow bands or patches.

The compounds which have been studied in its saliva have pharmacological properties relating to diabetes, Alzheimer’s disease and even HIV.^[18] This hormone was named exendin-3 and is marketed by Amylin Pharmaceuticals as the drug:Exenatide.^[19] One study done in 1996 revealed that it binds to cell receptors from breast cancer cells and may stop the growth of lung cancer cells.^[20]

Reproduction

The beaded lizard becomes sexually mature at six to eight years of age and mates between September and October. Males engage in ritual combat that often lasts several hours; the victor mates with the female.^[3] The female lays her clutch of 2 to 30 eggs between October and December, the clutch hatching the following June or July.^[11]

Young lizards are seldom seen. It is believed they spend much of their early life underground, emerging at two to three years of age after gaining considerable size.^[21]

Conservation

Mexican beaded lizard at the Louisville Zoo, Kentucky.

The beaded lizard is surrounded by myth and superstition in much of its native range. It is incorrectly believed, for example, that the lizard is more venomous than a rattlesnake, that it can cause lightning strikes with its tail, or make a pregnant woman miscarry by merely looking at it. As a result of this superstition, locals often kill the lizard on sight.^[21]

The seldom seen lizard is poached for resale through the illegal exotic animal trade. It does not reproduce well in captivity, and its scarcity means a high price for collectors. As a direct result, the beaded lizard is protected by Mexican law under the category A (Threatened), and it dwells within the range of several protected areas.^[22] In Guatemala it is protected by national legislation, and part of their range is within protected areas. It is listed on Appendix II of CITES.^[21]

Fewer than 200 individual animals remain in the dry forest habitat of the Motagua Valley and this subspecies of beaded lizard (Heloderma horridum charlesbogerti) was facing extinction due to local extermination and loss of habitat for agricultural purposes.^[21] A conservation effort has been launched known as Project Heloderma in order to preserve the semi-arid habitat of the Motagua Valley by The Nature Conservancy and partners such as ZOOTROPIC, CONAP, the IRCF (International Reptile Conservation Foundation), Zoo Atlanta, and the San Diego Zoo. This effort has been successful in getting the Guatemalan government to list the beaded lizard under the Convention on International Trade in Endangered Species (CITES) as an Appendix I animal, making it illegal to export the species.^[22]

Footnotes

References

• Ariano, D. 2008. Envenomation by a wild Guatemalan beaded lizard Heloderma horridum charlesbogerti. Clinical toxicology 46 (9): 897-899. [1]
• Ariano, D. y G. Salazar. 2007. Notes on the Distribution of the Endangered Lizard, Heloderma horridum charlesbogerti, in the Dry Forests of Eastern Guatemala: An Application of Multi-criteria Evaluation to Conservation. Iguana 14(3): 152-158.[2]
• Ariano, D. 2006. The Guatemalan beaded lizard: Endangered inhabitant of a unique ecosystem. Iguana 13(3): 178-183. [3]
• Beaman (1996). Heloderma horridum. 2006. IUCN Red List of Threatened Species. IUCN 2006. www.iucnredlist.org. Retrieved on 11 May 2006. Listed as Vulnerable (VU A2cd v2.3)
• Beck, Daniel D. (2005). Biology of Gila Monsters and Beaded Lizards (Organisms and Environments). University of California Press. p. 247. ISBN 0-520-24357-9. 
• Berkow, Robert, Ed. (1992). The Merck Manual, 16th Ed.. Merck Research Laboratories. ISBN 0-911910-16-6. 
• Cogger, Harold; Zweifel, Richard (1992). Reptiles & Amphibians. Sydney: Weldon Owen. ISBN 0-8317-2786-1. 
• Freiberg, Dr. Marcos; Walls, Jerry (1984). The World of Venomous Animals. New Jersey: TFH Publications. ISBN 0-87666-567-9.