Trachyderma horridum Wiegmann, 1829
The beaded lizard (Heloderma horridum) is a species of venomous lizard found principally in Mexico and southern Guatemala. It and its congener the Gila monster (Heloderma suspectum), are the only lizards known to have evolved an overt venom delivery system. The beaded lizard is larger than the Gila monster, but has duller coloration, black with yellowish bands of differing width depending on the subspecies. As it is 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 manufacting 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 species. The Motagua Valley subspecies (H. h. charlesborgeti) is one of the rarest lizards in the world, with a wild population of fewer than 200.
The beaded lizard has one close living relative, the Gila monster (H. suspectum), as well as many extinct relatives in the Helodermatidae, whose genetic 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. Because the helodermatids have remained relatively unchanged morphologically, they are occasionally regarded as living fossils. 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 indicate the beaded lizard is better placed in a separate family.
The species was first described in 1829 by Arend Wiegmann as Trachyderma horridum, but he renamed it Heloderma horridum six months later. 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.
- Mexican beaded lizard (H. horridum) (Wiegmann, 1829)
- Chiapan beaded lizard (H. alvarezi) Bogert and Martin del Campo, 1956
- Rio Fuerte beaded lizard (H. exasperatum) Bogert and Martin del Campo, 1956
- Guatemalan beaded lizard (H. charlesbogerti) Campbell and Vannini, 1988
Adult beaded lizards range from 57 to 91 cm (22 to 36 in) in length. They are substantially larger than the Gila monster, which only reaches lengths of 30 to 56 cm (12 to 22 in). The snout-to-vent length of a beaded lizard averages 33 to 48 cm (13 to 19 in). The average body mass of an adult beaded lizard is 800 g (1.8 lb), about 45% heavier than the average mass of a gila monster, with large specimens exceeding 2,000 g (4.4 lb). Maximum weight known is 4,000 g (8.8 lb) Although males are slightly larger than females, the lizards are not sexually dimorphic. Both males and females are stocky with broad heads, although the males' tend to be broader. The beaded lizard's scales are small, beadlike, and not overlapping. Except for the lizard's underside, the majority of its scales are underlaid with bony osteoderms.
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 it 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.
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. Their habitats are primarily in the desert, tropical deciduous forests and thorn scrub forests, but are found in pine-oak forests, with elevations from sea level to 1500 m. In the wild, the lizards are only active from April to mid-November, spending about an hour per day above the ground.
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 Chiapan beaded lizard (H. h. alvarezi) is found in the northern Chiapas and the depression of the Río Lagartero in Huehuetenango to northwestern Guatemala. The ranges of these three subspecies overlap, making them sympatric. 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. 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 northeastern Guatemala; less than 200 are believed to exist in the wild.
The beaded lizard is a specialized vertebrate nest predator, feeding primarily on bird and reptile eggs. A semiarboreal species, it is found climbing deciduous trees in search of prey when encountered above ground. It occasionally preys 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 do best on a diet of small vertebrates primarily mice and rats. Confiscated wild-caught specimens can be made to feed by using egg on the prey item.
The venom glands of the beaded lizard are modified salivary glands located in the reptile'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 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 human bites (eight in the past century) have resulted from prodding captive lizards with a finger or bare foot.
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.
The compounds in its saliva which have been studied have pharmacological properties relating to diabetes, Alzheimer’s disease, and even HIV. This hormone was named exendin-3 and is marketed by Amylin Pharmaceuticals as the drug exenatide. One study reported in 1996 revealed that it binds to cell receptors from breast cancer cells and may stop the growth of lung cancer cells.
The beaded lizard becomes sexually mature at six to eight years and mates between September and October. Males engage in ritual combat that often lasts several hours; the victor mates with the female. The female lays her clutch of two to 30 eggs between October and December, the clutch hatching the following June or July.
Young lizards are seldom seen. They are believed to spend much of their early lives underground, emerging at two to three years of age after gaining considerable size.
The beaded lizard is surrounded by myth and superstition in much of its native range. It is incorrectly believed, for example, to be more venomous than a rattlesnake, can cause lightning strikes with its tail, or make a pregnant woman miscarry by merely looking at her. As a result of this superstition, locals often kill the lizard on sight.
The seldom-seen lizard is poached and sold into 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. In Guatemala, it is protected by national legislation, and part of its range is within protected areas. It is listed on Appendix II of CITES.
Fewer than 200 lizards remain in the dry forest habitat of the Motagua Valley, and this subspecies of beaded lizard (H. h. charlesbogerti) was facing extinction due to local extermination and loss of habitat for agricultural purposes. A conservation effort has been launched known as Project Heloderma to preserve the semiarid habitat of the Motagua Valley by the Nature Conservancy and partners such as ZOOTROPIC, CONAP, the International Reptile Conservation Foundation, Lincoln Park Zoo, 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 as an Appendix I animal, making it illegal to export the species.
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- IRCF's Guatemalan Beaded Lizard Conservation Project
- Mexican Beaded Lizard at the LA Zoo
- Mexican Beaded Lizard at the Saint Louis Zoo