Mexican Beaded Lizard

The Mexican Beaded Lizard is one of only two venomouss lizards in the world, the other being the Gila Monster (Heloderma suspectum). It’s latin name consists of the two words Heloderma horridum. Heloderma, meaning studed skin, and horridum, meaning horrible (horrible studded lizard). This seems quite a fitting name for this unique animal, although I don't think they're horrible at all. Heloderm lizard's skin consists of several tiny beads called ostioderms. Each bead contains a tiny piece of bone that gives them almost an armor plated skin.

The lifespan of these lizards is said to be around thirty years, although in my opinion it’s more like fifty. For females, average adult size is 30in and three and a half to five pounds. For males, average adult size is 36in and five to six pounds. Exceptionally large males can reach 40in and close to ten pounds.

Most Beaded Lizards reach sexual maturity at two and a half to three years old. Once oviposition occurs, the incubation period is around 165 to 215 days. At about that time the hatchlings will pip their head out of the egg. It usually takes two or three days for them to come all the way out, absorbing their yolk sack and ingesting the liquid content of their egg. Newly hatched neonates are usually five to six inches and weigh around 40 grams. Until the animals are substantial in size, sexing is nearly impossible. Males generally have much broader heads and longer necks, with females having narroe lizards are only active from April to mid-November, spending about an hour

## Taxonomy

The beaded lizards have 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.

## Range and Habitat

Beaded lizards are 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 Mexican beaded lizard *H. horridum* is found in Mexico, from Sonora to Oaxaca. The Rio Fuerte beaded lizard (*H. exasperatum*) is found from southern Sonora to northern Sinaloa. The Chiapan beaded lizard (*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 species overlap, making them sympatric. The Guatemalan beaded lizard (*H. charlesbogerti*) is the only allopatric one, separated from the nearest population (*H. alvarezi*) by 250 km of unsuitable habitat. The Guatemalan beaded lizard is the most endangered of the species, 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.

## 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. 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 such as mice and rats. Confiscated wild-caught specimens can be made to feed by using egg on the prey item.

## Venom

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.

Some compounds in its venom have been shown to have pharmacological properties relating to the possible treatment of diabetes, Alzheimer's disease, and even HIV. One compound, a hormone named exendin-3, is marketed by Amylin Pharmaceuticals as the drug exenatide. A study in 1996 revealed that it binds to cell receptors in breast cancer cells and may stop the growth of lung cancer cells.

## Reproduction

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.