The Mexican beaded lizard (Heloderma horridum) is one of the two species of venomous beaded lizards. It and the other member of the same genus, the Gila monster, are the only lizards known to have evolved an overt venom delivery system. 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. 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.
Both males and females of this species are stocky with broad heads, although the males tend to be broader. The scales of these lizards are small, beadlike, and not overlapping. Except for the underside, the majority of their scales are underlaid with bony osteoderms. The base color of Mexican beaded lizards is black and marked with varying amounts of yellow spots or bands, with some individuals being all black in color. They have a short tail that is used to store fat so they can survive during months of estivation. Unlike many other lizards, this tail does not autotomize and cannot grow back if broken. Beaded lizards have a forked, pink tongue which they use to smell, with the help of a Jacobson's organ; they stick their tongue out to gather scents and touch it to the opening of the organ when the tongue is retracted.
Mexican beaded lizards are found in Mexico and southern Guatemala. They occur in the Pacific drainages from southern Sonora to southwestern Guatemala and two Atlantic drainages, from central Chiapas to southeastern Guatemala. These lizards live in tropical deciduous forests, thorn scrub forests, pine-oak forests and in tropical scrubland.
Mexican beaded lizards are only active from April to mid-November, spending about an hour per day above the ground. They are semi-arboreal reptiles that climb trees at night in search of prey. During the day, they hide in burrows or under rocks. Mexican beaded lizards use their venom to kill their prey, and also to weaken potential predators. When threatened, they will also produce hisses.
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.
Mexican beaded lizards are carnivores feeding primarily on bird and reptile eggs. They may also hunt small birds, mammals, frogs, lizards, and insects.
The mating season of Mexican beaded lizards occurs between September and October. During this time males engage in ritual combat that often lasts several hours; the victor mates with the female. Females lay clutches of 2 to 30 eggs between October and December. The incubation takes around 6-7 months. 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. They become reproductively mature at six to eight years of age.
Mexican beaded lizards are surrounded by myth and superstition in much of their native range. They are incorrectly believed, for example, to be more venomous than rattlesnakes or can cause lightning strikes with their tail. As a result of this superstition, locals often kill beaded lizards on sight. These seldom-seen lizards are also poached and sold into the illegal exotic animal trade. They do not reproduce well in captivity, and their scarcity means a high price for collectors. Threatened throughout its range by overcollection and habitat loss for agricultural purposes, the subspecies Guatemalan beaded lizard now is one of the rarest lizards in the world.
According to the IUCN Red List, the total population size of the Mexican beaded lizard is unknown. However, there is an estimated population of the Guatemalan subspecies (Guatemalan beaded lizard) consisting of around 350 mature individuals. Currently, this species is classified as Least Concern (LC) on the IUCN Red List but its numbers today are decreasing.
