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Heloderma horridumBeaded Lizard

By Nichol Stout

Geographic Range

Heloderma horridum is found throughout central and western Mexico from latitude 25, southward to northern Central America.

Habitat

Heloderma horridum is found in semi-arid rocky regions. The areas are sparsely vegetated canyon bottoms, open forest, and washes. These lizards are often found on rock ledges.

Physical Description

Adult female Mexican beaded lizards grow to about 76 cm in length and weigh 1.4 to 2 kilograms. Males are slightly larger, growing to 90 cm and weighing up to 4 kilograms The tail is the longest portion of the lizard, nearly 50% of the total length.

Beaded lizards have a cylindrical body with a long, thick tail. The head is wide and flat, and the legs are short and strong. Large, hard scales cover the top of the lizards' body, while the belly side is coated with soft scales.

The scales are bead-like and are predominately dark black or brown on the top and bottom of the lizard. Yellow spots are scattered along the tail and the neck. Younger individuals have thicker bands and larger blotches of yellow on their bodies when compared with the adults.

This species and the other species of Heloderma, H. suspectum (the Gila monster) are the only lizards known to be venomous. They have a few special grooved teeth to deliver their venom when they bite.

The grooved teeth are on the lower jaw bones, along with the venom glands, which are arranged at the rear edge. The venom passes from these glands through a channel to the roots of the grooved teeth. The venom is drawn into the wound by capillary action as the lizard chews its prey.

  • Sexual Dimorphism
  • male larger
  • Range mass
    1.4 to 4 kg
    3.08 to 8.81 lb
  • Range length
    75 to 90 cm
    29.53 to 35.43 in

Reproduction

The breeding season is in February and March and copulation lasts 30-60 minutes. About 2 months later, females lay 3-13 elongated eggs and bury them at a depth of about 12.5cm. The female then abandons the eggs. Incubation takes about 6 months. Each hatchling may be up to 20 cm long.

The Mexican Beaded Lizard has very few enemies besides humans, coyotes, and some raptorial birds. Having few natural enemies gives the hatchlings a greater chance for survival.

Lifespan/Longevity

Behavior

This species hides in self-dug or pre-existing burrows during the day and becomes active at night. When the lizard first leaves its burrow it moves slowly and clumsily but as the night progresses it becomes more aggressive. In this more active state, the Mexican Beaded Lizard can "turn and snap with the agility of an angry dog" (Ditmars, 1936).

The Mexican Beaded Lizard uses its venom not only to kill its prey, but also to subdue potential predators such as humans, coyotes, and raptors. The lizard also gapes and hisses to fend off its enemies.

Food Habits

In the adult stage, the Mexican Beaded Lizard is carnivorous. Its diet consists of small mammals, birds, lizards, frogs, insects, and eggs of birds and reptiles. All their prey is swallowed whole, except for eggs, which are broken first.

When food is scarce, the Mexican Beaded Lizard lives off fat reserves in the tail. Fat is stored in the tail of the lizard making it appear swollen. After the fat reserves are used up the tail appears thin again.

  • Primary Diet
  • carnivore
    • eats terrestrial vertebrates
    • eats eggs
  • Animal Foods
  • birds
  • mammals
  • amphibians
  • reptiles
  • eggs
  • insects
  • terrestrial non-insect arthropods

Economic Importance for Humans: Positive

Beyond its role in its ecosystems, this species has little positive effect on humans. Some animals are sold in the pet trade.

Economic Importance for Humans: Negative

Even though a bite from Heloderma horridum is not usually life threatening to humans, the wound must still be medically treated. This can be considered a negative impact upon health care costs for treatment. Unprovoked attacks by this species on humans are extremely rare, and may never occur. Nearly all documented bites occured when the victim tried to handle or disturb an animal.

Conservation Status

Humans are not only a threat to the Mexican Beaded Lizard because they kill them for fear of their venom, but are also destroying the habitat upon which these lizards depend.

Therefore Heloderma horridum is protected under the Convention on International Trade in Endangered Species of Wild Flora and Fauna (Appendix II) and are classified as "Vulnerable" by the International Union for the Conservation of Nature and Natural Resources (IUCN).

Other Comments

When the Mexican Beaded Lizard inflicts a vigorous bite, the victim goes through many stages. The first stage is the inability to sit or stand. The animal becomes drowsy and then experiences paralysis. Respiration becomes slow and labored until the heart exerts an increase of activity. The arterial pressure begins to take a great fall due to vascular dilation and the prey dies. Although the venom may have these effects on its small prey, bites are rarely fatal to humans in good health.

Contributors

Nichol Stout (author), University of Michigan-Ann Arbor.

Glossary

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

carnivore

an animal that mainly eats meat

chaparral

Found in coastal areas between 30 and 40 degrees latitude, in areas with a Mediterranean climate. Vegetation is dominated by stands of dense, spiny shrubs with tough (hard or waxy) evergreen leaves. May be maintained by periodic fire. In South America it includes the scrub ecotone between forest and paramo.

desert or dunes

in deserts low (less than 30 cm per year) and unpredictable rainfall results in landscapes dominated by plants and animals adapted to aridity. Vegetation is typically sparse, though spectacular blooms may occur following rain. Deserts can be cold or warm and daily temperates typically fluctuate. In dune areas vegetation is also sparse and conditions are dry. This is because sand does not hold water well so little is available to plants. In dunes near seas and oceans this is compounded by the influence of salt in the air and soil. Salt limits the ability of plants to take up water through their roots.

heterothermic

having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

mountains

This terrestrial biome includes summits of high mountains, either without vegetation or covered by low, tundra-like vegetation.

native range

the area in which the animal is naturally found, the region in which it is endemic.

scrub forest

scrub forests develop in areas that experience dry seasons.

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

terrestrial

Living on the ground.

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

venomous

an animal which has an organ capable of injecting a poisonous substance into a wound (for example, scorpions, jellyfish, and rattlesnakes).

References

Ditmars, Raymond L. 1936. The Reptiles of North America. Doubleday, Doran and Company Inc. Pgs. 87-89, 92

Foster and Cards. 1994. A Field Guide to Venomous Animals and Poisonous Plants. Houghton Mifflin Company. Pg. 12

Grzimek, Dr. H.C. Bernhard. 1975. Animal Life Encyclopedia. Volume 6 Pgs. 321-322 and 151-152. Van Nostrand Reinhold Company.

Halliday, Dr. Tim R. and Adler, Dr. Kraig. 1986. The Encyclopedia of Reptiles and Amphibians. Pgs. 86-94 and 105-106

http://www.scz.org/animals/l/beaded.html (Sedgwick County Zoo, 1995)

Angeli, S. 2004. "About Beaded Lizards" (On-line). "HorridumAngeli" Reptiles. Accessed June 16, 2004 at http://www.helodermahorridum.com/beaded_lizard.php.

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