Animal Diversity Web

Ge­o­graphic Range

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

• Biogeographic Regions
• nearctic
  • native

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.

• Habitat Regions
• temperate
• tropical
• terrestrial

• Terrestrial Biomes
• desert or dune
• chaparral
• scrub forest
• mountains

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.

• Other Physical Features
• heterothermic
• venomous

• 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

• Average lifespan
  Sex: female
  Status: captivity

  22.8 years

  Max Planck Institute for Demographic Research

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).

• IUCN Red List

  Least Concern
  More information

• IUCN Red List

  Least Concern
  More information

• CITES

  Appendix II

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.

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.