Mexican Beaded Lizard

They are the two well-known members of the same genus, Heloderma, and the only lizards with a long-established venomous bite. The Mexican beaded lizard is the larger of the two, reaching about 90 centimetres against the Gila monster's 55 or so, and it is longer-tailed and more slender in build. Its colour is mostly dark brown to black, sometimes with faint dull-yellow crossbands, whereas the Gila monster has a bolder, higher-contrast pattern of pink or orange and black. The beaded lizard lives in tropical dry forest and thorn scrub along the Pacific side of Mexico and into Guatemala, while the Gila monster ranges through the deserts of the southwestern United States and northwestern Mexico. Both are slow, secretive, egg-raiding predators.

Potentially very painful and unpleasant, but rarely life-threatening to a healthy adult, and almost never a risk to the public. The lizard is not aggressive, is slow-moving, and gives ample warning by hissing and gaping before it bites. When it does bite it clamps down hard and holds on, chewing venom into the wound through grooves in its lower teeth, so bites tend to be tenacious and hard to remove. Symptoms include severe burning pain, marked local swelling, weakness, nausea, sweating, and a sharp fall in blood pressure that can cause fainting. Deaths are exceedingly rare and historically involved people who were already unwell, intoxicated, or who had prolonged bites. Essentially all recorded serious bites are to keepers, collectors, or zoo staff handling the animals, not to people encountering wild ones.

Yes, and it is one of the best-known examples of a drug derived from animal venom. In the 1990s, researchers studying Heloderma venom isolated a peptide called exendin-4, which closely mimics a human gut hormone (GLP-1) that stimulates insulin release after eating but, unlike the human hormone, is not quickly broken down in the body. A synthetic version was developed as the drug exenatide (marketed as Byetta and Bydureon), approved in 2005 for type 2 diabetes, and it helped open up the now very large GLP-1 receptor agonist class of medicines, which also includes drugs later used for weight loss. The lizard makes the compound in its salivary venom glands; why it evolved a GLP-1 mimic is not fully understood, but it may help regulate the lizard's own metabolism between its infrequent large meals.

It is kept by specialist venomous-reptile keepers, and captive breeding occurs, but it is emphatically not a general pet. In most places, keeping a venomous lizard requires a permit, secure caging, and often proof of experience and a relationship with a hospital, because although the bite is seldom fatal it is medically serious and there is no specific antivenom. Beaded lizards are strong, long-lived (20 to 30 years), need a large heated enclosure with a deep substrate for burrowing and a big water container, and while some become calm they can deliver a sudden, tenacious bite. The isolated Guatemalan population is Endangered and protected, so any beaded lizard in the trade should be documented captive-bred stock of the Mexican forms.