Gila monsters (Heloderma suspectum) are one of only two poisonous lizards in the world. They produce venom in salivary glands in their lower jaws. The venom flows by capillary action along grooves in their teeth, giving the lizard a poisonous bite. The venom acts on the prey animal's nervous system, preventing the heart and lungs from working. For a healthy human, a bite from a gila monster will be very painful but not life-threatening.

Gila monsters are most active at night. They shelter from the heat of the day in rocky crevices or burrows abandoned by mammals. However, in northern parts of their range, the lizards are completely inactive for several months during the winter. Inactive individuals rely on fat stored in their tails to keep them alive when they cannot feed.

Gila monsters mate in springtime, and their copulation can last for over an hour. The eggs develop inside the females for about ten weeks. They then bury the eggs in areas that are often bathed in sunlight. The eggs incubate for up to ten months.

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