The black mamba is olive, khaki, or gunmetal, never black. Its name comes from the inky interior of its mouth, which it shows only as a last warning. This episode separates the legend from the biology of Africa’s longest venomous snake, an animal that averages two to three meters, weighs only a few pounds, tops out near 16 kilometers per hour, and returns to the same termite mound or rock crevice day after day. In the wild it seldom lets a human within 40 meters before fleeing.

The danger is real all the same. A cornered mamba can lift 40 percent of its body off the ground and deliver rapid, repeated bites, and its venom works without the swelling and tissue damage most people expect from a snakebite. Dendrotoxins overexcite the nervous system, alpha neurotoxins then block its signals, and collapse can follow within 45 minutes. The episode walks through fatal and survived bites, the puzzle of the dry bite, and a discovery that may turn this venom into medicine.

• Mongooses and honey badgers resist the venom because their muscle receptors have mutated so the alpha neurotoxins cannot bind.
• An average bite delivers over 100 milligrams of venom, while the lethal dose for a human is roughly 0.32 milligrams per kilogram of body weight.
• In 2008 a 28-year-old British trainee safari guide was bitten on the finger, felt almost nothing, sat through a lecture, and collapsed within the hour.
• A South African parks executive bitten in 1998 stayed calm, used a tourniquet, and still spent three days on a ventilator before recovering.
• Researchers decoding the venom in 2015 isolated mambalgins, peptides that relieve pain as strongly as morphine without respiratory depression, tolerance, or addiction.