Bored office workers yawn, and so do paratroopers seconds before they jump. This episode explores chasmology, the study of yawning, starting with the reflex itself: a slow inhalation, a peak of involuntary muscle stretching called the acme, then a fast exhalation, a sequence seen in fetuses on 4D ultrasound at 30 weeks. The airway dilates to three or four times its size, and the rumble in your head is the tensor tympani muscle in the middle ear. The old idea that yawning pulls in oxygen fails the test: changing oxygen or carbon dioxide levels leaves yawn frequency unchanged.

The leading explanation is brain cooling. At SUNY Albany, Gordon Gallup’s team found that people with cold packs on their foreheads, or breathing only through their noses, caught far fewer contagious yawns. Yawning also raises cortisol, which may explain the skydiver. Contagion follows emotional closeness, from kin to strangers, appears around age six, and even crosses to dogs. Elsewhere in the animal kingdom yawns are threats, courtship and jaw resets, while human cultures treated them as an open door for spirits and plague.

• A University of Pisa study ranking who catches yawns fastest: family, then friends, then acquaintances, then strangers
• Puppies only start catching their owners’ yawns at about seven months old
• Baboons, guinea pigs and Siamese fighting fish yawn to display weapons, while Adélie and emperor penguins yawn while courting
• Polydore Vergil’s account of people making the sign of the cross over yawning mouths during the plague
• George Washington’s civility rule to yawn privately behind a hand, and judges who can hold loud yawners in contempt