00:00 This episode of "Throughline" investigates the hidden life of rats, starting with a rat safari through New York City with rodentologist Bobby Corrigan.

04:33 Rats cause significant mental stress on humans and are found in almost every major city around the world, making them one of the most successful species on the planet.

09:19 New York City is primarily overrun by one kind of rat, the brown rat, and researchers are trying to determine the origin of these rats by comparing their DNA to rats from around the world.

14:20 The brown rat originated in East Asia thousands of years ago, likely in a region between Northern China and Mongolia, and began utilizing human food when agriculture began in China 11,000 years ago, leading to their commensal relationship with humans and eventual spread around the world through ships during the age of conquest.

19:19 Rats have been companions to humans for a long time, as they are intelligent, adaptable, and social creatures, but they also carry viruses, bacteria, fungi, and protozoans that make them unwelcome in cities, as seen with the spread of the plague in Europe in the 1300s.

25:40 In Victorian England, rats were a constant threat and were carried around by rich people in cages and forced to fight to the death in underground gambling rings, with Jack Black being one of the most famous rat handlers of the time.

31:10 Jack Black, a self-made man and rat catcher in Victorian England, caught rats alive using special bait and sold them as pets or for rat baiting, blurring the line between friend and foe and rebranding rats as more than just vermin.

37:03 In the 1960s, as fears about overpopulation grew, rats were no longer seen as just vermin, but as test subjects in laboratories that could help answer questions about overpopulation and the effects of living in crowded environments.

43:28 In an experiment with rats, it was found that overcrowding can lead to abnormal behavior, violence, and even extinction of the population, which parallels the perception of cities as dirty, crowded, and dangerous despite declining poverty and infant mortality rates.

48:33 Calhoun's rodent experiments were widely discussed and cited, but he began to question his own conclusions and believed that the rat dystopia he observed was not inevitable, leading him to redesign the physical space and try to create a harmonious, healthy environment with increased population density but without causing stress-related illnesses. Despite his efforts, the narrative of overcrowding and stress in dense environments persisted, leading to hysteria, but his lasting impact was the connection he drew between rats and humans.