Biologist Carlos Ruiz has spent a quarter-century working to save golden lion tamarins, the charismatic long-maned monkeys native to Brazil´s Atlantic Forest. Thanks to painstaking reforestation efforts, the population of these endangered monkeys was steadily growing until an outbreak of yellow fever hit Brazil in 2018, wiping out a third of the tamarins. Undeterred, Ruiz´s team devised an ambitious new experiment: This spring, they would start vaccinating many of the ...