Karen Strier knew something was wrong as soon as she entered the patch of Brazil’s Atlantic Forest in the southeastern state of Minas Gerais where she has been studying primates for 35 years. Instead of the usual deafening roar of howler monkeys, some of the most common monkeys in the region, there was an “eerie silence, like when something is wrong,” says the University of Wisconsin–Madison anthropologist. “It was stunning.” The animals had been silenced by the yellow fever virus, which had wiped out most of the local population of 500 howler monkeys.
