Dramatic decline in a titi monkey population after the 2016-2018 sylvatic yellow fever outbreak in Brazil

Platyrrhini are highly vulnerable to the yellowfever (YF) virus. From 2016 to 2018, the Atlantic Forest of southeast Brazilfaced its worst sylvatic YF outbreak in about a century, thought to have killedthousands of primates. It is essential to assess the impact of this epidemic onthreatened primate assemblages to design effective conservation strategies. Inthis study, we assessed the impact of the 2016-2018 YF outbreak on ageographically isolated population of Near Threatened black-fronted titi monkeys(Callicebus nigrifrons) in two Atlantic Forest patches ofthe Santuário do Caraça, MG, Brazil. Extensive pre-outbreak monitoring,conducted between 2008 and 2016, revealed that the home range and group sizes ofthe population remained stable. In 2016, the population size was estimated at53-57 individuals in 11-12 groups. We conducted monitoring and playback surveysin 2019 and found that the population had decreased by 68% in one forest patchand completely vanished in the other, resulting in a combined decline of 80%. Wediscuss this severe loss of a previously stable population and conclude that itwas highly likely caused by the YF outbreak. The remaining population is atrisk of disappearing completely because of its small size and geographic isolation.A systematic population surveys of C. nigrifrons, along other sensible Platyrrhini species, is needed to re-evaluate theircurrent conservation status.