COS 213-1 - Yellow fever causes a drastic decline in an island population of Alouatta guariba clamitans (Primates, Atelidae)

Abstract: Southern howler monkey (Alouatta guariba clamitans) is among the twenty-five most endangered primate species in the world mainly due to its high susceptibility to the yellow fever virus. Beginning in late 2016, Brazil faced the worst outbreak of yellow fever in recent decades, mainly reaching the south and southeast regions of the country, with drastic impacts on the population size of howler monkeys (Alouatta spp.). Here, we present the monitoring initiatives carried out with the population of southern howler monkeys at Ilha Grande, in the state of Rio de Janeiro-Brazil, one of the largest insular remnants of the Atlantic Forest (19 km²) that homes an isolated population of this species. Pre-yellow fever (2003-2005 and 2009-2010), the estimated population was 505 ± 211 individuals and abundance 0.40 groups/10km walked. Post-yellow fever, the attempt of estimate population size (using the same protocol) was unviable due to few encounter rates (n=2), however abundance is 0.06 groups/10km walked. Since 2019 until now, the field work effort addressed to look for southern howler monkeys’ groups totalizes 308,60 km of linear transection, approximately 50 km drone overflight (using a thermal camera), 103 interviews, and active search. We have found only 10 groups of 2 to 7 individuals, which were found mainly through local residents’ information. Even not being possible to estimate the magnitude, our results suggest a drastic population decline. Fortunately, it is now possible to observe some groups of this species with infants in restrict regions and the return of vocalizations, but the scenario is extremely worrying due to the considerable risk of a new outbreak. In front of the drastic scenario of population decline, we urgently need to address some gaps: (1) Is this population of southern howler monkeys viable in the long term? (2) How does this population differ from continental populations in terms of genetic heritage? (3) Did yellow fever cause historical population bottlenecks? We call upon scientists to help provide the answer to these questions and to support the long-term studies with this isolated population. Considering the difficulty of finding the remaining groups as well as the topography of the study area, which makes field work logistically challenging, we recommend using integrative approaches with residents (citizen science), which would allow us to increase sampling and monitoring efforts. Moreover, we highlighted the importance of ex-situ and in-situ management programs for southern howler monkeys’ conservation, including a vaccination program against yellow fever.