Between 2011 and 2016, approximately 50% of siamangs in the Way Canguk Research Area disappeared, including members of 7 of 12 habituated groups. Demographic data from 1998 to 2015 confirm that the population decline in the habituated groups reflects a larger trend in the local population. There was no evidence of hunting of primates in the area, and ecological data do not suggest substantial changes in food availability or predation pressure during this period. From 2011 to 2014, we monitored the habituated groups only intermittently, and most deaths or disappearances were not observed. However, in 2014-2016, we monitored some groups more intensively, and observed 2 individuals with symptoms including whitened skin on the face, hands, and abdomen, hair loss, swelling of the face, frequent scratching, and lethargy. One affected individual disappeared days after this observation, while the other survived. The spatiotemporal pattern of disappearances in the habituated groups was consistent with that expected if the deaths resulted from disease transmission among neighbouring groups. Thus, the available evidence, while preliminary, suggests that a local disease epizootic may have been a factor driving the population decline. We recommend that researchers establish monitoring protocols to better understand primate disease epidemiology and to guide conservation management.

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