Intact, under-patrolled forests harbor widespread prey but a male-biased tiger population in the Ulu Masen Ecosystem, Sumatra, Indonesia.

Conservation of threatened species is dependent on consistent population monitoring. We present the first status assessment of critically endangered Sumatran tigers (Panthera tigris sumatrae) and their prey in the Ulu Masen Ecosystem, Aceh, Indonesia. Our estimates of tiger habitat use are the first reported for a Sumatran ecosystem unprotected at the national level. During 6,732 trap nights accumulated over 23 months of camera-trap monitoring in 2020 and 2022, tigers were detected 39 times at 16 of the 52 stations. We identified 11 individual tigers but sex ratios were highly skewed: 8 males, 1 female, and 2 individuals of unknown sex. Cubs were not photographed either year and we did not observe evidence of tiger reproduction. Tiger habitat use (Ѱ = 0.52, SE = 0.15) was negatively influenced by human disturbance and positively influenced by elevation but those associations were not significant. Our study documents a widespread prey base but uncovers demographic characteristics of tigers indicative of heavy poaching pressures. We conclude that tiger-targeted protection is urgently needed to ensure the species' persistence in Ulu Masen which, together with the adjacent Leuser Ecosystem, represents the largest contiguous tiger conservation landscape remaining in Sumatra.