Searching for the forest ghosts: group counts and polyspecific associations of the endangered Rungwecebus kipunji in the Udzungwa Mountains, Tanzania

The IUCN-Endangered kipunji (Rungwecebus kipunji) is one of Africa's rarest primates, restricted to two isolated populations in Tanzania. The Udzungwa population is enigmatic, confined to a small area within a large, old-growth forest and consists of fewer than 100 individuals. Apart from baseline data from 2006, ecological knowledge remains scant. We present new data from three standardized sweep censuses conducted between 2013 and 2024 in Ndundulu forest along 42-45 linear transects of 2 km in length. Per census, only 4 to 6 kipunji groups were recorded, within a range of 10.42 km² which includes opportunistic sightings. Encounter rate averaged 0.04 groups/km, 8 to 12 times less than for the three other diurnal primates co-occurring there (Colobus angolensis palliatus, Cercopithecus mitis monoides, Piliocolobus gordonorum). Mean (SD) minimum group size was 17 ± 2 individuals, giving a crude and minimum abundance estimation per census that varied from 51 ± 6 to 102 ± 12 individuals. Despite occurring in high-quality, undisturbed and well-protected habitat, kipunji in Udzungwa show puzzlingly low density and a small range. Kipunji were observed in association with the other three diurnal primates in 43.8% of encounters, mainly forming dyads with Tanzania Sykes' monkeys (42.9%) and, less frequently, with Udzungwa red colobus (14.3%). However, Sykes' monkeys associated the least and were the only species whose relative abundance increased, suggesting niche dominance that may particularly affect the kipunji due to shared dietary preferences. We warn that kipunji persistence in Udzungwa is at risk and urge intensified monitoring and continued protection of the Kilombero Nature Forest Reserve.