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Tree hyraxes (Dendrohyrax spp.) are highly vocal, elusive nocturnal mammals that live in forests across much of Sub-Saharan Africa. Here, we studied how tree hyrax acoustic communication activity varies during the year in a moist montane forest in Taita Hills, Kenya. Our study increases the knowledge about the seasonal and diel variation in tree hyrax calling activity and on the impact of exogenous factors on the calling behavior of the species. A group of tree hyraxes called 700–2000 times during the night, and calling rate was highest during dry season months. Calling activity was bimodal, with peaks between 19.00 and 20.00 h in the evening and between 04.00 and 05.00 h in the morning; however, counter-calling bouts occurred throughout the night. Singing peaked in May, possibly coinciding with the species' mating season. Calling activity decreased during full moon phases, probably because the animals felt vulnerable to predation. Tree hyrax calling was suppressed by rain, as rainfall is a powerful acoustic competitor. Annual changes in night temperature, the background chorus of vocalizing orthopterans, or the competing calls of nocturnal primates did not reduce tree hyrax calling rates. The latter suggests that sympatric nocturnal mammals have adapted to each other's calls as an intrinsic feature of the soundscape of their forest habitat. [ABSTRACT FROM AUTHOR]