Cryptic Communication in a Montane Nocturnal Haplorhine, Tarsius pumilus.

Crypsis, including visual and auditory concealment, usually manifests in primates as an antipredator strategy. Other factors may also influence cryptic communication style, including habitat structure and phylogenetic history. Compared to less cryptic lowland Sulawesian tarsiers, montane pygmy tarsiers (Tarsius pumilus) exhibit a communication style that lacks scent marks and lower-frequency vocalisations. This study examines why auditory crypsis occurs in montane tarsiers more so than in larger tarsier species and presents the only known spectrograms of T. pumilus in the field. T. pumilus regularly exhibited calls with a dominant frequency of 60-80 kHz (n = 4) in both social situations (duet calls) and stressed contexts. These results indicate that highland, smaller-bodied tarsiers habitually communicate at high frequencies in contexts where Sulawesian and Philippine tarsiers use lower frequencies. While predation threats and habitat acoustics may influence the use of high-frequency vocalisations, this study found that T. pumilus shows an expected relationship between vocal frequency and body mass. These traits may represent a retention of primitive haplorhine traits rather than derived adaptations to a montane environment.