Acoustic communication of Taita tree hyrax

By Hanna Rosti26.12.2020 ( 6 )

Dendrohyrax sp.

This article focuses on incredible variable acoustic communication of Taita tree hyrax. If you are interested in other peculiar features of tree hyraxes please read this previous article of Taita tree hyraxes.

Taita tree hyrax is believed to be undescribed species for the science based on these calls. For nocturnal cryptic animals, that are difficult to observe, call analysis has been the typical method of making taxonomic evaluation. Each species have their typical calls, especially calls that are used for so called ‘advertising’. These are calls that are used to draw attention of other con-species.

Taita tree hyrax female. Life of tree hyraxes are almost completely unknown. We don’t know if both sexes are calling, or are they just males. Tree hyraxes are believed to be solitary, but continues calling is difficult to explain for animal that is believed to be solitary.

Basic territorial call that is heard frequently in the forests of Taita Hills is ‘strangled thwack’. This call is most likely call that tree hyraxes use to claim their territory. Tree hyraxes eat leaves, and they are dependent on tree cavities for daytime sleeping sites.

Strangled thwack is more than 100 dB call. Twist in the call, and noisy elements make sure that this call doesn’t go unnoticed.

Strangled thwacks are used in Taita Hills and in East Usambara Mountain. East Usambara Mountains are 160 km from Taita Hills. East Usambara Mountains are also part of Eastern Arch Mountains, and it is located in Tanzania.

Based on this very unique call type, Taita tree hyrax is believed to be tree hyrax species that is not yet described scientifically (Rosti et al. 2020).

*Strangled thwack call from Taita Hills. Horizontal axel time (s), vertical axel frequency (kHz) A) One strangled thwack call. Call duration about 1 s. Notice* *changes (frequency modulation) in the strongest frequency of the call.* *B) Sequence of strangled thwacks. Usually tree hyraxes use these calls in about 10 call sequences where they counter call with other tree hyraxes.*

Strangled thwack sequence from one animal

Usually tree hyraxes are counter calling with each other.

Taita Tree hyrax counter-calling

Another call type that tree hyraxes are using in Taita Hills is called hac. Hac call is used as territorial call. It is not quite as loud and frequency range is lower. Probably hac call is used when calling animal becomes little tired, or it want’s to use call type that is slightly less demanding.

*Spectrograms of hac calls, time (s) on horizontal axel, frequency (kHz) on vertical axel.* *Hac call has smaller frequency range and it can be continued much longer, sometimes sequence may have more than hundred calls.*

Songs from Taita tree hyrax

Taita tree hyrax sings songs. Songs have been previously found from rock hyrax (Procavia capensis) (Koren et al. 2008).

Singing mammals are very rare. Gibbons and whales sing. But I don’t know other singing mammals.

Singing Taita tree hyrax. Duration on horizontal axel (s) and frequency on vertical axel (kHz). These songs may last for more than 12 minutes.

Sound sample from Taita tree hyrax song.

Purpose of these songs are unknown. Probably they are used by males as they advertise their fitness for females that might be available for mating. In Taita Hills, remaining forests are packed with tree hyraxes and most likely all available territories are taken. It might mean that moving from one place to another could be difficult. Tree hyraxes may have to take a risk and come down to the ground where they are in greater danger to be hunted by genets. Acoustic communication is used as means to communicate with other tree hyraxes.