Tarsiers are among the world’s smallest primates. Only five inches tall, with big eyes and ears, some species from Borneo and the Philippines have been thought to be silent, though other species have calls that are audible to humans and used for sending out alarms about danger, or for social interactions. Researchers from Dartmouth University have now found that Tarsius syrichta, one tarsier species in the Philippines, uses ultrasonic vocalizations to communicate. Anthropology professor Nathaniel Dominy studied the Philippine tarsier’s hearing and vocalizations and describes their way of communicating as “extreme, and comparable to the highly specialized vocalizations of bats and dolphins, which are used primarily for echolocation.”
Dominy and other researchers suspected that the Philippine Tarsier did have some form of communication, as do the other species. Recent technical advances helped them to study tarsiers’ communication:
They found “an audible range that extended substantially into the ultrasound,” reaching a high of 91 kilohertz (kHz), “a value that surpasses the known range of all other primates and is matched by few animals.”
They also used a microphone and recording unit capable of registering sounds up 96 kHz. The upper limit of human hearing is generally set at 20 kHz, and frequencies above this limit are classified as ultrasound. In the field, the team recorded the sounds of 35 wild tarsiers from the islands of Bohol and Leyte with this equipment, documenting eight individuals giving out a purely ultrasonic call at approximately 70 kHz. The tone-like structure of the call resembles those of other tarsier species, but none were purely ultrasonic.
Noting that the tarsiers issued their ultrasonic calls when humans were near, the researchers speculated that they were voicing alarm.
In fact, Dominy and his colleagues suggest that “there may be selective advantages to vocalizations in the pure ultrasound” as these provide “private channels of communication with the potential to subvert detection by predators, prey, and competitors.” Tarsiers dine exclusively on insects such as moths and katydids, which emit sounds in ultrahigh frequencies. As Daniel Strain writes in Science, “because tarsiers’ perky ears are so sensitive, they may be able to intercept this chatter at night—then zoom in for the kill.”
In addition, because tarsiers’ “nails-on-a-chalkboard trills” are too high-pitched for predators such as birds to hear, their ultrasonic vocalizations make it possible to communicate without being noticed. Silence, or what seems like silence to most ears, can indeed be golden.
The researchers’ findings has been published in the Royal Society journal Biology Letters.
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Photo by Roberto Verso