Philippine Tarsier Has Bat-Like Pitch
February 8, 2012

Philippine Tarsier Has Bat-Like Pitch

One of the world´s smallest primates, the Philippine Tarsier (Tarsius syrichta), has the world´s highest pitched vocalization of any primate ever documented, according to a study published Wednesday.

That call, however, is so high-pitched that it is inaudible to human ears. It is a big voice for such a small creature, no bigger than the size of a man´s hand. It shrieks out the vocalization as a warning of danger or a call to dinner.

“Tarsiers are among only a handful of mammals that are known to communicate in the pure ultrasound,” Marissa Ramsier, lead author of the study, told Jennifer Viegas of Discovery News. The study is published in the Royal Society´s Biology Letters.

Ramsier, an evolutionary biologist for the Biological Anthropology Research Lab at Humboldt State University in California, noted the ironic discovery in an animal that has always been considered a silent nocturnal creature. “It turns out that it´s not silent. It´s actually screaming and we had no idea,” she told Stephanie Pappas of LiveScience.

The highest pitch a human ear can detect has a frequency of around 20 kilohertz. The Philippine Tarsier can emit calls up to 70 kHz and can hear up to 90 kHz. This hearing ability puts the creature´s hearing abilities in the same range as bats, and far beyond those of any other primate ever known. Even dogs, who tend to respond to higher pitches than humans can tune in to, only hear in the 22 to 23 kHz range.

These tiny, but shrilly creatures of the night are found only on islands of the Philippines. They are strictly nocturnal. They lack a tapetum lucidum, the layer of tissue in the eyes of animals such as cats that allows for strong night vision. Instead, the tarsier has giant, lemur-like eyes to navigate its nighttime surroundings.

“They´re most closely related to the group that includes monkeys, apes and humans, but in many ways they resemble lemurs and lorises,” Ramsier told Pappas.

Ramsier said these small animals made tempting subjects to investigate hearing capacity in the primate world. “People had pretty much thought that monkeys and other primates hear the way we do, but that was based on limited data,” Ramsier told Pappas.

But because the Philippine Tarsier is endangered and does not respond well to captivity, Ramsier and her colleagues had their work cut out for them. They were able to capture six tarsiers on the island of Mindanao and placed each one in a sound-muffling chamber, where they were exposed to noises of varying frequencies from a speaker inside the chamber.

The team placed non-invasive electrodes on the primates to measure brain response to the sounds. The test is more or less the same found in hospitals used to determine whether newborn babies have the full range of hearing, she noted.

Each test took about an hour, after which the tarsiers were released back into the wild, Ramsier said.

The team found that the tarsiers responded to pitches as high as 91 kHz -- far higher than the 65 kHz range of the Galago, previously believed to be the highest range for a primate.

But the team wanted to know if the primate only listened in high-pitched frequencies, or if it actually called out in ultrasonic noises as well.

Ramsier said another researcher, Texas A&M anthropologist Sharon Gursky-Doyen had been researching on the islands of Bohol and Leyte and “happened to notice that these animals were opening their mouths and she was hearing nothing coming out.” She happened to have a bat detector on hand and “she was able to get that vocalization on a recording,” Ramsier noted.

In all, researchers were able to capture the calls of 35 wild tarsiers using an ultrasonic microphone. They discovered eight of the animals cried out in pure ultrasound, ranging from 67 to 79 kHz, with most ranging right around 70 kHz.

“This is the first time that a primate has been shown to use vocalization that is only in the ultrasound, so this call doesn´t use anything in the lower frequencies that we can hear,” Ramsier said.

The Philippine Tarsiers join a select group of mammals that have the ability to use ultrasonic frequency calling. This group includes bats, rodents, cetaceans, and domestic cats. “Kittens produce a pure ultrasonic call from about 2-6 weeks of life when they are first exploring their environment, and a mother cat produces its own purely ultrasonic call in response to the kitten,” explained Ramsier to Discovery's Viegas.

Cats at these times of life can therefore communicate in ways not detected by their owners, “unless they follow them around with a bat detector,” she said.

Using ultrasonic frequencies to communicate could be a useful tool preventing detection from predators, prey and competitors. It could also enhance energetic efficiency and improve detection against low-frequency background noise, Ramsier said.

Other primates have ultrasonic elements to their calls, but the dominant frequencies are well within human hearing range, according to the authors.

Chris Kirk, an associate professor in the department of anthropology at the University of Texas, Austin, told Discovery News that this “study is important because it expands the number of primate species that concentrate a large part of the acoustic energy in their vocal communications within the ultrasonic range.”

The documented tarsier calls seem to conceal it from being detected by others within its auditory range, given the call seems to have “ventriloquial” properties, he said. “In fact, it looks an awful lot like the ℠seet´ alarm calls or (those of) passerine birds, but scaled up to a higher frequency range.”

Philippine tarsiers are odd primates, Ramsier said, so they may be unique in their ultrasonic abilities. But it´s also possible that other primates are talking on channels humans have yet to notice. “There could be a whole world of signals out there just waiting for us to hear them “¦ We just have to listen,” she concluded.


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