A huge-eyed little primate of the Philippines can communicate in pure ultrasound — issuing calls so high-pitched that human ears can't detect them.
Study researcher Marissa Ramsier noted the ironic discovery in an animal that has always been considered a quiet night creature. "It turns out that it's not silent. It's actually screaming and we had no idea," said Ramsier, an evolutionary biologist at Humboldt State University in California.
The shrillest noise a human can hear has a frequency of about 20 kilohertz. The Philippine tarsier can hear up to 91 kilohertz, and it cries out in the 70-kHz range. Those numbers put the tarsier's hearing abilities in the same range as bats and far beyond those of any other primate ever known.
Philippine tarsiers are found only on islands of the Philippines. They're one of the smallest primate species: When full-grown, they're about the size of a man's fist.
Though they are nocturnal, the tarsiers lack a tapetum lucidum, the layer of tissue in the eyes of animals such as cats that allows for strong night vision. Instead, Philippine tarsiers have giant, lemurlike eyes.
"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 LiveScience.
All this weirdness made Philippine tarsiers tempting subjects for investigating primate hearing, she said.
"People had pretty much thought that monkeys and other primates hear the way we do, but that was based on limited data," Ramsier said.
Tarsiers are tricky test subjects, though, because they are endangered and don't respond well to captivity. Ramsier and her colleagues had to be creative and quick. They captured six tarsiers on the island of Mindanao and placed each one in a sound-muffling chamber, where it was exposed to noises of varying frequencies from a speaker inside the chamber. Noninvasive electrodes measured their brain response to the sounds. It's more or less the same test that hospitals give to determine whether newborn babies have the full range of hearing, Ramsier said.
The tests took just about an hour each, Ramsier said, after which the tarsiers were released back into the wild.
Tarsiers were found to hear pitches as high as 91 kilohertz — far higher than the galago (also called bush baby), whose 65 kHz limit was thought to be the highest for a primate.
But hearing was just half of the equation. To find out if the tarsiers also could make ultrasonic noises, the researchers got out the recording equipment. On the islands of Bohol and Leyte, Texas A&M anthropologist Sharon Gursky-Doyen "happened to notice that these animals were opening their mouths and she was hearing nothing coming out," Ramsier said. "She had the foresight to get hold of a bat detector, and she was able to get that vocalization on a recording." [Hear a slowed-down recording]
All told, the researchers captured the calls of 35 wild tarsiers using an ultrasound microphone. They found that eight of the animals cried out in pure ultrasound, ranging from 67 to 79 kHz, with the most common frequency coming in around 70.
The results are being published today (Feb. 7) in the journal Biology Letters.
"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.
Other primates have ultrasonic elements to their calls, but the dominant frequencies have been well within human hearing range, Ramsier said. The only other mammals known to use ultrasonic communication are whales and dolphins, some bats and rodents and domestic cats (which communicate with their kittens in ultrasound). [10 Facts for Cat Lovers]
The researchers don't know for sure why tarsiers evolved such high-pitched cries, but the cries may act as a private line for tarsier-only conversations. By communicating in high pitches, the tarsiers may be able to keep predators from overhearing their "talk," Ramsier said. They also may be using their hearing skills to listen for insects, some of which produce their own ultrasonic sounds.
"It's this duel benefit," she said. "They can communicate without predators hearing them and also locate some potential food sources."
(Though bats use their ultrasonic cries for echolocation, Ramsier doubts the tarsiers can do the same.)
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," she said. "We just have to listen."
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Stephanie Pappas is a contributing writer for Live Science, covering topics ranging from geoscience to archaeology to the human brain and behavior. She was previously a senior writer for Live Science but is now a freelancer based in Denver, Colorado, and regularly contributes to Scientific American and The Monitor, the monthly magazine of the American Psychological Association. Stephanie received a bachelor's degree in psychology from the University of South Carolina and a graduate certificate in science communication from the University of California, Santa Cruz.