SYDNEY: A shy, wide-eyed and nocturnal species called the Phillipine tarsier is the first primate to be identified as having the ability to communicate in purely ultrasonic frequencies.
A new study published in Biology Letters today has revealed that the endangered Philippine tarsier (Tarsius syrichta) communicates using sound with a frequency greater than 20 kilohertz. The finding contradicts conventional thinking that all primate vocalisations are audible to humans.
“We found that the Philippine tarsier can hear higher pitched sounds than any other primate, and that it also has the highest pitched primate vocalisation ever documented! What we thought was a quiet species may actually be a species that has a variety of vocalisations that we had no knowledge of, simply because we could not hear them,” said first author and biological anthropologist Marissa Ramsier from Humboldt State University in Arcata, California.
“Although it is possible that the Philippine tarsier is unique in its ability, it is exciting to think about all of the animals, primates and non-primates alike, that may be communicating in ways that we have not yet realised. Many of my colleagues have noted silent mouth-opening behaviours in a wide range of species. There could be entire sets of signals out there waiting to be heard!” she added.
Tarsiers are good listeners
A select group of mammals including bats, rodents, domestic cats and aquatic cetaceans (whales, dolphins and porpoises) are known to communicate with pure ultrasound, but until now researchers were unaware that primates had the same capability.
Over the past four years, Ramsier and colleagues have studied the hearing sensitivity of over 20 primate species. “We actually know very little about how and why primate species vary in their hearing abilities. [It turns out that] even closely related species can vary in their auditory sensitivity, likely owing to differences in diet, competition, predator pressure, and habitat,” she said. “We decided to look at the tarsier because it is a very unique primate.”
The tarsier’s nearest relatives belong to a group of primates called Anthropoidea including monkeys, apes and humans. However, as a petite, nocturnal species that eats mainly insects, the tarsier has more in common with species in Prosimii, a distantly related primate group that includes lemurs. And unlike most nocturnal animals, the tarsier also lacks a tapetum lucidum, which is a layer of tissue many vertebrate animals have in their eyes, which creates an effect known as ‘eye shine’. Rather than maximising light detection in this way, tarsiers have the largest eyes relative to body size of any mammal.
Minimally invasive testing
The researchers tested the hearing and vocalisations of six individuals found in the wild using technology developed by the U.S. Navy Marine Mammal Program. The technology measures the response of the brain stem to auditory stimuli. The stimuli comprised a series of tones ranging in pitch and loudness, played through a speaker.
The neurologic response of the tarsiers was then measured using electroencephalography – the same technique used in medicine where electrodes attached to the head record brain activity. The minimally invasive technique enabled measurements to be performed within about an hour and without animal training, after which each individual was released back into the wild, unharmed.
Ramsier and her co-researchers discovered the tarsiers could hear sounds up to 91 kHz, around five times the hearing limit of most humans. They also recorded vocalisations with a dominant frequency of 70 kHz.