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.
“These findings are very surprising. I’m not aware of any other primate that can communicate at that level of ultrasound. The fact that the tarsiers make sounds that are truly in the ultrasonic range, with no overlap into frequencies audible to humans, that’s quite remarkable,” commented Gisela Kaplan, a specialist in animal behaviour at the University of New England in Armidale, New South Wales. “It’s an impressive finding, almost as impressive as the first echolocation research in bats.”
Private channel of communication
Exactly how the tarsier uses its exceptional auditory abilities is unknown, but they effectively provide access to a private channel of communication that is inaudible to most animals, predators included. While a multitude of possible uses include foraging for food, hiding from predators and locating offspring, the researchers did observe that the tarsiers made calls when humans were nearby. They suggest the calls are the secret sounding of an alarm.
“Detection is a disaster for small mammals, because there is always some predator there who would consider them the next meal. So if the sound an animal makes can be masked or made inaudible for the majority of ears, in evolutionary terms this makes good sense for survival,” said Kaplan.
Ramsier and co-researcher Nathaniel Dominy are considering further research to understand better tarsier communication. “Our findings on tarsiers are certainly exciting and unexpected, so we are definitely considering further exploring the acoustic ecology of the Philippine tarsier and other tarsier species. These are endangered animals, so the more we can learn about their ecology and behaviour, the better.”