One of the world’s smallest primates, which up to now had been mistakenly described as being “ordinarily silent”, has all along been using inaudible ultrasound to communicate.
No bigger than a man’s hand, the Philippine tarsier can hear and emit sounds at a frequency that effectively gives it a private channel for issuing warnings or ferreting out crickets for a night-time snack, a study published on Wednesday [February 8th] found.
Only a handful of mammals are known to be able to send and receive vocal signals in the ultrasound range, above 20 kilohertz (kHz), including some whales, domestic cats and a few of the many species of bats.
And few of these can squeal, screech or squawk at the same sonic altitudes as the saucer-eyed tarsier, Tarsius syrichta, researchers found.
Its finely-tuned ears are capable of picking up frequencies above 90 kHz, and it can vocalise in a range around 70 kHz.
By comparison, humans generally cannot hear anything above 20 kHz, and a dog whistle is pitched to between 22 and 23 kHz.
A team of scientists from the United States and the Philippines led by Marissa Ramsier of Humboldt State University in California gathered their inaudible results in two ways.
First they captured six of the docile nocturnal creatures and placed them inside custom-build sound chambers to test their sensitivity to high-pitched sounds.
After the experiments, the rare and endangered animals were returned unharmed to their natural habitat, on the Philippine island of Mindanao.
To measure the frequency of the tarsier’s ultrasound chatter, the researchers recorded another 35 specimens in the wild.
“The minimum frequency of the call – 67 kHz – is the highest value of any terrestrial mammal, excluding bats and some rodents,”
the study, published in the British Royal Society’s Biology Letters, said.
What advantages do the tarsier’s high-end vocal acrobatics confer? There are several, the researchers suggest.
One is being able to sound a silent alarm.
“Ultrasonic calls can be advantageous to both the signaller and receiver as they are potentially difficult for predators to detect and localise,”
the researchers explain.
The tarsier’s exceptional hearing may also facilitate acoustic eavesdropping on noises emitted by prey, which range from crickets and cockroaches – their staple diet – to the occasional moth, katydid or hatchling bird.
Finally, the study speculates, being able to communicate in ultrasonic ranges filters out all the low-frequency “noise” and hubbub of a tropical environment.
Tarsier’s have five-digit hands that eerily resemble – in emaciated form – their human counterparts.
Lacking the typical “night vision” of other nocturnal creatures, they also have – in relation to their body size – the largest eyes of any primate on Earth.
Abstract: “Primate communication in the pure ultrasound”
Marissa A. Ramsier (firstname.lastname@example.org), Dept of Anthropology, Humboldt State Univ., Arcata, CA 95521, USA
Andrew J. Cunningham, Dept of Human Evolutionary Biology, Harvard Univ., Cambridge, MA 02138, USA
Gillian L. Moritz and Nathaniel J. Dominy (email@example.com) Depts of Anthropology and Biological Sciences, Dartmouth College, Hanover, NH 03755, USA
James J. Finneran, US Navy Marine Mammal Prog., SSC Pacific, San Diego, CA 92152, USA
Cathy V. Williams, Duke Lemur Ctr, Duke Univ., Durham, NC 27705, USA
Perry S. Ong, Institute of Biology, Univ. of the Philippines, Diliman, Quezon City, Philippines
Sharon L. Gursky-Doyen, Dept of Anthropology, Texas A&M Univ., College Station, TX 77843, USA
British Royal Society Biology Letters
Few mammals — cetaceans, domestic cats and select bats and rodents — can send and receive vocal signals contained within the ultrasonic domain, or pure ultrasound (greater than 20 kHz). Here, we use the auditory brainstem response (ABR) method to demonstrate that a species of nocturnal primate, the Philippine tarsier (Tarsius syrichta), has a high-frequency limit of auditory sensitivity of ca 91 kHz. We also recorded a vocalization with a dominant frequency of 70 kHz. Such values are among the highest recorded for any terrestrial mammal, and a relatively extreme example of ultrasonic communication. For Philippine tarsiers, ultrasonic vocalizations might represent a private channel of communication that subverts detection by predators, prey and competitors, enhances energetic efficiency, or improves detection against low-frequency background noise.
Sources: Al Jazeera | British Royal Society Biological Letters | Haiti Chery (added photos)
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