One of the world’s smallest primates, the Philippine tarsier, communicates
in a range of ultrasound inaudible to predator and prey alike, according to
a study published on Wednesday.
No bigger than a man’s hand, Tarsius syrichta can hear and emit sounds at a
frequency that effectively gives it a private channel for issuing warnings
or ferreting out crickets for a nighttime snack, the study 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, which up to now had been mistakenly described as
being “ordinarily silent,” 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 can’t 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,” said the study,
published in the British Royal Society’s Biology Letters.
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.
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