Tarsiers are diminutive primates that live in the Philippines and other islands near Southeast Asia. They resemble the offspring of an unholy dalliance between Yoda and a squirrel, but with a kind of alien cuteness born of their huge eyes and clever “smile”. As with many other animals, their continued existence is threatened by habitat loss, and they are listed as endangered. They are small (only about 16 cm (6.5 inches) long), live in trees, and subsist on insects.

Now researchers have discovered another unusual characteristic: they communicate using ultrasound. Bats are the primary mammal group to use ultrasound; I don’t think any other primates can hear it, much less produce it. In fact, ultrasound’s rarity among mammals may be part of the toolkit tarsiers have exploited to be successful at survival.

So what exactly is ultrasound, and why is it a big deal for tarsiers to use it?

Sound, Pitch, and Music

As with our designation of types of light, sound is classified based on whether humans can perceive it or not. Sound travels in waves, and the sound frequency measures how many waves pass per second. One Hertz (Hz) is one wave per second; humans hear frequencies between roughly 20 Hz and 20 kilohertz (20,000 Hz, or 20 kHz), with smaller frequencies being interpreted as lower pitch than higher frequencies. Doubling the frequency produces a pitch one octave higher (using the term borrowed from Western musical conventions), so the average humans hears slightly less than 10 octaves of pitch. The “standard pitch” for musical instruments is based on the note A, defined to be 440 Hz; the alternating current used in the United States operates at 60 Hz, so many electrical appliances make humming sounds based off that frequency.

Most sounds don’t consist of single frequencies, and a note that only is one frequency sounds weird to many of us. Instead, many sounds have a fundamental frequency that determines their pitch, and overtones that determine the overall characteristic of the sound. (“Noise” often refers to sounds with a mixture of frequencies that lacks a well-defined fundamental pitch. White noise in particular is a mixture of sounds from all frequencies.) We humans can hear some sounds whose fundamental frequency lies below 20 Hz, but whose overtones fall within normal hearing range: some musical instruments like pipe organs and tubas regularly play pitches too low for us, but our brains reconstruct the note we can’t actually hear! Similarly, some sounds toward the high end of our register have overtones beyond our ability to perceive.

Infrasound and Ultrasound

Sounds with fundamental frequencies below 20 Hz are known as infrasound, and those with fundamental frequencies above 20 kHz are ultrasound. Many animal species communicate through infrasound, including whales, elephants, and tigers: these very low-pitched sounds have the ability to travel very large distances and can even pass around obstacles like rocks or trees. Elephants use infrasound to coordinate herd movements across many miles, while solitary tigers establish territorial boundaries. Ultrasound is used by some species of bats and rodents, though its most familiar application is in medicine: it penetrates most of our soft tissues nicely. By measuring the various ways it reflects and scatters off organs and other structures inside the body, doctors can gauge health of a fetus or measure blood flow through the heart, among other uses.

If you want more information about how sound travels around corners, please see my Double X Science post on the subject. To summarize briefly, for a wave to travel around an obstacle or through a gap coherently, it needs to have a wavelength comparable in size to the opening or barrier. Wavelength is analogous to frequency, but for spatial distance: it’s the physical length a wave requires to repeat itself. Frequency and wavelength are inverses: a large frequency means a small wavelength, and a small frequency means a large wavelength. That’s why infrasound is effective: a 17 Hz tone will have a wavelength of about 20 meters (about 66 feet), sufficient to travel around large obstacles.

Ultrasound, on the other hand, is more difficult to use for communication for two major reasons. First, ultrasound wavelengths are much smaller than typical gaps between trees, which means coherent ultrasound waves get broken up. The second reason is that sound travels through air, which is something known as a dispersive medium: different wavelengths of sound travel at slightly different speeds. To hear something clearly, it’s best for all parts of the sound to arrive at the same time, so the ear and brain can reassemble the original signal. Dispersion messes this up by making the various components of the sound arrive at slightly different times, so it becomes hard to pinpoint where the sound is coming from or even to tell a sound was there to begin with. The shorter the wavelengths, the stronger this effect is: while for infrasound dispersion is a minor annoyance, for ultrasound it’s a big problem. If you’re a small woodland critter, you might not be able to tell where your fuzzy friends are if ultrasound is the primary way you communicate.

Tarsier Ultrasound

Philippine tarsiers were previously classified by researchers as “ordinarily silent”: they occasionally would make shrill cries, resembling bat chittering calls, but few other audible signals. Because some bats do use ultrasound, that led the authors of the current study to test both for tarsier vocalization and hearing in the ultrasound. They found tarsiers can hear sounds as high as 91 kHz, and make calls with fundamental frequencies of 70 kHz—3.5 octaves above the highest pitch humans can hear! That’s fairly extreme ultrasound, even compared to other animals who communicate that way.

If ultrasound carries major disadvantages, why do tarsiers use it? Obviously it has some evolutionary benefit, and the authors of the study considered some possibilities. Though tarsiers have huge eyes, they are nocturnal: low-light conditions at night in their forest home make visual hunting of insects difficult. However, moths, katydids, and other prey emit ultrasound as they move through the woods, so maybe tarsiers track them through hearing. In addition, since things that might eat tarsiers don’t hear ultrasound, communicating using high frequencies might help them to remain undetected: if predators can’t hear them, they can stay hidden much better. Also, as the researchers point out, that part of the sound spectrum pretty much belongs to tarsiers alone: they would never confuse another tarsier’s cry for any other animal.

Tarsiers are hard to study, being both tiny and endangered. (Those facts alone make me go “awww…”.) It’s too early to say that tarsiers use ultrasound to hunt, in other words, or even to say that there is a survival advantage from it. However, evolution finds a way, both by selecting positively for traits (that actively help individuals in a species to survive), and allowing neutral traits to pass that might convey advantages down the line. Using ultrasound may be rare in animal species, but tarsiers have leveraged it in a way to help make them successful.

• Reference: Marissa A. Ramsier et al., “Primate communication in the pure ultrasound”. Biology Letters, doi:10.1098/rsbl.2011.1149