The Philippine tarsier was once considered one of the quieter primate species. Turns out that’s because we simply weren’t listening hard enough.
Tarsiers are one of the smallest species of primates, growing to only about the size of a man’s fist, and live exclusively on the Phillipine islands. What makes them special isn’t their enormous, lemur-like eyes, but the frequency of their shrill calls that fall well beyond the hearing range of humans—or any other primate for that matter. “It turns out that it’s not silent. It’s actually screaming and we had no idea,” Marissa Ramsier, an evolutionary biologist at Humboldt State University, told Live Sciences.
The human ear can pick up noises up to 20kHz. That’s paltry compared to the 91kHz sounds a Tarsier can hear and less than half of the 70kHz sounds a Tarsir can make. That’s 26kHz higher than the previous record holder, the 65kHz-hearing Bush Baby. In fact, those frequencies are very similar to the range that bats utilize.
A research team from Humboldt State made the discovery in the Phillipine wilderness—Tarsiers are highly endangered and struggle in captivity—by capturing six of the animals, placing them in sound-proof chambers, and conducting hearing tests. The Tarsiers were bombarded with noises of various frequencies while non-invasive electrodes picked up their brain activity. The team employed an ultrasonic microphone to capture the calls of 35 separate specimens and found that their calls range from 67 to 79kHz—ultrasound.
This adaptation may have evolved to allow the animals a discreet form of communication. Predators can’t track them from their calls if the predators can’t hear them in the first place. “It’s this duel benefit,” Ramsier said. “They can communicate without predators hearing them and also locate some potential food sources.” [Live Science]