The Tarsier Has an Eyeball Larger Than Its Brain and Talks in Pure Ultrasound

A tarsier is a tiny nocturnal primate that lives on a handful of islands in Indonesia, Borneo, and the Philippines, fits in a human palm, and looks — fairly — as if it has been assembled from spare parts. Each of its eyes is about 16 millimeters in diameter, which is roughly the volume of its entire brain. Because the eyeballs are too big to swivel in the sockets, the animal has to rotate its head instead, which it can do nearly 180 degrees in either direction.

Those eyes are tuned for catching insects in moonlight. Tarsiers hunt at night, leap up to several meters between trunks, and pounce on grasshoppers, beetles, lizards, and small birds. They are the only living primates whose diet is entirely animal — every other monkey, ape, and prosimian eats some plant matter; a tarsier will refuse it.

The oddest part is what they say. In 2012, a team led by Marissa Ramsier at Humboldt State recorded Philippine tarsiers in Mindanao and found them vocalizing at frequencies that peaked above 70 kilohertz, well above the human range and into bat territory. To a person standing next to one, the tarsier appears to be silently opening and closing its mouth. The bats and the tarsiers, apparently, are having a perfectly audible conversation that we are not invited to.