Tarsiers hunt at night without the reflective tapetum lucidum that brightens the eyes of most nocturnal mammals. They compensate with sheer aperture: each eyeball is about sixteen millimetres across, roughly the size of the animal's brain and close to it in weight, and is enlarged to a degree unmatched among mammals relative to body size. The bony orbit is correspondingly huge, which is one reason fossil tarsiiforms are recognisable from skull fragments alone.
Because the eyes fill their sockets and have very limited movement, the neck compensates. A tarsier can rotate its head roughly 180 degrees in each direction, giving it a full field of view from a fixed perch while clinging vertically to a stem.
A related discovery is ultrasonic communication. Philippine tarsiers were found to produce and hear calls with energy above the human hearing range, a private channel that had gone unnoticed because observers could not hear it. Whether the calls carry specific content, or mainly serve as contact signals, is not established.