The Bornean tarsier (Cephalopachus bancanus) is arguably the most extraordinary mammal you can find at Bako National Park. Weighing just 80–165 grams and small enough to sit in a human palm, it is the world's smallest primate. What it lacks in size it makes up for in anatomical extremes: its eyes are the largest of any mammal relative to body size, each one fixed in its socket and larger than its brain. To compensate for the fixed eyes, tarsiers can rotate their heads nearly 360 degrees.
Tarsiers are strictly nocturnal and strictly carnivorous — the only primate in the world that eats no plant material at all. Their diet at Bako consists of large insects, small lizards, geckos, and occasionally small snakes. Their ankle bones (tarsals, hence the name) are extraordinarily elongated, allowing them to leap up to 40 times their own body length between vertical stems in a single bound — a hunting technique that makes them formidably effective predators in the dark.
Finding a tarsier at Bako requires effort. Join the park's guided night walk (book at the park office, departs 20:00 nightly) or hire a local guide who knows the regular roost trees near the Lintang Trail. Tarsiers return to the same sleeping sites night after night — usually a vertical cluster of thin stems or a fig tree trunk. When the guide's torch catches a pair of reflective eyes 2 metres up in the undergrowth, the effect is immediately otherworldly. Do not shine a direct torch beam into their eyes for more than a second or two — their enormous eyes are exquisitely sensitive to light and prolonged exposure causes genuine distress.
The Bornean tarsier is classified as Vulnerable on the IUCN Red List. They are acutely sensitive to forest disturbance — their hunting requires dense, structurally complex vegetation — and they do not survive in degraded secondary forest. Bako's old-growth kerangas and mixed dipterocarp forest provides the habitat they need. A single individual occupies a territory of 1–2 hectares, so the park supports perhaps 50–100 individuals across its 27 km² — a small but stable population. Also watch for the flying lemur on your night walk, which uses similar resting sites at higher elevations.