The Sumatran tiger (Panthera tigris sumatrae) is the smallest and most endangered of the surviving tiger subspecies, with fewer than 400 left in the wild across the whole island. Gunung Leuser is one of the few places where a viable breeding population still exists, and where camera traps regularly record adults, juveniles, and cubs in the interior of the park.
Visitors will almost certainly not see one. The tigers are shy, low-density, and largely nocturnal, and the rainforest provides excellent cover. Even the park's own rangers and researchers see them only occasionally. What you can do — and what makes a trek here different from one in a less significant park — is be on ground where they exist, where the signs of their presence (claw marks on trees, prints in mud, the occasional kill) tell their own story. The serious threats to the tiger are habitat loss and snaring; supporting park-licensed guides funnels money to the people working to address both.