An isolated population of orangutans in the Batang Toru area of South Tapanuli was reported in 1939. The population was rediscovered by an expedition to the area in 1997, but it was not recognized as a distinct species then. Pongo tapanuliensis was identified as a distinct species, following a detailed phylogenetic study in 2017. The study analyzed the genetic samples of 37 wild orangutans from populations across Sumatra and Borneo and conducted a morphological analysis of the skeletons of 34 adult males. The holotype of the species is the complete skeleton of an adult male from Batang Toru who died after being wounded by locals in November 2013. The holotype is stored in the Zoological Museum of Bogor. The skull and teeth of the Batang Toru male differ significantly from those of the other two orangutan species. Comparisons of the genomes of all 37 orangutans using principal component analysis and population genetic models also indicated that the Batang Toru population is a separate species. The specific name, tapanuliensis, as well as the common name, Tapanuli orangutan, refer to Tapanuli, the hilly region in North Sumatra where the species lives.
Phylogeny
Genetic comparisons show that Tapanuli orangutans diverged from Sumatran orangutans about 3.4 million years ago, and became more isolated after the Lake Toba eruption that occurred about 75,000 years ago. They had continued sporadic contact that stopped between 10,000 and 20,000 years ago. Tapanuli orangutans diverged from Bornean orangutans about 674,000 years ago. Orangutans were able to travel from Sumatra to Borneo because the islands were connected by land bridges as parts of Sundaland during recent glacial periods when sea levels were much lower. The present range of Tapanuli orangutans is thought to be close to the area where ancestral orangutans first entered what is now Indonesia from mainland Asia.
Description
Tapanuli orangutans resemble Sumatran orangutans more than Bornean orangutans in body build and fur color. However, they have frizzier hair, smaller heads, and flatter and wide faces. Dominant male Tapanuli orangutans have prominent moustaches and large flat cheek pads, known as flanges, covered in downy hair. Both male and female Tapanuli orangutans have beards but with Bornean orangutans, only the males do. The Tapanuli orangutan differs from the other two existing orangutan species in several specific features:
their upper canines are larger;
they have a shallower face depth;
their tympanic tube is shorter;
they have a shorter mandibular joint;
they have a narrower maxillary incisor row;
the distance across the palate at the first molars are narrower;
there is a smaller horizontal length between the mandibular symphysis;
they have a smaller inferior torus; and
the width of the ascending ramus located in the mandible.
When comparing the Tapanuli orangutan with the Pongo abelii, the Tapanuli orangutan has a deeper suborbital fossa, a triangular pyriform aperture, and a facial profile that is more angled.
Behavior
The loud, long-distance call or 'long call' of male Tapanuli orangutans has a higher maximum frequency than that of Sumatran orangutans, and lasts much longer and has more pulses than that of Bornean orangutans. Their diet is also unique, containing unusual items like caterpillars and conifer cones. Tapanuli orangutans are thought to be exclusively arboreal as scientists have not seen them descend to the ground in over 3,000 hours of observation. This is probably due to the presence of Sumatran tigers in the area.
Habitat and distribution
Tapanuli orangutans live in tropical and subtropical moist broadleaf forests located south of Lake Toba in Sumatra. The entirety of the species is found in an area of about at elevations from. Tapanuli orangutans are separated from the island's other species of orangutan, the Sumatran orangutan, by just.
