A new species of orangutan
A third species of orangutan has been described from the island of Sumatra. The Tapunali orangutan Pongo tapanuliensis is found in a tiny area south of Lake Toba. With its restricted range and a population of just 800 animals, this species immediately became the most threatened ape species. Bornean orangutan P. pygmaeus and Sumatran orangutan P. abelii were separated as species in 1996. Now the population from Batang Toru, south of Lake Toba, is recognised as distinct on the basis of genetic data, skull and dental features. In general appearance there is little difference between this and the other Sumatran species: it is described as having frizzier hair, a prominent moustache and downy hair on the facial flanges of the males. Genetically this species is closer to the Bornean species than to other individuals on Sumatra. This will have arisen as a result of the isolation of different populations: first the Lake Toba isolating the northern Sumatran abelii around 3.4 million years ago and then the separation of tapanuliensis from the Bornean pygmaeus with habitat change on the Sunda shelf 674,000 years ago.
All recent studies list seven species of living non-human great ape: the three orangutans, chimpanzee Pan troglodytes, bonobo P. pansicus and eastern (Gorilla beringei) and western (G. gorilla) gorillas. Genetic studies indicate that there was gene flow between chimpanzees and bonobos possibly until 200,000 years ago, and between gorilla species until around 25,000 years ago. Given how recently there was gene flow between some of these and how similar they are morphologically considering these as 'species' may seem questionable.
The standard definition of a biological species is that it is a group that is reproductively isolated from similar groups, implying that they are unable to breed with related species, or that they produce sterile hybrids. This is not strictly accurate, as many clearly distinct species are able to produce hybrids and whilst many hybrids have reduced fertility, some may be fully fertile. This seems to be the case with the great ape 'species' as Bornean and Sumatran orangutans hybridised in zoos before their distinctiveness (and hence the need to conserve their genetic identity) was realised. There are also records of hybrids between eastern and western gorillas, and between chimpanzees and bonobos. Of these, the only species pair with notable morphological distinction is that of the chimpanzee and the bonobo. This is also the oldest split. It could be argued that the others are only separated as species because they are apes, and we naturally have a particular interest in the diversity of our close relatives.
Source: A. Nater et al. 2017. Morphometric behavioral, and genomic evidence for a new orangutan species. Current Biology 27(22):3487-3489. PLOSOne http://dx.doi.org/10.1016/j.cub.2017.09.047
Photo: Tim Laman. Published under the Creative Commons Attribution 4.0 International license.