Saturday 14 January 2017

Hoolock tianxing: A new species of Hoolock Gibbon from China and Myanmar.

Gibbons, Hylobatidae, are small Apes found across East, South and Southeast Asia. There are currently 19 species of Gibbons divided into four genera. Hoolock Gibbons, Hoolock spp., also known as Hoolocks or White-browed Gibbons, are found in the northwestern part of this total range, across parts of northeast India, Bangladesh, Myanmar and Yunnan Province, China. They are distinguished from other Gibbons by a number of genetic and morphological features, but most obviously by the presence of the distinctive white eyebrows that give the group its common name. The genus is currently split into two species, living on either side of the Chindwin River in Myanmar. Major rivers commonly provide barriers between Gibbon species, not simply because Gibbons cannot swim, but because the type of trees in river basins are not suitable environments for Gibbons. This has led to speculation that other river valleys may form barriers between undetected Gibbon species.

In a paper published in the American Journal of Primatology on 10 January 2016, a team of scientists led by Peng-Fei Fan of the School of Life Sciences at Sun Yat-sen University and Institute of Eastern-Himalaya Biodiversity Research at Dali University, describe a new species of Hoolock Gibbon from eastern Myanmar and southwestern China.

The new species is named Hoolock tianxing, where 'tianxing' means 'Skywalker' in Chinese, in reference to the unique brachiating motion of Gibbons. It was identified by morphological and genetic examination of museum specimens and living animals bred in Zoos and confiscated from smugglers, as well as photographs of wild animals and genetic material from stool specimens collected from the wild. In keeping with current Chinese law and the ethical guidelines of the American Society of Primatologists, no live animals were killed or collected during the study.

The new species can be distinguished on a number of morphological characteristics, but most obviously their fur, which is brown in the males and yellow in the females, with the white eyebrows of the males thinner and more widely spaced than in any other species and that of the females downturned and with more white between the eyes than any other species.

Male Hoolock tianxing. Fan et al. (2016).

The species was found in the areas between the Nmai Hka and Salween rivers (tributaries of the Irawady) in parts of Shan, Kayah and Kayin states in Myanmar and Yunnan Province in China. This area around Mount Gaoligong in the westernmost part of the Hengduan Mountain Chain, an extension of the Himalayas. This population has previously been thought to belong to the species Hoolok leuconedys, (the Eastern Horlook), which is considered to be Vulnerable under the terms of the International Union for the Conservation of Nature's Red List of Threatened Species, with an estimated population of 310 000–370 000 individuals. However the new species has a much more limited range and consequently a lower population. In addition it is subject to illegal hunting and destruction, degradation and fragmentation of its habitat throughout its range. It is estimated that about 200 of these Gibbons survive in China, with around 50 000 in Shan State in Myanmar and about 16 000 in Kayah and Kayin States. For this reason the new species is assessed to be Endangered under the terms of the Red List.

Female Hoolock tianxing. Fan et al. (2016).

Fan et al. also developed a genetic phylogeny for the new species, using mitochondrial genetic samples from 10 Gibbon species, two Gorilla species, two Chimpanzee species, two Orangutan species, three Macaque species, and Humans. This was given a chronological framework by the inclusion of fossil species thought to have been close to the branching points between modern lineages, namely Rukwapithecus fleaglei from the Miocene Nsungwe Formation of Tanzania, thought to be the oldest Great Ape fossil known, and the oldest Caterrhine (the group that includes Old World Monkeys and Great Apes) that can be placed in any modern group, Sivapithecus indicus from the Miocene Chinji Formation of Pakistan, which has been identified as a member of the Ponginae (the group that includes modern Orangutans), and Sahelanthropus tchadensis from the Late Miocene Nawata Formation of Toros Menalla in northern Chad, thought to be an Hominin (the group that includes modern Humans) that lived shortly after the split with the Chimpanzees. No fossil Gibbons were included in the study, as the relationships of the few fossil Gibbons known to modern Gibbons are far from clear. This study suggests that Hoolock tianxing split from its closest related species (Hoolok leuconedys) about 1.14 million years ago, in the Middle Pleistocene.

A juvenile male of Hoolock tianxing from Mt. Gaoligong jumping across trees. Lei Dong in Fan et al. (2016).

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