Now call the Gibbons!

There are not many facts in contemporary biology, more obvious than the proximity of Homo sapiens with other apes.

Since the publication of Darwin’s On the Origin of Species and more effectively (and provocatively), The Descent of Man, we have surely gone in the direction of corroborating these rudimentary speculations and provided a factual understanding of evolution. Experimentation and observation, both have equally driven us to historically choose the great apes, viz. chimpanzee (Pan), gorilla (Gorilla) and orang-utan (Pongo), as our guide species.

It is but natural to look for our evolutionary reflections in the closest of species. After all we have 98.7% genome similar to bonobos (Pan paniscus), which diverged from Homo sapiens about 7 million years ago (mya) [1]. Among behavioural studies also, bonobos and chimpanzees have been documented as similar to humans, with sexual idiosyncrasies in the former and brutal aggression in latter, reminding us of our kinship. Their African counterpart, gorillas, and orangutans in South-East Asia, with their respective splits from humans at 8 and 16 mya, close the node to the ancestry of the great apes or hominids.

Study of hominids has been highly informative and is still revealing wonders about our past. But now scientists have started looking at the other apes in the extended lineage, which diverged from hominids about 20 million years ago, known as gibbons or the lesser apes.

As the name ‘lesser apes’ suggests, gibbons are much smaller in size than the great apes. Their distribution is limited to South-East Asia; restricted by the Brahmaputra River in the west and Wallace line on the East. In this space they harbour the thick canopies of tropical rainforests and are known to be reluctant to come to ground. With primary mode of commute being brachiation (swinging from tree to tree using arms), their forelimbs are extraordinarily long and agile compared to somewhat rigid and short hind limbs.

Gibbons’ signature in the jungle is their loud call. Male and female produce elongated duets, with specific differences in their respective phrases. It has been hypothesized that their function could be territorial display or to show family bonding. A recent study has also highlighted the similarity between the rhythm of calls of Hylobates lar (white-handed gibbon) to human speech [2]. Another characteristic feature, which make gibbon separate from other apes, is their monogamous lifestyle. Sporadic promiscuity notwithstanding, all gibbon species are observed to have a single mating partner and take care of the young ones cooperatively. Even larger transgressions like “divorce” and “infidelity” are also observed, but still the general observations skew towards the side of social monogamy. Two reasons are speculated for this behaviour: to resist infanticide from other male and need of parental care for offspring [3]. Though one cannot take the risk of extrapolating the monogamy in gibbons to cultural monogamy in humans, yet gibbons form an interesting case by choosing different lifestyle than other apes.

Hylobatids, from gibbon’s family name Hylobatidae, also have a most intriguing chromosome number. While all great apes have 24 pairs and humans have 23, hylobatids have 19 – 26 pairs of chromosomes. Utilizing this as the criterion, the family is divided in four genera: Hoolock (19 pairs), Hylobates (22 pairs), Symphalangus (25 pairs) and Nomascus (26 pairs). These four genera are further classified in 20 species, with confusions in speciation abundant in the field.

This brings me to the two indigenous species of Hoolock gibbons found in the Northeast of India. Dense evergreen forests of northeast provide a haven to both the Eastern (Hoolock leuconedys) and Western Hoolock gibbons (H. hoolock). Though primarily the area lies in the distribution of Western hoolock, but Roing district of Arunachal Pradesh – which lies in the valleys of Lohit and Dibang rivers – is also reported to have Eastern hoolock gibbons [4]. As strictly diurnal primates, they start singing their duets early in the morning. They are chiefly frugivorous with small part of diet consisting on leaves, twigs etc. With a small home range of about 0.3 km², they forage for myriad of fruits available in the territory. They are seasonal breeders, with average birth interval of 3 years.

Unfortunately, Hoolocks are not very known to laymen in mainland India, despite its distinction of being the only ape in the country. But this ignorance is not the characteristic of the residents of the Seven Sisters, viz. the states of Assam, Arunachal Pradesh, Meghalaya, Nagaland, Manipur, Mizoram and Tripura. They have celebrated the ape’s presence for centuries. The word “Hoolock” comes from the Assamese word of these animals, “Holou”. This in turn is the onomatopoetic derivation of their loud calls, well-known to the tribes and other people living near forests. Each state and in some areas, each tribe has given a different name to these majestic animals: Huilang in Khasi, Yongmu in Meitei, Haukuk in Mizo, Jambli in Karbi. There are also multitude of myths and stories associated with gibbons. Tamang people believe that an early morning call of gibbon is harbinger of a clear sunny day; on the contrary Tangsas find hoolocks as inauspicious for festivals and hunting.

The research on the Hoolock gibbons in India is primarily concentrated on the aspects of population estimation and conservation. First reported to modern science in 1834, deciding their species status and basic behavioural research was majorly done by international experts like Groves and Geissmann. Dense forests in the region are difficult to explore, with virtually no ground absent of impenetrable undergrowth. Dr Anwaruddin Chaudhury, presently working with the Rhino Foundation, was among first biologists to break this barrier and publish data on reproduction and group size dynamics from the forests of Assam in 1990. Since then, other scientists, mostly native to the north-east, have provided distribution records for other states.

But once common, the Hoolock is presently classified as an endangered species according to IUCN (International Union for Conservation of Nature), 2008; a recent assessment is pending. Hoolock and other primates of north-east face many anthropogenic challenges. Poaching for bushmeat in some states like Nagaland and Manipur has virtually exterminated the animal from most forests, but some protected areas. Fragmentation due to cultivation and increasing human population have resulted in illegal encroachment of forest land and created small islands of original tree space, with few gibbon families trapped in it. Also, the discovery of minerals and oil in the area have prompted private players to enter the foray of financial competition in the area. This has put immense pressure on the ecology of region and can induce irreparable damage to the forests.

Fragmentation can also have subtle ramifications on the population of a trapped species. Geographic disconnect of such areas from other neighbouring jungles can result in a detrimental population dynamic. Mating between related individuals, due to unavailability of area to explore, gives rise to inbreeding depression, which reduces the amount of genetic variation in a population, alleviating the overall fitness of the constituent individuals. This, in turn, increases vulnerability to various genetic diseases. Small population size can also be susceptible to pathogenic diseases, due to less number of putative resistant alleles. Such aspects of population genetic variability are studied in the Laboratory of the Conservation of Endangered Species, a part of CSIR – Centre for Cellular and Molecular Biology, Hyderabad. Here, Dr G. Umapathy and colleagues have successfully shown the effects of fragmentation on endangered Lion – tailed Macaque (Macaca silenus), in Western Ghats. At present, the group is focusing on analysing their population genetics of Hoolock gibbons. This will help to assess the risk of their inbreeding and recommend appropriate suggestions for their conservation.

Other conservation efforts through surveys and public outreach are underway, thanks mainly to non-governmental organizations such as Aaranyak and Primate Research Centre, in Guwahati. Spearheaded by specialists like Dr Dilip Chetry and Dr Jihosuo Biswas, these and other smaller organizations conduct regular surveys with forest officials and volunteers to examine gibbons’ wellbeing as individuals and groups, rescue any injured individual and address human – animal conflicts, if any.

A significant part of their work is public outreach to make people aware of the importance of gibbon conservation and biodiversity as a whole.

A village named Barekuri, in Tinsukia district of Assam, has excelled in conserving Hoolocks, with many individuals living in harmony with humans. State governments also have taken initiatives to declare protected forest areas and tackling hunting and illegal timbre collection, though much more effort needs to be put.

With the publication of genome of another gibbon species, Nomascus leucogenys, in 2014 [5], it is high time we also try to understand the importance of our own gibbon, both as a model for evolution and for conservation of biodiversity. With hoolock’s guidance in issues like chromosome evolution and genome plasticity through retrotransposition, we can expect to uncover our own secrets, for both scientific and medical benefits. And possibly conservation! Although moral responsibility trumps economic concerns, gibbons are in fact essential for evergreen forest ecosystem through seed dispersal and balancing the ecosystem to provide sustainable timbre and other forest product supplies.

References:

1. Prüfer, Kay, et al. “The bonobo genome compared with the chimpanzee and human genomes.” Nature7404 (2012): 527.
2. Terleph, Thomas A., S. Malaivijitnond, and Ulrich H. Reichard. “An analysis of white‐handed gibbon male song reveals speech‐like phrases.” American journal of physical anthropology 166 (2018). 649
3. Opie, Christopher, et al. “Male infanticide leads to social monogamy in primates.” Proceedings of the National Academy of Sciences33 (2013): 13328
4. Das, J., et al. “First distribution records of the eastern hoolock gibbon (Hoolock hoolock leuconedys) from India.” Zoos’ Print Journal7 (2006): 2316-2320.
5. Carbone, Lucia, et al. “Gibbon genome and the fast karyotype evolution of small apes.” Nature7517 (2014): 195.

Author

Mihir Trivedi is a Ph.D. student at the CSIR – Centre for Cellular and Molecular Biology, Hyderabad, India. His work is concentrated on the evolution and genetics, pertaining to conservation of primates in North-east India. An avid traveler to historical and cultural sites, in his leisure time, he writes poems and short stories on his blog (https://heednoone.wordpress.com/) in Hindi. Reading books in a secluded nook remains his escape from reality and he is a wannabe science communicator intending to spread the excitement so often restricted to a lab, to the masses.

Editor

Arunima obtained her PhD from the University of Georgia. A computational structural biologist by training, she enjoys traveling, reading, and the process of mastering new cuisines in her spare time. Her motivation to move to New York was to be a part of this rich scientific, cultural, and social hub.

Photograph of Western Hoolock gibbon, and awareness program courtesy Mihir. Photograph of Lar gibbons courtesy Pixabay. Cover Image courtesy Wikipedia.

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