... ...

Episode 247: Sumatran Rhinos Barely Surviving

Listen on Apple Podcasts

With less than 80 animals left in the world, the Sumatran Rhino is one of the world’s most endangered rhino species. Only the Javan Rhino with estimates of maybe 60 animals is as close to extinction. Both Angie and Chris were very fortunate to meet up close one the these highly endangered mammals back in 2011. Harapan, one of 3 Sumatran Rhinos born at the Cincinnati Zoo, was held at a conservation center in Florida, and was returned to Sumatra in 2015 to help his species. While this species is on the brink of extinction, many are working hard to save what animals remain. We highlighted this in our interview of Dr. Barney Long (Episode 55) and his efforts to bring some of the Sumatran Rhinos under human care for a focused breeding program.

The Sumatran Rhinoceros (Dicerorhinus sumatrensis)

This small (the smallest rhino species in the world actually), hairy rhino hails from the dense, steamy tropical jungles of Indonesia, where it is clinging perilously to survival. Classed as critically endangered by the IUCN, the last population estimate puts their numbers at less than 80, spread out over several small, highly fragmented populations, most only existing in protected areas, under the watchful eye of armed guards. Also called the two horned rhino, this little shaggy beast is on a fast track to extinction, if efforts to save it fail.

Image from edgeofexistence.org

Evolutionary History: An ancient rhino in the modern age

The Sumatran Rhinoceros is one of five species living today, the other four being the white rhino (Ceratotherium simum), the black rhino (Diceros bicornis), the Indian or Greater One Horned Rhino (Rhinoceros unicornis) and the Javan Rhino (Rhinoceros sondaicus). All five species belong to the Perrisodactyla order and the Rhinocerotidae family, part of the Rhinocerotidea clade. Around 60-65 million years ago, the ancestors of our modern rhinoceroses diverged away from tapirs in Eurasia and North America, leading to a very diverse family consisting of many species. But by the time the Pleistocene Epoch (1,808,000 to 11,550) rolled around, most species had gone extinct, leaving ancestors of the five species on earth now, along with three other now extinct species, the Siberian Unicorn (Elasmotherium sibiricum), Mercks’ Rhino (Stephanorhinus kirchbergensis) and the Woolly Rhino (Coelodonta antiquitas).  The Sumatran rhino is the last surviving member of a family of primitive rhinoceroses, the Dicerorhinini that roamed the earth in the Miocene period 15-20 million years ago. 

A phylogenetic tree of Rhinocerotidae based of studies of their mitochondrial DNA. This clearly shows how closely related the Sumatran and Woolly Rhinos are, and how distinct from the other species of living rhino it is. It is also shows the close relationship between lowland tapirs (Tapirus terrerstris) and the domestic horse (Equus caballus). (Image from Glaubrecht and Neiber, 2017)

The Woolly Rhino is particularly relevant when we consider our hairy little friend the Sumatran Rhino. The Woolly Rhino lived in North-eastern Siberia 18500 years ago and went extinct at the end of the last ice age, which was around the time most of the Pleistocene megafauna, those huge mammals we can only imagine today, disappeared. The Sumatran Rhino is the closest living relative of the Woolly Rhino, meaning it has some of the most unique and ancient genetics present on earth today. A genetic study of the five rhino species and the three extinct species mentioned above confirmed this, and also found some other interesting elements in the rhino genomes. For instance, rhinos are well known to have very poor eyesight, and this has been traced to a mutation in a gene found in all species that codes for the structure of cilia in the eye. Cilia are hairlike projections found throughout the body but play a role in the photoreceptors of the eye, allowing the transport of vital molecules from one end to the other. Abnormal function of these structures would cause issues with eyesight, as seen in the rhino.

A Woolly rhino display in the Weston Park Museum, Sheffield (UK)

The researchers also found longstanding patterns of high inbreeding and low genetic diversity in all the rhino genomes. It was long thought that rhinos had poor genetic diversity due to their falling numbers from human activity, and while this is undoubtedly true, its possible that this may be partly natural and not just down to recent declines. This is great news for conservationists, as it means rhinos may have a better chance than they thought for survival, despite the severe declines they have gone through. 

Habitat and Ecology

Sumatran rhinos once roamed far but are now extinct in much of their former range, including Bangladesh, Bhutan, Cambodia, Vietnam, India and Malaysia. Today they exist only in a few scattered, isolated populations in three main locations; Way Kambas National Park, Aceh Province and the Leuser Ecosystem, spread between the Indonesian Islands of Sumatra and Borneo, hotspots of biodiversity, but unfortunately hotspots of looming extinction for many species too. 

These smaller rhinos’ trundle through dense, damp tropical evergreen rainforest and mountainous moss forests, using their horns and their stout bodies to carve paths through the seemingly impassable vegetation. These forests are incredibly diverse with up to 4000 species of fauna sharing a home with the rhinos. Once these forests stretched to cover up to 99% of the landmass of the Indonesian Islands, but today this has dropped to 55% with industrialised agriculture leading to the destruction and degradation of these biodiverse sanctuaries. 

Across what little habitat they have left, there are 3 recognised subspecies of the Sumatran Rhino:

  1. Dicerorhinus sumatrensis lasiotis (Northern Sumatran Rhino): almost extinct now, except for a possible small population in Myanmar.
  2. Dicerorhinus sumatrensis harrisoni (Eastern Sumatran Rhino): almost extinct, except for a very small number still living in East Kalimantan.
  3. Dicerorhinus sumatrensis sumatrensis (Western Sumatran Rhino): now living only in protected National Parks. 

They are browsers, enjoying a diet of fruit (figs and mango are particular favourites), leaves, bark, shoots (bamboo being a much-loved snack), vines and shrubs. They are also known to use salt licks to get the minerals that are missing from their herbivorous diet. 

The current (the red dots) and historical range (in black) of the Sumatran Rhino. Image from International Rhino Foundation

Reproductive biology: rhinos looking for romance 

Sumatran rhinos are solitary, except for when they have calves or they’re looking to breed. But matching up with a hot date is getting increasingly difficult for these rhinos, which has serious implications for their future in the face of creeping extinction. 

In the eighties and nineties, several rhinos were brought into a captive breeding programme to try and save the species. Scientists and zookeepers used all their vast rhino husbandry knowledge to kickstart the breeding programme, but after years, no little rhino babies were running about. Eventually, scientist Terri Roth (now the vice president of conservation and science at Cincinnati Zoo) cracked the problem. The rhinos do have a monthly cycle, but are only fertile for 24 to 48 hours, and to make matters even more complicated, are induced ovulators. This means that the rhino can only ovulate when the correct external signal is present; in this case, she needs a male around. 

Terri Roth of Cincinnati Zoo feeding one of her beloved Sumatran Rhinos.

Given how scattered the rhinos now are in the wild, it should come as no surprise that their numbers are tumbling. As populations decrease, it becomes less and less likely that males and females will bump into each other, and even if they do, what are the chances that a male will find a female in her 48 fertile hours and kickstart her ovulation? We don’t need a lot of complicated mathematical modelling to say the chances aren’t good. Researchers have encountered yet another problem in the fight against extinction; female rhinos have a very high rate of reproductive diseases and abnormalities, with as many as 50% of captured rhinos exhibiting these issues. This effectively renders them infertile, bringing the numbers of reproductively able adults down even lower. 

Singing for survival 

Another unique behaviour seen in the Sumatran Rhino is its song. All rhinos are known to vocalise to some extent, but these chatty little guys have been recorded in captivity squeaking, snorting, grunting, blowing and most unusually, singing! Not quite belting out a Mariah Carey song, but something similar to the haunting melodies of a humpback whale. This makes sense, as whales as close as sisters to rhinos on the family tree of life on earth. The songs of four captive rhinos at Cincinnati zoo were recorded and researchers found that their songs are actually high level infrasound, likely an adaptation to allow them to hear each other through their dense jungle habitat. What researchers don’t know, however, is why they sing. Some speculate it is to attract a mate, others hypothesise it is for some other type of communication between individuals. One thing that we do know from these studies is that the mournful call of these rhinos makes listeners very emotional, with several researchers being moved to the point of tears when a rhino decided to belt out its song. What is even more sad is the thought of those scattered rhinos out in the wild, singing into the wilderness, with no one to answer back. 

A seriously risky world for these singing sweethearts: what threats face these rhinos?

For these rhinos, loss of their jungle habitat is arguably the biggest threat facing them now. Logging and natural resource extraction has destroyed a huge amount of forest, driving these shy creatures into isolated pockets, making it near impossible for them to find each other and breed. Effectively, the rhinos are existing in islands of forest separated by vast tracts of human dominated landscapes. Even their foliage penetrating songs can’t reach that far. 

Poaching is a serious threat, although fortunately it is not at the same industrial scale as rhino poaching in Africa, where it is often led by organised gangs of poachers. As with the African species, the horns of the Sumatran Rhinos are in high demand as they are believed to have potent medicinal effects in Asian culture. In reality, just like the horns of African rhinos, Sumatran rhino horns are made of keratin, the same material that makes up our nails and hair. No doctor will recommend a dose of nail biting to cure any illness! 

Medicines seized by anti-wildlife trafficking agencies containing rhino horn. Image from the WWF

As mentioned above, the unique reproductive biology of the Sumatran Rhino doesn’t help its plight when successful breeding is the best thing the species can do to save themselves from extinction. With increasing reproductive abnormalities seen in female rhinos and low sperm counts in males, the future is looking even more uncertain. 

Because of these problems, Sumatran Rhinos are suffering from something called the Allee Effect. An Allee Effect occurs when a population of a species becomes so small that their chances of increasing their numbers becomes almost impossible. In the case of the Sumatran Rhino, this is mostly down to mate limitation; males and females almost never meet, and when they do the female may not be reproductively able, so it is highly unlikely new calves will be born to add to the population. This makes the species even more vulnerable to random events, such as forest fires, earthquakes and disease, or human factors like poaching and logging of essential habitat. This drives the species into a dreaded ‘extinction vortex’; a sudden and rapid decline towards a population of zero. 

Why should we care about these shy, unusual rhinos? Well, aside from their inherent right to live that all animals should have, and their super cute faces and shaggy hides of course, the rhinos have significant impacts on their tropical ecosystems in a variety of ways:
1.) The rhinos forge trails through dense vegetation using their horns, creating pathways that many other animals can use, including predators, who can use them to track and hunt prey. 

2.) Rhinos browse on a variety of vegetation including saplings. The removal of saplings, especially fragile, half dead ones, can help improve the composition of the forest. 

3.) Rhinos are big fans of fruit, and so disperse the seeds in their poop, helping to scatter them far and wide across the forest. 

4.) Sumatran rhinos have very poor thermoregulation and so they sleep in mud wallows during the day to help to cool them down. These mud wallows they create are frequently used by other species, and without the rhinos consistently using these wallows, they quickly fill in with leaves and dead plant matter. 

5.) Protecting these rhinos and their habitat protects countless other species. 

A species on the edge: what’s been done and what can we do now?

In 1984, the first meeting on the fate of the Sumatran Rhinoceros was held in Singapore. It was decided at this conference that more surveys needed to be completed to better identify population sizes and locations. The first ever Sumatran Rhino breeding programme was set up after this conference, as the scientists at the time decided it was the only chance to secure a future for the species. Between 1985 and 1992, 18 rhinos were captured from the wild and transferred into a breeding programme. This was an invaluable chance to gain a wealth of husbandry information, including the identification of an optimal diet for the rhinos. It was also the first opportunity researchers had to identify the Allee effect in the species, and the occurrence of uterine tumours and poor sperm count in the rhinos when their numbers fell very low. 

A second meeting was held in 1993 in Indonesia and concerned population and habitat viability. Mortality in captivity had become alarmingly high, and so further capture of wild rhinos was stopped. With the idea of improving this mortality rate, the Sumatran Rhino Sanctuary was constructed in Way Kambas National Park. Importantly, the special Rhino Protection Units (RPU), a contingent of armed guards specifically deployed to protect these precious animals, was deployed and are still a vital part of the conservation fight today. 

In 2013, the Sumatran Rhino Crisis Summit was held in Singapore to assess how the rhino’s recovery was going. At this point, it was decided the populations needed to be managed to allow a 3% increase every year. Currently, we sadly haven’t managed to meet this goal. The IUCN has released depressing data that states the chance of extinction in the next 3 generations is as high as 90%. Not great odds. 

It’s a real challenge to get accurate numbers of Sumatran Rhinos, as they are inherently shy and trekking through cloud forests to identify them is no walk in the park. Some scientists even worry that these censuses could be disruptive to the species, and disruption is something they don’t need any more of. Since 2013, more captive breeding capacity has been set up, and all the rhinos that were housed in Cincinnati Zoo have been successfully moved to the Sanctuary in Way Kambas. In 2012 and 2016, 2 very previous calves, Andatu and Delilah were born, and to this day are living happily and healthily in the sanctuary. Many reproductive experts are looking into Assisted Reproductive Technology (ART) to help boost numbers, with some promising early-stage results so far. Sadly however, since 2016 there have been no live births, only a few very tragic miscarriages. In an even further blow, in 2019, the last known remaining male in Malaysia, Tam, died. 

Ratu (whose name means Queen) and her calf Delilah in 2016. Delilah is known to be a real crooner; she loves to sing when her keepers hose her down! She’s already grown larger than her mother, although Ratu does have very short legs.

Where do we stand today in the fight against the extinction of the Sumatran Rhino? 

  • Way Kambas National Park: currently, there are seven animals in captivity and a small wild population resident in the park. Scientists there are studying ART and looking to capture more of the wild rhinos to supplement the breeding programme developed in the sanctuary. 
  • Kalimantan: In the Kelian Lestari Sanctuary there is currently one female named Patu, but unfortunately, she has severe reproductive abnormalities and scientists don’t have much hope she’ll be able to breed. Camera traps have revealed another female living in the area, and conservationists are hoping to capture her to add to the breeding programme. 
  • The Leuser Ecosystem: this is the stronghold of the last remaining wild population and their survival is balancing on a knifes edge, given how scattered and isolated they are. There are plans to capture several individuals for the breeding programme, but as with the other programmes and many other conservation projects, progress has been hugely slowed down by the COVID-19 pandemic. 

A Sumatran Rhino staring out over the electric fence at the border of the Captive Rhino Sanctuary in Way Kambas National Park. Sadly, this may be the only way to keep this species safe for the future.

Hope for the future?

Things look incredibly dire for the Sumatran Rhino, and it is all too easy to feel depressed about their fate in the face of stalled birth rates, poaching and catastrophic habitat loss. However, dedicated teams are fighting around the world and on the ground in Indonesia to save the species. With these organisations on the case, we can stop the forests becoming forever absent of these hairy little rhinos and their lovely songs. 

  • The International Rhino Fund has set up an International Partnership to bring together the brightest minds and most experienced experts to halt the slide towards extinction. Partners include Cincinnati Zoo (where the riddle of reproduction was cracked), not for profit organisation Save the Rhino International, Taronga Conservation Society Australia, Zoo Basel and Zoological and Botanical Garden Stuttgart. 
  • The Sumatran Rhino Rescue organisation has developed a multi-step plan to search for rhinos, rescue them from threatened populations and build breeding capacity to breed and protect the rhinos. 
  • The WWF work to monitor and manage remaining populations, rescue Sumatran rhinos from their highly threatened locations into sanctuaries and very importantly, engage with governments and local stakeholders to provide a long term, realistic conservation plan. Check out their Sumatran Rhino Rescue Alliance effort on their Sumatran Rhino webpage (https://www.worldwildlife.org/species/sumatran-rhino).
  • The Zoological Society of London has listed the Sumatran Rhino as one of their EDGE species. The EDGE programme stands for Evolutionarily Distinct and Globally Endangered, scoring species on the uniqueness of their genetics and conservation status, analysing whether adequate conservation attention is being given and what else can be done. 
  • And you! Yes, you reading this right now can help provide a future for the Sumatran Rhino:
  • If you can, adopt a rhino through the WWF. 
  • Share the podcast and all you’ve learnt about the Sumatran Rhino with your friends and family so more people are aware of their plight. 
  • Sign on to Stop Wildlife Crime: sign the WWF pledge on their website to make a stand against wildlife crime 


The EDGE score is used to prioritize species for conservation attention. The higher the value the higher priority the species is. The ED and GE scores are combined. The ED score is the Evolutionary Distinctiveness of the species, measured in millions of years, representing the unique evolutionary history existing in the species. The GE or red list category describes how close the species is to becoming extinct. The black lines on the bars above shows the median score for all the species in the taxonomic group (eg: in this case, all other species of rhino). As seen here, the Sumatran Rhino has significantly higher scores than all the other rhino species, especially due to its close link to the extinct Woolly Rhino and their fast-decreasing numbers in the wild.



Edge of Existence Programme (ZSL) (http://www.edgeofexistence.org/species/sumatran-rhinoceros/)

Flynn, R.W. and Abdullah, M.T. (1984) ‘Distribution and status of the Sumatran Rhinoceros in Peninsular Malaysia.’ Biological Conservation, 28(3), pp 253-273

Glaubrecht, M. and Neiber, M.T. (2017) ‘Museum specimens as Noah’s Arc of lost genes. The case of rhinoceros from Sumatra in the Zoological Musuem Hamburg.’ Evolutionary systematics, (1), pp 121-128

Groves, C. (2017) ‘The Sumatran Rhino is one of a kind.’ Pachyderm

Havmøller, R.G. Payne, J. Ramono, W. Ellis, S. Yoganand, K. Long, B. Dinerstein, E. Williams, A.C. Putra, R.H. Gawi, J. Talukdar, B.K. Burgess, N. (2015) ‘Will current conservation responses save the Critically Endangered Sumtran Rhinoceros Dicerorhinus sumatrensis?’ Oryx, 50(2)

IUCN red list (https://www.iucnredlist.org/species/6553/18493355)

Liu, S. Westbury, M.V. Dussex, N. Mitchell, K.J. Sinding, M.H.S. Heintzmann, P.D. Duchêne, D.A. Kapp, J.D. von Seth, J. Heiniger, H. Sánchez-Barreiro, F. Margaryan, A. André-Olsen, R. Cahsan, B. Meng, G. Yang, C. Chen, L. van der Valk, T. Moodley, Y. Rookmaaker, K. Bruford, M.W. Ryder, O. Steiner, C. Bruins-van Sonsbeek, L.G.R. Vartanyan, S. Guo, C. Cooper, A. Kosintev, P. Kirillova, I. Lister, A.M. Marques-Bonet, T. Gopalakrishnan, S. Dunn, R.D. Lorenzen, E.D. Shapiro, B. Zhang, G. Antoine, P.O. Dalén, L. and Gilbert, T.P. ‘Ancient and modern genomes unravel the evolutionary history of the rhinoceros family.’ Cell, 184(19), pp 4874-4885

Roca, A. (2020) ‘Evolution: Untangling the Woolly Rhino’s Extinction.’ Current Biology, 5(30) 

The International Rhino Foundation (https://rhinos.org/)

Van Strien, N.J. (1997) ‘Sumatran Rhino Conservation Plan.’ Report to the Conservation Management Division of the Leuser Management Unit

Von Muggenthaler, E. (2003) ‘Songlike vocalisations from the Sumatran Rhinoceros (Dicerorhinus sumatrensis), 4(83)

WWF (https://www.worldwildlife.org/species/sumatran-rhino)

Massive Thank You to Rachael Da Silva from the UK for this write up! Please follow her on Instagram and her wildlife artwork at tilly_mint08

September 29, 2021
Scroll to top