AboutZoos, Since 2008


Rein­tro­duc­tion and mix­ing Bornean orang­utans sub­species, a turn for the worse?

pub­lished 28 Feb­ru­ary 2016 | mod­i­fied 28 Feb­ru­ary 2016

Bornean Orangutans hybridizedRein­tro­duc­tion of genet­i­cally dis­tinct sub­species has led to hybridiza­tion in an endan­gered wild population

As their nat­ural habi­tats con­tinue to be destroyed, increas­ing num­bers of dis­placed endan­gered mam­mals are taken to sanc­tu­ar­ies and reha­bil­i­ta­tion cen­tres world­wide. The ulti­mate goal of these cen­tres is often rein­tro­duc­tion: to return these ani­mals to wild pop­u­la­tions. In a new study pub­lished online on 25 Feb­ru­ary in Sci­en­tific Reports, how­ever, Gra­ham L Banes and Linda Vig­i­lant of the Max Planck Insti­tute for Evo­lu­tion­ary Anthro­pol­ogy in Leipzig, Ger­many, cau­tion that such rein­tro­duc­tions can act as a form of genetic translo­ca­tion. By using genetic analy­sis to assess a sub­set of his­tor­i­cal rein­tro­duc­tions into Tan­jung Put­ing National Park, Indone­sia, they found that orang­utans from a non-​native and genet­i­cally dis­tinct sub­species were unwit­tingly released and have since hybridized with the Park’s wild pop­u­la­tion. As orang­utan sub­species are thought to have diverged around 176,000 years ago, with marked dif­fer­en­ti­a­tion over the last 80,000 years, the researchers high­light the poten­tial for neg­a­tive effects on the via­bil­ity of pop­u­la­tions already under threat.

When Biruté Galdikas and Rod Brindamour began their pio­neer­ing orang­utan reha­bil­i­ta­tion efforts at Camp Leakey in Tan­jung Put­ing National Park, Cen­tral Kali­man­tan, all orang­utans were con­sid­ered a sin­gle species. Over 14 years, from 1971 to 1985, they released at least 90 orphaned and dis­placed apes into the sur­round­ing wild pop­u­la­tion. Advances in mor­pho­log­i­cal and genetic stud­ies have since revealed two species of orang­utan, how­ever, on the islands of Bor­neo and Suma­tra. The Bornean orang­utan is fur­ther sub­di­vided into three dis­tinct, geo­graph­i­cally and repro­duc­tively iso­lated sub­species, which last shared a com­mon ances­tor in the Pleis­tocene and have dif­fer­en­ti­ated sub­stan­tially over tens of thou­sands of years.

Hybridiza­tion in the national park
Using genetic analy­ses and 44 years of data from Camp Leakey, Banes and Vig­i­lant worked with Galdikas to deter­mine the min­i­mum extent to which she released non-​native sub­species into the National Park. They found that Rani and Sis­woyo, two females that Galdikas res­cued from the pet trade, had orig­i­nally been cap­tured from north­ern West Kali­man­tan or Sarawak, and thus were of the non-​local sub­species Pongo pyg­maeus pyg­maeus. Since their release into Tan­jung Put­ing National Park, the pair have inter-​bred pro­lif­i­cally with males of the local sub­species, Pongo pyg­maeus wurm­bii, pro­duc­ing at least 22 hybridized descen­dants to date. These off­spring inher­ited a ‘cock­tail’ of genes that could not nor­mally occur in the wild.

There is no defin­i­tive evi­dence of out­breed­ing depres­sion among Bornean orang-​utans, but our find­ings are enough to cause seri­ous alarm
Gra­ham L. Banes, lead author, Divi­sion of Bio­log­i­cal Anthro­pol­ogy, Depart­ment of Archae­ol­ogy and Anthro­pol­ogy, Uni­ver­sity of Cam­bridge, UK »

Inter-​breeding ani­mals from two genet­i­cally dis­tinct pop­u­la­tions can some­times lead to ‘hybrid vigour’, in which off­spring reap the ben­e­fits of their par­ents’ indi­vid­ual qual­i­ties. This could explain how Rani came to found the biggest fam­ily of any female rein­tro­duced at Camp Leakey, with at least 14 descen­dants over three gen­er­a­tions. Though two died in infancy, the remain­der are pre­sumed to be alive and none are known to have required any vet­eri­nary inter­ven­tions. How­ever, “off­spring born to par­ents from two genet­i­cally dis­tinct pop­u­la­tions, which have not been in genetic con­tact for sig­nif­i­cant peri­ods of time, have also been shown to suf­fer poor health and repro­duc­tive suc­cess in a range of dif­fer­ent species,” said Vigilant.

Strik­ingly, in stark con­trast to Rani, Sis­woyo had fewer sur­viv­ing, healthy off­spring than any other female at the site, which might be linked to such ‘out­breed­ing depres­sion’. Her descen­dants are com­par­a­tively few, with only five first-​generation and three second-​generation off­spring. Two of her off­spring died in infancy, while infec­tion fol­low­ing the lat­ter preg­nancy resulted in Siswoyo’s own death ten days after the birth. Her only daugh­ter, Siswi, pro­duced a still­born off­spring, a daugh­ter that died in infancy, and a son that often needed med­ical inter­ven­tions. Siswi her­self has fre­quently required vet­eri­nary care, includ­ing major surgery to treat a per­fo­rated intestine.

Bornean orangutans hybridized offspring chartInter-​breeding between Rani (a) and Sis­woyo (b) with native males is known to have resulted in hybridized off­spring.
To illus­trate sub­se­quent intro­gres­sion, this dia­gram is based on the assump­tion that 50% of alle­les native to each sub­species are always uni­formly inher­ited: thus, by exam­ple, ‘Rimba’ is three-​quarters Pongo pyg­maeus wurm­bii and one quar­ter P. p. pyg­maeus. The fig­ure illus­trates exam­ples of the extent of admix­ture known to have occurred within and between these lin­eages over mul­ti­ple gen­er­a­tions. In prac­tice, as detailed in the text, Rani’s direct descen­dants number(ed) at least 14 hybridized and/​or intro­gressed indi­vid­u­als over 3 gen­er­a­tions; Siswoyo’s number(ed) at least 8 over 2 gen­er­a­tions. At least 9 of their com­bined descen­dants are males likely capa­ble of father­ing off­spring. Their repro­duc­tive out­put is unknown, but has inevitably resulted in fur­ther admix­ture in other lin­eages.
Banes, G. L. et al. Rein­tro­duc­tion of con­fis­cated and dis­placed mam­mals risks out­breed­ing and intro­gres­sion in nat­ural pop­u­la­tions, as evi­denced by orang-​utans of diver­gent sub­species. Sci. Rep. 6, 22026; doi: 10.1038/srep22026 (2016). Licensed under a Cre­ative Com­mons Attri­bu­tion 4.0 Inter­na­tional License.

Rein­tro­duc­tion only after genetic test­ing
“There is no defin­i­tive evi­dence of out­breed­ing depres­sion among Bornean orang­utans,” says Banes, “but our find­ings are enough to cause seri­ous alarm.” More than 1,500 orphaned and dis­placed orang­utans are cur­rently await­ing release from cen­tres on Bor­neo and Suma­tra, which missed a dead­line set forth by the Indone­sian gov­ern­ment to rein­tro­duce all their orang-​utans by the end of 2015. As their intake of dis­placed orang­utans increases, and as suit­able habi­tat for rein­tro­duc­tions declines, there have been sug­ges­tions that they hybridize Bornean orang­utan sub­species — either in iso­lated, ‘mixed’ pop­u­la­tions, or within exist­ing wild populations.

Banes and Vig­i­lant advo­cate genetic test­ing prior to all rein­tro­duc­tions of dis­placed ani­mals, in accor­dance with estab­lished inter­na­tional guide­lines. While their find­ings and rec­om­men­da­tions apply to a broad range of endan­gered mam­mals, Banes is espe­cially adamant that Bornean orang­utan sub­species be kept apart. “They might look roughly the same, but these orang-​utans from dif­fer­ent sub­pop­u­la­tions haven’t shared a com­mon ances­tor for tens of thou­sands of years. It may be that inter-​breeding them has no ill effects at all, but what if it does? Sud­denly, for the sake of short-​term wel­fare, we’ve com­pro­mised the via­bil­ity of wild pop­u­la­tions — and we can never take that back.”

(Source: Max Planck Gesellschaft news release, 25.02.2016)

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