The burning of large tracts of forests not only caused the haze but is also pushing the Sumatran orang utan close to extinction.
A species familiar to Singaporeans because of Ah Meng, the Singapore Zoo's best-known and most endearing ambassador, it is among the world's 25 most endangered primates, said a report launched at the Singapore Zoo yesterday.
Recent published data says there are only about 6,600 orang utans remaining in fragmented habitats in the central regions of Aceh, home to a majority of Sumatran orang utans. Overall, population numbers and habitats remain on a downward trend, noted the report.
Entitled Primates In Peril: The World's 25 Most Endangered Primates 2014-2016, the list was compiled by international primatologists from the Primate Specialist Group of the International Union for the Conservation of Nature's (IUCN) Species Survival Commission (SSC) and other international conservation and research organisations.
The list includes primate species from Madagascar, Africa, Asia and Central and South America. Asia alone contains 10 out of the 25 species, including the Hainan Gibbon, of which there are just 25 left in the world, all on Hainan.
These orang utans are also extremely vulnerable to extinction because of factors such as their slow reproductive rate. The females give birth to one infant every eight or nine years and... the loss of 1 per cent of females a year can place a population on an "irreversible trajectory to extinction", the report said.
The last time the orang utan was on the biennial list was in 2008, but it has been put back on the list because it is now considered to be in a "crisis situation".
Calling deforestation for oil palm a "terrible crisis", the chair of the IUCN/SSC Primate Specialist Group and one of the editors of the report, Dr Russell Mittermeier, said it continued to remain a major issue and this year's forest fires were exacerbated by the El Nino weather pattern.
"One of the most important habitats for them is the peat swamp forest… There's always pressure to convert these peat swamps to oil palm plantations and that's a disaster," he said at the launch.
These orang utans are also extremely vulnerable to extinction because of factors such as their slow reproductive rate.
The females give birth to one infant every eight or nine years - so over their lifetimes, one female may not have more than three - and previous studies have shown that the loss of 1 per cent of females a year can place a population on an "irreversible trajectory to extinction", the report said.
The only other orang utan species is the Bornean orang utan, which also differs slightly in appearance and behaviour.
A world leading primatologist, Dr Christoph Schwitzer, another editor behind the report, said he hoped the list would help draw attention to some of the lesser known primate species such as the Roloway monkey from Ghana and Ivory Coast, believed to be on the verge of extinction.
Apart from the need for national governments to do more, experts say zoos can also play an important role in raising awareness of these endangered species, especially in today's urbanised world.
Ongoing conservation efforts by the Wildlife Reserves Singapore - which manages the Singapore Zoo and other attractions - include rescue and rehabilitation programmes for orang utans in East, West and Central Kalimantan and North Sumatra.
There are currently eight Sumatran orang utans in the zoo, with 11 successful births since 1975.