“I heard a rustling in a tree near, and, looking up, saw a large red-haired animal moving slowly along, hanging from the branches by its arms. It passed on from tree to tree until it was lost in the jungle, which was so swampy that I could not follow it.”

These are the words of the great naturalist Alfred Russell Wallace, describing how he caught sight of his very first orangutan. Around two weeks later, Wallace found his second individual and, as you would expect for a 19^th century British explorer, he shot it dead.

During his fifteen-month stay in Borneo, Wallace ‘collected’ a further 28 orangutans and his tales of slaughter and science are vividly described in his famous tome, The Malay Archipelago (immortalised here by Google).

Wallace wasn’t the only explorer to shoot his way through Borneo’s orangutan population. Odoardo Beccari shot or saw at least 26 individuals in just over 5 weeks, while Emil Selenka collected around four hundred specimens over four years. All of these records attest to the fact that orangutans were relatively common in the late 19^th century, such that zealous Europeans had no problems in finding them.

The same can’t be said now. Field scientists working in Borneo rarely see a wild orangutan and when they do, they’re usually alone or in very small groups. You can travel down the very rivers where naturalists once described seeing orangutans many times in the same day, and find only nests.

Today, we might raise an eyebrow at the trigger-happy antics of Wallace and his contemporaries but, at the very least, they carefully documented what they did. And those tales, together with museum collections, have allowed Erik Meijaard from The Nature Conservancy in Indonesia to reconstruct the history of the Bornean orangutan since the 19^th century.

Meijaard studied records from 59 Bornean expeditions and found that the odds of encountering an orangutan on any given day have fallen by 6 times in the last 150 years. This downward trend stayed the same even after Meijaard accounted for the fact that expeditions have become shorter and involve fewer people.

In Wallace’s time, explorers relied on the skills of local trackers to find orangutans and the focus was very much on these prized animals. Today, scientists often survey orangutan populations by looking for their nests instead. However, when Meijaard only looked at expeditions that specifically set out to count as many orangutans as possible, he still found a sizeable drop between historic rates and modern ones.

Finally, it’s possible that orangutans have learned to avoid people because of the likelihood of getting shot by an intrepid European. Wallace’s accounts certainly suggest a less cautious attitude than one might expect. But Meijaard argues that orangutans, being largely solitary animals, have little opportunity to learn from the death of other group members. Nor would they learn from individuals who escaped 19^th century rifles, for very few did – these slow-moving and large apes were easily shot once spotted. So a more elusive temperament might contribute to the rarity of modern orangutans, but Meijaard thinks that it can’t fully explain it.

With all these possibilities considered and potentially ruled out, the most likely explanation for the downward trend is that it’s real: the ape’s population has actually declined. The genes of the surviving individuals support this conclusion. The genetic similarities between orangutans from the Bornean state of Sabah suggest that the population has fallen by around 10 times in the last one or two centuries. The big question is: why?

Surprisingly, it seems that deforestation hasn’t played a big role. It’s true that logging threatens the safety of orangutans today, but the decline in orangutan numbers was well underway some 120 years before logging kicked off. This industry really intensified during the 1960s and 1970s and during that time, orangutans didn’t suddenly become harder to see. Disease is another possibility, but one with little evidence to back it up.

For Meijaard, one explanation remains – hunting. Orangutans give birth to relatively few young and they have large generation gaps. As such, the adult population takes a long time to replenish. Even before Wallace and his chums arrived in Borneo, orangutans had already been severely hunted by nomadic humans, and been driven to extinction in some parts of Indonesia. Thousands of buried teeth in Borneo and Sumatra harken back to a time when these apes were hunted as commonly as wild pigs.

Once Europeans came on the scene, they weren’t just killed for food any more, but for scientific study, trophies, and the pet trade, while locals continued to kill them for traditional medicine, or as agricultural pests. Ironically, the colonial ban on head-hunting in Sabah may have made matters worse. By suddenly making large tracts of the jungle safe to travel in, the end of head-hunting tribes allowed Western hunters to spread to the jungle, shooting as they went.

Meijaard doesn’t think that his study is the final word on orangutan populations. In fact, he openly wishes that he had better data to work on and hopes that other scientists will take up the challenge. But he says that studies like these are important because they conservationists a better understanding of the real challenges facing a threatened species.

To work out how humans have affected a particular species, you need to know how that creature was faring before we came along. But usually, scientists assess the health of a species after a long period of exploitation and they end up using a baseline that has already been shifted. The result is what Meijaard describes as “historic amnesia”.

This is certainly the case for orangutans – it’s often said that this red ape has a low population density, even in parts of the forest that haven’t been disturbed by logging. The common wisdom says that the orangutan depends on fruit that is sparsely distributed, so a given patch of jungle can only hold so many individuals. This new study suggests that this isn’t true.

This has the potential to change not only our approach to orangutan conservation, but our understanding of their behaviour. Modern individuals are operating at much lower densities than their ancestors used to, and we need to bear that in mind when interpreting the way they act. How differently would they behave if 6 times as many orangutans lived in the same patch of forest?

Reference: PLoS ONE http://dx.doi.org/10.1371/journal.pone.0012042

More on orangutans:

• Orangutans are masters of conserving energy
• Orang-utan study suggests that upright walking may have started in the trees
• Scientists tickle apes to reveal evolutionary origins of human laughter
• Orang-utans use leaves to lie about their size
• Photo safari – Orangutans Part 1, 2, 3 and 4

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