Orangutans have the longest birth gap among mammals; a 2018 study explains how their 7.6-year wait for another baby, scarce forest fruit and solitary life may actually help them survive

Once widespread across Southeast Asia, orangutans are now confined to just three populations on the islands of Sumatra and Borneo, and they are disappearing faster than they can replace themselves. More than 100,000 Bornean orangutans were estimated to have been lost between 1999 and 2015, while fewer than 800 Tapanuli orangutans remain in the wild, according to WWF estimates and assessments cited by the organisation. Today, their total population is just over 11,900.

When their unusual reproductive biology is taken into account, that decline becomes even more concerning. Orangutans, the only great ape found in Asia, have one of the slowest reproductive rates among mammals, with females typically going more than seven years between births. A 2018 study led by Dutch biologist Maria A. van Noordwijk examined why these great apes reproduce so slowly and found that their long wait between babies is closely tied to their unusually high infant survival, late age at first reproduction and the difficult food conditions of Southeast Asian forests.

The study, “The slow ape: High infant survival and long interbirth intervals in wild orangutans,” was published in the Journal of Human Evolution in December 2018. Researchers combined demographic and birth data from seven long-term wild orangutan research sites across Sumatra and Borneo to get a clearer picture of how these endangered apes reproduce in the wild.


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Orangutans have a childbirth interval of 7.6 years

The researchers found an average closed interbirth interval of 7.6 years. They also found that orangutan infants had consistently high survival rates before weaning. Female orangutans had about a 94% chance of surviving until their first birth, which usually happens at around 15 years of age. The researchers described this as higher than what had been reported for any other mammal living under natural conditions.

The long gap between babies is not simply due to orangutans reproducing poorly. Their whole life history is unusually slow.

Why do orangutans have such a long gap in reproducing?

The environment in which orangutans live is one of the major explanations. The study points out that Southeast Asian forests generally produce less fruit than African forests. Orangutans depend heavily on fruit, but food availability can change dramatically. Some forests experience periods of “mast-fruiting,” when huge amounts of fruit become available, followed by periods when food is much harder to find.

Logically, that makes raising an infant a long-term investment. The researchers wrote that “lower productivity may translate into lower foraging yields”, which can restrict reproduction and growth. Meaning, when food is limited, an orangutan mother may not have the energy or resources to quickly produce and raise another baby.

There is another unusual part of orangutan life that may help explain it. Unlike many other great apes, orangutans are largely solitary and highly arboreal. Mothers have to carry and protect their young in the trees, where a fall can be deadly.

The study suggests that this lifestyle may reduce some causes of death. Being high in the trees can lower exposure to predators, while living mostly alone may reduce the spread of contagious diseases.

The babies survive, so mothers can afford to wait

The researchers found that orangutan offspring have remarkably high survival rates. That changes the usual evolutionary pressure to reproduce quickly.

The paper describes their life history as what can happen when a great ape faces “low unavoidable mortality.” If babies are very likely to survive, a mother does not necessarily need to have many offspring to maintain the population.

It helps explain the strange combination seen in orangutans: few babies, but a very high chance that those babies survive.

The researchers also found no major life-history differences between Sumatran and Bornean orangutans, despite earlier suggestions that orangutans in different parts of Southeast Asia might have different reproductive schedules because of differences in food scarcity.

But the slow life comes with a big risk

There is a downside to this strategy or adapted survival more. Expectedly, an orangutan population cannot recover quickly after losing large numbers of animals. If a population suddenly crashes because of habitat destruction, which is true in this case, for example, deforestation due to palm oil plantations, severe ecological change or other threats, producing replacements takes years.

The researchers therefore warned that the orangutan's slow life history makes it “highly vulnerable to a catastrophic population crash” when faced with drastic habitat change.

So the seven-year gap is not really a single biological quirk. It is the result of an entire lifestyle built around slow growth, late reproduction, intensive maternal care and unusually high survival.

So the Orangutan’s one liner strategy seems to be: have very few babies, invest heavily in each one and wait a long time before starting again.