杏吧原创

Orangutans have staggeringly low rates of infant mortality. They are better at keeping their offspring alive than people in most human societies throughout history.

鈥淲e see this incredibly high survival that鈥檚 higher than any [non-human] mammal that we know of so far,鈥� says Maria van Noordwijk at the University of Z眉rich in Switzerland.

The high survival rate is linked to orangutans鈥� lifestyle, which is more solitary than other apes. But it also puts them at risk, because it is tied to their low birth rate 鈥� meaning they would struggle to recover from a population crash.

Van Noordwijk and her colleagues compiled data on births and infant survival from five populations: two groups of and three of . There is also a third species, the Tapanuli orangutan described in 2017, but they have not been studied long enough to estimate survival rates.

Slow but steady

On average, female orangutans give birth once every 7.6 years: among humans, it is typically closer to 3 years. Female orangutans also did not give birth until they were on average 14-15 years old.

Infants had a very good chance of surviving. Overall 91 per cent survived until they were weaned. Breaking the data down by gender, the researchers discovered that 94 per cent of females survived into adulthood and gave birth themselves. The data were less reliable for males, which roam after weaning and so are harder to track.

This is a better survival rate than any other great ape, and is also better than African elephants and even a group of human hunter-gatherers. Children do have a higher survival rate in some populations, such as in modern Switzerland, but听听achieve the orangutan鈥檚 infant survival rate.

Alone in a tree

The key factor is that forests in South East Asia are unproductive, and fruit availability is erratic, says van Noordwijk. That means mothers must care for each youngster for many years. 鈥淚t can only work if you have low mortality,鈥� she says.

To ensure they get enough food without travelling too far, orangutans are solitary. This reduces competition and aggression, and protects them against disease. They also spend more time in trees than other great apes, protecting them from predators. 鈥淭ogether the protection against predation and disease makes it possible to have this high survival,鈥� says van Noordwijk.

However, their slow lifestyle puts them at risk. Humans are killing orangutans and logging is carving up their habitat. 鈥淚t takes a very long time to recover from a crash,鈥� says van Noordwijk. 鈥淚t鈥檚 one infant at a time.鈥�

The crucial thing will be to protect the largest possible habitats. 鈥淗aving large connected populations is the best way of making sure they鈥檒l hang in there,鈥� says van Noordwijk.

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