WORLD'S BEST MOTHERS - WHY ORANGUTANS TAKE THE PRIZE

Orangutan mothers may be best in world.

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

“We see this incredibly high survival that’s higher than any [non-human] mammal that we know of so far, says Maria van Noordwijk at the University of Zurich in Switzerland.

Orangutan infants are raised solely by their mothers. The high survival rate is linked to orangutans’ lifestyle, which is more solitary than other apes. But the factors that help explain the high rate– including a low birth rate – may actually make orangutans more vulnerable to a population crash.

Van Noordwijk and her colleagues compiled data on births and infant survival from two populations of Sumatran orangutans and three of Bornean orangutans. There is also a third species, the Tapanuli orangutan described in 2017, but they haven’t been studied long enough to estimate survival rates.

Infants had a very good chance of surviving. Overall, 91 per cent survived until they were weaned – which in orangutans sometimes doesn’t happen until the infant is 8 years old.

What’s more, the researchers discovered that 94 per cent of weaned females made it to adulthood and gave birth themselves. The team couldn’t determine survival rates for males after weaning, as they roam and so are harder to track (Journal of Human Evolution, doi.org/cwfg).

This is a better survival rate than for any other great ape. It is also better than in some human populations. While children do have a higher survival rate in many countries today, not all achieve the orangutan’s level of success.

Counter-intuitively, the key factor to the high survival rate is that forests in South-East Asia are unproductive, and fruit availability is erratic, says van Noordwijk. 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. “Together the protection against predation and disease makes it possible to have this high survival,” says van Noordwijk.

However, their slow lifestyle puts them at risk. Humans are killing orangutans and logging is carving up their forest habitat. “It takes a very long time to recover from a crash,” says van Noordwijk. “It’s one infant at a time.”

The crucial thing will be to protect the largest possible habitats. “Having large connected populations is the best way of making sure they’ll hang in there,” says van Noordwijk.

Written by Michael Marshall in "New Scientist Magazine", USA,vol. 240 no.3202, November 3-9, 2018, excerpt p.8. Digitized, adapted and illustrated to be posted by Leopoldo Costa.