An orangutan mother will not give birth again until she’s finished providing milk to her previous offspring. Nursing can take a long time and vary across seasons, as we found in research published today in Science Advances.
Primate mothers, including humans, raise only a few slow-growing offspring during their reproductive years.
Differences in infant development have a profound effect on how many children a female can have over the course of her life – the key marker of success from an evolutionary vantage point.
Great apes have a high-stakes strategy. Chimpanzee mothers nurse their offspring for five years on average, twice as long as humans in traditional small-scaled societies.
Orangutans have been suspected of having even longer periods of infant dependency, although determining just how long has been a particular challenge for field biologists.
Living high up in dwindling Southeast Asian forests, these apes are adept at evading observers. Their nursing behaviour is often concealed, particularly while juveniles cling to their mother or rest together in night nests.
Teeth tell the story
I have spent the past few decades studying how orangutans and other primates form their teeth. Amazingly, every day of childhood is captured during tooth formation, a record that begins before birth and lasts for millions of years.
Teeth also contain detailed dietary, health and behavioural histories, allowing biological anthropologists an unprecedented window into the human past.
I’ve also teamed up with researchers Manish Arora and Christine Austin, at Icahn School of Medicine at Mt Sinai in New York, who have pioneered methods to map the fine-scaled elemental composition of teeth, as well as primate lactation expert Katie Hinde at Arizona State University.
We have shown in a previous study that tiny amounts of the element barium are an accurate marker of mother’s milk consumption. Like calcium, barium is sourced from the mother’s skeleton, concentrated in milk, and ultimately written into the bones and teeth of her offspring.
Once animals start nursing after birth, their teeth show increases in barium values, which begin to decrease when solid food is added to the diet. These values drop further to pre-birth levels when primates stop nursing and are weaned.
We’ve recently used this approach to explore the nursing histories of wild orangutans in collaboration with orangutan expert Erin Vogel at Rutgers University. In order to do so, I borrowed teeth housed in natural history museums from individuals that had been shot many years ago during collection expeditions.
Orangutan teeth show a gradual increase in barium values from birth through their first year of life, a time of increasing consumption of their mother’s milk. After 12-18 months, values decrease as infants begin eating solid foods consistently.
But surprisingly, barium levels then begin to fluctuate on an approximately annual basis. We suspect that this is due to seasonal changes in food availability. When fruit is in short supply, infants appear to rely more on their mother’s milk to meet their nutritional needs.
Another surprising finding is that nursing may continue for more than eight years, longer than any other wild animal.
This information is the first of its kind for wild Sumatran orangutans, as they have been especially difficult to study in their native habitat. Previous estimates from two wild Bornean orangutans suggested that juveniles nurse until about six to eight years of age.
Rather than spending so much time and energy breastfeeding their children, human mothers in traditional societies transition their infants onto soft weaning foods around six months of age, tapering them off milk a few years later.
Humans also benefit from having help such as older siblings and grandparents who lend a hand with childcare and enable women to energetically prepare for having their next child.
Orangutan mothers have it hard by comparison. They live alone in unpredictable environments with limited nutritional resources. In order to survive they use less energy than other great apes, raising their young more slowly.
Female orangutans begin reproducing around age 15 and can live until 50 years old in the most favourable of circumstances. They bear new offspring every six to nine years, producing no more than six or seven descendents over their lifetime.
Having a long nursing period and slow maturation makes orangutan populations especially vulnerable to environmental perturbations.
Recent work has also implicated poor habitat quality and the pet trade as additional factors in their rapidly declining numbers, which is underscored by their critically endangered status.
Research on collections housed in natural history museums provides timely evidence of how remarkable orangutans are, how much information we can retrieve from their teeth, and why conservation efforts informed by evolutionary biology are critical.
Conservationists and environmental scientists are used to bad news. So when there’s some really good news, it’s important to hear that as well.
While the battle is far from over, there has been a series of breakthroughs in the long-running battle to protect the imperilled Leuser ecosystem in northern Sumatra, Indonesia – the last place on Earth where tigers, orangutans, rhinoceros and elephants still live alongside one another.
The government of Aceh Province – which controls most of the Leuser ecosystem and has been subjected to withering criticism for its schemes to destroy much of the region’s forests for oil palm, rice and mining expansion while opening it up with a vast road network through the forest – has agreed to a moratorium on new land clearing and mining.
This is huge news, and it’s clear that both the international community and Indonesia’s federal government have played big roles in making this happen. Indonesian President Joko Widodo deserves a great deal of credit for this accomplishment, which he has been pushing for many months, not just in Aceh but elsewhere in Indonesia too.
It is the culmination of an almost three-year battle by the Alliance of Leading Environmental Researchers and Thinkers (a scientific group I founded and lead) as well as many other dedicated researchers and conservationists.
Set in stone?
Moratoria can always be cancelled or weakened, but the chances of that happening seem increasingly remote. In a speech at last month’s signing of the Paris climate agreement in New York, Indonesia’s environment and forestry minister, Siti Nurbaya, underscored her commitment to the Leuser moratorium.
It seems unlikely that she would make this statement at such a high-profile event if there were any significant possibility that the moratorium will collapse.
And the news gets even better. Last week, Aceh’s deputy governor, Muzakir Manaf, declared that he will provide full support for ground-level measures needed to enforce the moratorium.
That is critical, for two reasons. First, it shows that the Aceh government is strongly behind the moratorium. Second, a moratorium is just a piece of paper unless there is real on-the-ground enforcement to ensure that illegal land-clearing, poaching, mining and other activities don’t continue unabated.
Limiting palm oil
A final piece of good news is that Nurbaya has confirmed her intention to halt completely the granting of new permits for oil palm plantations in state-owned forests right across the country.
To be clear, this doesn’t mean that oil palm plantations won’t keep expanding in Indonesia. There are thousands of existing permits encompassing many millions of hectares of native forest. Indeed, Indonesia has previously announced plans to clear a further 14 million hectares of native forest by 2020, mostly for oil palm and wood-pulp production.
But at least it means that the avalanche of new oil palm permits is coming to an end, for which both Widodo and Nurbaya deserve credit.
Not over yet
The fight to conserve Indonesia’s mega-diverse forests is far from over. The nation’s plans for massive road, dam and mining projects – many in forested areas where they can open a Pandora’s box of problems such as illegal poaching, logging and forest burning – is enough to frighten even the most sober of observers.
But for today, at least, we can celebrate a very significant victory for conservation, and give credit to the many people who have worked to raise the profile of Leuser, including the actor Leonardo DiCaprio, who visited recently.
Few have had more impact than Ian Singleton, director of the Sumatran Orangutan Conservation Program. In a recent interview, Singleton laid out a remarkably compelling and detailed argument for saving Leuser, and for the surprisingly limited economic benefits its exploitation would generate for the local Sumatran citizens.
The economic and environmental think-tank Greenomics Indonesia also deserves a big round of applause for its efforts to facilitate this groundbreaking achievement.
But while we’re congratulating ourselves and others, we shouldn’t forget to keep a close eye on Leuser to ensure the promised moratorium really does take effect, and that one of the most important wild places in the world still survives.
This is an edited version of a blog post that originally appeared here.
The link below is to an article on Orangutans in Sumatra and the threat posed to them by deforestation, and a fire that has burnt unchecked there as a result of logging.