The tropical peat-swamp forests in Central Kalimantan, Borneo are home to the largest surviving populations of the endangered red apes. The only truly arboreal great apes, orangutans have no significant predators besides humans. Loss of habitat is surely the greatest threat to their continued survival, but the quality of the habitat they have left is also a concern. Orangutans don’t just need forests to survive; they need the right kind.
Lacking any top-down population control from predators, the main factors that predict how many orangutans can survive in a given patch of forest is how much nutrition that forest provides for them. In other words, their population size is controlled from the bottom up. Areas with lots of fruit available allow the orangutans plenty of energy. Mothers with enough food to support their gestation and lactation need less time to recover between pregnancies. Those living in areas without adequate nutrition need more time to find more food before they’re ready to reproduce again. It should be obvious that a struggling population needs to have as many babies as possible if they’re to survive, especially amidst all the stresses that our species has placed on them.
Within a single forest, different areas can offer different nutritional opportunities – even if they look identical to the casual human observer. The Sabangau Forest is a typical peat bog. The only nutrient input it gets is from rain. Because it has such a limited source of nutrients, its plants ought to offer up fewer nutritional resources to orangutans. Just 39 miles away is the Tuanan research station, situated on land with much shallower peat. That allows plants to soak up nutrients more easily from the dirt that lies underneath it. Because it’s also near a river, the area experiences seasonal flooding, which offers an additional influx of nutrients beyond the ones that fall down in precipitation. It’s perhaps not surprising then that Sabangau supports around 2.3 orangutans per square kilometer, while Tuanan supports nearly twice as many.
To find out if nutrient availability really is the limiting factor – if it explains why Tuanan supports more individuals than Sabangau – Rutgers University anthropologist Erin R. Vogel and her colleagues compared the nutritional contents of orangutan foods at the two sites. In all, they assessed 39 food items from 22 plant species that were eaten at both sites.
Just as they predicted, Tuanan offered better nutrition than did Sabangau. That’s despite the inclusion of the same food items on the menu in both spots. More specifically, Tuanan orangutans gobbled up more “metabolizable energy” than did Sabangau overall because there was more metabolizable energy per “plant item.” That means the orangutans’ increased caloric intake at Tuanan was not due to differences in feeding rates or in the mass of the foods they ate, but instead because the food was packed, bite for bite, with better nutrition. Even during the low-fruit season, Tuanan orangutans ate an average of 819 more calories each day; during the high-fruit season, they consumed 2,345 more calories than those living in Sabangau every day.
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That’s all despite the finding that Tuanan orangs actually spent less time feeding on fruit than did their Sabangau counterparts. And the Sabangau apes didn’t spend more time foraging than Tuanan ones because more fruit was available; the Tuanan forest was actually slightly more productive. Instead, they focused on fruit (as opposed to leaves) more in an effort to “make up” for the reduced nutritional quality of their food.
As a result of the mismatch between the orangutans’ energy requirements and energy intake, Sabangau orangutans operated on a nutritional deficit 84% of the time over the four years they were observed. By contrast, Tuanan orangutans’ energy expenditure exceeded their intake just 20% of the time. Because they eat so well during the high-fruit season, the Tuanan population is better positioned to survive the low-fruit season.
Vogel and her team also discovered that the Tuanan orangutans had more overlapping home ranges than did the Sabangau ones. While their individual ranges didn’t change in size, they suspect that the nutritional quality of the Tuanan forest better allowed the orangutans there to coexist without competing over access to resources.
Next, Vogel says that she plans to compare the fecundity of the two orangutan populations to see if the nutritional differences she’s observed carry over into reproductive ones. If they do, as she suspects, then it will be “important to incorporate careful habitat quality assessment and maintenance into future conservation action plans for this endangered species,” she sa