They might swing in the canopy, but orang-utans expend fewer calories by mass than your average human couch potato. In fact, the only placental mammal that expends less is the sloth, according to of Washington University in St. Louis, Missouri, and his team, who analysed the apes’ urine (Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.1001031107).
An animal’s daily energy use usually increases in direct proportion to its mass. The heavier it is, the more energy it needs. But Pontzer’s team thinks the irregularity of food availability in the orang-utans’ habitat in south-east Asia has led the animals to evolve an especially thrifty metabolism. The endangered apes depend on a diet of ripe fruit, but the rainforest’s supply can suddenly plummet, forcing them to survive sometimes months at a time without adequate food.
While thrift is helpful in waiting out the forest’s fickle fruit cycles, it doesn’t help replenish dwindling numbers. Orang-utans take 12 years to reach adult size and have a gap of seven years between offspring, the longest of any primate. However, there could be a silver lining in that degraded forests, once considered unsuitable, may actually be a suitable habitat for the apes.
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“If we drive this species to extinction,” Pontzer says, “the extra shame would be that it is actually a species that has evolved to be a survivor in really tough conditions.”
