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Primate keeper Dannielle Stith at the Oakland Zoo in California explained that gibbons have very little body fat, and can't swim.
"Our exhibit is surrounded by a mote, which they avoid because they don't like water," Stith said.
She added that they have "super dense hair," having hundreds of additional hairs per inch than humans do. The thick coat helps to keep their skin dry in moist rainforest habitats, but is another impediment to swimming, Stith explained.
Researchers in Asia have struck species gold of late by exploring remote rivers. Kai He, for example, said that a recently discovered species of Myanmar snub-nosed monkey, Rhinopithecus strykeri, was found to be isolated from its sister species, Rhinopithecus bieti, by the Salween River.
"Rivers in southwestern China and northern Myanmar have played an important role in shaping speciations, not only in primates, but also other terrestrial species," he said.
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It remains to be seen how our own species might have been shaped by barriers like large rivers, particularly early in human evolution when our ancestors had more body hair and were better adapted to life in the trees.
Gibbons are not too far down on the human family tree either, given that they are closely related to chimpanzees, gorillas and orangutans. They actually share many features with us, according to the Tree of Life web project, since they have no tail, maintain a semi-erect posture, have an adaptation that allows them to turn their hands palm-side upwards, have a big brain and more.
Some might even say that gibbons connect too well with us, which could explain why they are so coveted in the illegal pet trade. In some U.S. states, it is even still legal to own a pet primate.
"They're very cute," Stith admitted, admiring her own vocal charges at the zoo.