Secret Night Lives of Primates Revealed

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Most primates are active either in the day or night, but camera traps are revealing that some monkey and chimp day

dwellers also go out at night for things like pool soaks and

snacks.

The latest to be snapped unaware is the Guizhou snub-nosed

monkey, Rhinopithecus brelichi. Once thought

to be exclusively diurnal, this Asian monkey's nightlife is documented in the

latest issue of the journal Primates.

"Our camera trap photos showed Guizhou snub-nosed monkeys

moving in trees at night," lead author Chia Tan told Discovery News. "We

believe the monkeys were on their way to search for food."

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Tan, who works at the San Diego Zoo Institute for Conservation

Research, and her colleagues Yeqin Yang and Kefeng Niu spied the monkeys after

setting out the camera traps. Guizhou snub-nosed monkeys are endangered, with

just a single global population of 700-800 individuals restricted to

Fanjingshan in southwest China.

The monkeys were active during both day and night all year

round, but they went out more at night during the early spring and autumn.

"We think the monkeys are extending their activity beyond

daylight hours to increase feeding, and the highly sought after food items are

young leaves in spring and fruit and seeds in autumn," Tan explained.

She added, "It makes sense that the monkeys take advantage

of these super nutritious foods to maximize their reproduction and survival.

Spring and autumn are critical times for the monkeys; they are the birthing and

mating seasons, respectively."

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Since their forest home is often foggy, the researchers

suspect that the monkeys may have evolved the ability to see under low light

conditions. Poor eyesight along with night predators, such as the clouded

leopard, would make for a potentially disastrous combination, but the monkeys

seem to have mostly overcome it.

Other primates that are active during the day have been

found to have nightlives too. The well-named owl monkey, for example, is known

for its nocturnal ways.

Humans who have swum on a hot summer's night will also

appreciate what was discovered about our closest living primate relatives.

"A recent camera trap study conducted in Fongoli, Senegal,

revealed nocturnal behavior — pool soaking — in savanna chimpanzees," Tan said.

Eduardo Fernandez-Duque, an associate professor of

anthropology at the University of Pennsylvania, told Discovery News that Tan's

team used a "novel approach to the study of primate activity."

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He added, "The article combines the use of some new

technology with traditional approaches to learn that the snub-nosed monkeys,

traditionally considered diurnal, may show some nocturnal activity under

certain circumstances. Their proposition that this flexibility in activity

patterns may be associated to the temperate environment is reasonable."

Humans, of course, fall into the day and/or night-living

primate category too. This is often a survival tactic, as people with late

shift jobs could attest. It's possible that our distant relatives were 24-7

individuals as well.

"It is difficult to infer a full range of behavior from

fossilized species, but I would not be surprised if some of our earliest human

ancestors were regular night owls," Tan said. "They would, under certain

environmental conditions, be up at night. Party anyone?"

(Images: Chia Tan)