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."
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."
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."
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)
