Chimpanzees in a national park in Uganda have been filmed raiding farmers' fields inside the park's borders, augmenting their natural diet with the farmers' planted corn.
Crop raids by chimps are a long-standing problem through the animals' range, but the new incursions into cornfields had one significant difference -- they happened in the dead of night.
Normally, chimps are strictly diurnal, or daytime, animals that avoid nighttime activity because their historic main predators, leopards, hunt at night.
With the decline of leopards, and with snares set by humans replacing them as the main threat, the chimpanzees have shown how adaptable they can be, researchers say.
Sabrina Krief with the National Museum of Natural History in Paris, France, and colleagues used motion-sensing video cameras to capture chimps in nighttime crop raids in and around Uganda's Kibale National Park.
A 6-foot-wide and 6-foot-deep trench had been dug around the cornfields to keep out marauding elephants, but that didn't stop the chimps, which used a convenient fallen tree that had bridged the trench to make repeated forays into the plantation.
In a 20-day period, they used the natural bridge over the trench to conduct 14 separate raids of the cornfields, they researchers say.
Such nighttime incursions are a new behavior for the chimps, they say.
"No previous studies reported crop raiding after sunset," the researchers noted in reporting on their study in the journal PLOS One.
Another surprise, they said, was that rather than small groups seen previously, usually just two or three chimps, the nighttime raids averaged up to eight animals, often including mother chimps with their young in tow.
Females were often seen leading the invasions, they said.
And instead of quickly eating the food at the site of the raid, the chimps were seen leaving the area with ears of corn gripped between their teeth or carried in their hands, obviously intent on eating them later and at a different location.
The new behavior is seen as evidence the chimps are learning to co-exist with human neighbors and even take advantage of them as farmlands increasingly encroach on their habitats, the researchers said.
The chimps' demonstrated ability to move effectively in the dark is a puzzle, they say, since the animal have no special adaptations allowing them to see at night.
"As of today, the nightlife of chimpanzees has been neglected, and we have probably missed some interesting activities," the researchers wrote.
