Chimpanzees in Uganda’s Kibale National Park are supplementing their diet with maize from a plantation within the park’s borders. While crop raids are a well-known problem throughout the chimps’ range, these animals were filmed venturing into the fields in the dark of night—a first for chimpanzees.

Wildlife is often a problem for people living on the edges of parks and preserves. One study found that crop raids by chimpanzees and monkeys in Rwanda caused losses equivalent to 10 to 20 percent of a farmer’s income. Chimps have been recorded eating parts of 36 different crop species, from bananas and papayas to lemons and coffee. It seems little is off the menu for a hungry chimpanzee.

A crop raid, though, can be a dangerous activity for a chimp. While the animals can be scared off by throwing stones or banging pots, some people have resorted to harsher measures, killing chimps to deter potential thieves. With chimpanzees already dwindling in numbers because of habitat loss, poaching and disease, the endangered animals hardly need another source of human conflict.

To better understand the crop-raiding behavior of chimps in the northern part of Kibale, Sabrina Krief of the National Museum of Natural History in Paris and her colleagues set up camera traps on the edge of a maize plantation in February 2013. The maize fields were separated from the forest by a 6.5-foot-wide, 6.5-foot-deep trench that was dug to keep out elephants. But the trench had been bridged by a fallen tree, and there were chimp footprints and corn debris nearby.

Over a 20-day period, chimps were caught on video using the tree as a bridge to make 14 raids into the fields. These raids were unusual, the researchers report today in PLOS ONE. There were more animals per outing—an average of eight instead of three—and the groups often included mothers with young. Females sometimes led the raids. And rather than eating the food on site, the chimps often left the fields with ears or stems of corn in hand or gripped in their teeth.

Most notable was that several of the raids occurred after twilight, in full dark. “No previous studies reported crop raiding after sunset,” the researchers note. Until now, it had seemed that chimpanzees preferred to venture into fields during late afternoon or close to sunset. When raiding during the day, though, the animals screamed and barked and showed signs of anxiety, such as letting loose feces. Under the cover of darkness, the chimps displayed fewer signs of anxiety and less vigilance. They also showed less hesitation at entering a field and seemed less inclined to leave quickly.

Chimps don’t have any special adaptations to see at night, so the researchers aren’t sure how the animals are able to effectively move around in the dark. The scientists are also unsure whether the chimps were deliberately taking advantage of the dark or were simply raiding when the corn was at its peak. “As of today, the nightlife of chimpanzees has been neglected, and we have probably missed some interesting activities,” the scientists write. Because the park is in an equatorial zone, nighttime lasts for about half of each day all year round, so that’s plenty of unaccounted time.

However, while the ill-gotten gains probably help the chimps find enough food in a shrinking habitat, raiding maize fields might also have a significant downside: Chimps in this area of Kibale have a high frequency of congenital deformities, which, the researchers write, “may be related to the exposure to chemicals while crop raiding.”