Camera traps have caught wild chimpanzees in the act as they carried out night-time raids on farmland.
The footage, captured by researchers from the Museum of Natural History in Paris and the Uganda Wildlife Authority, shows the chimps adapting to human pressure on their habitat.
The team says this is the first record of 'frequent and risky' nocturnal raids to find food.
It is published in the journal Plos One.
The researchers carried out their study in Kibale National Park in Uganda, setting up their camera traps on the edge of the park.
'[It's] surrounded on the outside by smallholder farms, forest fragments and tea estates,' the researchers said in their paper.
They think that humans encroaching on chimp habitats may have 'promoted' the animals' foraging trips into cropland. But the scientists were still surprised by how daring the chimps' raids were.
Major threats to wild chimps
- Habitat destruction by 'slash-and-burn agriculture'. Deforestation across West and Central Africa has severely reduced chimpanzee habitats. It is estimated that more than 80% of the region's original forest cover has been lost;
- Logging, oil and gas mining also pose a threat, especially through road building to access remote areas. This can degrade and and fragment chimp habitats;
- Poaching: Low population densities and slow reproductive rates mean that hunting can quickly wipe out local populations. Chimps are hunted for meat, the pet trade and even medicinal purposes. Farmers may also try to protect their crops and livelihoods by setting traps for animals that raid their land;
- Disease is a leading cause of death in chimpanzees. Because chimps are so similar to humans, they succumb to many diseases that afflict humans. And as the number of encounters between chimpanzees and humans increase, this increases the risk of transmission of diseases including common human respiratory infections. Ebola is also a major threat to wild chimpanzees and gorillas
Source: International Union for Conservation of Nature
Groups of about eight chimpanzees took part in each raid; these groups included vulnerable animals, such as females with clinging infants.
They would raid during the day as well, but during night-time raids the chimps stayed longer in the maize fields. They also showed fewer signs of vigilance and anxiety, such as looking around or scratching themselves roughly - a recognised signal of stress.
Dr Catherine Hobaiter, an expert in chimp behaviour from the University of St Andrews, said the observations were fascinating.
Habitat loss, she explained, was a much greater threat for the chimpanzees than natural predators.
'It forces chimps to explore new food sources, like human crops,' she told BBC News.
'Raiding fields is extremely dangerous - chimps may be attacked or even killed by people defending their crops, but by raiding at night [these chimps] seem to have reduced this threat.'
Despite how adaptable the chimps have shown themselves to be, Dr Hobaiter says the behaviour is also worrying.
'Such a dramatic change suggests the chimpanzees are responding to a very strong pressure to obtain the basic foods they need to survive - a response to the widespread destruction of their natural forest home.
'While it might be working for now, this won't be a long-term solution.
'As local people become aware of these nocturnal raids they may try to defend their fields in the dark, and the risks of conflicts escalating and injury to both chimps and people is likely to increase,' she said.
'From a conservation perspective, the only long-term solution is the protection of the remaining forests.'
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