These days wild monkeys on the Son Tra Peninsula in the central city of Da Nang, a top tourism destination in Vietnam, wait until garbage is dumped in landfills behind the local Linh Ung Pagoda, then begin scavenging the piles for leftover food.
Their search continues even when workers prepare to burn the trash.
Open coconuts are a favorite find.
One monkey drinks from a bag of soup.
Having got used to searching for food in trash dumps, the monkeys are not afraid of humans.
"The fact that the monkeys are no longer searching for food in their natural habitats indicates a big change in their behavior," said Bui Van Tuan, an expert with the GreenViet Biodiversity Conservation Center.
"Food from the trash dumps might be spoiled or contaminated, which can make the monkeys sick as their immune systems aren't equipped to deal with these kinds of food," he added.
"Many monkeys have lost their fur which might be due to different reasons, including food from the trash dumps that is too salty," said Tuan, warning that the condition could spread within the monkey colony and even to other species in the Son Tra Nature Conservation Center.
Visitors to the Son Tra Conservation Center ignore a sign asking them not to feed wild animals.
This has prompted the monkeys to rely on humans for food instead of finding it on their own in the jungle, said Tran Thang, head ranger for Son Tra and Ngu Hanh Son districts.
"Visitors are making a mistake, not knowing that the monkeys being friendly means they are losing their natural food foraging habit," he said.
By feeding the monkeys, visitors can also catch diseases from them, said Nguyen Duc Vu, head of the management board of the Son Tra Peninsula and Da Nang beaches.
Authorities would continue warning visitors in Son Tra not to feed the monkeys, he added.