Why Killer Sloth Bears Became Hyper-Aggressive to Humans

The aggression of sloth bear's could be explained by the species' early coexistence with tigers, a new study suggests.

Sloth bears are considered one of the most dangerous animals in Asia. Although an estimated 20,000 or fewer remain in the wild, the bears are notorious for attacks on humans, which have increased in recent years—the species kills over a dozen people each year, according to a 2020 National Geographic report.

Human-wildlife conflicts occur globally, however, most predators do not attack unless provoked—for example, an animal may attack if it feels that its territory is threatened. And although numbers are on the rise, deaths remain relatively rare.

However, researchers have long suspected that sloth bears have more of a tendency to attack, when compared with other species. In June, a sloth bear killed and mauled a man and woman in India before feasting on their remains for hours.

Sloth bear
A stock photo shows a sloth bear. They are one of the most dangerous animals in Asia. gnagel

India-based conservation non-profit Wildlife SOS embarked on a study to asses why the species is prone to such behavior.

The study, published on Nature.com, assessed sloth bear conflicts occurring in the Indian state of Karnataka and the Deccan Plateau—a preferred habitat of the bears, comprised mostly of rocky scrub forests. The study recorded 180 sloth bear attacks in the region.

Researchers noted that the bears' aggression could be down to evolutionary factors. They may be more prone to "innate defensive-aggressive responses to surprise (sudden)" encounters, as they co-evolved with tigers, the study said—a predator that will prey on sloth bears opportunistically.

Sloth bears are not often hunted by other animals. The study noted that cubs may be preyed upon by wolves or leopards, however, their only "natural predator," and only realistic opponent, is the Bengal tiger. According to tiger scat studies, 2 percent of a Bengal tiger's diet may be comprised of sloth bear.

"Since [sloth bears] cannot outrun tigers and cannot climb trees fast enough to escape tigers, sloth bears have evolved an aggressive behavior to fight tigers off," M. Swaminathan, a senior biologist on the study, told Newsweek. "While tigers can still have the upper hand over sloth bears who share their habitat, their aggressive behavior sometimes has tigers thinking that they just aren't worth all the trouble."

The study said that tigers no longer live on the Deccan Plateau, however, the large array of caves in the area "undoubtedly historically benefited sloth bears, perhaps facilitating a higher density than would have been otherwise attainable."

Although aggressive tendencies potentially stem from their evolution, the study noted there are other factors at play when it comes to conflict.

In the Deccan Plateau region, researchers found that sloth bear attacks were most common in the winter months. At this time of year, more local people become active in the forest, as the monsoons have come to an end.

Similarly, previous studies conducted in central India have found that attacks are more frequent during monsoon season, which is typically when local people are active outside farming and protecting crops.

The study noted that there are a higher number of attacks when human activity is at its highest levels in their habitats.

"We conclude that the seasonal activity of bears plays a much smaller role on attack rates than the seasonal activity of humans. Consistent with findings in other studies, human incursion into bear habitat is the primary factor responsible for precipitating conflict," the study read.

Despite their reputation, Kartick Satyanarayan, co-founder and CEO of Wildlife SOS, told Newsweek that sloth bear attacks happen when the animal feels threatened.

"There has been an increase in human-sloth bear conflict wherein, in some cases, sloth bears behave aggressively toward humans when threatened," Satyanarayan said. "Human activity and encroachment both within and outside protected areas result in high frequencies of human-sloth bear conflict and restrict the movement of sloth bears and other wild animals among fragmented forest patches.

"This species is also threatened by habitat loss and poaching for body parts," he said.

"In various regions, people fear these animals due to their aggressive and unpredictable nature. Oftentimes, sloth bears are libeled as 'problematic' or 'nuisance-causing' animals. Local communities respond to crop damage by attacking sloth bears in retaliation. Such incidents not only lead to injuries and in some cases mortality but inculcate insensitivity and negative perception about wildlife."