A 15-year-old male elephant at Birsa Munda Biological Park allegedly turned aggressive without provocation and killed its mahout on Sunday morning, an unprecedented incident in Jharkhand that has left zoo officials in Ormanjhi, 20km from the state capital, thoroughly bewildered.

Animal welfare advocates who are against keeping large mammals in captivity consider space crunch unhealthy enough for elephants to effect sudden changes in their behaviour. Experts also say that once bull elephants reach their reproductive phase or musth — normally at 10 to 15 years of age — the level of testosterone can increase up to 60 times, leading to exceptional aggression.

Birsa zoo officials are yet to make an official statement on why Ramu, the tusker, lost his cool and killed mahout Mahendra Singh, 56, who had raised him since he was a baby.

Sources said the tragedy took place around 8am, when the zoo had not yet opened to visitors. After the mahout’s death, the authorities decided to keep the park closed for the day. Visiting hours at the 104-hectare Birsa zoo start from 9am and end at 5pm while Monday is the weekly off.

“Singh had taken Ramu out for his routine walk on the premises. All of a sudden, the elephant lifted the mahout with his trunk and flung him to the ground. Singh was rushed to Medanta hospital, which is nearby, but he succumbed to grievous head injuries,” said a zookeeper. He added that Ramu’s behaviour was rather odd because Singh had raised him since childhood and they loved each other.

Director of Birsa Munda Biological Park S.K. Gupta said the attack on the mahout was unprecedented in Jharkhand zoo history.

“We are baffled why Ramu suddenly became so aggressive and that too towards a man he knew so well. Ramu had been fine in his enclosure before being taken out for the walk. After the mahout died, we closed the park,” Gupta said, adding that a decision on whether to reopen after Monday’s break would be taken later.

Zoo vet Dr Ajay Kumar sounded equally perplexed.

“Singh was a veteran mahout. Had he noticed something wrong with Ramu, he would not have taken the elephant out for a walk in the first place. Also, Ramu has been calm since his fleeting phase of aggression. I offered him sugarcane and he ate it. What exactly happened will be probed. We will speak to other zookeepers,” the vet said.

Mukti Roy of Asian Nature Conservation Foundation, Bangalore, which works for conservation of Asian elephants, said it was difficult to say why Ramu behaved in such a manner without knowing the circumstances. “An elephant may get angry if its mahout is drunk or didn’t feed him on time or making him do things he didn’t like,” Roy added.

Ormanjhi thana OC Santosh Kumar said it was fortunate the incident took place before visiting hours.

“There were no people inside. Families, including children, frequent the zoo on weekends. Things could have gone out of control,” he said, adding that the mahout’s body had been sent to RIMS for autopsy.