A spike in testosterone and two female elephants on heat are the likely reason a baby elephant pinned its trainer against a pole, says Taronga Zoo.
The findings of an investigation into the elephant attack which nearly crushed zoo keeper Lucy Melo to death last October were made public on Tuesday.
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Early puberty caused elephant attack
An internal investigation by Taronga Zoo has found that unusually high testosterone levels in elephant calf, Pathi Harn, caused his boisterous behaviour that left his trainer pinned to a pole in October 2012.
Acting general manager of research and conservation, Dr Rebecca Spindler, said neither the zoo nor the trainer nor the elephant calf Pathi Harn had been at fault.
"We've confirmed two of our female elephants were coming into oestrus at roughly the same time," Dr Spindler told reporters at the zoo.
"More surprising is that when we examined Pathi Harn's testosterone, he went through a spike in testosterone that day that was higher than any of our males, including our adult male."
Dr Spindler said the two-year-old calf had developed at a faster rate than most baby elephants.
"Pathi Harn, we now know, is a very unusual elephant," she said.
"He's developed much more quickly than anyone could have expected.
"It's entirely possible this spike in testosterone contributed to the sudden changes in behaviour we saw that day."
She said it appeared Pathi Harn had no ill will towards his trainer.
"We're not sure if the two females and the spike in testosterone caused him to act that way, but it was out of the ordinary for Pathi Harn and even for elephants his age to have those levels of testosterone."
Most elephants do not display any indications of testosterone until they are four years old, Dr Spindler said.
Tuesday also marks Ms Melo's return to the zoo on a full-time basis.
She spent 12 days in a Sydney hospital in October after she was pinned to a bollard by Pathi Harn, formerly known as Mr Shuffles, during a routine training exercise.
Ms Melo was conscious when paramedics first arrived, but lapsed into unconsciousness and had a cardiac arrest for about five minutes.
After a spell of convalescence at Sydney's Royal North Shore Hospital, Ms Melo was given the all clear to return home.
While recovering in hospital, she suggested the zoo's review include a blood test that examines Pathi Harn's testosterone.
Dr Spindler said the zoo had used Ms Melo's advice and experience with elephants to try to understand what happened between the trainer and the calf.
"We're a mixed family. The keepers and the elephants interact extraordinarily closely," she said.
"All the elephants greeted Lucy when she came back. The elephants moved up towards her with low rumbles, and greeted her very warmly."
The zoo said it would implement additional safety measures to accommodate for Pathi Harn's hyperactivity.
"We've never seen the like of this before," Dr Spindler said. "I can liken the early changes in behaviour in Pathi Harn as a little bit like your toddler getting up and showing the first signs of walking.
"So you 'baby-proof' the exhibit, which is what we started doing after it happened.
"We now understand that some of the most difficult parts of managing him is his unpredictable behaviour."
Ms Melo and the zoo's other keepers will continue to work with Pathi Harn on an accelerated training program.
"I just wanted to personally thank everyone for their ongoing messages of support," said the trainer in a video message on Tuesday.
"It has meant to much to me and it has truly helped me in my recovery.
"I want to reassure everyone that I am well and truly back at work.
"Looking after these elephants is my priority."