Guwahati, July 16: Six months from now, Swati would have celebrated her birthday at the Assam zoo.

But that was not to be, as the 21-year-old tigress Swati passed away of old age today in the early hours, leaving behind a pall of gloom.

Swati was the oldest tigress at the zoo and was brought from Mysore zoo in 2005.

"She had actually become weak because of old age and last night she did not have the strength to get up. She passed away around midnight," Tejas Mariswamy, divisional forest officer of the zoo, told The Telegraph.

The zoo now has four tigers, all female. The average lifespan of a tiger varies between 14 and 16 years.

Mariswamy said the zoo staff were upset at hearing about the death of Swati as many of them, especially the animal keepers, used to look after the tigress.

"They were attached to the big cat and they fondly remembered the day when she had come to the zoo in 2005. They had developed a personal bond with her," he said.

The aged tigress's diet was boneless chicken.

The oldest tiger in a zoo in the country was Guddu, 26, who died in January 2014 at Kanpur zoo. The tiger was brought to Kanpur zoo in 2001 from Chandigarh when he was 13.

Mariswamy said Swati had 11 cubs of which five were born in Mysore. Of the six born in Assam, one is in the Assam zoo, one in Tata Zoological Park in Jamshedpur and two in Raipur zoo, Chhattisgarh. Two others died.

"We are trying to get details of the cubs from Mysore," he said.

Narayan Mahanta, who was the divisional forest officer of the zoo when Swati was brought in, said it was sad that she has passed away but all credit should go to the zoo for keeping her healthy and alive all these years.

"We had rescued a male tiger from Tezpur, named him Aniruddha and brought him to the zoo. He needed company. We searched everywhere and found a matching female in Karnataka. Her name was Swati," he said.

Aniruddha passed away a couple of years ago.

Mahanta said after taking approval from the Central Zoo Authority, Swati was brought to Assam zoo. We had a lot of apprehension whether they would match. But after some time, both of them accepted each other," he added.

Mahanta said the zoo was trying to procure another tiger. A male Royal Bengal tiger died in the zoo on June 27.