"Life expectancy for those in captivity is far below than those living freely," said a zoo official. "The average age of animals in captivity ranges between 16 and 65 depending on the species," he added.

The zoo authorities have become cautious of aged animals after the death of a rhino a few months ago

"We have already drawn the attention of the Ministry of Fisheries and Livestock about these animals, if the animals die it will be natural death, we wont be responsible for it," said Mafizur Rahman, Curator of the Zoo.

The animals ear-marked with age are pairs each of mandrills, elephants and giri baboons; and one each of sambur, tapir, zebra, gayal, horse, chimpanzee, olive baboon, gharial, crocodile, black bear, hyena, great Indian horn bill and ostrich.

Some of them have reached the average age much earlier and are not a pleasant sight to visitors at the zoo. On a visit to the zoo, it was found that these animals were looking exhausted and fatigued and looked hardly alive. Caretakers say that they sleep most of the time and very rarely move.

"When we try to feed them, they do not respond and there is nothing we can do to cheer them up," said a caretaker who is in charge of the monkey family. The only chimpanzee at 26 has reached its peak.

It was alleged that apart from these animals deteriorating because of age, authorities are not aware if an animal is seriously ill until it dies.

With the death of some prominent animals, most cages are empty. People are less interested in visiting the zoo under these circumstances, said a zoo official.

"My children were eager to see the Chimpanzee. They knew it to be an agile animal, but to their disappointment they found a sleeping chimp," said Nurullah from Jatrabari, who came to the zoo with his children.

The hospital has no provision for diagnosis or quarantine and the veterinary surgeon does not get sufficient time to treat animals properly, especially because they are sent to the hospital at the latter stages.