A tigress killed a man at theBirsa Biological Park here on Wednesday after the youth jumped into the big cat's enclosure, an official of the zoo said."When the youth, in his twenties, jumped into the enclosure of the tigress around 11.30 am, she attacked the man on his neck, killing him instantly," the official said.The zoo was closed for some time following the incident, he said.One of the man's friends, who reached the zoo after he came to know about the incident, told the police that the deceased was "mentally not stable", the official added.