1991 is a big year for the San Diego Zoo. The zoo is 75 years old and special activities are scheduled throughout the year.

The $11.5 million Gorilla Tropics Habitat will open March 23. The exhibit, located behind the flamingo lagoon near the zoo's entrance, will house six gorillas in an environment that includes hills, streams, trees and jungle plants.It also includes a walk-through rain forest and aviary.

Zoos all over the world are giving animals to the San Diego Zoo in honor of its birthday. The zoo has received koalas from Australia, Komodo dragons from Indonesia, Burmese takins and Corsac foxes from the U.S.S.R.

As part of the celebration the zoo will be giving away souvenir pins throughout the year.

Plants sculpted to look like animals will be on display during the Topiary Festival May 23-27.

The zoo is conducting a nationwide memorabilia search. Submit anything you have from the zoo that's old. The items will be displayed at the zoo.

The world's largest LEGO gorilla will be assembled by a master builder May 24-26 and on display for the rest of the year.

Children will be admitted free during October, the zoo's birthday month. Everyone gets in free Oct. 7, Founder's Day.

The zoo's history began with the 1915-16 World's Fair in Balboa Park. The fair had a row of caged animals including lions.

Dr. Harry Wegeforth was driving near the park and heard the roar of a lion. He thought it would be splendid if San Diego had a zoo. That evening he met with friends and founded the Zoological Society of San Diego.

The zoo has grown into one of the world's most respected.

Among the youngsters born to the zoo in recent years: twin Sumatran tigers named Maya and Durga who were introduced to the Tiger River exhibit a year ago December. Keepers said the cubs were timid at first, staying close to the entrance before exploring the exhibit. Once they reached the river, they played in the water and attacked the bamboo.

A male sun bear cub was born on Sept. 17, 1989. His birth was only the second birth of a sun bear in a zoo. The cub, named Stanley, weighed one pound at birth. His mother was protective of him and would curl around him and growl whenever she sensed a threat.

Sun bear mothers teach their cubs carefully. They are also strict disciplinarians. They don't hesitate to swat their cub if it's doing something dangerous or isn't paying attention.

On Aug. 28, 1989, the zoo had its first birth of a mountain tapir. The mountain tapir is considered one of the rarest mammals in the world. The baby weighed 11 pounds at birth. He eventually will weigh 300 to 400 pounds.

He will gradually lose the white spots and stripes he was born with. Mom, dad and baby are on display at the zoo every day.

Among the zoo's most exotic animals is a spotted cuscus from New Guinea. The animal is a small, snow-white marsupial that lives in the mountains of New Guinea. There is only one other spotted cucus in a U.S. zoo.

Admission to the zoo is $10.75 for adults. It includes the children's zoo and a ride on Skyfari. Bus tours of the zoo are extra.

Admission for children is $4.

Gates are open from 9 a.m. to 4 p.m. in winter and 9 a.m. to 5 p.m. in summer.

The zoo is located in Balboa Park near downtown San Diego. For information call (619) 234-3153.