This summer the Denver Zoo will borrow a brown koala from the San Diego Zoo, not for breeding but to attract more visitors. Speaking of how briefly the koala will be in Denver, zoo director Clayton Freiheit called the loan ''a limited engagement.'' It was as though he were speaking of a movie booked into a cinema. He began to correct himself for the way that sounded, then fell silent.
Rather than beg alms of broke municipalities or rely more heavily on private donors, zoos from San Diego to New York have taken up the top hat and cane. Show business is helping zoo business, and it's a rare zoo administrator who apologizes for it. ''If zoos remain dull, they won't attract visitors, and without more visitors, zoos are in trouble,'' said Tim O'Sullivan, deputy director of the Bronx Zoo in New York.
Two years ago the city of Denver cut its money to the zoo by 59 percent, said Mr. Freiheit, forcing the zoo to double its admission price. Then, so as not to lose attendance, the zoo rolled out a multimedia advertising campaign and borrowed a white Bengal tiger from the zoo in Omaha.
The result was a record-setting increase in attendance and revenues. This year the zoo has contracted with a San Diego bird trainer to bring in a wildlife show and will continue the program begun last summer of hosting the Denver Symphony for outdoor performances on the zoo grounds.
Summer concerts are traditional in European zoos, but here, other attractions are being created for that highly prized game, the hometown American spender.
''We want people to come back to the zoo,'' said James Doherty, curator of the Bronx collection. ''Nobody wants to leave Manhattan. We're up against all the great cultural attractions in the city . . . but if all the people who came to the zoo as kids would just come back and see how much it's changed, we know they'd be delighted. And so would we.''
Having little space for animal-performance shows, the Bronx Zoo brings in extra visitors with holiday events. At Easter, when eggs begin to appear in nests, the zoo shows off its bird collection; at Halloween it naturally turns to bats and owls. ''We do have an elephant show,'' Mr. Doherty acknowledged, ''though we prefer to think of it as an elephant behavior show. For the past two summers, two times a day, our trainer has shown what three elephants can do in the way of pulling and pushing - the sort of things that elephants have historically been trained to do. We certainly don't make fools of them: no hats, no glitter, no flourish.''
The same idea prevails at the animal shows in the San Diego Zoo. But elsewhere some glitter has begun to show: performances by Chinese acrobats, pop-rock concerts in the Wild Animal Park. Last year the zoo appointed a new general manager, Terry A. Winnick, who, for the previous nine years, had designed and directed the tours at Universal Studios. The zoo horticulturalist promptly quit in a dispute with Winnick, and other employees grumbled that the zoo was becoming an amusement park.
When a siamang recently escaped its moated enclosure in the new Heart of the Zoo complex, hundreds of visitors gathered to root for the tree-swinging ape as it eluded the keepers' darts and nets. Within minutes the keepers were circulating a joke: ''I'll bet Winnick put the siamang up to this. Anything to please a crowd.''
''I do consider myself to be a showman,'' said Winnick, ''in the sense that I am interested in putting on the best kind of experience for the greatest number of people.''
Charles Bieler, the zoo director, said that Winnick had been hired for his expertise in architecture and crowd control, not for show business. And further, he said, the acrobats and the concerts were parts of the necessary strategy for the zoo to hold its own against other crowd pleasers in the region.
''Twenty years ago, the zoo, Disneyland, and Knott's Berry Farm were the major attractions,'' he said, but now many others compete for the ''leisure dollar.'' And although the zoo will not engage in a ''rollercoaster war,'' building merely sensational attractions, it will work harder to please its traditional patrons.
''Our studies have shown us some things about why people come to zoos,'' he said. ''The primary reason: It's something the family can do as a whole. No. 2: to entertain visitors. And No. 3: to see the animals.'' He added that the zoo is justified in striking up the band for more families and tourists because the income will go toward improving the grounds and maintaining zoological research.
''Are we becoming too commercial?'' asks Charlie Hoessle, director of the St. Louis Zoo. ''We think it's a matter of survival. Zoos have become survival centers where many species must be bred to produce sufficient numbers to keep them from extinction. The conflict is that zoos must finance these activities from their general coffers. . . . So, in order to serve the animals, the zoos must also do what they can to promote themselves and their animals.''
The Brookfield Zoo in Chicago, the first to put animals and visitors on an equal footing with moated, barless enclosures, has gone even further in equalizing man and beast. Last Mother's Day, the zoo named two lowland gorillas ''Mothers of the year,'' and invited the public to visit other motherly animals as they were given special feedings in honor of the day: a cake of oats to the Clydesdale, laurels to the giraffe.
''More than being a menagerie any longer, zoos have to get people to feel they are welcome and a part of the activity,'' said Pat Bares in Brookfield's marketing office. ''Still, we're so busy with promotion, it sometimes hits me: Did I think of an animal today? Sometimes I haven't.''
Bieler, one morning in his woodsy new office, definitely had animals on his mind. ''We think we've got a baby polar bear,'' he told a visitor. ''It somehow got separated from its mother, and we've got a very strong line on bringing it down. I hope we can. It would add to our collection. And, of course, it would bring a lot of people in, too.''