MYSTERIES MULTIPLY FOR ZOO OFFICIALS.

Byline: Michael Coit Daily News Staff Writer

A baby boom has hit the Los Angeles Zoo's chimpanzees, deepening the mystery - who are the fathers or father?

Not to be outdone by the unexpected birth of Toshi in January, chimps Gracie and Regina are pregnant, surprised zoo officials said Wednesday.

``It just seems like we can't stop them from having babies,'' said Vicki Bingaman, the chimpanzee keeper.

Gracie is due later this month, and Regina will deliver in about three months, zoo officials said.

The good news in this unfolding story is that there is room in the new exhibit for Toshi and the two others. ``They weren't planned, but they are wanted,'' Bingaman said.

Now as a safeguard, all of the chimps have been placed on birth control, even though the males of breeding age have undergone vasectomies.

Exactly who fathered Toshi can't be determined for at least an additional six months, when mother Yoshiko and she can be separated for DNA testing.

In the case of Gracie and Regina, handlers suspect Shaun, one of three males vasectomized to prevent breeding. Yoshiko's mate might have been Jerrard, another one of those vasectomized, zoo officials said.

The reason for limiting the number of births is to prevent too many from becoming closely related, said Michael Hutchins, conservation and science director for the American Zoo and Aquarium Association in Bethesda, Md.

Under the Species Survival Plan called for by the association in 1991, all participating zoos must regulate their chimpanzee breeding.

Hutchins said he knows the Los Angeles Zoo is trying as best it can.

``But there are sometimes biological limitations. This is the luck of the draw,'' he said. ``Abstinence would be a tough sell to a chimpanzee.''

The three vasectomized in 1996 were Judeo, Shaun and Jerrard. The oldest male, Toto, was not vasectomized because he never produced offspring and showed no interest. Glenn is believed too young to breed, and Ripley is too young, said Dr. Cynthia Stringfield, zoo veterinarian.

Only Ripley, who still nurses, is not a suspect in the three recent pregnancies.

``If it's Toto, then everyone will be thrilled because he's a wild-caught male. Genetically, he is extremely valuable,'' Bingaman said.

Although only 4, Glenn might be a long shot candidate because sexuality is accelerated in captivity. Bingaman noted that chimpanzees typically don't reproduce in the wild until age 12, yet Regina delivered Yoshiko at age 7.

So it's back to the pill for the female chimpanzees.

``We thought we had a fail-proof method with the vasectomies,'' Stringfield said.

Then Toshi was born, and two more babies are on the way.

``My first reaction was disbelief. Then it was excitement, and then it was realization of the path ahead,'' Stringfield said.

Bonnie, Pandora, Nan and Joanna have been taking birth control pills in daily yogurt feedings since Feb. 21, and Yoshiko will begin in April. Only baby Toshi is not on the plan.

The pills are just like those women take and should have a similarly high success rate, Stringfield said.

Hormone implants and vasectomies have not been as successful as birth control pills, Stringfield said.

Zoo officials noted there were no pregnancies when birth control pills were given beginning in 1991 following the recommendation of the national zoo association.

But the zoo switched to hormone implants because keepers had difficulty keeping young chimps and males from eating the pills in the older, much smaller enclosure. In the $5 million habitat that opened in August, keepers can separate or catch the females when they're alone, Bingaman said.

Three chimpanzees were born when the implants were used between 1992 and 1994. There also were two miscarriages and one death during birth, Bingaman said.

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Photo: (Color) Regina is one of two pregnant chimpanzees at the L.A. Zoo.

Andy Holzman/Daily News