Six Flags Discovery Kingdom in Vallejo is preparing for the rare birth of a walrus calf in the next few weeks.

There have been no more than 11 recorded walrus calves born in North American zoological facilities since 1931, according to Michael Muraco, the theme park's animal care director.

Park officials are cautiously optimistic that the union of male walrus Sivuqaq and female Uquq will be a success. The odds of a calf's survival are about 50 percent.

There are only 17 walruses currently living in U.S. zoological parks and oceanarium facilities, and the three 16-year-old Pacific walruses, including Siku, another female, at the Vallejo park are among the few that are still viable breeders, Discovery Kingdom's communications manager Nancy Chan said.

The park acquired the three orphaned walrus calves in 1994. They were recovered from a federally authorized Native Alaskan subsistence hunt near Gambell, Alaska on St. Lawrence Island in the Bering Sea.

Six Flags Discovery Kingdom was one of only a few zoological institutions in the country authorized by the U.S. Fish and Wildlife Service to take in and care for beached and stranded Pacific walrus calves, Chan said.

Little is known about reproduction in walruses, but Muraco's wife, Holley Muraco, a marine mammal reproductive physiologist, has shed some light on why they rarely reproduce in captivity.

She determined sunlight is the primary regulator of the male reproductive season, Chan said.

As the three walruses matured, it was believed breeding would occur naturally, but the two females' breeding periods did not coincide with Sivuqaq's seasonal, sunlight-dependant mating period, or rut, Chan said.

Holley Muraco tracked Sivuqaq's production of sperm and captured the walruses' mating on video.

"The pregnancy was confirmed via ultrasound in the fall -- even though we were fairly confident of the situation through our tracking of the significant changes in hormone levels and other physiological changes," Muraco said.

She also studied and documented courtship behaviors.

"Pacific walruses remain one of the least studied and most mysterious pinniped species with a complex reproductive biology adapted to life in the Arctic," Muraco said.

"Although they live long, healthy lives in zoos and aquariums, they don't reproduce very often. My studies have focused on understanding why so that we can maximize reproductive success and answer key questions about their reproductive biology. Hopefully, this information will be useful for zoos and aquariums, and also assist wild animal management," she said.

Muraco monitored Uquq's progesterone level, which increased tenfold during her reproductive cycle. That and different physiological changes indicated Uquq was most likely pregnant.

It is believed the walrus gestation period is 18 months but this has never been proven. By documenting Uquq's pregnancy from conception to birth, the gestation period will be verified.

Alaska Natives know a walrus is pregnant only in late winter or early spring because they are fatter than nonpregnant females, Michael Muraco said.

"That is the only time period -- very late pregnancy -- that outward appearance gives any indicators of pregnancy," he said.

A fetus would not normally appear on an ultrasound until about a month before birth but Holley found a live, moving fetus as early as five months ago, Muraco said.

Chan said the birth is expected in a matter of weeks.