Watching the two female Masai giraffes at the Los Angeles Zoo, it is easy to imagine them loping across the savannas of their native Africa. Like their kin in the wild, the slender giants browse, chew their cud and rub their long necks together.
One is pregnant. The other is in a state unknown in nature: She is on birth control.
While the arrival of a baby elephant or panda is often a major media event, preventing unwanted pregnancies is also a key part of captive breeding programs today.
Because of better nutrition and healthcare, and more naturalistic exhibits that allow greater access to others of their species, zoo animals reproduce like never before and routinely live into old age. As a result, contraception has become crucial to keeping zoo populations from exploding.
"It's a huge part of our job now," said Cynthia Stringfield, senior veterinarian at the L.A. Zoo.
In some countries, surplus animals are put down. But because Americans won't tolerate such treatment, most North American zoos practice a form of planned parenthood, aided by a growing pharmacopeia of birth-control agents and techniques.
During most of the 20th century, spaying and castrating were the standard ways to keep animals from reproducing. But zoos now avoid those practices whenever possible. Such surgeries permanently banish the individual from the breeding pool and can cause undesirable physical and behavioral changes.
Lions whose testes have been removed gain weight and lose their manes. Castrated elk develop weirdly shaped antlers. A chimp who has been neutered may have trouble bonding with his fellow males.
Separation of males and females keeps animals from breeding, but disrupts the normal relationships among social animals. By using the new contraceptive implant Deslorelin, the L.A. Zoo can keep its yellow-footed rock wallabies together, while preventing them from "breeding like bunnies," Stringfield said.
While not foolproof, birth-control options for zoo animals have surged in the last decade, from sustained-release implants to a vaccine that makes eggs resistant to sperm.
But birth-control decisions remain complex. In addition to efficacy and safety, zoos must take into account reversibility, cost and possible impact on a fetus or nursing young, Stringfield said.
She regularly consults the website of the American Zoo and Aquarium Assn.'s Contraceptive Advisory Group, which for 15 years has collected data on a dozen contraceptives in nearly 300 species, from bats to dolphins.
The original animal birth-control device was probably the desert diaphragm devised centuries ago by Arabs who inserted small stones into the vaginal canals of camels. When the L.A. Zoo wanted to keep its camels from reproducing, it temporarily sterilized its male with the synthetic hormone Lupron. Used to treat human endometriosis, the drug would have cost $48,000 annually, had it not been donated by the manufacturer.
After consulting the association's contraceptive website, Stringfield put a gorilla on human birth-control pills. The ape had been on an anti-fungal medication that could have caused birth defects, and the pill seemed like the safest option.
With endangered and rare animals, genetic concerns often drive contraception. The sex lives of all 50 Masai giraffes in North American zoos, including L.A.'s four, are managed by a Species Survival Plan, developed by a committee of the zoo and aquarium association.
A reproductive future has been worked out for each animal, based on complex calculations of which pairings will maximize genetic diversity and long-term survival of the world's tallest land animal.
To avoid inbreeding, the planners first translate the relatedness of any two animals into a kinship number. For instance, .125 indicates first cousins, .25 a brother and sister, .5 a parent and offspring.
"It's kind of like computerized dating," said Jay Kirkpatrick, a veterinarian at ZooMontana, who pioneered the use of a contraceptive vaccine that keeps wild horses from overrunning Maryland's Assateague Island National Seashore.
In the wild, many animals have incest-avoidance mechanisms that minimize inbreeding. For instance, young male elephants usually leave their birth group, as do young male lions, mandrills and Belding's ground squirrels.
But captive animals aren't free to find a new neighborhood. As a result, zoos have seen considerable inbreeding and its attendant genetic ills.
In the late 1970s, biologist Katherine Ralls found that inbreeding harmed 41 out of 44 populations at the National Zoo in Washington, D.C; a third of the offspring of siblings died.
When possible, zoos intervene. A recent contraceptive implant will increase the odds that Elinya, a young yellow-footed rock wallaby, does not breed with her father, also at the L.A. Zoo.
