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Jon Ballou is a studmaster.

Studmaster of the golden lion tamarins, to be exact. From his office at the National Zoo, Dr. Ballou decides which golden lion tamarin—or “GLT”—is going to breed, when, where, and with whom. And not just for the 10 or so GLTs that reside at the zoo. Ballou oversees the sex lives of all 480 tousle-headed Leontopithecus rosalia rosalias in captivity around the world.

Basically, Jon Ballou plays God.

“With a small “g,’ ” he insists.

Ballou’s role is critical to the survival of the GLTs, which have been reduced to a population of less than 600 in their shrinking natural habitat, the Atlantic coastal rain forest of Brazil. Due to the wild tamarin’s uneasy future, zoos and private GLT owners banded together to form an international Species Survival Program (SSP), ceding control of their animals’ mating habits to Ballou. The GLT program has become a model for some 70 SSPs that carefully control and orchestrate the reproduction of threatened populations.

But endangered species aren’t the only zoo animals mated with scientific precision. If the National Zoo, or any zoo for that matter, wants certain animals to breed, it must employ a host of behavioral and husbandry techniques. When such “persuasion” is not adequate, advanced reproductive techniques—everything from hormone therapy to artificial insemination to in vitro fertilization—are brought to bear. And if reproduction is not desired, it can be prevented using methods as elementary as separating mates, or as sophisticated as Norplant-style contraceptive implants.

Being a monkey matchmaker is no small responsibility. Ballou is charged with keeping the gene pool of the golden lion tamarins as diverse as possible. To do that, he must overcome decades of inbreeding. Until15 years ago, zoos tended to encourage as many offspring as possible, responding to a perceived need to propagate endangered species and the more mercantile impulse to have baby animals on exhibit. In fact, the majority of the GLTs in captivity today are direct descendants of a pair that lived in the National Zoo’s monkey house 20 years ago.

But when the harmful affects of inbreeding tamarins—such as early mortality and congenital heart defects—were documented around 1980, a global effort was launched to improve the stock.

Now, zoos and the private members of the SSP fill out questionnaires every year on the gender, lineage, offspring, dates of birth and death, and health characteristics of each of their tamarins. Ballou compiles the results in a “studbook” that allows him to determine the blood-line and genetic characteristics of each animal without having to perform costly DNA tests. The information is then fed into a computer program that helps pair off the GLTs.

The goal is to couple one set of rare GLT genes with another of equal genetic value. The need to diversify is so great, says Ballou, that “we don’t intentionally select against abnormalities unless it is extremely deleterious.” For example, many tamarins have a hole in their diaphragm that can lead to an early death—but if the animals are otherwise valuable, they are still bred.

Selecting which GLT matches will enhance the gene pool is the hard part. Once a decision has been made to mate a tamarin in Washington with, say, one in Hong Kong, the animal is shipped overseas using the same sky-kennels that you might use for your dog or cat. That’s all there is to it. When the prospective mates are put together, says Ballou, “90 percent of the time they reproduce.”

In fact, GLTs breed so well that zoos must take definitive steps to make sure that only the monkeys they want to reproduce do, and then only as often as necessary. That’s where family planning comes in. Traditionally, population management relies on housing animals in combinations that won’t reproduce, or separating animals during the breeding season. But the GLT population, like that of more and more zoo animals today, is largely controlled by contraception.

In other words: monkey Norplant.

Surgically implanted progesterone may be a relatively new phenomenon in humans, but the cattle industry has been using it for decades. And the same progesterone that works for a cow can work for a kangaroo or for a human. “Progesterone is progesterone,” says Dr. JoGayle Howard, of the National Zoo’s New Opportunities in Animal Health Sciences (NOAHS) Center. “The only difference between species is the dose.” In fact, says Ballou, when progesterone was first used on GLTs in 1985, the vets simply took a capsule designed for a tiger and, based on body weight, sliced off what they presumed would be a tamarin-sized dose.

It’s no coincidence that the original dose was a tiger’s, for it is with big cats that the National Zoo’s NOAHS Center—a mobile laboratory team that transports scientists and sensitive equipment all over the world to help endangered animals—is pioneering new methods of birth control. Researchers have even developed a “male pill” for big cats, an anti-sperm drug called bisdiamine. “We’ve known about this for years, but you can’t take it with alcohol—so forget about getting human males to take it,” laughs Howard.

Whatever their other vices, lions and tigers and bears don’t drink. But even for animals vanity is an issue.

“We still want the big mane,” says Howard, explaining that veterinarians, like human males, fear less testosterone could result in a less-than-manly appearance. For example, while loweringa lion’s testosterone can end “aggression problems,” it can also reduce secondary sex characteristics. In other words, no testosteroneno mane, no maneno crowd appeal, and no crowd appealno crowd.

Tamarins breed like, well, rabbits. But to propagate species that are endangered and hard to mate, both “basic research and the latest technology” must be brought to bear, says Howard.

Take the cheetah. For many years, this animal was virtually impossible to breed in captivity. Worse, says Howard, nobody had any idea why. Females would fail to “cycle” or go into heat. Even when they did, “the males weren’t interested. We’d put them together and nothing would happen.”

So gradually vets and keepers developed techniques to encourage mating. “We keep them active, and move them around to different females. If nothing happens after two years, we step in,” says Howard. And step in they do. Males are given testosterone to make them randy. Females are similarly injected with hormones to induce ovulation. Howard also found that female cheetahs needed to be exposed to more than one male to put them in the mood. “They seem to want a choice,” smiles Howard. So the cheetah equivalent of cocktail parties were instituted.

And if husbandry methods and hormone therapy don’t work, there’s always artificial insemination (AI).

At first, Howard tried nonsurgical insemination. The sperm was pumped directly into the cat’s vagina. The result? After 20-odd attempts, no pregnancies. “So I tried the same thing in domestic cats,” says Howard. The house cats, too, failed to get pregnant. Eventually, Howard discovered that putting male cats—domestic or large—under anesthesia reduces sperm mobility. For cheetahs, low mobility was further complicated by a low sperm count, the result of inbreeding.

Sperm is milked from all male animals by way of electrical stimulus. Delicately put, if you are going to prompt donation in this manner from a very large carnivore, anesthesia is a necessity. So Howard had to find some way to make the most of the sperm she had.

She found an answer in laparoscopic artificial insemination. In this procedure, a female cheetah is anesthetized and prepared for surgery. A half-inch incision allows Howard to wend the laparoscope—a fiber-optic telescope—directly into the uterus. Using the laparoscope to guide her, Howard then uses a needle inserted into the cheetah’s uterus to release the sperm near the egg. So far, 40 percent of the cheetahs that have undergone AI have been successfully impregnated.

A room a few doors down from Howard’s office functions as a sperm bank that contains the ejaculate of valuable animals from all over the world. “That’s panda sperm, and that’s cheetah sperm,” says Howard, pointing to a big blue vat. One advantage of frozen sperm (liquid nitrogen is used for transport) is that it can be used long after an animal’s death. In fact, sperm can be milked from a corpse within 24 hours after death. Sperm banks also allow zoos to screen for FIV (feline immuno-deficiency virus), a deadly cat virus that operates much like AIDS.

With AI and mobile labs, zoos no longer have to treat captive and wild populations as separate breeding pools. Instead, vets use both to diversify the species’ genetic base. This has become increasingly important as inbreeding ravages the wild populations of certain species trapped in small geographic pockets—i.e., national parks and reserves. With AI, animals in the wild can be tranquilized, brought to an animal MASH unit, milked for sperm or inseminated, and released.

Once AI proved successful, other advanced human fertility techniques were quickly adapted to large cats. On July 18, 1994, two other NOAHS doctors injected the sperm of a rare Siberian tiger into the egg of another. The actual fertilization took place in a petri dish; the fertilized egg was then put back into the uterus of the female tiger. On Nov. 5 that year, at Omaha, Neb.’s Henry Doorly Zoo, the world’s first test-tube tiger was born.

Using in vitro fertilization, Howard hopes to implant females that are not genetically valuable with the fertilized eggs of rarer cats who are unable to carry a pregnancy. And unlike human surrogates, cats don’t sue for custody. “They don’t know the difference,” she notes.

For the most part, zoo reptiles are bred by private suppliers who can mate the animals better, faster, and cheaper. But there are exceptions, the biggest of which is the Komodo dragon. The National Zoo was the first to breed the world’s largest monitor lizard outside Indonesia—but not without great difficulty.

The zoo tried to mate Komodos for many years without success. Officials didn’t know if the lizards should be housed together or brought together only to breed. In 1988, the zoo placed a new lizard couple in a specially designed exhibit and tried again. “We had no idea how long it would take for her to lay her eggs,” says Dr. Dale Marcellini, head of the herpetology department. Lay them she did, but on three occasions keepers “found eggshells that had been passed through—one of them ate her eggs. The male would eat them for sure, and the female, she don’t care,” he laughs.

The next time the keepers noticed the female was “getting fat and burrowing,” they put down more dirt so she could bury her eggs. In 1992, the zoo was rewarded with a clutch of some 20 eggs, which keepers took out of harm’s way. Incubation was also a matter of guesswork. “Sometimes luck, planning, and skill come together and you are successful,” says Marcellini. After nine months, the clutch was hatched, and many of the offspring given to other zoos.

The remaining young presented another conundrum. “The biggest problem is we can’t tell what sex they are until they’re really big,” says Marcellini. Before the next clutch hatched, the herpetology department tried to analyze the egg whites to determine the sex of the offspring. If the technique proves successful, it will be applied to other reptiles that are hard to “sex.”

Normally, wild Komodos have a high infant mortality rate. “They’d have to,” says Marcellini, “otherwise we’d be up to our ankles in Komodos.” Now that the zoo has bred a few clutches and provided other zoos with the lizards, the Komodo dragon will soon join the ranks of species whose captive populations must be kept down.

And the next studmaster will be chosen.