Actually, two: Naga, an 8-foot, 150-pound male Komodo dragon, and Sobat, a 6-foot, 60-pound female.
Mr. Arnett's job: get these endangered lizards to breed.
''To introduce Naga to Sobat, who had never seen or smelled each other before, was fraught with danger,'' says Mr. Arnett, area supervisor of reptiles, amphibians and fish, and the Komodo dragons' primary keeper at the Cincinnati Zoo.
It was dangerous not only for the keeper - another male dragon three times tried to disembowel Mr. Arnett with its sharp, serrated teeth - but for the animals. In their native habitat (three islands in the Indonesian archipelago), 40 percent of a Komodo dragon's diet is other dragons.
''A small female, a third the size of a large male, will rip a male to shreds if she does not want to breed with him,'' Mr. Arnett says.
But he was even more concerned about Sobat. She was on loan from the National Zoo, at that time the only U.S. zoo to successfully breed the reptiles.
''I was there, if (Naga) tried to hurt her. This was the only female (Komodo) in the Western Hemisphere. At that point she was the most valuable reptile in the world, probably. It had to work.''
For a week, the reptiles were kept in separate areas, unable to see each other, but aware of each other through smell. Then came the introduction, which went smoothly.
After her eggs were fertilized, Sobat exhibited typical Komodo dragon behavior: She began attacking her mate. Zoo keepers had to rescue Naga.
Sobat laid eggs in January 1993. At the time, only the National Zoo had incubation equipment needed to hatch them. So the precious cargo flew in business class on a Delta jet, strapped into a seat next to a zoo keeper.
Eight months later, 14 of 24 eggs hatched.
A second clutch that Sobat laid in Cincinnati began hatching early in 1994. Those were incubated here, and 18 of 24 eggs hatched.
All told, the Sobat-Naga union produced 32 dragons. Cincinnati and the National Zoo each got 16, and most of Cincinnati's are now on loan to other zoos.
''If I ever get another clutch, I think I can hatch them all,'' Mr. Arnett says, given what he now knows about incubation temperature and humidity.
But neither Sobat nor Naga will be permitted to breed again, Mr. Arnett says, because of a need to diversify the Komodo dragon gene pool. To that end, he expects in August to bring back two pairs of Komodo dragons from a zoo in Indonesia.
On a cool morning, David Oehler stands in tall, dewy grass in a spacious pen at the Cincinnati Zoo's Mast Breeding Farm. A white-naped crane approaches the zoo's curator of birds.
''This is great for the birds,'' Mr. Oehler says. ''Not great for the public.''
Visitors are not permitted at the Clermont County farm. The pens - which occupy an area larger than a football field and can accommodate up to eight pairs of Asian cranes - were designed solely for the propagation of the endangered birds.
Although cranes reproduce well in captivity, Mr. Oehler says, ''they need a lot of space,'' which is in short supply at the 67-acre zoo in Avondale.
The farm provides privacy, lush grass that is allowed to grow long during the breeding season, and plenty of insects, which the birds devour for protein.
Such an environment no doubt has contributed to the farm's success in breeding white-naped, Manchurian and hooded cranes, Mr. Oehler says.
In addition, the farm now has one extremely rare Siberian crane. That male is awaiting a mate.
Introducing a new pair of cranes can be tricky, especially if one bird is overly aggressive. ''There's a lot of personality going on here,'' Mr. Oehler says.
A pen is divided; a male on one side, female on the other. A portion of the fence is covered by a shade cloth, but the birds can see each other through another part. Keepers then watch for a tell-tale sign that the birds are ready to be united: synchronized walking.
Like a pair of figure skaters, they mimic each other's movements.
When that happens, ''we know we've got a pretty good pair bonding,'' Mr. Oehler says.
Three times a week, Terri Roth performs a rectal ultrasound test on Emi, a 1,700-pound Sumatran rhino. It is dirty, smelly work. And crucial, perhaps, to the survival of a species.
Killed by poachers for their horns, Sumatran rhinos number less than 400 in the wild. Of the 19 in captivity, only two females and one male are in the United States, all at the Cincinnati Zoo.
Mr. Maruska calls the hairy Sumatran rhino ''a living fossil'' because it has changed relatively little in 30 million years. No Sumatran rhino has been bred and born in captivity this century.
''We're putting a lot of emphasis on it,'' Dr. Roth says. ''Because basically, this is the last chance.''
In the past, Emi has rebuffed Ipuh, the male. That has exacerbated the typically combative rhino courtship, sometimes resulting in injury to Emi and putting a halt to breeding attempts until she heals.
Dr. Roth began working with Emi in December, two months after she was hired to direct the Cincinnati Zoo's Carl H. Lindner Jr. Family Center for Research of Endangered Wildlife (formerly the Center for Reproduction of Endangered Wildlife).
CREW has an international reputation for its pioneering work in reproductive technology. That work will continue, but Dr. Roth says she also will focus on understanding how a captive environment affects reproduction.
She has been using ultrasound since February to better understand the Sumatran rhino's reproductive cycle.
''We're not sure why, if this female is coming into estrus (or heat), she won't stand for the male. One of the questions early on was, maybe she's not even cycling. Now, we can eliminate that. She does appear to be having regular cycles where the ovaries produce eggs and ovulate.
''Now the question is, maybe we were just introducing them at the wrong time. So when she wasn't receptive, he'd get mad.''
Typically rhinos breed 24 to 48 hours around the time of ovulation, Dr. Roth says. By tracking Emi's cycle via ultrasound, Dr. Roth hopes to be able to predict when the rhino will ovulate.
''So that's when we'd really focus on introducing the animals, and see if then she's receptive and will let him breed her.''
Emi, who arrived two years ago on loan from the Los Angeles Zoo, munches apples and bananas in a stall in the Wildlife Canyon building as Dr. Roth uses a probe to bounce sound waves off the rhino's reproductive tract.
''This is her ovary,'' Dr. Roth says, eyeing a TV monitor. ''That dark circle is a follicle on her ovary.'' The machine measures the follicles, so Dr. Roth can tell whether they are growing.
The ovaries of the zoo's other female appeared inactive, so Dr. Roth no longer monitors her.
If natural breeding doesn't work, artificial insemination is a possibility, Dr. Roth says. But so far, researchers have been unable to get sperm from the male.
Why, one might wonder, would any zoo want to breed bugs?
''We're trying to educate people about the natural world,'' Randy Morgan says. ''How can you say you're doing that if you're omitting one of the most important groups of animals on the planet?''
Humans would cease to exist without insects, notes Mr. Morgan, Cincinnati Zoo's associate curator of entomology. They pollinate plants, serve as decomposers and play a critical role in the food chain.
More than 100 species of animals, mostly insects, are displayed in the zoo's Insect World. Usually, more than half the insect species are in some stage of breeding.
And for good reason. Some come from faraway places, so replacing them isn't easy. And many have life spans of only a few weeks or months, so an assembly-line production keeps displays full.
Breeding insects is not unlike breeding other small animals; they must be well fed and have a clean, comfortable living area.
Case in point: bullet ants.
Four times in the 1990s Mr. Morgan has excavated bullet ant nests in Peruvian rain forests. In the process, he felt the ants' sting - 30 times as powerful as a typical bee or wasp sting - more than a dozen times. Each sting left his arm useless for several hours.
During the fourth excavation, Mr. Morgan procured a queen. He also made detailed measurements of nest architecture.
Once the inch-long insects were back in Cincinnati, he tried various combinations of heat, humidity and nest chamber sizes. He also experimented with the ants' diet.
Unlike most ants, bullet ants didn't care for honey water, but they loved sugar water. And, ''They really got really excited over termites.''
A sign that the ants are thriving: The colony, which went on display last July, has nearly tripled in size.
Mr. Morgan says he knows of no other zoo in the world that has successfully collected, bred and displayed the ants.
Early this century, zoos couldn't keep gorillas alive.
''A famous quote from the director of the Bronx Zoo in 1908 was, he didn't think a gorilla would ever be kept in captivity, never mind breeding,'' says Dan Wharton, director of the Central Park Wildlife Center in New York and chairman of the SSP for western lowland gorillas.
But the outlook changed with improvements in veterinary medicine and hygiene. By the late 1960s and early 1970s, zoos were learning that gorillas need not be kept in sterile, tiled environments.
Today in North America, a healthy population of western lowland gorillas numbers 351, and is growing 2 percent to 3 percent a year. Cincinnati is the U.S. leader in gorilla births with 45.
''For animals to become breeders, the biggest thing is that they have to feel at home in their environment,'' says Mike Dulaney, the zoo's general curator. ''We're trying to make more naturalistic exhibits, and to have natural (social) groups.''
Cincinnati Zoo's Gorilla World houses 14 western lowland gorillas, 12 of which were born here. They can explore an open-air compound. Or, if they need a break from the public, hide behind rock work.
In the wild, one adult male leads a group that includes several females. That, too, has been replicated at the zoo.
Diets also are carefully controlled, because overweight animals aren't effective breeders.
Another consideration: the gorillas' psyche. Even in the best of conditions, simply putting males and females together does not guarantee success. ''They have personalities,'' Mr. Dulaney says.
The zoo has been trying unsuccessfully to get a silverback named Colossus to breed since he arrived here four years ago from a Florida zoo. Because he was born in the wild, his genes are unrepresented in the captive gorilla population.
''It's something that just may take time,'' Mr. Dulaney says. ''Perhaps now if he starts interacting with youngsters, a paternal instinct could kick in.''
And youngsters are plentiful, thanks to Chaka, a male on loan from the Philadelphia Zoo. He has fathered seven offspring since arriving here in 1994.
