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The Mating Game

Mating sounds simple. It isn't. Finding the right match for a Zoo animal takes hard work and hard science.

By Brittany Grayson

Cuddly bear cubs, awkward elephant calves, fluffy bird chicks, and even precocious newly hatched amphibians are common sights at the Smithsonian’s National Zoo. Visitors flock to admire the newest arrival and to exclaim at its endearing awkwardness.

Births are often a triumph for zoos. Reproducing in captivity indicates that animals are healthy and well cared for. Sometimes such births are even important on a more global level; species that are endangered or even extinct in the wild augment their numbers by breeding in the protected environment of a zoo.

Births at the Zoo, though, are usually more than just a product of a simple gestation period. This is demonstrably true in the cases of artificial insemination, where Zoo reproductive scientists and veterinarians have had to inseminate the female, watch her closely for signs of a pregnancy, and then monitor her through pregnancy and birth. It is less obvious, though, that every birth at the Zoo takes hours of forethought and planning before conception even occurs. Almost none happen effortlessly.

It sounds counterintuitive at first. If a zoo has an endangered species on the premises, why not breed as many as possible? Just put a bunch together in an exhibit and let them handle the rest, churning out as many babies as they can. The reality, though, is that such a plan is not feasible for most zoos. And in many cases, it turns out, it is not even a good idea.

The Evils of Inbreeding

A key problem with letting zoo animals do what comes naturally is the risk of inbreeding. Zoo scientists Jon Ballou and Kathy Ralls conducted one of the first studies of inbreeding in zoos. What they found was that as a population became more inbred, infant mortality increased. Inbreeding can also lead to animals’ having fewer babies, dying younger, and suffering from more diseases and abnormalities. The cause lies in the animals’ own DNA.

Each organism born from sexual reproduction (most vertebrates and zoo animals) receives two copies of every gene in its body: one copy from its mother and one from its father. That means that an animal has two chances to get a healthy version of every gene, rather than a defective one. When parents are closely related, though, the chances are higher that an animal will get two copies of a defective gene, which gives it a greater risk of inheriting abnormalities and diseases. The more inbred a population is, Ballou and Ralls discovered, the more frequently harmful traits appear. “The genes are already there in the population,” Ballou explains. “Inbreeding just uncovers them.”

Their study began on dorcas gazelles but quickly expanded to other organisms. Since then, researchers at other zoos all around the country have found similar results. Zoos started realizing that they had a problem on their hands.

Zoo scientists have been highly successful at breeding golden lion tamarins. (Mehgan Murphy/NZP)

At the National Zoo, Jon Ballou and Devra Kleiman delved deeper into the problem by focusing on golden lion tamarins. They knew the family tree of the current zoo population of golden lion tamarins, including roots that went back to wild ancestors. The family tree, however, was not very helpful to look at. A snarl of black lines, zigzagging back and forth across the page, it looked more like a tangle of blackberry brambles than a royal lineage.

Ballou wanted to find a way to use that family tree to produce a population of golden lion tamarins that was as outbred  (the opposite of inbred) as possible. “We had this mess,” Ballou explains. “We spent years trying to figure out how best to do it. And it turned out to be simple.”

The process does not look simple in its mathematical form, but the concept is elegant. Ballou and his team looked at each tamarin and, using the family tree, figured out how distantly it was related to every other tamarin in the population. This “mean kinship” allowed them to calculate which tamarins were the least related to each other. Breeding tamarins that were least related to the population overall would result in the most genetically diverse, and healthy, population possible.

Ralls and Ballou took another big step forward by organizing a meeting of geneticists from around the country and asking them to give advice on how the captive population should be managed. This meeting took place at the National Zoo’s campus in Front Royal, Virginia. Working together, population biologists and zoo scientists agreed that they needed to preserve 90 percent of a population’s genetic diversity for at least 200 years for that population to be considered healthy and successful.

Lifeguards in the Gene Pool

Scientists gradually convinced zoo directors that no one zoo had enough animals of each species to meet their goal of maintaining sustainable populations of wild animals, both in zoos and for conservation and reintroduction efforts.

The Association of Zoos and Aquariums (AZA), the body that oversees and accredits North American zoos, took the lead in solving this problem, by creating Species Survival Plans (SSPs). Loosely based on the recommendations from the meeting at Front Royal and Ballou’s golden lion tamarin breeding scheme, SSPs call for looking at animals at every zoo, and breeding the least related to generate a healthy, self-sustaining population.

That seems a herculean task, requiring massive amounts of data and expertise. Not to mention massive numbers of animals. Paul Boyle, senior vice president for conservation and education at the AZA, explains, “What really drove the creation of the SSPs was the realization that even zoos with a large number of animals had nowhere near enough animals to be able to ensure that level of diversity over the long term. You should really have at least 100 individuals in the breeding population, but the more you have, the better off you are. That really drove the realization that the zoological community needed to work as a community, not as a collection of individuals, to ensure these animals’ survival.”

In 1981, the first SSP was established, and by the late 1980s, many zoo animals, especially charismatic or endangered species, were being managed under this umbrella. “Before that,” Boyle explains, “zoos viewed themselves as islands with their local populations. What we have now is truly amazing. Every one of the accredited zoos and aquariums, each with its own collection, manages the collection as if all animals in all accredited zoos and aquariums in North America are actually one single collection, almost a national zoo.”

This pooling of zoos’ resources has given them many more options in deciding how they breed and manage their populations. It has also given all the animals a better chance at maintaining healthy, sustainable populations in captivity, and at providing individuals for eventual reintroduction programs.

This pooling of zoos’ resources has given them many more options in deciding how they breed and manage their populations. It has also given all the animals a better chance at maintaining healthy, sustainable populations in captivity, and at providing individuals for eventual reintroduction programs.

Each species managed by a Species Survival Plan has a species coordinator who in effect looks after every animal of that species in AZA-accredited zoos. That person keeps records of the family tree of each animal in the zoo population, going back as far as possible. Then, with the help of population biologists at the AZA’s Population Management Center, the species coordinator uses software based on Jon Ballou’s original golden lion tamarin program and looks at each individual animal in the zoo population. He or she tries to match up animals that are genetically most appropriate for each other, taking facts such as age, location, and breeding history into consideration.

Actually running the software to generate the breeding recommendations is not extraordinarily difficult, according to Ballou—“It would take me five minutes to show you how to do it”—but making breeding recommendations is as much an art as a science. “What looks like an optimal pairing in the genetics may actually not be,” he says, “due to the animals’ location or temperament. A lot of the animals that are valuable are that way precisely because, for whatever reason, they’ve never bred before, despite being given the opportunity.”

Complicated Questions

The breeding guidelines offer zoo scientists and keepers objective information and a way to make science-based decisions about which of their animals they breed or ship to another zoo for breeding. It also prevents zoos from letting individual animals overbreed. Some animals are naturally good at breeding in captivity, but Ballou explains these animals should not be allowed to dominate the gene pool: “The problem with not having scientific breeding recommendations is that the ‘best’ breeders often have the most offspring.” These breeders ‘flood the market,’ so that in the next generation, it becomes difficult to find animals that are not related to each other to breed.”

Allowing breeding to take place without any human intervention could also lead to animals “domesticating” themselves, by adapting to captivity, according to Ballou. Natural selection, which occurs when animals choose their own mates and breed as the opportunities present themselves, produces animals best suited for their current environment. In zoos, those that breed the most, and leave the most descendents in the next iteration of the gene pool, would be the animals that are the most tolerant of life in captivity.

“It may be that the best breeders in captivity wouldn’t be the best breeders in the wild. They may be the ones that tolerate humans, or a captive diet, better than other animals,” Ballou says. Effectively, the wildness would be bred out of zoo animals, which is exactly the opposite of what SSPs hope to achieve. SSPs strive to avoid breeding for any one trait, or for any suite of traits; they aim to preserve as many traits in the gene pool as possible. You never know what genes will turn out to be important to these animals—for their health, reproduction, or eventual survival if they are reintroduced into the wild.

What about animals that do not breed well in captivity, though? That is where the reproductive science comes in. As Brandie Smith, one of the National Zoo’s senior curators, notes, “You can put two animals together, and they can say, ‘No, thanks!’” Indeed, in some cases, due to personality clashes or a lack of familiarity with breeding situations, animals can actually hurt each other.

Artificial insemination helped Zoo scientists breed white-naped cranes. (Mehgan Murphy/NZP)

That is what happened with a pair of endangered white-naped cranes at the Smithsonian Conservation Biology Institute in Front Royal. A male and a female, both hand-reared by humans, had never bred, and so both were extremely valuable genetically. With only about 5,000 white-naped cranes left in the wild and 60 in North American zoos, every unique gene counted. Because of their human upbringing, neither bird knew how to behave as a proper crane—and cranes have very specific courtship behavior. In this case, keepers ended up collecting semen from the male and artificially inseminating the female, producing the much-desired and highly valuable chick, who was reared by her grandparents.

Of course, artificial insemination is not always possible or practical. Scientists only know how to successfully artificially inseminate a fraction of zoo species, though they are working on more every day.

But what about the opposite: How do you keep the best breeders from overbreeding? The answer is that many zoo animals are on contraceptives. Zoo contraception can include oral pills, subdermal patches, hormonal injections, or simple separation during fertile periods.

Contracepting an endangered species may sound counterintuitive, but it is one of the best tools for keeping zoo animal populations healthy. Smith explains, “Contraception means we can keep animals in their natural social environments. It prevents breedings we don’t want, while allowing animals to live in a more natural social setting.”

Building an Ark Together

More SSPs continue to be established every year. This spring, the AZA is reviewing the SSP system, analyzing its strengths and looking at what areas could improve. Boyle, who is helping to lead this effort, says that, in general, the SSP program is an awe-inspiring concept.

“The notion of cooperative animal management is truly an amazing outcome,” he says. “All these zoos and aquariums have their own collections, and what they’ve agreed to do, with no law saying it should happen, no coercion, is to work together because they all recognize the power these amazing collections have to connect people to the wild. More and more, we’re dependent on the power of zoological collections to fill people’s minds with the wonder of these amazing animals.”

The more zoos inspire people, zoo staffs hope, the more species, and the more of the planet’s biodiversity, they may be able to save. It is an uphill battle and, like trying to fit some 5,000 species of mammals onto one boat, a daunting one. The scientists, however, do not seem to be considering giving up.

“The planet is falling apart,” Smith says, in the manner of one rolling up her sleeves and getting down to work. “Everything needs to be saved.”

—Brittany Grayson is a web content editor and science writer for Friends of the National Zoo.

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Smithsonian Zoogoer 39(1) 2010. Copyright 2010 Friends of the National Zoo. All rights reserved.