A Question of Breeding
Scientists are trying to play matchmaker for species threatened by lack of genetic diversity
- Sharon Begley
- Feb 01, 1991
For hours on end, she tussles with her towel and bats her ball like a furry Pele, taking a break for avid sucks from the bottle proffered by her caretakers at the Henry Doorly Zoo in Omaha, Nebraska. Mary Alice, tipping the scales at 20 pounds last summer, looks and acts like your basic adorable feline. But she is much more, a rare Bengal tiger, of which 200 survive in the wild and 700 live in zoos. And she is a yellow and black symbol of hope that, through sophisticated breeding techniques, she and animals similarly posed to disappear from the planet may be saved.
Mary Alice is the first tiger born through in vitro fertilization. She and two brothers (which did not survive) were born by Caesarean section after sperm from on Bengal male were mixed with eggs from two female Bengals. The embryos were then implanted in a Siberian tiger at the Doorly Zoo. The procedure promises to increase tigers' genetic diversity by facilitating matings between animals separated by vast distances. For wildlife biologists, that goal is as pressing as increasing absolute numbers of endangered species.
Even after dwindling to fewer than a dozen individuals, a species can rebound — but only if the survivors differ enough in the genes they carry to adapt to a changing environment. If they are too inbred — if there have been too-frequent matings between related individuals — then the consequent loss of gene types could threaten survival. According to conservation biologist Thomas Foose of the international Captive Breeding Specialists Group (CBSG) in Minneapolis, a sort of SWAT team for captive breeding, "Within a few decades, between 2,000 and 2,500 terrestrial vertebrates will be in trouble — in large part because of a loss of genetic diversity."
Although molecular techniques for measuring genetic diversity have so far been applied only to the cheetah, biologists suspect many creatures suffer from inbreeding. Giant pandas isolated by habitat loss on China's mountaintops, northern white rhinos in Zaire and black-footed ferrets of the American West are only a handful of the creatures threatened by a lot of genetic heterogeneity as well as by a decline in numbers.
To stop the genetic hemorrhaging, scientists are doing sophisticated captive breeding and tracking wild populations at risk of inbreeding. They are even beginning to think of the wild as a global zoo in which animals will one day be managed as closely as those in captivity. The reason: As the human population swells, and as people rob habitat from wildlife, there may come a time when creatures able to survive in nature without close management will be the exception rather than the rule. Few conservation groups have been enthusiastic about captive breeding, preferring to emphasize preservation of habitat as a tool for preventing extinctions. But the two strategies are increasingly difficult to separate.
Genes are the cards nature deals. Without a full deck, a species is like a poker player dealt all spades: doing great for now, but unable to adapt to a new game. Although in theory "there is nothing special about a great deal of genetic variation," says population biologist Michael Soule of the University of California, in reality a species without enough variety, the raw material of evolution, may be doomed — even though it may be finely adapted to the present. Consider, for instance, naked mole-rats, the most inbred mammals known. Although they now thrive in colonies similar to those of bees beneath the soils of East Africa, without genetic variation they are highly susceptible to disease or environmental change.
Animals can lose genetic diversity in several ways. With each generation, a stochastic process called genetic drift erases some gene forms from the population. But in most cases of genetic homogeneity, "the animals are not on the brink of extinction for natural reasons," argues Soule. "It is because of humans." Habitat destruction and human settlement can isolate populations, cutting them off from an influx of new genes and eliminating an important mechanism for avoiding inbreeding: dispersal of littermates. When mankind replaces savannah with parking lots, siblings are physically prohibited from finding well-separated territories.
Nowhere is the danger of genetic homogeneity more apparent than in the cheetah. Using biochemical techniques, researchers led by Stephen O'Brien of the National Cancer Institute have found that genetic variation from one wild cheetah to the next hardly exceeds that of inbred lab rats. The cheetahs have only 1 percent to 10 percent of the genetic variability of other felines. In O'Brien's experiments, they accepted skin grafts from non-relatives as if from an identical twin.
Cheetahs display the classic symptoms of inbreeding. Their sperm concentration and motility are markedly lower than in domestic cats, and 71 percent of the sperm are abnormally shaped, compared to 29 percent in domestic cats. They are susceptible to disease, possibly because in every cheetah studied, the group of genes that controls the immune response is identical. The consequences can be tragic. In 1982 and 1983, nearly half the cheetahs in an Oregon wildlife park died from feline infectious peritonitis — which rarely kills more than 10 percent of the house cats it infects. The cheetahs simply did not have the genetic bank on which to draw to produce infection-fighting antibodies.
The cheetah's troubles probably date to 10,000 years ago. Scientists theorize that something —climate apocalypse, plague, habitat destruction or hunting — forced the cheetah through a "genetic bottleneck, making inbreeding inevitable. The term connotes an event in which so many individuals die that the few survivors retain only a fraction of the original genetic palette. But it is somewhat of a misnomer: Even small populations can be genetically viable. "Within the limits of what we know, no species is doomed unless it gets down to two or three individuals," says Ulysses Seal, chairman of the CBSG.
Just seven individuals can carry more than 95 percent of the variation. That's why, says Soule, "Even with only a dozen ferrets or condors, we actually have most of the genes" — because these animals have not suffered a genetic bottleneck. But those genes will evaporate into history unless the species expands quickly and unless its individuals mate so as to foster genetic heterogeneity. For some reason, the cheetah did not. Its plight illustrates the threat facing the world's other endangered creatures.
"Loss of genetic variation is inevitable in small populations," says zoologist Georgina Mace of the Zoological Society of London. That means almost every species in captivity. Maintaining genetic diversity becomes ever more crucial as zoos not only breed rare species but try to return some animals to their native habitats. To date, zoo-born golden lion tamarins have been released in a Brazilian preserve; more than 250 Arabian oryx are roaming Jordan, Saudi Arabia and Oman; Andean condors are soaring over South America; and red wolves are romping in North Carolina. All are also breeding. If these and other species are to survive after generations in captivity, they must face the future with a full complement of their genes. In the simplest case, the best route to genetic diversity may be literally to import fresh genes.
This past summer, for instance, the Fossil Rim Wildlife Center in Glen Rose, Texas, brought in a male cheetah from Amsterdam. "His arrival is a major victory in the fight to increase the genetic diversity of the species," says Kelley Snodgrass, cheetah curator at Fossil Rim. "With enough small pockets of the animals around the world, we think we can keep their lack of genetic diversity from pushing them into extinction."
Such gene trades are organized under "Species Survival Plans," overseen in North America by the American Association of Zoological Parks and Aquariums but carried out by member zoos. The idea is to determine the best matings to preserve the genetic vitality of the species. SSPs now cover about 60 beasts in North America, from the Arabian oryx and Bali mynah to the spectacled bear and Sumatran tiger — and about 150 worldwide. Each involves anywhere from two zoos to 85 (for tigers), and serves as a sort of family-tree master guide for choosing matings that will most effectively increase a species' genetic diversity.
All captive animals risk losing genetic diversity without genes from the wild. But since capturing rare breeds is decidedly frowned upon, the next best strategy is to dip into the genes in someone else's zoo. Without cooperation among zoos, the world's captive animals would be like creatures isolated on mountaintops, unable to reach each other and doomed to inbreed.
To find the ideal sire and mother, SSP experts scour animals' pedigrees. Then they calculate the degrees of relatedness and decide which should mate. By factoring in life span, age of reproduction, litter size and age distribution in the existing population, curators can also determine how many animals are needed for species survival. Ulysses Seal of CBSG calculates, for instance, that cooperating zoos need 160 breeding adult Siberian tigers plus about 90 younger ones; in contrast, there must be 1,000 black-footed ferrets for a viable population to thrive.
SSPs may also try to equalize family size; if no two animals contribute more than their share of genes to the next generation, diversity is maximized. For the same reason, each male and each female should be given the same chance for breeding success. Often, that means disrupting natural behavior: Although only the alpha male gorilla will mate in the wild, in captivity all the males might be given a chance at fatherhood.
Over time, loss of genetic variation in captivity is inevitable. By the mid-1980s, every Speke's gazelle in American zoos was descended from one of four animals imported in 1969. By 1979, 80 percent of the young were dying before they could reproduce, in part because of inbreeding. In a crash survival program, mammal curator Bruce Read of the St. Louis Zoo and geneticist Alan Templeton of Washington University devised a drastic plan. Short of doing DNA analysis, they had no way of knowing which animals carried certain unwanted recessive genes (a matching of recessive genes is what gave your child blue eyes despite your brown eyes). So the team mated only close relatives in order to bring together pairs of lethal recessive genes; offspring carrying them died and partly eliminated those genes from the population.
Then the team mated least-related animals to maintain as many diverse genes as possible. Today, more than two dozen Speke's gazelles are scattered around four American zoos, and the species, almost extinct in the wild, seems assured of a future. The quality of that future, of course, depends on how one views the desirability of captivity. Some animal enthusiasts say that if animals can live only in zoos, they would be better off not living at all. The animals themselves have not been polled on the question.
Though conservationists caution against captive breeding as a replacement for saving habitat, many biologists see the two tactics as unavoidably related. "Vertebrates shape their habitat and thus influence other species, and indeed the whole ecosystem," says Seal. Without the "keystone" species (the one that shapes its habitat most strongly), other animals and plants may not make it.
Moreover, there can be political reasons a habitat needs its inhabitant. In Brazil, the government expanded the protected reserves of the golden lion tamarin only after American zoos returned a few dozen of the tiny primates to the area. After red wolves became extinct in the wild, they were bred in the Tacoma Zoo and, in 1987, introduced into a 100,000-acre North Carolina refuge; they attracted such local support that authorities committed another 200,000 acres to the refuge. "We do not want captive breeding to become an excuse not to do something about habitat," says Seal. "But high-profile species often make it easier to expand the resources for that species in the wild."
Says Foose, "Most of the natural world is becoming a mega-zoo; the numbers of so many species are so small and the populations so fragmented that they will suffer the same problems as captive populations." To save them, biologists may have to transplant their zoo techniques to the wild. If researchers identify a small population in which only a few males are breeding, thus reducing genetic diversity, "you can arrange to have new males brought in," says Foose. "This has been done with black rhinos." Giant pandas, too, could benefit from such management. Of the 700 to 1,500 remaining in China, no population numbers more than 75 or so individuals.
High-tech reproduction, on the other hand, won't be used widely for conservation until scientists know more about species' basic reproduction. Still, researchers plan to try to produce another Mary Alice. And David Wildt, a National Zoo physiologist, is optimistic about using in vitro fertilization to introduce genes from wild tigers to captive animals. Says Seal, "If you have separated populations of tigers, you can't just drop one in on another group; he'll get killed. So the ability to move genes without moving animals is enormously important."
For all the gloom and doom, scientists say that a population bottleneck is not a death sentence. In the past, small groups of inbred animals have recovered. The elephant seal was down to about 20 individuals off Mexico and California at the beginning of this century, for instance. But after protective legislation, the numbers rebounded until today the huge beasts frolic by the tens of thousands.
"Even if there are only a few animals left, there is no such thing as a lost cause," argues population biologist Soule. "There is only the loss of hope and the shortage of funds. With enough hope, financial support and proper management, we can save almost anything." As the creatures of this world teeter on the edge of disappearance, that hope will be severely tested.
Sharon Begley is a science writer for Newsweek.