Are high levels of genetic diversity absolutely necessary, however? Low levels of genetic diversity have been linked to poor health, decreased reproductive success, and even physical abnormalities, such as the crooked tail that was frequently observed among Florida panthers prior to their hybridization with panthers from Texas. Some species, however, have extremely low levels of genetic diversity but little measurable consequence to their ability to survive. Polar bears, for example, have extremely low levels of genetic diversity, but they have had the same tiny amount of diversity for at least the past 100,000 years. During that time, polar bears survived two ice ages and the present warm interglacial period. Nonetheless, as the habitat to which they are very specifically adapted disappears, their lack of genetic diversity may be their downfall: the more genetically diverse a population is, the greater the chance that this diversity will recombine in new ways, resulting in phenotypes that can adapt to surviving in a different environment.
Clearly, genetic diversity and the adaptive potential that genetic diversity provides are important, and a healthy population cannot be made up entirely of genetic clones. While it won’t be the simplest solution, the most likely solution to the diversity problem will be to engineer cells taken from different individuals and use multiple cells to create a genetically diverse population. When editing the genomes within these cells, we will need to be certain that the edits are made in both chromosomes of every cell that is used. That way, the population will be genetically identical at these particular loci, and the target phenotype will be maintained even after the population is released into the wild.
While genetic diversity will be important to consider when creating our resurrected population, we must also keep in mind that diversity is not the only factor that determines whether a species is sustainable over the long term. If we were to survey genetic diversity among living primates and use this information to decide which primate species is most in need of protection, the result would shock most of us. The primate with the least amount of genetic diversity is … us. Humans have almost no genetic diversity, whereas other primates, including chimpanzees and gorillas, are doing just fine. Engineering genetically diverse populations will be important for de-extinction, but, ultimately, it will not be as important as finding a stable, healthy, and sufficiently large wild space into which our population can be released.
FROM THE BIRTH OF ONE TO THE REARING OF MANY
The second phase of de-extinction involves not only creating multiple individuals, but also rearing and nurturing these individuals, moving them out of captivity, and establishing populations in the wild. Ideally, this second phase would culminate with the establishment in the wild of multiple genetically robust, healthy, self-sustaining populations that are resilient in the face of environmental change. Phase two is certainly not going to be easier than phase one.
As a start, the babies must develop into adults. They must develop both physically and behaviorally, taking on the characteristics that they have been engineered to express. Most likely, several generations will be born and raised in captivity before a sufficient number of individuals are available to be released into the wild. Populations living in captivity, possibly for decades, need not only to survive, but they must also learn how to live. The individuals that make up these populations need to learn how to feed and protect themselves, how to interact with others, how to avoid predation, how to choose a mate, and how to provide parental care to their own offspring. Understanding how a species might fare in captivity is therefore an important consideration in deciding whether a species is a good candidate for de-extinction.
Humans have lots of experience raising and breeding organisms in captivity. For decades, we have raised animals in zoos, farms, breeding centers, and even our own houses. This experience has taught us that species differ in how they respond to captivity. Some species thrive—they are healthier, live longer, and have more offspring than wild-living individuals of the same species. Other species suffer terribly—they have shorter life expectancy, rarely reproduce, and even develop psychological disorders such as the repetitive swaying or pacing that is often observed in polar bears in zoos. Understanding how our candidate species for de-extinction is likely to fare in captivity will be critical to the success of our project.
When rearing genetically engineered animals, it will be important to keep in mind that some of the traits observed in these animals—both physical and behavioral traits—may be a consequence not of their edited genomes but of the pressures of life in captivity. One fascinating example of how different selection pressures in captivity can change the way an animal looks involves a wild population of Russian silver foxes. In 1959, Dmitry Belyaev, a Russian biologist who would later become director of the Institute of Cytology and Genetics, Russian Academy of Sciences, took 130 wild silver foxes and began breeding them on a farm near his institute in Novosibirsk. With each generation, he allowed only the foxes that appeared to be the tamest to breed. After only four generations, foxes began to wag their tails as their keepers approached. Over the course of just a few decades, his population of wild silver foxes was transformed into a population of animals that whined, wagged their tails, and jumped into the arms of and licked the hands of their keepers. The behavioral transformation was fascinating, but so was their physical transformation. The younger generations included animals with floppy ears, rolled or shortened tails, and coat coloration that is not seen in the wild but is reminiscent of other domestic animals.
Animals that are born and raised in captivity tend to look and act differently than their wild cousins. Physical differences appear in animals that are bred in captivity, including having shorter intestinal tracts and brains that are different sizes from those of wild-bred individuals. Traits that advertise sexuality also tend to be less noticeable among captive-bred individuals, which can influence their ability to find and compete for a mate in the wild. The behavioral differences between captive-bred and wild-bred individuals may be even more troubling. Animals in captivity don’t need to learn how to avoid predation, for example, and the absence of social conflict and the unnatural social structures that form in captivity can lead to changes in defensive or sexual behaviors. Without sufficient space or stimulation, some species become overwhelmed by stress in captivity and develop acute psychological disorders, such as wall licking by captive giraffes and self-mutilation by captive big cats and bears. Stress also affects the animals’ physiology, often reducing their fertility or stopping reproduction entirely.
Captive breeding also leads to unintended genetic changes, which may complicate the interpretation of genome-editing results. Without the need to find food, avoid predators, or fight off disease, selection pressures on animals living in captivity are relaxed. Instead of favoring traits for survival in the wild, captive breeding favors traits that increase reproductive success in captivity. This is not ideal when the goal is to release these populations into the wild.
Given the problems that so many species experience in captivity, it would be useful to have some way to predict a priori how a species might fare during this stage of de-extinction. It is tempting to guess how an extinct species might fare in captivity based on how well their living relatives fare. However, data from zoos and other captive-breeding facilities show that even close evolutionary relatives can differ considerably in their response to captivity. Among cetaceans, for example, captive Fraser’s dolphins and Dall’s porpoises damage their bodies by throwing themselves against the sides of enclosure pools and refuse to eat, while bottlenose dolphins and finless porpoises seem happy and playful in captivity, with reproductive rates that are sometimes higher than those observed in wild populations.
Those species that do fare well in captivity, from an animal welfare perspective at least, tend to be those that flourish in close proximity to humans. They include species that are sometimes characterized as “invasive,” such as rats and mice, and species that do well in urban environments. They are species tha
t deal well with disturbance and are flexible in their reaction to predators and new resources. In a sense, species that do well in captivity are those that are not particularly likely to be extinct in the first place.
YET ANOTHER SET OF MAMMOTH CHALLENGES
Elephants, unfortunately, are among those species that do not fare well in captivity. Both African and Asian elephants live longer in the wild than they do in zoos. Elephants in zoos are prone to obesity, arthritis, and infections, particularly in their feet. Even worse, both living species of elephant struggle to reproduce in zoos. Their ovulation cycles become abnormal and unpredictable, and they have low fertility rates and high infant mortality rates. Many elephants living in zoos also show signs of psychological distress, including repetitive swaying behavior, hyper-aggression toward other elephants, and a propensity to kill their infants. These animals are provided with food, water, and medical care, and yet it seems clear that their most basic needs are not being met.
Elephants are known to be intelligent, social, wide-ranging animals and have needs that are very difficult to satisfy within the confines of most enclosures. There is little reason to suspect that the physiological and psychological needs of elephants whose genomes contain some small fraction of mammoth DNA will differ considerably from those of elephants whose genomes have not been edited. If elephants are going to be used in future de-extinction projects, sincere efforts will be necessary to improve the well-being of elephants in captivity and elephants released into the wild. This includes both careful consideration in the design of any enclosure in which these animals will breed and live, and the establishment of sufficiently large numbers of these animals in the wild to satisfy them socially and intellectually.
The challenges of captive breeding are likely to vary considerably among species. For example, species that migrate annually over long distances may be particularly unsuited to captive breeding, as sufficient space for this behavior will be exceedingly hard to replicate in a captive setting. If migratory paths are learned by interaction with a social group, how should scientists replicate the process by which these behaviors are learned?
Passenger pigeons were not migratory birds. However, they did fly long distances in order to find forests with sufficient numbers of fruiting trees to sustain their large flocks. Fledgling passenger pigeons learned this behavior by opportunistically joining flocks as they passed overhead. In his presentation at the TEDx event in March 2013, Ben Novak presented a plan to teach resurrected passenger pigeons how to find food. He proposed painting hundreds or thousands of homing pigeons so that they looked like passenger pigeons and training these painted homing pigeons to fly over the breeding colonies. These “surrogate flocks,” as he called them, would attract the attention of the fledglings, who would follow their instincts and join the flocks. The surrogate flocks would ferry the young passenger pigeons between feeding sites that Ben intended to set up across the northeastern United States. As the passenger pigeon population grew, Ben would gradually use fewer and fewer surrogate birds in his flock, until eventually only passenger pigeons remained, complete with behaviors taught to them by Ben via his trained, painted flock of homing pigeons.
The combination of captivity-induced stress, reproductive problems, genetic consequences of different selection pressures, and lack of appropriate social interactions in captivity perhaps explains why captive-breeding programs for conservation—those that aim to rear endangered species in captivity and eventually release them into the wild—have been so variable in their success. Some strategies to resurrect extinct behavioral traits in captivity, such as that proposed by Ben Novak to train passenger pigeons to find food, are so far-fetched that they might actually work. There is no doubt, however, that captive breeding will be another tall hurdle for de-extinction.
Yet, captive breeding may not be as high a hurdle as the step that would come next: releasing these genetically modified organisms into the wild and allowing them to fend for themselves.
CHAPTER 10
SET THEM FREE
The California condor was once found as far north as British Columbia, as far south as Mexico, and as far east as New York. The large-bodied bird fed on the remains of even larger-bodied ice age animals, including mammoths and horses. When these animals began their gradual decline toward extinction, so did the California condor. Eventually, the California condor was restricted to California, where it survived by scavenging the remains of large marine mammals, including whales and seals. Elsewhere across its former range, it disappeared.
As human populations boomed along the California coast during the nineteenth and twentieth centuries, the California condor did not fare well. A program to preserve their habitat was established in the 1930s, but it had little impact on declining condor populations, and by 1982, the total population of California condors had reached a startling low of twenty-two individuals. In a last-ditch attempt to save the condors from extinction, a partnership was formed between the US Fish and Wildlife Service, the Los Angeles Zoo, and the San Diego Wild Animal Park. This partnership established a captive-breeding program for California condors. The program began with several eggs and chicks taken from wild nests and a single wild adult. A few years later, a controversial decision was made to relocate all remaining California condors from the wild into the breeding program. The hope was to conserve as much genetic diversity as possible while that diversity was still around.
California condors have a slow reproductive cycle compared with other birds. They breed for the first time between ages six and eight, after which a breeding pair will produce one fertilized egg every year or two. The program attempted to increase the reproductive output of the captive population by implementing a trick known as “double-clutching.” Female condors can be duped into laying a second and sometimes a third egg if the first eggs are removed from the nest. When the first egg was laid, breeders would move it to an incubator so that another egg could take its place in the nest.
Double-clutching worked for the condors, in that many breeding females did in fact lay more than one egg. As this first round of eggs hatched in the incubators, however, a new problem surfaced. Who would raise the hatchlings? Who would teach them how to be California condors? Some of the incubator-hatched eggs were placed with foster condor parents and this worked out well. However, there were too few potential foster condors to place each hatchling with a condor parent. The breeders would have to rear some of the baby condors themselves.
Rearing by humans is tricky, as too much close contact with humans during these early life stages can lead to imprinting—an unhealthy trust of humans on the part of the of the baby chicks. Chicks that are too trusting would be at a disadvantage after release into the wild. Humans can be pretty nasty, after all.
So, the human breeders became puppeteers. They watched videos of real condor parents interacting with and feeding their young. They learned and adopted an appropriately strict, condor-like parenting style, which they implemented as best they could using puppets that resembled the heads of adult condors. Chicks fed by puppets also participated in a mentoring and socialization program, in which they spent time in an aviary with condor-raised chicks and other adult condors.
The first captive-bred California condors were released into the wild of southern California approximately five years after all wild condors were taken into captivity. By the end of 2010, the number of California condors had increased from twenty-two to around four hundred, approximately half of which were living in the wild.
AND … RELEASE
By many measures, the California condor captive-breeding program has been a success. More condors are living freely in California today than would be alive anywhere had the breeding program not been established. The path to success has, however, been circuitous and expensive, and condors are by no means in the clear with regard to extinction risk. Many of the problems encountered by the condor program are directly applicable to de-extinction and are worth considering here.
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sp; First, given the challenges that the condor program experienced while trying to build the condor population, are there some species for which captive breeding will simply be too slow to be successful? And, for those species that do have slow reproductive rates, is there any way to speed up the process? The developmental interval between the first and second generations of elephants is very long. Sergey Zimov told me that this is what he finds most troubling about the mammoth cloning and engineering projects. I had a chance to talk with Zimov recently at a conference, just after he delivered an impassioned speech about how mammoths would transform his Pleistocene Park. Keeping his voice low, he admitted to me that he would actually prefer woolly rhinos to mammoths. Pointing to his long gray beard, he conceded with visible dismay that an animal capable of reproducing at age five (like a woolly rhino) rather than at age fifteen (like a mammoth) was more within what he considered to be his personal time-frame. Mammoths, he said, would be for his children to introduce to the park.
Obviously, de-extinction projects will proceed more quickly with species that reproduce frequently and have many babies at once. In the first meeting to develop the passenger pigeon de-extinction project, one of the traits that was suggested as a first target of genetic engineering was the number of eggs laid per mother per year. Passenger pigeons laid a single egg once a year. It was proposed that we try to double this, and make each bird lay two eggs at a time. Two eggs a year would certainly facilitate the early stages of a passenger pigeon–rearing project, as it would mean more animals to work with while the population was small. But, if one egg at a time led to billions of passenger pigeons, I’m not certain that we want to genetically engineer them to reproduce at twice their normal rate. Perhaps a simpler solution would be to practice double-clutching during the early stages of captive breeding, as they did in the condor project. After the passenger pigeon population grew to a reasonable size, we could simply revert to leaving that first egg in the nest to be reared by its parent.
How to Clone a Mammoth Page 19