De-extinction: Guidelines for Species Revival

Bethany Gately, Animal Science
Mark Pastore, Natural Resource Conservation
Mark Salhany, Environmental Science

De-extinction: Guidelines for Species Revival

Jurassic Park, an island attraction full of resurrected dinosaurs, is often what comes to mind when we think of extinct species being brought back to life. This widely acclaimed tale evokes excitement and curiosity at the prospect of coming face-to-face with creatures long extinct, but viewers understand that the premise is fictional and that dinosaurs will never again roam the earth. Today, however, the prospect of reviving extinct species is no longer science fiction. Advancements in technology have now made de-extinction, the term used to describe the revival of extinct species back to living animals,  a scientific possibility. While current de-extinction projects are not looking to resurrect Tyrannosaurus rex, scientists are hoping to revive species they feel left the world too soon. De-extinction should not be taken lightly however, as the return of each species comes with its own set of obstacles and consequences. Instead, scientists interested in the de-extinction of a specific species should check each potential applicant against a defined system of evaluation that dictates whether or not the species is ripe for revival. These criteria will act as an insurance policy so that species will only be brought back if they can be successfully reintroduced and have limited negative impacts. With this system in place, de-extinction can be carefully used to bring back animals we had a hand in destroying, but that are ready for return.

De-extinction Benefits

Humans have had an enormous detrimental impact on the natural world. Due to our ever increasing population growth and resulting industrial needs, we have marked up over 80% of the earth’s surface and caused major damages (“The Human Condition,” n.d.). Deforestation destroys forests and natural resources, exhaust and industry emissions have irreversibly altered the air we breathe and the atmosphere that surrounds us, and our waterways suffer greatly from pollution and overfishing (“The Human Condition,” n.d.).We are suffering from our own actions, but the species we displaced in the process are suffering far more. The number of endangered and extinct animals increases each year, and as a result the world’s biodiversity decreases (“Species disappearing,” 2004). The new “Go Green” movement encourages people to partake in sustainable living practices so that we can prevent additional trauma to our natural world. The process of de-extinction is one more way we can help restore what has been lost. By reviving extinct species, especially the ones that we played a major role in destroying, we can uphold our moral obligation to right what has been wronged.

In addition to upholding an ethical imperative, de-extinction will prove to be biologically and scientifically advantageous. On a biologic level, restoring lost species will increase biodiversity that was reduced due to their demise. Conservation biologist Stewart Brand (2013a) also argues that bringing back extinct keystone species would help restore ecological richness to the environment. For example, Brand notes that the grasslands sustained by the grazing wooly mammoth were “replaced with species-poor tundra and boreal forest” in their absence, and their return would “bring back carbon-fixing grass and reduce greenhouse-gas-releasing tundra” (Brand, 2013a, para.13). Though reviving the wooly mammoth may be advantageous ecologically, there are several additional components that factor into its potential for revival. As we will discuss later, it turns out that the wooly mammoth does not meet the criteria for de-extinction.

On a scientific level, bringing back particular species would mean we could once again study them for research purposes. For example, the Southern gastric-brooding frog (Rheobatrachus silus), which went extinct in the 1980s, was quite an interesting specimen because of its reproductive habits (Hines, 2003). The female frog would swallow whole her fertilized eggs, and shut off her gastric secretions so that her stomach could become a pseudo-womb (Hines, 2003). There, the eggs would mature into tadpoles and then eventually into froglets (Hines, 2003) . When they were fully developed, the mother would open her mouth and out hopped the baby frogs (Hines, 2003). Scientists were extremely interested in studying this creature, but they went extinct before much experimentation could be completed, likely due to a fast-spreading chytrid fungus that affects amphibians (Retallick, McCallum, & Speare, 2004). Paleontology professor Michael Archer from the University of South Wales is hoping to revive the Southern gastric-brooding frog so we can learn more about its incredible reproductive and enzyme-regulating abilities (Yong, 2013). Further research could result in improved techniques for treating human ulcers and other stomach-related health problems. But in order to make any headway, the animals must first be brought back to life.

Scientific Process

De-extinction involves taking DNA from the extinct species and implanting it into cells of a living relative. This can be done using Somatic Cell Nuclear Transfer (SCNT), a common form of cloning. In this process, the nucleus of a host egg is inactivated using UV radiation, and the nucleus of the extinct animal is put into the enucleated egg and allowed to divide/multiply (Stocum, 2013). The resulting embryo contains the DNA of the initial nucleus, therefore giving life to the extinct species. This technique was used in the case of the Southern gastric-brooding frog, where Professor Archer and his team used SCNT to implant a Southern gastric-brooding frog nucleus into the egg of a living relative, the great barred frog (Mixophyes fasciolatus)

(Grunbaum, 2013). Though the development was arrested at the blastula stage for unknown reasons, Archer is certain that with additional work and research they will get the cells to gastrulate (turn in on each other) and differentiate into organs to create a tadpole and eventually a frog (personal communication, November 15, 2013) . The end goal is to create a few specimens using this technique, and then breed them to each other to naturally produce additional specimens.

First De-extinction

The first official de-extinction event came in 2003 when scientist Alberto Fernández-Arias (2013), head of the Hunting, Fishing, and Wetland Department in Aragon, Spain worked with his team to create the successful birth of an extinct baby Pyrenean ibex, known colloquially as the bucardo. The bucardo was a type of wild mountain goat, native to Iberian Peninsula (Acevedo & Cassinello, 2009). The last bucardo, a female named Celia, was captured, tagged, and a small piece of ear DNA was taken for cloning purposes by Fernández-Arias (2013) and his team. When a passerby found Celia deceased, crushed under a tree in 2000, the bucardo become officially extinct (Gray & Dobson, 2009). Scientists then set out to restore the bucardo to the mountains using cloning techniques.

Using tissue from Celia, Fernández-Arias (2013) implanted her DNA into a host goat egg from a Spanish ibex (a close relative of the bucardo), and the resulting embryo was implanted into the host goat mother. The pregnancy was carried to term and out came a living, breathing, full-bred bucardo, confirmed through DNA testing to be genetically identical to Celia’s original cells (Folch et al., 2008). Though sadly the offspring only survived for 10 minutes due to a respiratory birth defect, this was the first known de-extinction (Folch et al. 2008). Though the success was short-lived, the birth of an extinct animal was an amazing scientific advancement. Fernandez-Arias (2013) and his team are certain they will be able to overcome the remaining obstacles and the bucardo will roam the mountains once again.

Now that de-extinction is scientifically feasible, it is important to analyze the several factors that limit its efficacy. The current list of species successfully revived (discounting the 10-minute bucardo revival) using de-extinction is nonexistent, and that is due to the many hurdles that must be overcome before extinct species are walking the earth again. The cloning processes required for revival come with their fair share of complications, a common one of them being birth defects, as shown with Celia’s revived offspring (Fernández-Arias, 2013). The revival of the Southern gastric-brooding frog is still at the embryonic blastula stage of development, and is a long way from hopping around and growing babies in its stomach (M. Archer, personal communication, November 15, 2013). But the amazing technological advancements of the last ten years show great promise for the next ten to come. Instead of hindering our potential by assuming that the process will forever be arrested at its current stage, we should consider what implications may occur not if, but when, we indeed revive a species.

Downsides and Rebuttal

There is a large population of naysayers who believe that even when the technology is fully understood and usable, that extinct animals should still stay extinct. Biologist David Ehrnfeld of Rutgers, The State University of New Jersey, argues that de-extinction is “very expensive and not going to achieve any conservation goal as far as I can see,” (as cited in Lewis, 2013, para. 16). Professor Archer makes a good counter argument however, in claiming, “I can’t think of what cloning Dolly must have cost and now it’s a routine technique,” (as cited in Yong, 2013, para. 26). Archer also argues that de-extinction allows for improvements one better than conservation: restoration. He claims humans have an “implicit moral obligation to try to undo the damage we’ve done,” and de-extinction can help make that happen by bringing back species we helped destroy (personal communication, November 15, 2013). Award-winning science writer Carl Zimmer (2013) claims some people feel scientists are “playing God” by reviving animals that have passed on. Archer retorts, “I think we played God when we exterminated these animals,” (as cited in Zimmer, 2013). Another critic of de-extinction, Glenn Albrecht, director of the Institute for Social Sustainability at Murdoch University in Australia, argues, “Without an environment to put re-created species back into, the whole exercise is futile and a gross waste of money,” (as cited in Zimmer, 2013). Archer has carefully considered these factors, which is why he focuses his energy on specifically the Southern gastric-brooding frog. When questioned about the availability of frog habitat upon their return, he answered, “[the frog’s] original mountain rainforest habitat remains as close to pristine as a habitat can be” (Archer M., personal communication, November 15, 2013). He also claims that if habitat for an extinct species does not currently exist, but can be recreated, we should not discount them as a de-extinction possibility. Considering the mantra “wildlife is no longer safe in the wild,” Archer argues, “perhaps artificially constructed and maintained ecosystems will be a necessary strategy for the future” (personal communication, November 15, 2013). It is clear that de-extinction proponents such as Archer carefully consider the drawbacks before pursuing a species for de-extinction.

There are many questions to consider before resurrecting and re-introducing an extinct species. These include:  How scientifically feasible is it to genetically recreate, and then raise the species to reproductive adulthood? How will we introduce the species back into the wild once they are revived? How will the animals cope with the changed climate and their affected habitat? How will their reintroduction affect current species? They died out once, how can we assure they will not disappear again? How will the public react to the scientific processes required (i.e. cloning) and the reappearance of long lost species? The feasibility of revival is different depending on the species, and all these factors should be taken into consideration before choosing a species for de-extinction.

Compromise

The way to quell the naysayers but not prohibit de-extinction completely is to find a compromise. Just because we have the scientific capability of reviving a specific species does not mean we should. Instead, there should be a system of evaluation in place that dictates whether or not certain species should be revived. Each aspect of their de-extinction process must be considered, including everything from how to physically revive them to how the world will react to their return. Even if one species is not adequate for revival, that does not mean de-extinction processes should be stopped for all species. Species that meet all of the defined criteria can be a perfect candidate for de-extinction.

First Criterion: Potential of Physical Revival

The first criteria to be considered for reviving an extinct species is how easily the animal can be physically brought back to life. The feasibility of generating a living specimen is dependent upon several factors. The length of time since the species walked the earth is perhaps the most relevant, because it usually correlates with the ability to salvage an intact genome. The wooly mammoth went extinct between 3,000 and 10,000 years ago, and as such there are no pieces of intact DNA from which to recreate a live specimen (Lewis, 2013). For de-extinction to be possible, scientists must reconstruct the genome, harvest host eggs from an elephant (a feat that has not yet been accomplished anywhere in the world), and perform somatic cell nuclear transfer to create an embryo with mammoth DNA (Lewis, 2013). Each of these steps are very time-intensive, and two of them have never even been done before. The fact that the wooly mammoth does not meet the first criterion should be enough material alone for the case against its de-extinction.

In a more recently extinct species, such as the Southern gastric-brooding frog, there are frozen samples of intact frog DNA that can be used for de-extinction purposes. No additional genome construction is necessary. The frog also lays eggs, so growing fetuses are easy to incubate in a lab, and generation time is much faster. But mammals also have a chance at de-extinction, as shown in the case of the bucardo. Though short-lived, this success proved that an extinct mammal can be brought back to life using cloning techniques. The fact that Celia’s cloned offspring had birth defects is not uncommon, as cloned animals often exhibit deformities, but there is hope (Brand, 2013b). The exact same cloning technology was used in 2003 to reproduce an endangered Javan banteng, a species of wild cattle native to Southeast Asia, from a normal domestic cow (Brand, 2013b). Scientist Robert Lanza used Javan banteng DNA frozen since 1980 to impregnate a domestic cow with a Javan banteng embryo (Brand, 2013b). The pregnancy was carried to term, and the resulting baby was a thriving, full-bred Javan banteng, which is still alive today (Brand, 2013b). This shows that in addition to being useful to the de-extinction cause, SCNT can be a useful tool in conservation biology by helping to increase numbers of endangered species. Successes such as these show that the bucardo and similar species have a great chance at revival.

Second Criterion: Re-extinction Prevention

The second criterion on the list is whether the initial cause of extinction can be prevented upon the species’ return to the wild. Species do not instantaneously disappear, so it is important to analyze the reasons they went extinct in the first place. Only if the cause of extinction can be assured to not happen again, should the species be a candidate for de-extinction. Again, the Southern gastric-brooding frog is a prime example. The frog died out due to a rapidly spreading chytrid fungus, so even if it was revived and re-introduced, it would likely die out again from a fungal infection (Retallick, McCallum, & Speare, 2004). In this specific case, in order to successfully bring the frog back, a fungus-resistant gene should be inserted into its genome. That way as the frog reproduces, the resistance would be spread throughout generations, and the species could live amongst the fungus that grows in its natural habitat. If scientists can make them fungus resistant, it is likely the Southern gastric-brooding frog would flourish upon revival.

The species that went extinct due to the actions of humankind are the ones we must seriously consider before re-introduction. If the species was swept out due to poaching, who is to say that they will not be hunted to extinction again? For example, the passenger pigeon was once the most prevalent bird in North America until poachers hunted it into disappearance in the late 1980’s (Archer, 2013). Scientists are very close to reviving the bird, but in order for the species to propagate they have to ensure hunters will not destroy it again. Project Passenger Pigeon is an organization dedicated to the restoration of the passenger pigeon (Project Passenger Pigeon, 2012). They realize that in order to prevent hunting them back to extinction, that the public must be educated about their usefulness and place in our world. Most poignantly perhaps is the thought that once the most prolific bird does not exist on our world anymore, and humans are to blame (Passenger Pigeon Project, 2012). If we can so quickly kill off such an abundant species, how much damage will be done in the years to come? According to Project Passenger Pigeon (2012):

The passenger pigeon’s story is proof that even common species can be lost forever if we do not interact with them in a sustainable manner…We hope to inspire people to think about their own role within the larger biotic community and to develop curiosity and wonder for the complexity, mystery, and uniqueness of the species we share the planet with. (para.7)

If measures can be taken to protect the passenger pigeon from humans, such as education and advocating for legislative protection, it has a chance at survival.

Third Criterion: Habitat Availability

A third criterion for guiding species revival should be the availability of necessary habitat. This is a shared understanding among de-extinction experts. As Jorgensen (2013) states, “species should be target[ed] for de-extinction only if … the habitat requirements of the species are satisfied” (Jorgensen, 2013). The International Union for Conservation of Nature (IUCN) also recognizes that, “background studies to allow identification of the species’ habitat requirements… should be undertaken before [beginning] the technical work on re-creating the species” (Jorgensen, 2013). The environment these species once lived in has likely changed since they went extinct, which would make reintroduction difficult (Grunbaum, 2013). As mentioned previously, Professor Michael Archer believes that even if habitat does not currently exist, that appropriate habitat could be created in anticipation of their return (personal communication, November 15, 2013). He presents an interesting argument, but because habitat recreation would take time, energy, and resources in addition to physically bringing the species back, having to rebuild habitat may be a strike against a de-extinction candidate.

Fourth Criterion: Overall Impact

A final criterion for the revival of species should examine the total impact that a potential candidate would have on the environment selected for its reintroduction. In other words, this criterion would serve as an inquiry into whether or not a species would bring benefit or detriment to an environment in purely ecological terms. This is an important point of focus when considering species revival, as very rarely will a de-extinct species fall into the exact same ecological position that it occupied prior to extinction, regardless of how well its environment has avoided change. For example, some species considered for reintroduction may now be considered to be invasive within their historic environments and as such, these species should be subject to further scrutiny (Koebler, 2013). On the other hand, some species may bring with them restorative effects on their habitat upon reintroduction such as increased genetic diversity or increased predation of local invasive or detrimental species (Brand, 2013b). As a result these candidates should have increased de-extinction priority. Of course, there are a multitude of intertwined factors that determine an organism’s total impact (both harmful and restorative) on any given environment, and furthermore, some of these factors are elusive and difficult to predict.

Of these factors, one key question to consider would be where the organism actually fits in the local food chain and how this might create shifts in ecosystem balance. Life ultimately evolves to utilize and fit into energy niches and to introduce a foreign organism into this localized energy equation, extinct or otherwise, could cause great changes to the overall balance of this equation (Moyle, 2013). This of course depends on the degree in which the reintroduced species is actually able to thrive in its environment upon reintroduction. In the case of large herbivores or omnivores such as the wooly mammoth, reintroduction may lead to the mass stripping of certain types of vegetation which could pose a danger to a wide range of smaller communities. This could decrease the probability of survival for many native and endemic species that are dependent on this specific vegetation for their primary source of energy. However, to illustrate the ambiguity of these impacts and as mentioned in an earlier section, the large scale stripping of certain types of vegetation could in turn decrease competition faced by other key species of flora, especially in nutrient-scarce environments, many of which may be carbon or nitrogen-fixing in nature and thus very restorative to their ecosystem (Brand, 2013b). As Brand (2013b) describes, large plant eaters such as the wooly mammoth as well as the aurochs (extinct ancestors of modern day cattle) both helped to keep populations of nutrient exhausting plant species at bay, enabling forests and plains to maintain highly diverse meadows and grasslands. Through all of the ambiguity, overall it is important to understand that regardless of species’ historic position in any given ecosystem, upon reintroduction their degree of fitness may differ greatly from when they originally lived and thus necessary tests must be first conducted to determine viable candidates.

Testing the System: Gastric-brooding Frog

With the guidelines outlined, the goal is for scientists to be able to pick a possible species for de-extinction, and check it against the criteria. To continue with our example, let us check the Southern gastric-brooding frog against our defined criteria. The first step is to determine how easily it can be physically revived. Archer (2013) and his team made great strides in creating a frog embryo in lab, but the development arrested at the blastula stage. The testing is currently stopped until their next trials in early 2014, and until then they will continue to examine and assess the embryonic blastulae they have managed to produce to date (M. Archer, personal communication, November 15, 2013). Once the in-lab recreation has been achieved, the frog is essentially home-free. Because the frog was first wiped out by a chytrid fungus, if a chytrid-resistant gene can be inserted into its DNA, the initial cause of extinction will be overcome. As Archer notes, the frog only went extinct in the 1980’s and much of its habitat still exists as it did 40 years ago (personal communication, November 15, 2013). Additionally, because of its recent extinction date, the surrounding plants and animals should not be adversely affected by the frog’s return. As for the sociological impact, we would greatly benefit from being able to study the live frogs again. Their ability to turn off gastric acids may lead to great advancements in stomach treatments (Yong, 2013). As such, it is doubtful that there would be much societal resistance. Because the Southern gastric-brooding frog meets each of the criteria for species revival, it would be a great candidate for de-extinction.

Testing the System: Wooly Mammoth

The wooly mammoth on the other hand is a much more difficult sell. Physically resurrecting the animal will prove extremely challenging, as it has been extinct for thousands of years and no intact DNA remains. This means that scientists must reconstruct the genome before any additional developments can occur. If they are able to overcome this obstacle, they must then harvest an elephant egg and then using SCNT successfully produce a live specimen. Considering how long the world has lived without them, the wooly mammoth would have a large impact on its new environment. This would likely either displace current species, or the mammoth itself would not thrive. For these reasons it makes sense to strike down the notion of reviving the wooly mammoth. De-extinction efforts would be more useful elsewhere.

Criteria Implementation

The possibility of de-extinction is a new and exciting prospect. But just as with any new technology, there are still obstacles to overcome and consequences not yet understood. To utilize the benefits but reduce the possibility of problems, a set of evaluation criteria can be used to select species for de-extinction. This will ensure that only species ready for revival will make a return. Steward Brand, one of the biologists at the forefront of the de-extinction cause, founded the Long Now Foundation which supports forward-thinking scientists interested in “fostering long-term responsibility” (Long Now Foundation, 2013, p.1). A major component of this foundation is the nonprofit project Revive & Restore: Genetic Rescue of Revived and Extinct Species (Long Now Foundation, 2013). On their website, they have a page dedicated to “Revival Criteria” in which they list several of the criteria we have created. Questions such as, “Has the original cause of extinction been detected and resolved?” and “Is there habitat for the animals to return to?” are analyzed before a candidate is chosen (“Revival criteria,” 2013, para. 6). It is reaffirming to learn that real-world scientists are on our same page. De-extinction can be a great scientific tool if the kinks are worked out and all drawbacks are considered. As long as each species is tested against, and passes, the guidelines for revival, resurrecting extinct species will be a wonderful addition to the world of science.

References

Acevedo, P. & Cassinello, J. (2009). Biology, ecology and status of Iberian ibex Capra pyrenaica: a critical review and research prospectus. Mammal Review, 39(1), 17-32. doi:10.1111/j.1365-2907.2008.00138.x

Archer, M. (2013). Michael Archer: How we’ll resurrect the gastric brooding frog, the Tasmanian tiger. [Video file] Retrieved from http://www.ted.com/talks/michael_archer_how_we_ll_resurrect_the_gastric_brooding_frog_the_tasmanian_tiger.html

Ashlock, L. (2013). Species for de-extinction [Illustration]. Retrieved from http://ecowatch.com/2013/09/05/de-extinction-we-could-revive-species/

Brand, S. (2013a). The case for reviving extinct species. National Geographic News. Retrieved from http://news.nationalgeographic.com/news/2013/03/130311- deextinction- reviving-extinct-species-opinion-animalsscience/ rptregcta=reg_free_np&rptregcampaign=20131016_rw_membership_n2p_us_sm_w#.

Brand, S (2013b). Stewart Brand: The dawn of de-extinction. Are you ready? [Video file]. Retrieved from http://www.ted.com/talks/stewart_brand_the_dawn_of_de_extinction_are_you_ready.html

Clothier, S. (Producer), & O’Neill, P. (Director). (2002). The end of extinction: Cloning the Tasmanian tiger [TV Documentary]. Australia: Becker Entertainment.

Fernández-Arias, A. (2013, March 15). The First De-extinction: Alberto Fernández-Arias [Video file]. Retrieved from http://longnow.org/revive/tedxdeextinction/the-first-de-extinction/

Folch et al. (2008).  First birth of an animal from an extinct subspecies (Capra pyrenaica pyrenaica) by cloning. Theriogenology 2009; 71: 1026-1034. Retrieved from http://media.longnow.org/files/2/REVIVE/Therio%2071-2009%20Bucardo%20Cloning.pdf

Gray, R. & Dobson, R. (2009). Extinct ibex is resurrected by cloning. The Telegraph. Retrieved from http://www.telegraph.co.uk/science/science-news/4409958/Extinct-ibex-is-resurrected-by-cloning.html

Grunbaum, M. (2013). Back from the dead: Should scientists bring extinct species back to life? Science World, 70, 1. Retrieved from http://scienceworld.scholastic.com/Earth-Science-News/2013/07/Back-from-the-Dead

Hines, H. (2003). Southern gastric-brooding frog. Retrieved from
http://www.arkive.org/southern-gastric-brooding-frog/rheobatrachus-silus/

The Human Condition: Our imprint deepens as consumption accelerates. (n.d.). Retrieved from http://www.nationalgeographic.com/earthpulse/human-impact.html

Jorgensen, D. (2013). Reintroduction and de-extinction. BioScience. 63(9). Retrieved from http://www.bioone.org.silk.library.umass.edu/doi/pdf/10.1525/bio.2013.63.9.6.

Koebler, J. (2013). Reviving extinct animals could lead to invasive species from the past. USNews. Retrieved from http://www.usnews.com/news/articles/2013/04/04/reviving extinct-animals-could-lead-to-invasive-species-from-the-past.

Lewis, T. (2013, August 26). De-extinction experts see benefits in resurrecting extinct animals, but critics abound. The Huffington Post. Retrieved from http://www.huffingtonpost.com/2013/08/26/de-extinction-extinct-animals-life_n_3816271.html.

The Long Now Foundation. (2013). Retrieved from http://longnow.org/

Moyle, P. (2013). Ch. 7: Niche and habitat. Retrieved from http://marinebio.org/oceans/conservation/moyle/ch7.asp

Project Passenger Pigeon. (2012). Retrieved from http://passengerpigeon.org/casestatement.html

Retallick, R.W.R., McCallum, H., Speare, R. (2004). Endemic infection of the amphibian chyrtrid fungus in a frog community post-decline. PLOS Biology, 2(11, 351. doi:10.1371/journal.pbio.0020351

Revival criteria. (2013). Retrieved from http://longnow.org/revive/candidates/revival-criteria/

Species disappearing at an alarming rate, report says. (2004). Retrieved from http://www.nbcnews.com/id/6502368/#.Upvlj2TwJ7S

Stocum, D. (2013). Somatic cell nuclear transfer (SCNT). Encyclopedia Britannica. Retrieved from http://www.britannica.com/EBchecked/topic/1382860/somatic-cell-nuclear-transfer-SCNT/

Yong, M. (2013, March 15). Resurrecting the extinct frog with a stomach for a womb. National Geographic. Retrieved from http://phenomena.nationalgeographic.com/2013/03/15/resurrecting-the-extinct-frog-with-a-stomach-for-a-womb/

Zimmer, C. (2013). Bringing them back to life. National Geographic, 233, 28-33, 35-41. Retrieved from https://search.proquest.com/docview/1328046896/abstract?accountid=14572