by Arianna Lord
Earlier this year, the Columbus Zoo and Aquarium performed artificial insemination on two of their female polar bears in hopes that the procedure would produce cubs. Polar bears greatly rely on Arctic sea ice for hunting, traveling, mating, and resting. However, because of the continued shrinking of sea ice due to climate change, polar bears have been listed as a threatened species since May 2008. Assisted reproductive technologies (ARTs) – procedures used to address infertility or manage animal populations – are thus increasingly being used to bolster population numbers and protect endangered species, like polar bears, from extinction.
Inbreeding: A problem with small and fragmented populations
Climate change and habitat destruction impact many species, leading to population fragmentation and decreased population sizes. If a population is too small, individuals end up breeding with close relatives. Within a population, genetic variation increases with its size. This is because every individual has a unique genome and when individuals breed, unique combinations of parental genes are passed to offspring in the next generation. The more closely related two individuals, the more similar their genomes are. When two individuals with similar genomes breed, it increases the probability of homozygosity by descent (having two copies of a given gene that are descended from a single ancestral source). This reduces survival rates and fertility in offspring, a phenomenon known as “inbreeding depression,” further perpetuating the issue of small population size in endangered species (Figure 1).
Reducing the impact of inbreeding and minimizing the loss of genetic diversity is key for conservation. One approach to help grow population numbers of at-risk species, with the goal of increasing both genetic diversity and the number of successful offspring per generation, is implementing ARTs. ARTs can allow the introduction of genetic diversity into a population from individuals that cannot breed naturally, individuals that are geographically separated, or even those temporally separated if gametes have been specially preserved (Figure 2).
ARTs include artificial insemination, in vitro fertilization (IVF), and cloning, amongst others. ARTs were originally developed for breeding animals like cattle, horses, or sheep for human consumption and use. These days, however, we see this technology being applied to a wider range of organisms. There are many challenges to implementing ARTs in wildlife, largely due to the diversity in anatomy, physiology, and cellular mechanisms involved in reproduction between species. The physiology and reproductive system of a polar bear, for example, is very different from that of another mammal like a human, let alone that of a marsupial or reptile. Furthermore, while there have been years of research, time, and money invested into the livestock and human fertility fields, there are few researchers dedicated to understanding the physiology and reproduction of any other species – never mind rare, endangered animals. While the fundamentals of the biotechnology used in ARTs remain the same, a steep learning curve awaits scientists who hope to apply ARTs to new species, as techniques must be adapted to each animal’s unique biology. While there are certainly challenges, implementing such approaches can have a positive impact on the conservation of endangered species. Such benefits are perhaps best illustrated by the work done with the black-footed ferret.
Black-footed ferrets: an ART success story
The black-footed ferret historically inhabited the great plains of the western United States. However, due to habitat loss, dwindling food sources, and disease, they were declared extinct in the wild in 1979. Two years later, a previously unknown population of 18 black-footed ferrets was discovered. Conservationists captured these individuals with the goal of setting up a captive breeding program to grow population numbers until reintroduction was possible. Only seven of the captured ferrets bred successfully. All black-footed ferrets today are descended from these seven individuals, making the genetic variation somewhat limited. Fortunately, forward-thinking scientists took tissue samples from some of the remaining original individuals that did not breed, and cryo-preserved them in the San Diego Zoo Wildlife Alliance Frozen Zoo.
While there was not enough species-specific knowledge to apply ARTs in black-footed ferrets at the time, this “genome banking” – the preservation of an individual’s genetic variation – meant that as technology improved, the genetic variation represented in the non-breeding individuals could eventually be reincorporated back into breeding populations (Figure 3). In addition to natural breeding, the program has used sperm preserved for up to 20 years for artificial insemination. As of 2022, there are over 300 individuals now living in the wild that have been released from the program. What is perhaps most exciting is that in 2020, scientists were able to successfully clone one of the original non-breeding females from 1981 by using a cell line established from the preserved tissue samples. Scientists have estimated that her genome alone contains three times more genetic diversity than what is present in the current population. Thus, once the new individual reaches breeding age, she will hopefully be able to introduce much needed genetic diversity into the small population.
Assisted reproductive attempts in polar bears
Unfortunately, ART implementations are not always successful. Captive breeding programs for polar bears have seen moderate levels of success from natural conception, but there has yet to be a successful birth via artificial insemination. However, scientists continue to attempt artificial insemination, such as the most recent attempt at Columbus Zoo, because they are hopeful to increase the genetic diversity of captive populations by using banked sperm from male bears that have not sired any offspring, males who have since died, or to facilitate breeding between animals in different locations without the need for translocation. Polar bear reproduction is a complex process which involves “delayed implantation.” While polar bears typically breed in the spring, the embryo does not implant in the uterus until four to six months have passed. Growing a cub is incredibly energy intensive, so for the embryo to implant and develop, the mother must be in excellent health. However, as there is no pregnancy test for polar bears, there is no way to know if implantation is successful. Scientists must wait until the expected time of birth, months later, to see if their efforts were successful. While these repeated unsuccessful attempts may be discouraging, from each attempt scientists are able to learn more about polar bear reproductive physiology and increase the future chance of success.
ARTs are valuable – but not the sole solution
New ideas are continually being announced in this field, including marsupial-specific ART projects, such as implementing IVF in koalas or the more ambitious de-extinction attempt to resurrect the Tasmanian tiger species, which has been extinct for almost a century. While ARTs are very promising, there is still a long way to go for these technologies to become more widely used with greater success. This primarily relies upon improving our understanding of reproductive physiology in species of conservation concern. In this regard, we are somewhat at the edge of a scientific frontier with much to be explored. However, it is also important to note that for many species, unless efforts are made to prevent the causes of declining wild population numbers and habitat loss, these technologies have only limited value. There is only so much that boosting a population’s size or genetic diversity will do if the ecosystem that the animals rely upon is not also protected. While not every conservation effort or ART project may be a success, each one allows researchers to gain valuable information that increases our understanding of how this technology can be applied across species. The implementation of assisted reproductive technologies for wildlife conservation is an exciting field that will continue to grow with our desire to minimize biodiversity loss.
Arianna Lord is a third-year graduate student in the department of Organismic and Evolutionary Biology at Harvard University.
For More Information:
- Read more about why a multi-pronged approach is essential for saving endangered species.
- Learn more about the San Diego Zoo Wildlife Alliance Frozen Zoo here.
- Read more about the Black-Footed Ferret Project.
- Could freezing koala sperm save the species?
- Learn more about how we plan to de-extinct the Tasmanian Tiger.
This article is part of our special edition on diversity. To read more, check out our special edition homepage!