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The Frozen Zoo is a place where endangered species can be preserved and stored, providing a glimmer of hope for their future.
Environment Science

The Frozen Zoo is a place where endangered species can be preserved and stored, providing a glimmer of hope for their future.


In a lab located under a wildlife park spanning 1,800 acres in San Diego, California, Marlys Houck notices a man in a uniform holding a blue, insulated lunch bag containing small fragments of eyes, trachea, feet, and feathers.

“Oh,” she whispers. “These are today’s samples.”

The bag in question holds small pieces of animal tissue that were obtained from natural deaths at the zoo. The current samples consist of a leaf frog and a starling.

James Boggeln, a volunteer at the zoo, is seen holding a bag which he then passes to Houck, the curator of the “Frozen Zoo” laboratory. She and her team will begin the process of transforming these tissue fragments into a store of research and conservation materials for the future. The tissue will be placed in flasks where enzymes will break it down, and the lab members will gradually incubate it over the span of a month, producing a large quantity of cells that can be frozen and revived for later use.

The Frozen Zoo, now approaching half a century since its establishment, boasts the distinction of being the world’s foremost and most extensive collection of living cell cultures. With over 11,000 samples encompassing 1,300 distinct species and subspecies, including rare specimens and even three extinct species, it remains an invaluable resource for preservation and research.

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Currently, the Frozen Zoo is managed by a team of four women. They oversee a large assortment of labeled vials, with names like “giraffe”, “rhino”, and “armadillo”. These vials are carefully stored in large circular tanks that are filled with liquid nitrogen. In a time where the environment and diversity are in crisis, preserving species by freezing them provides a glimmer of hope for the future.

The effort at hand has consistently been fulfilling, however, a rapidly increasing risk of extinction has created heightened tension for Houck and her colleagues. The clock is ticking to preserve the samples in the Frozen Zoo before they vanish from existence outside of the laboratory. These women who are responsible for this task view it as their obligation to secure the future.

The task can be meticulous – collecting specimens from birds, mammals, amphibians, and fish all involve distinct procedures. However, given the significance of the work, Houck speaks of it with a sacred respect.

She is anxious about the responsibilities of her position. In the past, a malfunction resulted in the zoo’s 300 samples and a year’s worth of work being lost. According to Houck, her main priority is to ensure the safety of the frozen samples, but she also experiences happiness and pride as it is a privilege to be able to carry out this task.

Gather items without fully comprehending why you are doing so.

The founder of the zoo, Kurt Benirschke, was a German American pathologist who established it in 1972. He began with collecting animal skin samples in his lab at the University of California, San Diego and eventually relocated it to the San Diego Zoo. During this period, the technology to utilize the samples for more advanced research was not yet available. However, Benirschke often referenced the words of American historian Daniel Boorstin: “You must gather things without fully understanding the reasons behind it.”

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The quote remains displayed on a poster in the Frozen Zoo, where Houck extracts vials from large liquid nitrogen tanks that resemble oversized silver thermoses. The tanks are compressed at a chilling temperature of -320F, which halts the movement or alteration of cells, keeping them in a dormant state. Once thawed, the cells can be brought back to life as if no time had elapsed, even after decades or centuries.

Every species is unique and certain groups prove to be more difficult to protect than others. The Frozen Zoo initially focused on preserving mammal species, but later expanded to include cryobanking for birds, reptiles, and amphibians. According to Houck, the success rate for mammal preservation is nearly 99%. However, the success rate for amphibians was only about 1% for a while, but has since increased to possibly 20-25%. Preservation of birds has shown to be relatively successful.

The nitrogen tanks have racks that can hold 100 vials, with each vial containing between 1 and 3 million living cells. These cells could be from a variety of endangered species such as a giraffe, lemur, or the rare vaquita. They hold potential answers to current and future issues.

In the future, the cells may have the potential to revive completely extinct species – although that is not the primary objective. Instead, the material is primarily utilized to preserve current struggling species. In 2020, the Frozen Zoo successfully cloned a black-footed ferret, which is an endangered species in the United States, using cryopreserved DNA. In a similar success, frozen cells that were cryopreserved 42 years ago were used to clone two critically endangered Przewalski’s wild horses. This reintroduction of genetic diversity into the existing population will increase its resilience to potential illnesses or environmental threats. One of the cloned foals was given the name Kurt in honor of the Zoo’s founder.

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The work that San Diego’s Frozen Zoo is part of a global movement to cryobank everything from animals to seeds. Today there are around a dozen wildlife-based cryobanks around the world, mostly located in North America and Europe.

Sue Walker, the head of science at Chester Zoo and co-founder and vice-chair of UK nonprofit Nature’s Safe, believes that the work being done in San Diego is extremely groundbreaking. The cryobank, which gathers live cells, sperm, and eggs, has the potential to revolutionize the field in the future. Walker envisions a time when these cells can be transformed into pluripotent stem cells, which can then be used to generate sperm and eggs.

Reworded: In a perfect scenario, safeguarding species in their natural habitats would be possible; however, this is not the reality. According to one expert, the pace at which species are disappearing is outpacing scientific efforts to track them. Therefore, our best approach is to preserve genetic material for the future.

Obtaining permits for importing animal tissues from other countries can be challenging. The goal is to establish the ability to store these tissues through cryobanking locally, specifically near conservation centers in Africa, South America, and Southeast Asia. However, this requires developing the necessary skills to handle and preserve these cells in a consistent manner, which is a costly and intricate process. Despite the challenges, Walker emphasizes the importance of this work.

left: portrait of woman in lab coat. right: toy rhinos, rhino stuffed animals and photos of rhinos on shelves

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She believes it is necessary to try everything in order to preserve the few remaining species that are in danger of dying out. It’s about holding onto optimism.

Rewording: Methods of traveling through time to both the past and future.

Taking care of cell cultures can feel like manipulating time. While conducting research on rhino chromosomes, Houck stumbled upon a vial labeled with her predecessor Arlene Kumamoto’s handwriting. These cells were frozen in the same month that Houck graduated high school. Reflecting on this, Houck couldn’t believe that Kumamoto’s actions from the past were now benefiting her own studies. It begs the question, what advancements are we making now that will have a lasting impact in the future?

Julie Fronczek, who has been a part of the zoo staff for 24 years, pauses her work under the microscope to share her theory about why women are at the forefront of the Frozen Zoo project. According to her, it is because they prioritize the care and nourishment of the cells, treating them as living beings and catering to their needs. She likens it to taking care of babies.

The zoo team annually adds approximately 250 to 350 individual specimens. According to Houck, the recently arrived leaf frog is of great importance. However, the starling that also arrived is not as crucial. These decisions greatly impact her as she must consider the need to culture and preserve each new animal, as well as the limited space in the large containers. Furthermore, she must take into consideration the number of specimens already present and potential opportunities for obtaining more of the species. She expresses the desire to not reject any incoming samples, as they are all valuable.

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The collection includes three extinct species: the po’ouli or Hawaiian honeycreeper, Rabbs’ tree frog, and the Saudi gazelle. They’re holding a collective breath as they watch for more species in their collection to go extinct. “The next will probably be the white rhino and the vaquita,” Houck says.

The holding room for the tanks is currently at maximum capacity, but the tanks themselves are not yet full. Therefore, the team is still diligently working on culturing and preserving cells that could potentially save endangered animals in the future.

In the upcoming years, the laboratory should modernize its techniques – the manually labeled vials will eventually be able to be scanned as barcodes. It also requires new scientists to join and closely monitor the developing collection of frozen specimens.

Houck expresses that we all feel a strong sense of guarding for the frozen zoo and its importance, as well as the legacy of Dr Benirschke. She hopes that we can inspire the next generation to continue its preservation.

Source: theguardian.com