Koala Sperm Lasts In The Fridge, Not The Freezer


Stephen Luntz

Stephen has a science degree with a major in physics, an arts degree with majors in English Literature and History and Philosophy of Science and a Graduate Diploma in Science Communication.

Freelance Writer

koala and baby
The discovery that koala sperm keeps really well at cool temperatures could lead to more healthy babies like this one. Apple2499/Shutterstock

The discovery that koala sperm is exceptionally long-lasting at 5ºC (41ºF) marks an important step to preserving the genetic diversity of the beloved marsupial. It also has scientists pondering why koala sperm is so unusual.

Although koalas are overpopulated in some areas, eating their favorite trees bare, across most of their range they are struggling through a combination of deforestation, disease, and being killed by cars and dogs. Under such circumstances, conservation organizations and wildlife parks are struggling to maintain their genetic diversity.


In other species, genetic health can be preserved by freezing semen and using it to inseminate appropriate females. However, according to University of Queensland PhD student Bridie Schultz, marsupial sperm, in general, does not recover well after freezing, and koala sperm is particularly bad.


Bridie Schultz with two juvenile study subjects. University of Queensland

However, some years ago, in an accident reminiscent of Fleming's discovery of Penicillin, a researcher left a vial of koala semen out overnight. “The next morning he noticed that the sperm were still swimming,” Schultz told IFLScience, raising hopes for their viability.

Schultz narrowly missed the breeding season, so has yet to try inseminating a female koala. However, she has used a number of tests for sperm viability, including DNA fragmentation, mitochondrial membrane potential (which determines how much energy sperm have to swim), and the state of their acrosomes.


Even after 45 days the sperm just keep swimming, showing every sign of being ready to do their work.

“This length of chilled storage substantially exceeds the capacity of any other known mammalian sperm to survive outside the body without cryopreservation (freezing),” Schultz said in a statement. “It has only been done in two fish species – rainbow trout and halibut.”

Quite why koala sperm lasts so long at cool, but above freezing, temperatures, is not yet known. Schultz told IFLScience “we've got a couple of theories,” but rules out their eucalyptus oil diet acting as a preservative.

The work has many potential applications, including making research easier. Schultz has demonstrated it even applies to sperm extracted from the testicles of koalas recently killed in car accidents. It should also help maintenance of koala populations in international zoos, where it is much easier to fly over some chilled sperm than a grumpy male koala (non-grumpy male koalas not really existing).


However, potentially the most important aspect of Schultz's findings lies in the potential to help related species. So far no other marsupial sperm has shown the same longevity, but few have been tested in depth. “We're really keen to try it with hairy-nosed wombat sperm,” Schultz told IFLScience. “And we also want to look at quolls.”

Studying the semen of Phascolarctos cinereus is, Schultz admitted to IFLScience, “a niche market.” However, she explained: “I always wanted to work in animal reproduction, but there wasn't a course, so I studied human IVF and then got called by the University of Queensland and told they had a koala project.” Living the dream.

You didn't think we were going to deprive you of another photo of the ultimate outcome of this research, did you? Apple2499/Shutterstock


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