Many koalas from Kangaroo Island (KI) only have one testicle and are a poor sample to regenerate declining mainland populations, an expert in wildlife ecology has warned.

Key points: A lack of genetic diversity has led to testicular aplasia for KI koalas

A lack of genetic diversity has led to testicular aplasia for KI koalas Limited ancestry is also behind oxalate nephrosis, or kidney damage

Limited ancestry is also behind oxalate nephrosis, or kidney damage The director of the wildlife park where the surviving KI koalas have been taken says it's a good chance to stage manage their recovery with a better genetic makeup

The warning comes as marsupials orphaned by bushfires are brought to the Adelaide Hills to start a new colony.

Koalas are a pest on Kangaroo Island after being introduced as an ark population of 18 in the 1920s from French Island, a population that was itself introduced at the turn of the century in response to mainland devastation caused by the fur trade.

Unlike mainland populations, KI koalas are free of chlamydia and have a low rate of koala retrovirus (similar to AIDS) and, up until December and January's bushfires, had spiralled out of control to an unsustainable population of about 50,000.

"People think, 'Oh, you have lots of koalas over there and they're in decline in other parts of Australia, so why don't you just take them across?'," Associate Professor Mathew Crowther from the University of Sydney said.

"But the problem is there's not a lot of genetic diversity there.

"You have a sub sample bred from a sub sample and lot of koalas in parts of Victoria and South Australia have what they call testicular aplasia, where they only have one testicle."

A 2002 study found testicular aplasia in koalas resulted in the remaining testicle being significantly larger than normal, which effectively compensated for the lack of a second testicle.

Some affected koalas had no testicles at all and were therefore sterile.

Dr Crowther said an introduced population on an island could suffer from inbreeding and genetic drift, where the frequency of a gene form called allele was affected randomly and led to large changes in populations.

"A lot of people talk about inbreeding being bad because it affects population growth, but it obviously didn't on French Island and Kangaroo Island because they grew a lot," he said.

Several SA Government programs on KI have sought to sterilise or translocate its koala population to protect the local environment from over-browsing, despite koala populations being in decline in parts of Australia.

Bushfire orphans the 'golden children'

The bushfires of December and January are estimated to have decimated KI's koala population by up to 90 per cent.

A bushfire approaches the town of Parndarna on Kangaroo Island earlier this month. ( Instagram: Trent Lawson/tmanadventure )

Some 12 koalas orphaned by the fires have been housed at Cleland Wildlife Park in the Adelaide Hills where they are to form a new ark population — free of disease.

Park director Chris Daniels said the new colony was known as the "golden children" and could be used for rewilding purposes and to improve the genetics of koalas across SA, KI, and Australia.

"The program involved the military to help with feeding them because it had to be done under the strictest of quarantine conditions," Professor Daniels said.

"If they are true golden children, we cannot expose them to any potential chlamydia or any other infective diseases."

Koalas breeding just fine with one testicle

Professor Daniels said testicular aplasia in SA koalas had not affected their ability to breed.

"I'm not sure exactly about the proportion of them having one testicle, but let me assure you that testicle works," he said.

"It's interesting in that, just because they are genetically bottlenecked, in the sense they're very similar because they came from few individuals, it does not mean they are not able to reproduce and establish sustainable populations.

"We've seen this now with quite a lot of animals that have come down to a few hundred, such as species of birds for example, cheetahs, some types of rhino."

An orphaned koala makes itself at home at Cleland Wildlife Park. ( Supplied: Nicole Mankowski )

Kidney problems leading to death

Professor Daniels added that a lack of genetic diversity had also led to problems such as oxalate nephrosis, which left deposits of calcium oxalate in a koala's kidneys that could lead to dehydration and death.

"That condition can come on at any time and seems to be triggered by stress events sometimes, but different animals don't get it at all, or get it occasionally or late in life, sometimes severely or sometimes mildly," he said.

"There's a whole mass of issues around that but it seems to be related to the fact that they have only had a handful of ancestors."

He said the pilot program was an opportunity to "stage manage" the species' recovery with better genetic makeup.

"So there's opportunity for crossbreeding and to outbreed to deal with genetic issues, as well as dealing with their disease-free status," Professor Daniels said.

"A special colony is really useful for understanding all of those things."

Koalas aren't koalas

But Dr Crowther said it was also worth considering the differences in koalas from varied habitats.

Queensland and northern NSW koalas are smaller and have bigger ears than SA koalas. ( Supplied: Coomera Conservation Group )

"Victorian koalas, which is basically what the KI koalas are, are up to 14 kilograms in size," he said.

"You'll be lucky to get a 10kg koala in Queensland or northern NSW.

"And their ears are different: they are relatively small for their body compared to Qld and NSW and their fur is a different consistency."

He said northern koalas dissipated a lot of heat through their larger ears to deal with their local climate.

"KI koalas are not well adapted to other areas and they have that low genetic diversity," Dr Crowther said.

"There are many reasons why you wouldn't use them to supplement mainland populations."