Chester Zoo has marked a major conservation milestone, with the birth of a rare Goodfellow tree kangaroo joey - one of the world's most threatened marsupials.

The newborn male arrived to parents Kitawa (mum) and Kayjo (dad) as part of an international conservation breeding programme that's ensuring the continued survival of the species.

It's just the second time experts have bred the species at the conservation zoo, with only two zoos in the UK currently caring for the rare animals.

Now, the zoo has released seldom-seen images revealing the secret world inside the kangaroo's pouch and confirming that the joey is healthy male.

With the all-clear given, zookeepers say they will soon begin choosing a suitable name for the "special arrival."

The joey, which now weighs 1.85kg but was no bigger than a jellybean at birth, has spent the first months of his life developing inside Kitawa's pouch.

Experts inserted tiny endoscopic cameras into the pouch to confirm Kitawa's pregnancy and carefully monitor her joey's development.

"When people think of kangaroos, they rarely imagine small, fluffy animals living high in the treetops," said Matthew Lloyd, a tree kangaroo expert at the zoo.

"With so little known about tree kangaroos, Kitawa's joey is a particularly special arrival, and represents a major step forward in understanding and protecting this remarkable species from extinction."

With the use of cameras to track the joey's development inside the pouch made possible just a few years ago, researchers are already learning crucial information about the early stages of life inside the pouch.

Matthew added: "We don't have a name for the little one just yet, but our choice will be influenced by communities in Papua New Guinea who live alongside tree kangaroos and are now part of efforts to protect their forest homes."

Goodfellow's tree kangaroos are found only in the forests of Papua New Guinea and are classed as endangered by the International Union for Conservation of Nature, with populations declining due to hunting and widespread habitat loss.

The successful birth follows the pairing of Kitawa and Kayjo, who are part of a European conservation breeding programme designed to maintain a healthy, genetically diverse population in in conservation zoos.

Scientists helped pinpoint the best time for the two kangaroos to be paired, which is crucial to breeding success, by using hormone monitoring carried out in the zoo's on-site science laboratory - the only facility of its kind at a zoo in Europe.

David White, team manager at Chester Zoo, added: "Goodfellow's tree kangaroos don't breed quickly, as a joey is a huge energy investment for the mother.

"This is one of the reasons they are disappearing from the forests of Papua New Guinea - they just can't keep up with numbers being lost to hunting and deforestation.

"That's why every birth like this is incredibly important. It's been a real team effort, from zookeepers using their expert animal knowledge, to vets and scientists closely monitoring hormone levels and behavioural changes to understand the best times of year to introduce the parents."

Kitawa can be seen in the zoo's Islands zone, where visitors may spot the joey popping his head out of her pouch as he becomes more confident.

Tree kangaroos are known for spending long periods resting in trees, often sleeping in a head-down position to help shed rain from their fur in their tropical forest homes.