A blind jelly bean at birth, the baby koala promptly climbs unaided into its mother's pouch. Six months later it pops its head out.
Facts about koalas and koala babies
A koala is not a bear, although it looks like everyone's favourite teddy. It is a tailless marsupial.
Marsupials are a sub-class of pouched mammals found in the Americas, where they are known as opossums.
In Australia kangaroos and koalas join possums as important marsupials.
Marsupials arrived in Australia about 55 million years ago. Australia was then still attached to Antarctica and South America.
Antarctica was not yet ice and the great southern land was called Gondwana.
The koala family's evolution in Australia dates back some 30 to 40 million years, when it separated from its closest relative, the Australian wombat. Their common ancestor was possibly a burrowing animal which started to climb trees to access more or better food.
Koala babies born 19mm long
blind, hairless and earless
One defining property of mammals is that they give birth to live young.
But whereas placental mammals, like humans and real bears, give birth to fully developed young, marsupial neonatals are largely undeveloped when born and grow to independently viable life in their mothers' pouches.
Koalas breed between November and March. The female koala gives birth to one (occasionally two) young after around 35 days gestation.
A very young koala is sometimes called a pinkie (or pinky), being small, pink and hairless.
At birth the pinkie, or baby koala, weighs less than 0.5 grams (0.02 oz) and measures less than 2 cm (not much more than half an inch).
It is about the size of a jellybean, considerably smaller than in the picture on the left.
Baby Koala in Pouch
The newborn baby koala cannot see or hear but it has strong forelimbs and an acute sense of smell.
Unaided by its mother, immediately after its birth it pulls itself up from her cloaca, through her thick fur, and finds its way into the vertical opening which is located toward the bottom of her pouch.
Inside the pouch it latches on to one of the two teats there, which swells in its mouth, effectively anchoring the tiny neonate.
It will spend 13 weeks attached to the teat. At the end of this time it has a body weight of about 50 grams (less than 2 oz).
After 22 weeks its eyes will open. At 24 weeks it will develop teeth. Between 5 and 6 months old the baby koala is fully furred and will stick its head out of the pouch for the first time.
In this time it will have grown into a joey - the cute young furry animal with fluffy ears we all respond to.
The milk that baby koalas receive from their mothers during the time in her pouch changes as they grow.
The milk that the young pinkie receives is high in carbohydrates and low in proteins and lipids. As the joey grows, the carbohydrates in the milk gradually decrease while the protein and lipids increase.
The baby koala's first semi-solid food is also provided by its mother. It is a green, jelly-like substance called pap. It is derived from the contents of her caecum, the large back-intestine of the koala which digests the fibrous bulk of the leaves it eats.
Through this pap the mother passes on micro-organisms from her intestine which the baby koala will need to digest eucalyptus leaves. It is also a rich source of protein.
Pap is not koala poo! It is generated on demand by the baby koala, which leans downwards out of its pouch (thereby pulling the pouch backwards) and licks its mother's rectum. After passing faeces for some time, the substance produced by the mother changes and becomes semi-liquid. The joey starts to feed.
It will feed like this between several days and a few weeks, just before leaving the pouch for good.
Even after it no longer fits into the pouch the koala baby may still drink from the mother's teat which, for some time, extends out of the pouch so that the joey can reach it.
Mother and Joey in Tree
The joey first leaves its mother's pouch at about 7 months old. At 30 weeks it weighs about 0.5 kg (just over 1 lb).
It clings on to her belly as she moves around but returns to the pouch to feed.
By the time it is 9 to 10 months old the joey weighs 1 kg (2.2 lb) and has permanently left the pouch.
It flattens itself against the mother's back while she searches for food, or sits there when she is feeding.
The joey still returns to the mother's belly to sleep or if the weather is wet and cold.
Koala babies are fully weaned at about 12 months. At this stage the young koala will weigh a little over 2 kg (about 4.5 lb). A mother may speed up the weaning process by becoming quite aggressive towards the joey, particularly if she has a new baby in her pouch.
Koala food preferences are set by the mother's diet
Baby koalas learn to eat eucalypt leaves while riding on their mothers' backs.
This means that they first learn to eat the leaves of the trees favoured by the mother and present in her home range.
Koala babies seem to develop a preference for the same trees, and will continue to prefer them over others when they establish their own home range.
This appears to be particularly so for young female koalas who generally establish a home range close to or overlapping that of their mother.
This proximity makes it likely that the same trees will be available to them, and the young female can continue to be a highly specialised feeder.
Young male koalas tend to go further afield when they leave home. This, combined with the fact that they will almost certainly be smaller than the older koalas with whom they compete for food, means that they may have little choice but to change their food preferences. At the very least they may need to become more generalist feeders in order to survive.
After weaning a young koala will often stay within its mothers territory, but in a separate, pseudo 'home range' for some time.
Koala in its home range
Female baby koala bears tend to leave their mother's territory of their own accord to set up their own home range nearby.
Males are often evicted by their mothers at around two.
Male koalas travel further than females, searching both for mates and for a home range.
Every area appears to have an alpha male, larger than the others, who achieves and maintains his dominance through body weight.
But almost any older male will be bigger than a two year old.
A male koala can breed from around 18 months but, until he reaches his peak weight at around 4 or 5, the other bigger males will usually prevent his getting access to females.
They will also prevent his moving into a home range of his own, so the male koala often spends up to three years wandering as a transient until his body weight peaks. During this time he is at the lowest rung of koala power and may be forced to occupy less than optimal habitats, sometimes to the extent of needing koala hospital care.