Koala and kangaroo are an example of the incredible versatility of adaptation of the marsupial mammals of Australia
The trees of the jungle were always a place of refuge and expansion for the marsupials. As foliage became sparser, due to climate warming, they were forced to colonise the plains and grasslands. But some of the close relatives of those who live on the pastures returned to the trees they had originally come from, and this is the result.
The ornate tree kangaroo is one of the seven species of kangaroos that learnt to climb. As this group of animals spread into new areas, the ornate variety was the one which reached furthest north, colonising the lands of New Guinea. The kangaroos were the result of millions of years of adaptations in order to survive out on the open plains.
Why, then, did they return to the trees, with the enormous, renewed effort to adapt that that implies?
There is no clear answer. Probably in order to get at the tempting foods which were out of reach, high up in the trees. When the slow evolutionary changes began, those kangaroos had no competitors for the leaves at the tops of the trees. The plains were very extensive, and the food sources scarce, so new types of kangaroos began to appear, with different features. Stronger front legs, powerful claws, and finally, the ability to move each one of its four legs independently, something which no kangaroo down on the ground is able to do.
At the same time, the different kangaroos that had invaded the plains were in competition with each other. But, as the climate became increasingly temperate, and there was ever more open land, the kangaroos, which had developed a digestive system capable of assimilating the tough grasses, continued to expand.
And at the same time, some trees also adapted to the increasingly temperate climate of Australia, and one in particular, the eucalyptus, was so successful that it spread throughout the continent, from north to south, creating a new type of forest. And as soon as this new habitat appeared, there was a marsupial ready to take advantage of it.
The koala was able to colonise the eucalyptus forests thanks to an adaptation which would seem impossible – the ability to feed on its leaves. The leaves of the eucalyptus tree are a combination of low-quality food, indigestible material, and active poisons. Any animal that could adapt and make use of these leaves would have absolutely no competitors. And that is precisely what the koala did.
Taking care of the young is especially complicated in this world of toxin-laden foods. After completing their development in the mother’s pouch, breast-feeding for six long months, the time comes for the cubs to be weaned, and start feeding on the eucalyptus leaves. At this time, the mother excretes special soft faeces, a kind of pulp of half-digested leaves, which she uses to feed her young. As well as nourishing them, these faeces inoculate the young with the microorganisms they will need for this difficult digestion. From then on, they will become independent from their mothers.
For most part of the day, the koalas rest among the branches of the trees. This is part of their metabolic strategy. As the food is very poor, nothing better than to rest for twenty hours a day, and be able to function with the least expenditive of energy. Their specially-adapted digestive system takes care of the rest. Five hundred grams of these poisonous leaves a day are enough to keep a koala going.
On the ground, the koalas are clumsy and vulnerable. Their feet, which are designed for climbing, are not made for walking on four legs. So, they only leave the safety of the branches to move from one tree to another, in search of more food, or a female ready to mate.