Getting a Koala-ty Diet
When You Specialize, You Must Adapt to Fit Your Niche
The koala, Phascolarctos cinereus (sometimes called, inaccurately, the koala bear as it is a marsupial and not an actual bear at all), is one of the classic diet specialists in the animal kingdom. It eats eucalyptus leaves and very little else. In fact, the genus Eucalyptus contains over 700 species of trees and shrubs (all native to either Australia or nearby Southeast Asia) but the koala shows a specific preference for about 30 of those. However, this diet poses a set of unique challenges that the koala must face.
In general, plant matter is not highly nutritious. Those that do consume it will usually focus on fruits, seeds, pollen, or nectar. These substances have more easily accessible and digestible sugar compounds and some are even high in protein. The greenery, on the other hand, contains much less consumable energy sources. The sugars are in the form of long chains of cellulose (the substance that makes up plant cell walls). Most animals cannot break the individual glucose bonds in cellulose so that they can be absorbed and used for fuel. Even the ones that do usually need some friendly assistance.
You may remember some of these terms from my gut bacteria story during Season 2 on Symbiosis. Koalas do not have a multi-chambered stomach to aid in their digestion like in cattle or deer. Instead, they retain plant material in their hindgut and allow it to ferment there with the help of symbiotic bacteria. Wild koalas can retain plant material in this portion of their digestive tract for up to 100 hours to allow it to be broken down.
Despite the lack of overall nutritional value, the koala does not make up for it in terms of volume consumed. A koala will eat about 400g of eucalyptus leaves per day, between three and ten percent of its body weight. In contrast, the Canada goose, Branta canadensis, is well-known for its voracious appetite and can consume up to 1,800g of grass per day, a third of its body weight (most of which is quickly expelled as feces). Instead of increasing its intake, the koala has developed a way to decrease it overall energy output. They will sleep for twenty hours a day, twice as many as a sloth (a group of animals whose own name is a reference to their supposed laziness).
As if the nutritional content (or lack thereof) weren’t enough of a problem, the koala also faces an issue similar to the clay-eating parrots we talked about a few weeks ago. Eucalyptus leaves contain disinfectant oils that are toxic in large quantities as well as a variety of other related toxic chemicals. While koalas and related herbivorous marsupials have developed a tolerance for these chemicals, they often have to make leaf consumption choices based on smell to avoid the more potent toxins.
Overall, the unique flora and fauna of Australia exist in large part because of their isolation from other populations. Current DNA and fossil evidence indicates that its marsupials are descended from an ancestral lineage inhabiting South America when it was part of a supercontinent called Gondwana, which formed approximately 600 million years ago and began breaking up around 200 million years ago. This isolation has led to a wide variety of unique diet strategies like that of the koala. As the populations became more isolated over millions of years, they began to adapt to foraging in their new habitat. Those habitats continued to change and, depending on the nature of those changes, evolutionary balance can tip towards generalists or specialists.
Phascolarctos cinereus is the only living member of its taxonomic Family. The climatic changes of the Miocene dried out much of the continent, reducing the coverage of rainforests that used to be the main living area of the koala’s more generalist relatives. Deep forests gave way to more open eucalyptus woodlands. In these conditions, a generalist was now at a disadvantage, but those individuals that could exploit the growing new resource could flourish. Remember again, that these diets and foraging behaviors do not occur in a vacuum. Today’s koala feeds as it does because of a climatic and environmental shift that occurred millions of years ago. A new niche opened up while others closed and those individuals best suited to the new situation were able to survive and pass on their genes.
These stories are like onions in some ways: peel back a layer, and additional layers with additional questions emerge. That is part of why I enjoy sharing them with all of you. As always, please consider sharing this with others whom you think might enjoy it and let me know what kinds of stories you are interesting in reading about in the future.