This enhanced video resource examines how the koala’s unique diet of eucalyptus leaves affects their behavior.
- Why did people once think that koalas got intoxicated from their eucalyptus gum leaf diet?
- What is a marsupial? Can you think of any other marsupial species? Where do they live? What might this suggest?
- How does a baby koala—or “joey”—feed for the first six months of its life, and how is it prepared for an adult koala diet?
Few animals are as closely identified with their diet as the Australian koala is with eucalyptus gum leaves. 50,000 years ago, the koala’s ancestors consumed a broad diet of tropical vegetation in what was then a rainforest. As the climate cooled, this diversely lush ecosystem was replaced by more homogenous forests of tough eucalyptus trees better suited to the new, drier environment. The fact that eucalyptus gum leaves were low in nutrition, almost indigestibly fibrous, and contained toxic compounds meant that very few animals consumed them, allowing eucalyptus forests to thrive and spread over much of what would become southern and eastern Australia. Koalas gradually adapted to fill an ecological niche by taking advantage of this plentiful—if low quality—food source by developing sharp claws to climb tall eucalyptus trees, powerful teeth to grind the fibrous leaves into a paste, a liver capable of filtering out the toxins, and an extended digestive system to extract their few nutrients.
Even as these physiological adaptations allowed the eucalyptus tree to become the koala’s sole food source, the poor quality of this diet means that koalas must spend over five hours a day eating up to two and a half pounds of the tough leaves. They spend the remainder of each day virtually motionless, sleeping and dozing high in the trees. This lethargic behavior once inspired speculation that koalas were actually drugged by narcotic toxins in their eucalyptus diet, but we now understand that their behavior is a means of energy conservation required by a very slow metabolism. Indeed, the extreme inactivity of the modern koala has had a uniquely unfortunate effect on the size of its brain: it has shrunk dramatically, now filling only 40% of a cranial capacity which once held a much larger brain of the type required by a more active animal.
The diet of koala infants—known as “joeys”—is yet another oddity. Although often referred to as “bears,” the koala is in fact a marsupial—an animal which, like its fellow famous Australian species the kangaroo, carries its young in a pouch. Born hairless, earless, and blind, a joey immediately crawls into its mother’s pouch and attaches itself to one of two teats. After about six months of feeding on milk, it begins to emerge and explore the larger world, at which point it expands its diet to include “pap”—a specialized form of the mother’s feces, enriched with the microbes necessary to inoculate the joey’s gut and permit the digestion of eucalyptus leaves. After about six more months of a milk and eucalyptus diet, the joey is weaned and ready to go off on its own.
National Science Education Standards
Life Science – Content Standard C
As a result of their activities in grades 5-8, all students should develop understanding of
- Reproduction and heredity
- The characteristics of an organism can be described in terms of a combination of traits. Some traits are inherited and others result from interactions with the environment.
- Regulation and behavior
- All organisms must be able to obtain and use resources, grow, reproduce, and maintain stable internal conditions while living in a constantly changing external environment.
- Regulation of an organism’s internal environment involves sensing the internal environment and changing physiological activities to keep conditions within the range required to survive.
- Behavior is one kind of response an organism can make to an internal or environmental stimulus. A behavioral response requires coordination and communication at many levels, including cells, organ systems, and whole organisms. Behavioral response is a set of actions determined in part by heredity and in part from experience.
- An organism’s behavior evolves through adaptation to its environment. How a species moves, obtains food, reproduces, and responds to danger are based in the species’ evolutionary history.
- Populations and ecosystems
- A population consists of all individuals of a species that occur together at a given place and time. All populations living together and the physical factors with which they interact compose an ecosystem.
- Populations of organisms can be categorized by the function they serve in an ecosystem. Plants and some microorganisms are producers—they make their own food. All animals, including humans, are consumers, which obtain food by eating other organisms. Decomposers, primarily bacteria and fungi, are consumers that use waste materials and dead organisms for food. Food webs identify the relationships among producers, consumers, and decomposers in an ecosystem.
- The number of organisms an ecosystem can support depends on the resources available and abiotic factors, such as quantity of light and water, range of temperatures, and soil composition. Given adequate biotic and abiotic resources and no disease or predators, populations (including humans) increase at rapid rates. Lack of resources and other factors, such as predation and climate, limit the growth of populations in specific niches in the ecosystem.
- Diversity and adaptations of organisms
- Millions of species of animals, plants, and microorganisms are alive today. Although different species might look dissimilar, the unity among organisms becomes apparent from an analysis of internal structures, the similarity of their chemical processes, and the evidence of common ancestry.
- Biological evolution accounts for the diversity of species developed through gradual processes over many generations. Species acquire many of their unique characteristics through biological adaptation, which involves the selection of naturally occurring variations in populations. Biological adaptations include changes in structures, behaviors, or physiology that enhance survival and reproductive success in a particular environment.
As a result of their activities in grades 9-12, all students should develop understanding of
- Biological evolution
- Species evolve over time. Evolution is the consequence of the interactions of (1) the potential for a species to increase its numbers, (2) the genetic variability of offspring due to mutation and recombination of genes, (3) a finite supply of the resources required for life, and (4) the ensuring selection by the environment of those offspring better able to survive and leave offspring.
- The great diversity of organisms is the result of more than 3.5 billion years of evolution that has filled every available niche with life forms.
- The millions of different species of plants, animals, and microorganisms that live on earth today are related by descent from common ancestors.
- Matter, energy, and organization in living systems
- The chemical bonds of food molecules contain energy. Energy is released when the bonds of food molecules are broken and new compounds with lower energy bonds are formed. Cells usually store this energy temporarily in phosphate bonds of a small high-energy compound called ATP.
- The complexity and organization of organisms accommodates the need for obtaining, transforming, transporting, releasing, and eliminating the matter and energy used to sustain the organism.
- The distribution and abundance of organisms and populations in ecosystems are limited by the availability of matter and energy and the ability of the ecosystem to recycle materials.
- Behavior of organisms
- Organisms have behavioral responses to internal changes and to external stimuli. Responses to external stimuli can result from interactions with the organism’s own species and others, as well as environmental changes; these responses either can be innate or learned. The broad patterns of behavior exhibited by animals have evolved to ensure reproductive success. Animals often live in unpredictable environments, and so their behavior must be flexible enough to deal with uncertainty and change. Plants also respond to stimuli.
- Like other aspects of an organism’s biology, behaviors have evolved through natural selection. Behaviors often have an adaptive logic when viewed in terms of evolutionary principles.