Welcome to Murray Logan's Koala Research Page
The ability of an animal to utilise particular resources (diet) depends upon an ability to balance its requirements (demand) for energy and nutrients with its ability to aquire and assimilate (supply) energy and nutrients. However, neither the supply nor the demand of energy and nutrients remain static throughout the life of an animal. Many factors have the potential to disrupt the balance between supply and demand, and therefore the ability of an animal to utilise a particular diet throughout life will depend on the animals ability to maintain the balance of supply and demand under conditions of nutritional stress.
Koalas (Phascolarctos cinereus) are small-medium sized (< 15 kg) arboreal folivores that feed almost exclusively on a Eucalyptus diet which is high in fibre, low in available energy and nutrients, physically difficult to process, highly defended chemically and at times toxic. Data from a range of mammalian herbivores would suggest that the poor diet quality, compounded with the allometric constraints of small body size should preclude exclusive utilization of mature Eucalyptus foliage.
The ability of koalas to utilize such a poor quality diet is attributed to their low metabolic requirements and highly developed caecum and proximal colon in which fine digesta particles are selectively retained for microbial fermentation, whilst the larger, less fermentable particles are more rapidly eliminated thereby alleviating the gut filling effects that are particularly limiting in combination with small body size. Nevertheless, koalas are thought to be highly stressed energetically and thus the balance between the supply and demand of nutriments is often considered to be within fine tolerances.
The abitity of koala teeth to reduce the food material depends on the integrity of their shearing surfaces. However, the actions of processing their food material, along with the abrasive properties of the food material, cause the teeth to wear down over time. Thus, by reducing the rate and extent of nutrient and energy intake, tooth wear has the potential to impose severe limitations on the feeding ecology and life histories of koalas. The figures below illustrates both unworn and unworn teeth. Despite this however, earlier work that investigated the size of food particles present in different regions of the gut of koalas found that in general, there were no differences in the number and size of particles in the small intesting of koalas with worn teeth compared to those with unwarn teeth. Differences were only apparent in individuals that had extreme wear. This suggests that up to a point, koalas are able to compensate (at least in part) for the lower food processing effectiveness (associated with tooth wear).
There are many costs imposed on the mammalian mother including the need to produce milk that is rich in energy and nutrients, transport and protection of the young, reduced mobility and agility, as well as some dependent thermoregulatory demands. In fact, lactation is considered to be the most energetically demanding mammalian life history stage. As a result of the poor quality of an exclusively eucalypt diet, lactating female koalas appear unable to utilise fat stores as a means of satisfying their extra energetic requirements. This is believed to be the reason that juvenile koalas have abnormally slow growth rates. Presumably, all the extra requirements must be met instantaneously and therefore, the lactating female koala must be able to compensate for the extra demands.
If koalas are to endure nutritional stresses, anatomical, physiological and behavioural characteristics must be flexible enough to make allowances for these influences so as to restore or maintain the supply-demand balance. Consequently, the characteristics that enable koalas to utilize a poor quality diet probably reflect the difficulties in maintaining the balance between supply and demand on a poor quality diet during conditions of extreme nutritional stress rather than purely reflecting the balance itself. That is the characteristics represent adaptations to compensate for the extremes rather than adaptations for regular conditions.
The aim of this research was to investigate the compensations and consequences of two aspects of nutritional stress (tooth wear and lactation) on the feeding behaviour and activity patterns of free-ranging koalas. Specifically, I wanted to investigate how these factors influenced food intake, chewing rates and number of chews per mouthful as well as the other activity patterns (timing and duration of major activities). As tooth wear and lactation represent two different strains on the supply/demand balance, these two stresses should also offer some insight into the generality and flexibility of the koala's nutritional compensation mechanisms. Such insights can be then used to speculate on the interactivity of the mechanisms and thus the ability of koalas to compensate for multiple concurrent nutritional stresses.
