Bark chewing reveals a nutrient limitation of leaves for a specialist folivore
Au, J, Youngentob, KN, Clark, RH, Phillips, R & Foley, WJ 2017, Journal of Mammalogy, vol. 98, no. 4, pp. 1185-1192.
Koalas of the subalpine Monaro region in New South Wales appear to have developed the unusual adaptation of chewing the sodium-rich bark of Eucalyptus mannifera to meet their nutritional requirements within a landscape that is otherwise lacking in the mineral micronutrient.
The koalas of the Monaro region live at heights of 900 – 1300m above sea level, which are at the upper elevation limit for the species. Foliar sodium may be low in this region given its high elevation and position in the rain shadow of the Australian Alps. To attempt to explain observations of koalas chewing on E. mannifera bark during the spring, the authors of this study compared the mineral contents of this bark with the bark and leaves of other eucalypt and non-eucalypt species, and with E. mannifera leaves. The sodium content of the foliage of the preferred food tree, E. viminalis, was also compared across different elevations to determine whether subalpine trees contain less foliar sodium than those at lower elevations. Surveys revealed that koalas chewed on the bark of no trees other than E. mannifera, which are patchily distributed throughout the habitat. Compared to the bark of E. mannifera, the bark and leaves of all other surrounding tree species, including the leaves of E. mannifera itself, had significantly lower concentrations of sodium. Sodium concentration of E. viminalis leaves was found to decrease as elevation increased, indicating their unsuitability for sustaining the sodium requirements of koalas in the Monaro region. Sodium concentrations of the chewed bark were such that a koala in the Monaro region could obtain approximately the same amount of sodium from eating 10g of E. mannifera bark as that from 200g of eucalypt leaves.
Herbivores other than koalas have also been reported to feed on refractory plant parts or non-vegetative sources to supplement intake of minerals lacking in preferred foods. Mineral micronutrients are essential for sustaining animal life, and sodium, in particular, is critical to nerve function and electrolyte balance. A lack of sodium can have negative effects on growth rate, muscle strength and reproduction. To mitigate these effects, koalas in the Monaro region seem to have developed a unique adaptation to supplement their intake of this important mineral. The alternative solution would be simply for koalas to consume greater amounts of eucalyptus leaf. This option, however, is prohibited for this species by their gut capacity, slow digestion rate and inability to handle excessive amounts of the potentially toxic compounds of the leaf.
Given this observed capacity for nutrient availability to shape koala behaviour, the authors suggest that future research efforts explore how these important yet patchily-distributed E. mannifera trees influence social interactions and population dynamics in the Monaro region. Furthermore, the potential association between bark chewing in spring and increased sodium requirements during lactation could be explored. Finally, it is vital that these ‘chewing trees’ in the Monaro region are conserved given their role in maintaining the resident koala population.
Summarised by Joanna Horsfall
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