Bark chewing reveals a nutrient limitation of leaves for a specialist folivore
Mineral micronutrients are critical for basic physiological function, and variable availability of minerals over the landscape can influence foraging decisions. Sodium is essential for nerve function and osmotic balance; however, it can be limiting in some environments, such as those at high elevations. Koalas (Phascolarctos cinereus) living in subalpine regions have been observed eating Eucalyptus mannifera bark, an unusual food choice for a folivore. We hypothesized that sodium may be deficient in leaves at high-elevation sites, and that the bark from trees could be a potential sodium source. We compared the mineral content of eucalypt bark and leaves in 3 areas where koalas chew bark and in leaves of a preferred food tree of koalas, E. viminalis, across a range of elevations. Individual chewed trees were rare compared to non-chewed conspecifics and patchily distributed. Bark from chewed E. mannifera trees had significantly higher concentrations of sodium than plant parts from non-chewed neighboring conspecifics and other Eucalyptus species trees. We also found that E. viminalis foliage had significantly less sodium at higher elevations than lowland populations. We propose that koalas have developed the unusual bark-eating behavior to meet sodium requirements in an otherwise sodium-poor landscape. Given the physiological importance of sodium, behavioral adaptations such as these may be critical to the ability of mammalian herbivores to survive in nutrient-deficient landscapes.