Many arboreal (tree dwelling) Mammals are reliant on leaves as a food for at least part of the year, and some feed exclusively on such fodder. However leaves leaves are difficult food, as, unlike fruits or nectar, these are parts of the plant which the plant does not want eaten, and many plants put considerable effort into making their leaves as unpalatable as possible, minimising the amounts of nutrients stored in the leaves, defending them physically with tough fibres or spines, and packing them with toxins such as phenols or tannins. Leaf-eating Mammals avoid these defences by carefully selecting both which leaves and which parts of the leaves they consume, and choosing different food sources and feeding methods at different times of the year.
The Japanese Giant Flying Squirrel, Petaurista leucogenys, is a large arboreal Rodent (adults often reach 50 cm plus a meter-long tail) with a wing-membrane between its fore- and hindlimbs which enables it to glide between trees. It is found in sub-alpine forests and boreal evergreen forests on Honshu, Shikoku and Kyushu islands in Japan, as well as in Guangdong Province in China. It has a varied diet, which includes a variety of fruits, nuts, and flowers, but also includes leaves. Usefully, it is prone to detaching leaves before partially consuming them, then allowing partially consumed leaves with distinctive feeding traces to fall to the forest floor, where they can be picked up by interested scientists.
In a paper published in the journal Ecology and Evolution on 15 June 2017, Mutsumi Ito of the Department of Biology at Tokyo Metropolitan University, Noriko Tamura of the Tama Forest Science Garden, and Fumio Hayashi, also of the Department of Biology at Tokyo Metropolitan University, describe the results of a three-year study of the leaf-feeding behaviour of the Japanese Giant Flying Squirrel in the Tama Forest Science Garden.
A Japanese Giant Flying Squirrel, Petaurista leucogenys. 飯能に棲むいきものたちのこと.
Ito et al. collected leaves of two Oak species along a 2 km census route between one and five times a month between May 2013 and November 2015. These were the deciduous Sawtooth Oak, Quercus acutissima, and the evergreen Tsukubanegashi Oak, Quercus sessilifolia.
They found that while the leaves of the evergreen Quercus sessilifolia were the preferred food of the Squirrels in winter, they were seldom eaten in summer when the leaves of the deciduous Quercus acutissima were available. The leaves of the evergreen Quercus sessilifolia were almost always eaten apically, i.e. from the tip, while the leaves of the deciduous Quercus acutissima were eaten basally, or centrally, i.e. from the base or centre, with the tip being avoided.
Three types of leaf debris eaten by the Japanese Giant Flying Squirrel (Type A, apically eaten; Type B, basally eaten; Type C, only centrally eaten). The total length (Lt) and width (Lw) of intact leaves are measured, as well as the remaining length for basally (La) and apically (Lb) eaten leaves, and the maximum width of the centrally eaten circle (Ld) of leaf debris. All leaves shown are the evergreen Quercus sessilifolia. Ito et al. (2017).
Examination of the leaves in the laboratory revealed that those of Quercus acutissima had a far higher sugar content that those of Quercus sessilifolia. This explains the preference of the Squirrels for these leaves when they are available, as most Mammals preferentially select food that has a higher sugar content (is sweeter) as this relates directly to the energy available from the food. However, the leaves of Quercus acutissima also have far higher levels of phenols, toxic chemicals that Mammals generally avoid (though this is not the first time that leaf-eating Mammals have been shown to overcome an aversion to leaf toxins if the sugar content is high enough). Importantly the phenols in the leaves of Quercus acutissima were found to be concentrated around the tips of the leaves, which the Squirrels were consciously avoiding, showing that they were capable of adjusting their feeding behaviour to avoid toxins in a seasonally available food.
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