The giant panda’s seasonal activity is almost stable. It has opted for bulk processing, and perhaps because of this, it is extremely sensitive to the mineral and nutritive content of all parts of the bamboo. The animal is constantly monitoring the leaf, stem and shoots of bamboo, weighing up the advantages of each, and making decisions to eat one part, or rejecting another. It continuously alters its behavior to extract the best nutrition pay-off from bamboo.
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| Feeding on umbrella bamboo shoots –note the pale-coloured discarded sheath littering the feeding site |
The giant panda recognizes three ecological seasons in its food year: spring (April to June), summer/autumn (July to October) and winter (November to March). During these three major divisions of the year the giant panda normally feeds on specific parts of the bamboo, or even changes its diet from one bamboo species to another.
In Wolong the panda’s favorite bamboo is arrow bamboo. Panda spend 85 percent of the year above 2600m feeding on arrow bamboo from July all the way through to March. From July to October their food is almost exclusively the leaves of this species; no stems are consumed. By November, young stems are beginning to feature in the diet, along with leaves, a trend which continues until spring. At the end of this period, leaves suddenly drop out of the diet, the panda selecting mainly old stems to eat.
Appendix: Why does the giant panda eschew the protein-rich leaves in spring?
Some arrow bamboo are consumed during spring, but most giant panda leave the higher elevations at this time and descend into the stands of umbrella bamboo. Spring is the time of maximum shoot production for most bamboo, and between May and June the giant pandas in Wolong feed almost exclusively on the emerging shoot of this species. Arrow bamboo shoots are found only as a causal item in the diet. So pressing is the urge to eat umbrella bamboo shoots that the pandas will leave their accustomed haunts to sample this seasonal fare.
Appendix: Advantages of feeding on umbrella bamboo shoots
Giant pandas do not feed randomly on the shoots. Generally speaking, they leave shoots less 0.9 cm in diameter, and concentrate on shoots with diameters of 1 cm or more. They also search for food along the edge of the umbrella bamboo clumps, where the extra light allows plumper, juicier shoots to grow. Height is also important, pandas tend to ignore short shoots less than 25cm high. The ‘bigger shoots provide more energy’ explanation probably partly explains this behaviour, but the reasons behind the rejection of short shoots are a little more complex.
When they emerge from the ground umbrella bamboo shoots are covered by a protective hairy sheath which is unpalatable and painful for humans to consume. Giant pandas seem to feel the same way. The giant panda take pains to remove the sheath from the shoot before eating, normally by holding the shoot aslant in its mouth and tearing the sheath off with a pulling and twisting motion of its paws and mouth. But sheath removal is a relatively costly energy activity. It turns out that the shoots the giant pandas select to feed on give the best return; in short shoots the ratio of digestable core to indigestable sheath is 1:2 by weight. This ratio is reversed with long shoots, with core outweighting sheath by a healthy behaviour is finely honed to maximized food intake and minimize energy expenditure.