Appendix: Why does the giant panda eschew the protein-rich leaves in spring?

This is a puzzling behavior: leaves contain the highest leaves of protein, followed by branches and stems. Arrow bamboo leaves also have the best balance of amino acids. The main explanation of such behavior seems to be that the leaves become unpalatable to the animals at this time of the year because of a build-up of silica. Compare the percentage of leaves in 339 droppings throughout the year with the amount of silica present in the leaves of arrow bamboo over a twelve-month period (see table), it clearly shows that giant pandas are aware of silica levels (by smelling at the leaves) and reject leaves during those when the silica levels are high.