Nepenthes lowii, a pitcher plant in southeast Asia, primarily relies on insects as a source of food, but in winter, as the insect population declines, it has to supplement its diet by digesting the droppings of a species of tree shrew. Droppings, however, are not reliably available in the plant’s habitat, and Nepenthes lowii requires substantial amounts of these in order to survive the season. One possible explanation is that tree shrew droppings contain significantly more nutrients and amino acids than the insects the plant typically consumes.
Which of the following, if true, most seriously calls into question the explanation given for the plant’s winter diet?