Physiological status is a stronger predictor of nutrient selection than ambient plant nutrient content for a wild herbivore

There is generally a close relationship between a consumer's food and
its optimal nutrients. When there is a mismatch, it is hypothesized that
mobile herbivores switch between food items to balance nutrients, however,
there are limited data for field populations. In this study, we measured
ambient plant nutrient content at two time points and contrasted our
results with the nutrient ratio selected by wild female and male
grasshoppers (Oedaleus senegalensis). Few plants were near O.
senegalensis’ optimal protein:carbohydrate ratio (P:C), nor were
plants complementary. Grasshoppers collected earlier all regulated for a
carbohydrate-biased ratio but females ate slightly more protein. We
hypothesized that the long migration undertaken by this species may
explain its carbohydrate needs. In contrast to most laboratory studies,
grasshoppers collected later did not tightly regulate their P:C. These
results suggest that field populations are not shifting their P:C to match
seasonal plant nutrient shifts and that mobile herbivores rely on
post-ingestive mechanisms in the face of environmental variation. Because
this is among the first studies to examine the relationship between
ambient nutrient landscape and physiological state our data are a key step
in bridging knowledge acquired from lab studies to hypotheses regarding
the role ecological factors play in foraging strategies.