Question

In what order are stored energy sources in the body exploited? i.e glycogen, fats, proteins etc?

Answer 1

To some extent this depends on the species you are discussing. The canonical answer is that glycogen is used, then fats, then proteins - but some animals, particularly marine mammals, have been known to use proteins for energy before fats under certain circumstances. If they burn their fat reserves they burn their insulation, and if they get rid of all their insulation it will be worse for them than if they burned some of their proteins.