Empty stomachs contract, causing both hunger pangs and the secretion of chemical messages that travel to the brain to serve as a signal to initiate feeding behavior. this is an example of

This is an example of a physiological mechanism.  A physiological mechanism is an automatic  mechanism that governs our body using our bodily organs to stay alive and  functioning. Other examples of physiological mechanisms include those we use  for sleeping and regulate body temperature.

desert perennials often survive by remaining dormant during dry periods of the year, then springing to life when water becomes available. most annual desert plants germinate only after heavy seasonal rain, then complete their reproductive cycle very quickly.