Body Behavior is an organism's response to stimulus, that is, its observable reaction to a change in conditions.
Ethology is the study of behavior.
The field of study called behavioral genetics studies the genetic component of behavior. For instance, if a fruit fly has a certain mutant gene, males will only court other males, and not females. Another mutant in fruit flies makes the fruit fly paralyzed if it sees something moving fast in front of it. Some breeds will fetch a thrown object, some won't, etc. Instinct and learning form a continuum.