▁N erv ous ▁System ▁on ▁the ▁Body ▁suggests ▁that ▁emot ional ▁expression ▁occurs ▁when ▁nerv ous ▁energy ▁has ▁passed ▁a ▁threshold ▁and ▁needs ▁to ▁be ▁released . ▁ ▁Dar win ▁saw ▁emot ional ▁expression ▁as ▁an ▁out ward ▁communication ▁of ▁an ▁inner ▁state , ▁and ▁the ▁form ▁of ▁that ▁expression ▁often ▁car ries ▁beyond ▁its ▁original ▁adapt ive ▁use . ▁For ▁example , ▁Dar win ▁remarks ▁that ▁humans ▁often ▁present ▁their ▁can ine ▁teeth ▁when ▁s ne ering ▁in ▁ra ge , ▁and ▁he ▁suggests ▁that ▁this ▁means ▁that ▁a ▁human ▁ancest or ▁probably ▁util ized ▁their ▁teeth ▁in ▁ag gress ive ▁action . ▁A ▁domestic ▁dog ' s ▁simple ▁tail ▁w ag ▁may ▁be ▁used ▁in ▁subt ly ▁different ▁ways ▁to ▁convey ▁many ▁mean ings ▁as ▁illustrated ▁in ▁Dar win ' s ▁The ▁Expression ▁of ▁the ▁Em ot ions ▁in ▁Man ▁and ▁Anim als ▁published ▁in ▁ 1 8 7 2 . ▁ ▁A ne cdot al ▁approach ▁Ev idence ▁for ▁emot ions ▁in ▁animals ▁has ▁been ▁primarily ▁an ec dot al , ▁from ▁individuals ▁who ▁interact ▁with ▁p ets ▁or ▁capt ive ▁animals ▁on ▁a ▁regular ▁basis . ▁However , ▁critics ▁of ▁animals ▁having ▁emot ions ▁often ▁suggest ▁that ▁anth rop omorphism ▁is ▁a ▁motiv ating ▁factor ▁in ▁the ▁interpretation ▁of ▁the ▁observed ▁behav i ours . ▁Much ▁of ▁the ▁debate ▁is ▁caused ▁by ▁the ▁difficulty ▁in ▁defining ▁emot ions ▁and ▁the ▁cogn itive ▁requirements ▁thought ▁necessary ▁for ▁animals ▁to ▁experience ▁emot ions ▁in ▁a ▁similar ▁way ▁to ▁humans . ▁The