Such symmetries can account for the reciprocity reported for fish, vampire bats (which regurgitate blood to their buddies), dolphins and many monkeys.

• 这种对称关系可以解释研究人员观察过的鱼类、吸血蝙蝠（牠们会呕血喂食哥儿们）、海豚，以及许多种猴子的行为。