 matern separ rhesu monkei studi involv separ monkei mother-infant pair period week measur behavior infant reunion mother mother reunion infant emot disturb respons separ drastic decreas plai complex social behavior separ clear infant-mother separ produc emot disturb human macaqu infant pattern respons separ speci studi monkei infant-mother separ indic sheer physic separ crucial aspect matern separ monkei undoubtedli factor associ separ mother vital human children account absenc rariti detach stage human respons pattern separ monkei infant consider similar respons human children infant monkei separ mother 