Postnatal experiences and genetic effects on squirrel monkey social affinities and emotional distress

Most nonhuman primate research on risk factors underlying vulnerability to stress has focused on early psychosocial experiences in various species of macaques. To test for genetic and experiential effects on emotional vulnera bility in randomly bred squirrel monkeys, here we combined a paternal half- sibling analysis with three postnatal rearing protocols that altered aspect s of maternal availability. In one condition offspring were periodically re moved from natal groups, whereas differences in maternal availability were produced in two other conditions by manipulating the effort: required of la ctating mothers to successfully locate food. After completion of these prot ocols at 21 weeks of age, social affinities, maternal separation induced pe ep-calls, and plasma levels of cortisol were assessed from 29 to 37 weeks o f age. Significant postnatal rearing effects and the lowest heritabilities were detected in peak elevations of cortisol measured ii day after the remo val of mothers from otherwise undisturbed groups. Individual differences in cortisol 3-7 days later revealed negligible postnatal rearing effects and the highest heritabilities (h(2) similar to .70), as offspring sired by cer tain fathers failed to return to the preseparation level found in undisturb ed natal groups. paternal half-siblings that responded with long lasting in creases in cortisol spent more time near their mother in undisturbed groups and exhibited long-lasting increases in separation induced peep-calls. The se findings concur with human twin studies that suggest genetic and experie ntial factors contribute to individual differences in vulnerability to emot ional distress. (C) 1999 Academic Press.