The present study used a cross-fostering procedure to investigate the effects of early traumatic experience on vocal expressions of emotions in rhesus macaques (Macaca mulatta). The subjects of the study were 12 juvenile females: six were born to abusive mothers and reared by nonabusive controls, and six were born to controls and reared by abusive mothers. The cross-fostering took place within 24-48 hr after birth. Vocalizations were recorded from the subjects in their social groups during their first 2 years of life. Abusive mothers maltreated their adopted daughters in the first 2-3 months after birth with patterns similar to those previously shown with their biological offspring. Abused females produced proportionally more noisy screams compared to controls. While controls used noisy screams during contact aggression and tonal screams during non-contact aggression, the screams from the abused animals appeared to be distributed equally across contexts. Acoustical analyses revealed that the screams of the abused females were less modulated and had lower fundamental frequencies compared to the screams of controls. Taken together, these results suggest that traumatic experience in the first few months of life can have long-term effects on vocal emotional expression in rhesus macaques.
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