Attachment and early rearing: longitudinal effects in chimpanzees (Pan troglodytes)

Abstract

Between the years of 1991 and 1995, two different chimpanzee nursery rearing strategies were employed by caregivers and research staff at the Yerkes National Primate Research Center. One of these strategies included, in addition to the basic care provided by both nurseries, an additional 4 hours of human contact for 5 days each week. This human contact was provided by caregivers instructed in the behavior of chimpanzee mothers toward their offspring and instructed to emulate that behavior as much as possible. Various measures of cognitive and motor development were taken during the first year of the nursery-reared chimpanzees' lives; additionally, a modified version of the Strange Situation Test, used to measure attachment, was used to assess the chimpanzees' attachment style to their primary human caregiver. Based on these measures, chimpanzees reared in the standard care nursery (without the additional human contact) were significantly more likely to exhibit disorganized attachment styles towards their human caregiver; additionally, the standard care chimpanzees displayed less advanced motor and cognitive development. The responsive care chimpanzees (reared with the additional human contact) developed cognitively and in terms of motor function at a faster rate than the standard care chimpanzees; they also exhibited less coping skills. After one year of rearing in these two nurseries, all the chimpanzees were reared in conspecific social groups and the differential nurseries were terminated. In 2011-2012, 22 out of 49 of the original chimpanzee subjects were reassessed in an attempt to determine of long term effects of these differential rearing styles could still be detected. Chimpanzees that were identified as exhibiting disorganized attachment at one year of age exhibited significantly higher rates of abnormal behavior as compared to those that did not exhibit a disorganized attachment style at one year of age. Chimpanzees reared in both nurseries exhibited significantly higher rates of abnormal behavior, solicitation of, and attendance to humans as compared to chimpanzees that were mother-reared. Additionally, chimpanzees reared in either nursery were rated by survey respondents as exhibiting significantly higher human orientation and significantly lower subjective well-being as compared to mother-reared chimpanzees. Finally, trends found in the data consistently indicated that chimpanzees reared with more extensive human contact (responsive care) exhibited higher rates of abnormal behavior, solicitation of, and attendance toward humans as compared to standard care chimpanzees. Trends also indicated consistently that responsive care subjects were scored higher on human orientation and lower on subjective well-being by survey respondents. Sign tests were conducted to explore these differences and consistent support was found for these trends as significant. Further research should be conducted to explore welfare-related issues as related to differential nursery rearing strategies for chimpanzees.