Neurobehavioural integrity of chimpanzee newborns: comparisons across groups and across species reveal gene–environment interaction effects

The aims of this article are to describe the neurobehavioural integrity of chimpanzee newborns, to investigate how early experiences affect the neurobehavioural organization of chimpanzees, and to explore species differences by comparing chimpanzee newborns with a group of typically developing human newborns. Neurobehavioural integrity related to orientation, motor performance, arousal, and state regulation of 55 chimpanzee (raised in four different settings) and 42 human newborns was measured with the Neonatal Behavioral Assessment Scale (NBAS), a semistructured 25-min interactive assessment. Thirty-eight chimpanzees were tested every other day from birth, and the analyses revealed significant developmental changes in 19 of 27 NBAS scores. The cross-group and cross-species comparisons were conducted at 2 and 30 days of age. Among the four chimpanzee groups, significant differences were found in 23 of 24 NBAS scores. Surprisingly, the cross-species comparisons revealed that the human group was distinct in only 1 of the 25 NBAS scores (the human group had significantly less muscle tone than all the chimpanzee groups). The human group was indistinguishable from at least one of the chimpanzee groups in the remaining 24 of the 25 NBAS scores. The results of this study support the conclusion that the interplay between genes and environment, rather than genes alone or environment alone, accounts for phenotypic expressions of newborn neurobehavioural integrity in hominids.