What defines successful integration into a social group for hand-reared chimpanzee infants?

Abstract

Hand-rearing of captive great ape infants is sometimes necessary but can have negative behavioral consequences. Modern hand-rearing protocols, including early integration into a diverse group of conspecifics, appear to reduce the negative consequences of hand-rearing, but the process of integration is not well studied. We investigated six potential metrics of success during the introduction of two hand-reared chimpanzee infants into a troop of nine other chimpanzees at the Maryland Zoo in Baltimore. Little aggression was observed and the infants continue to be maintained in the troop at publication. As we predicted, during the introduction the hand-reared infants showed consistent levels of stress-related behaviors, participated in affiliative interactions with all available partners, and acted, received, and mutually engaged in these interactions. Solitary behaviors by these infants were similar to a mother-reared infant in the same group. Each infant also formed a relationship with a specific female that involved nest-sharing, carrying, retrieval, and intervening to reduce risk to the infant; these relationships could be classified as allomothering because they involved maternal behavior but occupied significantly less of the infants’ time than a maternal relationship. Contrary to our prediction, the hand-reared infants therefore spent significantly less time in social behavior than a mother-reared infant of the same age. In addition, the hand-reared infants continued to show strong social preferences for each other as introductions progressed and to direct a low but consistent number of nonfeeding social behaviors to humans. The successful introduction of hand-reared infants appeared to involve adding conspecific social relationships to the infants’ social repertoire, but not eliminating social interactions directed at humans.

Appendix: Hand-rearing and introduction process

During hand-rearing, both chimpanzees received 24 h/day responsive care (as in Bard 1996) from one or more of six MZiB staff members and 10 volunteer assistants, including daily protected contact (PC) socialization with the chimpanzee troop through mesh. Observations of PC interactions and a consideration of individual history led MZiB staff to begin integration by introducing Rozi and Asali to the two oldest females in the troop on 12 January 2006. While the adults were attentive to the infants, both infants were distressed, screaming and moving away from the adults for several hours. Over the next few days, the infants became more comfortable, eventually allowing themselves to be groomed and carried. From this point forward, the infants were treated like other members of the troop except that they were bottle-fed through the mesh. Jambo was born into this group (hereafter the nursery group) on 20 January 2006 and stayed with her mother.

Gradually, other MZiB chimpanzees were added to the nursery group. Two adolescent female chimps were added on 31 January 2006. The older adolescent, daughter of the oldest and alpha female of the troop, showed moderate interest in and behaved appropriately toward both infants, while the subordinate female showed inappropriate behaviors, often grabbing and carrying infants roughly. Because this created tension among others in the group, the subordinate female was removed at the end of the first day of introduction. The nursery group therefore consisted of two adult females, one adolescent female, and three infants.

On 28 March 2006, the alpha male was introduced to the nursery group during the day and separated overnight. After 2 weeks of daily introductions he was allowed to remain with the nursery group overnight. One week later, following an overnight altercation with an unknown aggressor that resulted in moderately serious injury to Rozi, the alpha male was removed from the nursery group. After this event, the nursery group was returned to six animals. Data collection began on these six animals on 5 July 2006.

The remaining three female chimpanzees were added to the nursery group on 3 and 5 September 2006. The youngest adolescent was introduced first, on 3 September, followed by the remaining two adolescents on 5 September. All females showed interest and behaved appropriately toward both infants. The last two females introduced (including the one removed from the nursery group in January 2006) each paired with an infant in a relationship that included play, defense, carrying and nest-sharing behaviors. After these introductions, the nursery group included a total of nine animals: two adult females, four juvenile females, and three infants.

Finally, on 19 February 2007, the alpha and subordinate males were added to the nursery group. No problems were observed. Both males interacted appropriately with the infants; the subordinate male frequently played with both infants. This completed the introduction with all 11 MZiB chimpanzees in one group.