The effects of four nursery rearing strategies on infant behavioral development in rhesus macaques (Macaca mulatta)

Abstract

Nursery rearing is the single most important risk factor in the development of severe forms of abnormal behavior, such as self-biting, in rhesus macaques. This practice is common in research laboratories and typically involves continuous pair housing of infants without maternal contact. We examined the effects of variation in peer socialization on the behavioral development of rhesus infants by exposing 32 newborn infants to 4 different socialization routines: continuously paired; intermittently paired; continuously paired rotationally (partners rotated within the group once a week); and intermittently paired rotationally. Analyses revealed that infants paired intermittently exhibited 'floating limb' and self-biting behavior at significantly higher frequencies than those reared by using any other strategy. Results also suggested that continuous pairing was most effective in reducing the development of abnormal behaviors (that is, self-bite and floating limb), whereas intermittent pairing significantly reduced partner clinging and geckering. A principal component analysis revealed that floating limb behavior and self-biting are strongly associated. Self-biting began as early as 32 d of age, and a negative binomial regression on data of floating limb and self-biting revealed that early development of floating limb behavior predicts self-biting behavior later in development. Despite the significant effects of rearing strategies on the frequency of abnormal behaviors, we note that animals in all 4 treatment groups developed these traits to some degree. We suspect that the solitary incubator environment may be a trigger for the development of abnormal behaviors.

Figure 1.

Ethogram of the behaviors reported in this study.

Figure 2.

Effects of rearing condition on the frequency of floating limb and self-biting behavior. Mean frequency was calculated over the entire study period. Different letters indicate groups that were significantly different from one another. Bar, SEM; CP, continuous pairing; CRP, continuous rotational pairing; IP, intermittent pairing; IRP, intermittent rotational pairing.

Figure 3.

Effects of sex and rearing condition on the frequency of social play behavior. Mean frequency was calculated over the entire study period. Different letters indicate groups that were significantly different from one another. Bar, SEM; CP, continuous pairing; CRP, continuous rotational pairing; IP, intermittent pairing; IRP, intermittent rotational pairing.

Figure 4.

Effects of sex and rearing condition on the frequency of partner clinging. Mean frequency was calculated over the entire study period. Different letters indicate groups that were significantly different from one another. Bar, SEM; CP, continuous pairing; CRP, continuous rotational pairing; IP, intermittent pairing; IRP, intermittent rotational pairing.

Figure 5.

Effects of sex and rearing condition on the frequency of gecker vocalizations. Mean frequency was calculated over the entire study period. Different letters indicate groups that were significantly different from one another. Bar, SEM; CP, continuous pairing; CRP, continuous rotational pairing; IP, intermittent pairing; IRP, intermittent rotational pairing.

Figure 6.

Principal component analysis showing that floating limb and self-biting are highly associated with each other, as are motor stereotypies and self-stimulation. Stereotypic rocking (rock) shows dissociation from other motor stereotypies. Comp, component.