Stephen Anderson and Dr. James Higley, Psychology
Introduction
Temperament is defined as a pattern of behaviors that remain stable and show continuity over time and across different contexts (Clarke & Boinski, 1995). These behavioral patterns can be observed early in life, and can serve as a predictor of future behaviors (Weinstein & Capitanio, 2008). Temperamental differences are largely a result of interaction effects between genetic and environmental factors. These interactions can be complex, and there is a great need to better understand the nature of those interactions (Clarke & Boinski, 1995).
The purpose of this project (which is still ongoing) is to better understand the features of early rearing environments that have a large impact on observed temperamental differences. Because temperament acts as a foundation for future behavior, it is important to understand the impact of early rearing environments on predicting future behavior.
Method
Researchers at the California National Primate Research Center have developed a BioBehavioral Assessment (BBA) used to establish temperament ratings in infant rhesus macaques (Gottlieb & Capitanio, 2013). The BBA is comprised of standardized behavioral and physiological tests that take place over the course of a 25-hour period. As a part of the BBA, infants were evaluated on 16 different temperamental traits. Results are compiled into four temperamental scales through factor analysis. These four temperamental scales have been named vigilance, nervousness, gentleness, and confidence.
Subjects (N=262) were reared in 18 different field cages that house up to 150 rhesus monkeys of varying age and sex. These field cages are intended to reflect a troop-like setting characteristic of wild rhesus monkeys. Temperament ratings from infants within specific cages were compared to infants from other cages in order to determine cage differences in temperament who were born in the same year. We analyzed data from 2010 due to the larger number of infants born in each cage during that year.
In addition to evaluating temperament among infants by cage, another set of analyses was performed using infants who were raised by a foster mother in order to determine fostering effects. Subjects (N=18) were separated from their mothers shortly after birth, and were raised by a foster mother. Infants were switched shortly after birth with another infant. Before the exchange, mothers were injected with a general anesthetic so they would not be aware of the exchange of infants. After recovering, mothers interacted with infants as if they were their own and raised them throughout their early lives.
Results
A multivariate ANOVA were used to determine whether the cage an infant was raised in could predict temperament ratings. Therefore, home cage was the independent variable, with temperament ratings being the dependent variables. Results showed that cage as a variable could predict temperament ratings for all four temperament scales (confidence, p<.01; vigilance, p<.01; gentleness, p<.01; nervousness, p<.01). The effects of cross-fostering were assessed by comparing mean temperament ratings for both fostered and infants who stayed with their mothers. A one-way ANOVA was used to generate results. We found that fostered infants as a group were more likely to display behaviors associated with vigilance and gentleness (p<.01 and p<.05 respectively) in comparison to all other infants born in 2010.
Discussion
The first step in trying to identify and understand environmental influences on temperament was to establish that temperament could be predicted based on which field cage an infant was raised in. While this analysis was necessary, our findings could not distinguish between whether genetic or environmental factors were the mechanism behind observed cage differences. Because of this, we also decided to evaluate monkeys who had been cross-fostered and raised in a different environment.
Our findings suggest that rhesus monkey infants who were raised by a foster mother differ behaviorally from other infants, though it is not clear what mechanism would drive the difference we observed. Something about the process of being raised in a different environment, away from a biological mother, may lead to predictable changes in behavior. This is especially interesting considering the supposition that neither the mother nor the infant is aware of the exchange.
While our results have some interesting implications, our findings need to be replicated using a better experimental cross-fostering design. Future research should also focus on identifying any potential mediators or moderators that could explain the behavioral differences seen between fostered animals and those raised by their biological mothers.
Conclusion
The goal of our research is to better understand the early environmental influences on aggression. We hoped this preliminary research would better prepare us to study aggressive behaviors. Currently, we have designed and begun implementation of the major portion of our research. We are evaluating older subjects who participated in the BBA as infants, and trying to identify whether their temperament ratings in infancy are good predictors of their current aggressiveness, and whether there are other environmental factors that are better predictors of current aggression.
References
- Clarke, A. S., & Boinski, S. (1995). Temperament in nonhuman primates. American Journal Of Primatology, 37(2), 103-125. doi:10.1002/ajp.1350370205
- Gottlieb, D. H., & Capitanio, J. P. (2013). Latent variables affecting behavioral response to the human intruder test in infant rhesus macaques (Macaca mulatta). American Journal Of Primatology, 75(4), 314-323. doi:10.1002/ajp.22107
- Weinstein, T. R., & Capitanio, J. P. (2008). Individual differences in infant temperament predict social relationships of yearling rhesus monkeys, macaca mulatta. Animal Behaviour, 76(2), 455-465. doi:10.1016/j.anbehav.2008.01.024